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This is to certify that the
thesis entitled

Tidal breathing flow—volume loop analysis as
a test of pulmonary function in the exercising horse

presented by

Bruce A. Connally

has been accepted towards fulfillment
of the requirements for

MS degree in Large Animal

Clinical Sciences

     

Major professor

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TIDAL BREATHING FLOW-VOLUME LOOP ANALYSIS AS A TEST OF
PULMONARY FUNCTION IN THE EXERCISING HORSE

By

Bruce A. Connally

A THESIS
Submitted to
Michigan State University
in partial fulﬁllment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Large Animal Clinical Sciences

1993

ABSTRACT

TIDAL BREATHING FLOW-VOLUME LOOP ANALYSIS AS A TEST OF
PULMONARY FUNCTION IN THE EXERCISING HORSE

By

Bruce A. Connally

The usefulness of tidal breathing ﬂow-volume loops to evaluate pulmonary
function was investigated in six Standardbred horses during treadmill exercise. Tidal
breathing ﬂow-volume loops (TBFVL) are a graphical representation of airﬂow rate
versus tidal volume for each individual breath. These TBFVL were obtained from horses
exercising at speeds corresponding to 75% of maximum heart rate and maximum heart
rate. Measurements were recorded in each horse before and after ovalbumin-induced
allergic lung disease. Moderate obstructive lung disease, characterized by a significant
increase in pulmonary resistance and a decrease in dynamic compliance, was observed
while the horses were at rest. We found that in horses with airway obstruction
exercising at 75 % or 100% of maximum heart rate, the quantitative indices describing
TBFVL shape and size were not markedly different from normal horses exercising at

similar speeds.

I dedicate this thesis to Pat, Clint, and Matt.
It is their support and their love that has given me the strength
to make this change in my career.

iii

 

ACKNOWLEDGMENTS

I wish to express my sincere appreciation to Dr. Fred Derksen, my major adviser,
for his guidance during the course of this study. I also wish to thank the members of my
committee, Dr. Ed Robinson, Dr. Bob Bowker, and Dr. Judy Marteniuk, for their
constructive criticism and editorial assistance in the preparation of this thesis. A special
thanks is extended to Cathy Berney for technical assistance and to Vicki Kingsbury for

thesis preparation.

iv

TABLE OF CONTENTS

LIST OF FIGURES ...................................... vii
LIST OF TABLES ...................................... viii
LIST OF ABBREVIATIONS ................................ 1x
INTRODUCTION ....................................... 1
CHAPTER 1: LITERATURE REVIEW ......................... 3
The maximum expiratory ﬂow-volume curve ................... 3

The equal pressure point ............................. 6

Wave speed limitation of ﬂow .......................... 7

Viscous limitation of ﬂow ............................ 9
Analysis of the ﬂow-volume curve ....................... 9

Clinical usefulness of the MEFV curve .................... 11
Maximum expiratory ﬂow-volume curves during exercise ......... 13

Tidal breathing ﬂow—volume loops .......................... 14
Clinical exercise testing ................................ 19
Exercise-induced pulmonary hemorrhage ...................... 23
Pathologic changes ................................. 25
Pathogenesis ..................................... 26
Treatment ...................................... 31
Conclusion ...................................... 33

CHAPTER II: TIDAL BREATHING FLOW-VOLUME ANALYSIS
AS A TEST OF PULMONARY FUNCTION

IN THE EXERCISING HORSE .............................. 35
Materials and methods ................................. 35
Sensitization of horses ............................... 35
Induction of disease ................................ 36
Pulmonary function measurements ....................... 36

Rapid incremental exercise test ......................... 37

Tidal breathing ﬂow-volume loops ....................... 37
Experimental protocol ............................... 38
Statistical analysis ................................. 39

vi

Results ........................................... 39
Pulmonary function tests ............................. 39
Flow-volume loops ................................ 43
Heart rate ...................................... 52
Endoscopic examination ............................. 54

Discussion ........................................ 54

SUMMARY AND CONCLUSIONS ........................... 58
REFERENCES ........................................ 59

 

 

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

LIST OF FIGURES

A maximum expiratory ﬂow-volume (MEFV) curve from a normal
subject (left). Values that compose this curve represent maximum
ﬂow at corresponding volumes. At right are three representative
isovolume pressure-ﬂow curves from the same subject. Curves A,
B, and C were measured at the volumes represented by corre-
sponding points on the MEFV curve. Transpulmonary pressure is
the difference between pleural pressure and mouth pressure (from:
Hyatt, 1986).

Pulmonary resistance (RI) (mean 1 SEM) measured before (base-
line) and after (5 hours and 1, 3, 5, 7, 14, and 21 days) aerosol
ovalbumin administration. * = signiﬁcant difference (p < 0.05).

Tidal volume (VT), respiratory frequency (f), and minute ventila-
tion (V II;) (mean j: SEM) measured before (baseline) and after (5
hours and 1, 3, 5, 7, 14, and 21 days) aerosol ovalbumin adminis-
tration. * = signiﬁcant difference (p < 0.05).

Representative ﬂow-volume loops measured before (solid line) and
after (broken line) aerosol ovalbumin administration. The upper
loops are measured at 75 % of HR“. The broken line is ﬁve
hours after ovalbumin administration. The lower loops are
measured at HR“. The broken line is one day after ovalbumin
administration.

Heart rate (mean ;t SEM) measured at treadmill speeds corre-
sponding to 75 % HR“ before (baseline) and after (5 hours and 1,
3, 5, 7, 14, and 21 days) aerosol ovalbumin administration. * =
signiﬁcant difference (p < 0.05).

vii

41

42

51

53

Table 1

Table 2

Table 3

Table 4

Table 5

Table 6

Table 7

Table 8

LIST OF TABLES

Pulmonary resistance (RI) and dynamic compliance (Cm) before
(baseline) and after (5 hours, and l, 3, 5, 7, 14, and 21 days)
ovalbumin administration (mean i SEM).

Rate and timing of the respiratory cycle measured at speeds
corresponding to 75 % of HR“, before (baseline) and after (5
hours, and 1, 3, 5, 7, 14, and 21 days) aerosol ovalbumin
administration.

Airﬂow indices taken from ﬂow-volume loops measured at speeds
corresponding to 75 % of HR1m before (baseline) and after (5
hours, and 1, 3, 5, 7, 14, and 21 days) aerosol ovalbumin
administration.

Ratios of ﬂow indices derived from ﬂow—volume loops measured
at speeds corresponding to 75 % of HR,m before (baseline) and
after (5 hours, and l, 3, 5, 7, 14, and 21 days) aerosol ovalbumin
administration.

Rate and timing of the respiratory cycle measured at spwds
corresponding to 100% of HR“, before (baseline) and after (1, 3,
5, 7, 14, and 21 days) aerosol ovalbumin administration.

Airﬂow indices taken from ﬂow-volume loops measured at spwds
corresponding to 100% of HR“, before (baseline) and after (1, 3,
5, 7, 14, and 21 days) aerosol ovalbumin administration.

Ratios of ﬂow indices derived from ﬂow-volume loops measured
at speeds corresponding to 100% of HR“, before (baseline) and
after (1, 3, 5, 7, 14, and 21 days) aerosol ovalbumin adminis-
tration.

Flow-volume loop variables measured at speeds corresponding to
75% (75% HR“) and 100% (100% Hle) of maximum heart
rate, which changed signiﬁcantly after ovalbumin administration
(mean -l_- SEM).

viii

40

45

46

47

48

49

50

COPD

EELV

MEFV

MEFVL

LIST OF ABBREVIATIONS

dynamic compliance

chronic obstructive pulmonary disease
end expiratory lung volume

expiratory ﬂow at 12.5 % tidal volume
expiratory ﬂow at 25 % tidal volume
expiratory ﬂow at 50% tidal volume
exercise-induced pulmonary hemorrhage
respiratory frequency

forced vital capacity

heart rate

maximum heart rate

inspiratory ﬂow at 12.5 % tidal volume
inspiratory ﬂow at 25 % tidal volume
inspiratory ﬂow at 50% tidal volume
isovolume pressure-ﬂow curves
maximum expiratory ﬂow-volume
maximum expiratory ﬂow-volume loops

maximal voluntary ventilation

x
peak inspiratory ﬂow
pulmonary resistance
tidal breathing ﬂow-volume loops
expiratory time
inspiratory time
expiratory volume
minute ventilation
inspiratory volume
maximal oxygen uptake
tidal volume

work rate

INTRODUCTION

Respiratory abnormalities have been recognized as a common cause of poor
performance in horses (Morris and Seeherman, 1991; Sweeney, 1991; MacNamara et a1. ,
1990). Exercise-induced pulmonary hemorrhage (EIPI-I) and chronic obstructive
pulmonary disease (COPD) are examples of two speciﬁc conditions that have been
investigated and are characterized by airway obstruction (Sweeney, 1991; MacNamara
et al., 1990; O’Callaghan et al., 1987e). Pulmonary function tests currently in use to
evaluate airway caliber include the measurement of pulmonary resistance (RL) and
dynamic compliance (Cdyn) in the standing horse (Derksen et al. , 1982a). Because the
measurement of RL and Cdyn is insensitive and only detects severe airway obstruction,
the usefulness of these tests in the detection of potentially restricting lung disease is
limited.

Maximum expiratory ﬂow-volume loops (MEFVL) were ﬁrst described by Hyatt
et al. in 1958, and have proven to be the most sensitive test of pulmonary function in
human medicine (Bass, 1973; Knudson et a1. , 1976). A subject must inhale to total lung
capacity and subsequently exhale forcefully to residual volume to generate MEFVL. The
technique is noninvasive and sensitive but requires patient cooperation to produce a

maximal respiratory effort.

2
Tidal breathing ﬂow-volume loops (TBFVL) have been used to evaluate airway

caliber in dogs, horses, and human infants who were unable or unwilling to perform the
forced vital capacity necessary to generate MEFVL (Amis and Kurpershoek, 1986;
Petsche et al., in preparation; Godfrey et a1. , 1983). Because tidal breathing airﬂow and
pressure changes in the airway are small, this technique is less sensitive in the detection
of airway obstruction than the analysis of MEFVL.

I hypothesize that TBFVL obtained during exercise may be more sensitive in the
detection of airway obstruction due to the greater airﬂow rates achieved with exercise
(Belknap et a1. , 1989). The objective of this study was to evaluate TBFVL generated
during exercise as a measure of pulmonary function in normal horses and in horses with

induced airway obstruction.

CHAPTER I

LITERATURE REVIEW

The maximum expiratory ﬂow-volume curve
Forced expiration to assess pulmonary function is common in human medicine.
The maneuver is called a forced vital capacity (FVC). The patient is required to inspire
maximally, then exhale as rapidly and completely as possible. Measurement of ﬂow and
volume of air during this maneuver can provide information about expiratory airﬂow
limitation. The FVC maneuver is a useful measurement of pulmonary function because
of the effort independence of maximal expiratory ﬂow at all but near-maximal lung
volumes (Hyatt et a1. , 195 8). The relationship between pressure and ﬂow at a given lung
volume are shown in Figure 1 (Hyatt, 1986). The measurements were recorded with the
subject in a body plethysmograph to record lung volumes.
The three curves at the right of Figure l are isovolume pressure-ﬂow curves
(IVPF) from a normal subject. The curve on the left shows a maximum expiratory ﬂow-
volume (MEFV) curve. This curve can be constructed by plotting the maximal ﬂows
from the IVPF curves against the lung volumes at which they are measured. The
expiratory ﬂow at point A is measured at a point of high lung inﬂation. This ﬂow is
directly related to increases in transpulmonary pressures. Thus expiratory ﬂow near total

lung capacity is dependent on the subject’s effort.

4
At points B and C on the MEFV curve, the degree of lung inﬂation has

decreased. The IVPF curves that correspond to these points show that ﬂow increases
with pressure until a maximal ﬂow is reached. At that point further increases in pressure
do not result in any signiﬁcant increases in ﬂow. Over the volume range corresponding
to approximately the lower 80 % of the vital capacity, the IVPF maxima do not require
the subject to exert maximum effort but cannot be exceeded with maximum effort,
resulting in measurements that are quite reproducible at these volumes. As can be seen,
the maximum ﬂow is a function of lung volume. As such it becomes essential to specify
the lung volume in relation to maximum ﬂow. A three-dimensional graphic representa-
tion of pressure, volume, and ﬂow has been constructed by Fry and Hyatt (1960). The
surface of this graph can be used to analyze the FVC but is difﬁcult to construct and

complex to interpret.

Expir. flow (Us) A Expir. ilow (.l/S)

 

 

 

 

 

 

i I I I (*lF/HIIIII Ilrti')

 

o 1 2 3 4 603025201510'505101520
Volume (liters from TLC) Transpulmonary pressure
(cm H20)

Figure 1. A maximum expiratory ﬂow-volume (MEFV) curve from a normal subject
(left). Values that compose this curve represent maximum ﬂow at corresponding
volumes. At right are three representative isovolume pressure-ﬂow curves from the same
subject. Curves A, B, and C were measured at the volumes represented by correspond-
ing points on the MEFV curve. Transpulmonary pressure is the difference between
pleural pressure and mouth pressure (from: Hyatt, 1986).

The equal pressure point

The mechanism of ﬂow limitation during the MEFV maneuver has been studied
extensively. The concept of an equal pressure point was introduced by Mead et al. in
1967. This theory stated that once ﬂow was limited at a particular lung volume, there
was an area in the intrathoracic airway where intrapleural pressure was equal to
intrabronchial pressure. The elastic recoil pressure of the lung was the driving pressure
from the alveolus to this equal pressure point. Airways downstream from this equal
pressure point would be compressed because intrabronchial pressure would be less than
intrapleural pressures, resulting in a narrowing of the airway.

This model of ﬂow limitation is based on the principle of a Starling resistor. This
device is a compressible tube passing through a chamber with adjustable pressure.
Pressure differences between the input side of the collapsible tube and the chamber
pressure determines ﬂow, not the pressure drop between the input and output ends of the
tube itself. This pressure-ﬂow relationship is similar to the ﬂow of water over a
waterfall. Flow is determined by the pressure drop between the head of the river and
the edge of the waterfall. The downstream segment can have no effect on the rate of
ﬂow. Pride et al. (1967) used this principle to describe ﬂow limitation in the lung as a
number of Starling resistors connected in parallel. If the critical transmural pressure
applied over the collapsible airway is considered to be zero, this model is the same as

the equal pressure point concept.

Wave speed limitation of flow

The analyses based on an equal pressure point or the Starling resistor concept
does not completely explain ﬂow limitation mechanisms. Dawson and Elliott (1977)
concluded that the lung must obey the laws of ﬂuid mechanics and thus cannot carry a
greater ﬂow than the ﬂow for which the ﬂuid velocity equals wave spwd at some point
in the system. This wave speed is the speed at which a disturbance travels in a compliant
tube ﬁlled with ﬂuid. The wave speed (c) in a compliant tube with an area (A) that

depends on a lateral pressure (P), ﬁlled with a ﬂuid of density (p), is given by:

AdP)-:'

 

where dP/dA is the slope of the pressure-area curve for the airway. Maximal ﬂow
(me) is equal to ﬂuid velocity at wave speed (c) multiplied by airway area (A). The

resulting equation for maximal ﬂow is:

AdP)%

 

The ﬂow-limiting site or choke point occurs where the ﬂuid velocity reaches the value

of the local wave speed. Even this is not completely predictive because airway area,

8

compliance, and wave speed are functions of transmural pressures. Therefore the
maximal ﬂow that an airway can carry is a function of lateral pressure within the airway.
A more intuitive explanation of wave speed limitation has been provided by Hyatt

(1986):

Essentially one wishes to understand how the wave-speed mechanism
produces plateaus on IVPF curves. The critical point to keep in mind is
that on the plateau, ﬂow is independent of the downstream pressure; this
is the so-called ”waterfall" or Starling resistor effect. In the isovolume
condition at low ﬂows, wave speed is high because the airways show little
compression (area large) and are on the ﬂat part of their pressure-area
characteristic (they are stiff). A drop in pressure at the outlet will be
transmitted upstream at a speed that is diminished only slightly by the
countering effect of airﬂow velocity. As flow increases, the speed at
which a downstream pressure disturbance (a drop in pressure) can be
transmitted upstream will fall because 1) wave speed is decreasing and 2)
the opposing effect of the airﬂow velocity is increasing. When the speed
(predicted by the wave-speed equation) no longer exceeds the speed of
expiratory ﬂow, the pressure disturbance can no longer propagate
upstream and affect the ﬂow. In essence the downstream pressure change
is no longer seen by the upstream driving pressure and ﬂow becomes
independent of, or uncoupled from, downstream pressure. When this

occurs, the system is ﬂow limited and the waterfall phenomenon and a

9

choke point develop. Further decreases in downstream pressure only
compress the airways downstream of the choke point and, of course, have

no effect on the ﬂow.

The concept of ﬂow limitation at wave speed is related to the equal pressure point
concept described by Mead et a1. (1967). The location of the choke point caused by
wave speed limitation is anatomically very close to the equal pressure point (Dawson and
Elliott, 1977). Dynamic airway collapse is a local phenomenon due to a pressure drop
caused by wave speed limitation. The airway segment upstream from the choke point
remains ﬁxed during me and the cross sectional area of the airways remains constant.
Downstream from the choke point there is a large pressure drop where energy is

dissipated due to the inability of ﬂow to increase.

Viscous limitation of ﬂow

The wave speed ﬂow limitation model is not as useful at low lung volumes
(Dawson and Elliott, 1977). If airway diameters and ﬂows are small, the limiting ﬂow
is essentially established by the coupling between ﬂow limitations due to ﬂuid viscosity

and tube compliance before limitation at wave speed becomes signiﬁcant.

Analysis of the ﬂow-volume curve
Flow-volume curves are usually quantiﬁed by measuring Vmax at various volumes
in the vital capacity. Examples are 75 % , 50% , and 25 % of vital capacity remaining to

be exhaled. These curves are not based on total lung capacity of the subject and are thus

10
termed free-ﬂoating curves (Hyatt et al. , 1979). Because of the difﬁculty in measuring

total lung capacity, the free-ﬂoating curves are most commonly used.

Deﬁning normal values for MEFV curves has remained difﬁcult. Variations due
to age, sex, size, and race have been reported by Knudson et al. (1976). Intersubject
variability may also reﬂect differences in mechanical properties of the lung. Green et
al. (1974) reported that lung static recoil contributed little to the variability between
individuals and concluded that the major variability in maximum ﬂows was attributable
to the airways. Green et al. (1974) argued that the most important airways are
intrathoracic trachea and the large bronchi, and reported substantial between-individual
differences in airway size and function that were independent of lung and parenchymal
size. These differences may be due to disproportionate but physiologically normal
growth within the organ. This phenomenon has been termed dysanaptic growth. It was
also suggested that this variation in airway-parenchymal relationships between apparently
healthy individuals might have an inﬂuence on the pathogenesis of airway disease.

When volume measurements are taken at the mouth, gas compression may also
result in distortions that do not actually reﬂect lung volume changes. Ingram and
Schilder (1966) have reported pressures of greater than 100 cm H20 that may result in
gas compression values of over 10% .

Another difﬁculty in interpretation is presented when repeated MEFV curves on
the same subject result in moderate variation in VC, peak ﬂows, and slope. Peslin et a1.
(1979) evaluated eight methods for comparing successive MEFV curves. They concluded

that the most reproducible and unbiased data can be obtained from composite curves

11
obtained by superimposing the breaths either at total lung capacity or along the

descending limb.

Clinical usefulness of the MEFV curve

Hyatt and Black (1973) state that the MEFV curve is a more valid and reliable
way of studying expiratory ﬂow events than is the spirogram, which plots ﬂow against
time during a FVC maneuver. The MEFV curve provides direct visualization of the
reduction in Vm at all lung volumes, a value which is not as directly visualized on the
volume-time plot of the spirograph. Hyatt and Black (1973) also describe reduction in
Vmu as the major mechanical expression of the obstructive process.

It is easy to inspect a series of MEFV curves to evaluate their general reproduci-
bility. If there is doubt about a patient exerting sufﬁcient effort to achieve Vmu, an
esophageal balloon can be placed to determine if adequate pressures are being produced.
If a subject is unable to produce a continuously forced expiratory effort due to lack of
coordination or comprehension, an approximation of the MEFV curve may be attempted
by superimposing repeated submaximal efforts at various levels of lung inﬂation (Hyatt
and Black, 1973). Another means of deﬁning MEFV curves in the noncompliant patient
is to record a series of coughs from which the MEFV curve can be constructed (Hyatt
and Black, 1973).

Maximum expiratory ﬂow-volume curves have been used to evaluate patients with
asthma (Bass, 1973; Chan-Yeung, 1973; Jordanoglou and Pride, 1968). Jordanoglou and
Pride ( 1968) were able to differentiate asthma from severe ﬁbrosis of the lung but were

unable to distinguish asthma from bullous emphysema. Maximum expiratory ﬂow at

12
50% of vital capacity was decreased in both asthma and bullous emphysema. Chan-

Yeung (1973) also demonstrated a signiﬁcant decrease in maximal expiratory ﬂows at
50% of vital capacity and also at 60% of total lung capacity, even when patients were
asymptomatic between asthmatic episodes. In three of nineteen subjects, asthmatic
reactions were only detected by these measurements.

Bass (1973) was more successful in using MEFV curves to differentiate between
clinical conditions. He reported patterns of abnormality for bronchial asthma as well as
COPD, chronic bronchitis, and pulmonary emphysema. Flow limitation in large airways
reduced the maximum expiratory ﬂow at all lung volumes. Flow limitation in medium-
sized airways decreased maximum expiratory ﬂow rates primarily in the mid portion of
the MEFV curve, while ﬂow limitation in small airways reduced maximum expiratory
ﬂow at low lung volumes. Chronic bronchitis and bronchial asthma had similar
reductions of ﬂow in the mid portion of the MEFV curve. Differentiation of these two
conditions was possible following isoproterenol administration. Only in patients with
bronchial asthma did values revert toward normal. Pulmonary emphysema patients had
more severe expiratory ﬂow limitation than patients with either chronic bronchitis or
bronchial asthma.

Hyatt and Black (1973) used MEFV curves to evaluate the effect of bronchodila—
tor drugs. Maximum expiratory ﬂow was compared at the same lung volume on curves
recorded before and after therapy. If total lung capacity was measured, the curves were
aligned in terms of absolute volume. If total lung capacity had not been measured,
alignment was done at points of maximum inspiration. Response to therapy was readily

visualized with this approach.

13

Maximum expiratory ﬂow-volume curves during exercise

Attempts have been made to measure MEFV curves during or immediately
following exercise in normal subjects (Olafsson and Hyatt, 1969; Stubbing et al. , 1980)
and in subjects with obstructive lung disease (Potter et al., 1971; Stubbing et al., 1980).
Olafsson and Hyatt (1969) studied the interrelationships among transpulmonary pressure,
ﬂow, and volume during exhausting exercise. They then compared expiratory
transpulmonary pressures during exercise with ﬂow-limiting pressures measured at rest.
No relationship was found between the magnitude of transpulmonary pressure and
exercise limitation. Also, transpulmonary pressures in excess of the pressures necessary
to achieve maximal airﬂow were not observed in these normal subjects.

Younes and Kivinen (1984) added support to the ﬂow limitation concept by
studying lung elastic recoil and inspiratory muscle fatigue at maximal exercise in normal
subjects. They concluded that elastic recoil of the lung does not change with maximal
exercise and that the measured decrease in Cd,“ of the lung was not observed except at
high levels of exercise. At near-maximal levels of exercise, these subjects’ end-
inspiratory volume was within 1.2 L of total lung capacity. Younes and Kivinen (1984)
further state that this greater degree of lung inﬂation makes the lung much stiffer than
it would be at a lesser lung volume.

Stubbing et al. (1980a) evaluated pulmonary mechanics during exercise by
adapting a cycle ergonometer to a whole body plethysmograph. They were unable to
document a change in MEFV curves of healthy subjects during exercise. The ﬂow rates
during tidal breathing at exercise did not exceed the MEFV curve. Like the ﬁndings of

Olafsson and Hyatt (1969), even moderately severe exercise did not generate pressures

14

greater than those required to produce maximum ﬂow. Stubbing et al. (1980a) also
measured a decrease in static compliance, indicating a change in the elastic recoil
properties of the lung. It was suggested that this reduction in static compliance may be
due to the increase in pulmonary capillary blood volume during exercise, resulting in a
less pliable lung.

Maximum expiratory ﬂow-volume curves have also been recorded during
(Stubbing et al., 1980b), and immediately after exhausting exercise (Potter et al., 1971)
in subjects with airﬂow limitation. Potter et a1. (1971) found that the expiratory pressure
during exercise in patients with obstructive lung disease exceeded ﬂow-limiting pressure
measured at rest, and Stubbing et al. (1980b) found pressures to increase with exercise
work load, but the MEFV curves were not altered signiﬁcantly by these increased

pressures.

Tidal breathing ﬂow-volume loops

Some subjects, such as animals or human infants, cannot be induced to perform
a forced vital capacity maneuver. In these subjects MEFV loops obviously are not
applicable. Partial expiratory ﬂow-volume maneuvers using various techniques have
been performed in an attempt to quantify ventilatory mechanics in these subjects.
Godfrey et al. (1983) utilized a technique for thoracic compression to generate partial
expiratory ﬂow-volume loops in human infants with lung disease. They compared the
partial expiratory ﬂow-volume loops with airway resistance and lung volume measure-
ments obtained from whole body plethysmography, and the effects of inhaling a

helium/oxygen gas mixture to partition the airway obstruction between large and small

15

airways. In several instances they were able to detect small airway obstruction without
changes in airway resistance or total lung capacity.

Another technique for obtaining ﬂow-volume loops in infants was utilized by Wise
et al. (1980). A crying vital capacity maneuver was performed by " gently stimulating
the infant to cry into a snugly-ﬁtting rubber-rimmed face mask." Infants with hyaline
membrane disease had reduced "crying vital capacity" when compared to normal infants.
The authors suggested that the low expiratory ﬂows may occur due to 1) loss of lung
volume and diminished recoil pressures; 2) increased resistance at the vocal cords due
to vocal tone during crying; and 3) partial collapse of major intrathoracic airways due
to elevated pressures developed during crying.

Tidal breathing ﬂow-volume loops analysis has been performed on dogs (Amis
and Kurpershoek, 1986) and horses (Petsche et al. , in preparation) at rest to evaluate
airway obstruction. Because these TBFVL are effort dependent, absolute values for ﬂow
and volume as well as breathing pattern used are difﬁcult to standardize. Abnormalities
of TBFVL associated with ﬂow limitation might not become apparent until pulmonary
ﬂow reserves are dramatically reduced (Amis and Kurpershoek, 1986). In this case the
animal may be approaching the MEFV curve during tidal breathing.

Amis and Kurpershoek (1986) reported that absolute values for ﬂow and volume
in dogs were affected greatly by body size and amount of effort. They concentrated on
the shape of the TBFVL to eliminate body size differences. The measurements proved
to be insensitive due to the large variability in results obtained.

Petsche et al. (in preparation) used TBFVL to examine horses with recurrent

airway obstruction (heaves). Horses with severe airway obstruction, as demonstrated by

16

increase in RL and decrease in Cdyn of the lung, had increased peak expiratory ﬂow early
in expiration compared to normal horses. Horses with obstructive lung disease had
TBFVL that differed from normal TBFVL by the absence of a biphasic pattern on
inspiration or expiration, even when the obstruction did not result in an increase in RL
or decrease in Cdyn'

Tidal breathing ﬂow-volume loops have also been measured during exercise in
humans and animals. Stubbing et al. (1980a) measured TBFVL during exercise in
normal adult males and compared the results to MEFV curves obtained when the subject
had reached a steady state at each work load increment. The tidal breathing expiratory
ﬂow rates measured during exercise did not exceed the MEFV curve. In two of twelve
subjects the ﬂow during tidal breathing at the highest work load reached the MEFV
curve.

Art and Lekeux (1988) studied exercise-induced changes in equine breathing
pattern by analyzing TBFVL in ponies at rest and during a standardized exercise. They
found that rest and exercise TBFVL varied between ponies but remained relatively
constant for a given animal. Ratios relating peak expiratory ﬂow to peak inspiratory
ﬂow, and expiratory ﬂow at 50% and 25 % of tidal volume to inspiratory ﬂow at the
same intervals were signiﬁcantly increased. The other indices did not change
signiﬁcantly except for peak expiratory ﬂow/expiratory ﬂow at 50% of tidal volume,
which was decreased.

Lumsden et al. compared the TBFVL at rest and during strenuous exercise for

evaluation of upper airway obstruction in the Standardbred horse. They found signiﬁcant

17

inspiratory ﬂow limitation during exercise using TBFVL. The same loops were found
to produce no useful information when the horses were at rest.

The use of TBFVL during exercise to evaluate obstructive lung disease has been
more limited than the use of MEFV curves. Potter et al. (1971) compared TBFVL
measured while individuals were exercising on a treadmill to MEFV curves measured
before exercise. In normal subjects the exercise TBFVL rarely impinged on maximum
inspiratory or expiratory ﬂow-volume curves except at low volumes. Patients with
obstructive lung disease reached maximal expiratory ﬂow predicted from their MEFV
curves before heart rate (HR) was maximal, prompting the authors to conclude that
exercise capacity was limited by ventilation.

Stubbing et al. (1980b) also compared TBFVL during exercise to MEFV curves
in patients with chronic airﬂow obstruction. They had the patients exercise within a
whole body plethysmograph to facilitate measurement of vital capacity, MEFV curves
and TBFVL at each exercise increment. The MEFV curves did not change as a result
of exercise, and tidal breathing ﬂows equalled but did not exceed the maximal ﬂows at
similar lung volumes.

Pollmann and Homicke (1987) evaluated TBFVL during exercise in horses with
reduced performance due to pulmonary emphysema or bronchitis. Tidal volume and
inspiratory ﬂow rates were increased in horses with obstructive lung disease compared
to healthy horses. The authors concluded that respiratory airﬂow of horses with COPD
differs from that described for human subjects with obstructive pulmonary disease.

It is possible that the increased ﬂow rates found by Pollmann and Homicke (1987)

may not be physiologic, but may be a function of the placement of the TBFVL on the

18
volume axis. Potter et al. (1971) and Stubbing et al. (1980b) both were able to report

an increase in end expiratory lung volume (EELV) in their patients during exercise.

Stubbing et al. (1980b) states:

The MEFV curve deﬁnes the limits of ﬂow at a given lung volume
and subjects who breathe along the same FV curve at rest as during
maximal expiratory efforts should only be able to increase their total
ventilation by either increasing inspiratory ﬂow, allowing more time for
expiration, or by increasing the end-expiratory volume, allowing greater

expiratory ﬂow.

The regulation of EELV during exercise has been evaluated in healthy subjects
by Henke et al. (1988). They were able to demonstrate a reduction in EELV with
increasing exercise intensity. This reduced EELV during exercise was thought to aid
inspiration by optimizing diaphragmatic length and permitting elastic recoil of the chest
wall. Babb et al. (1991) also demonstrated this exercise-associated reduction in EELV
in healthy subjects but found that patients with mild-to-moderate airﬂow limitation
responded quite differently. As predicted by Stubbing et al. (1980b), Babb et al. (1991)
found that patients with airﬂow limitation actually increased EELV to 58 % of total lung
capacity, whereas controls decreased EELV to 45 % of total lung capacity. They
concluded that the increase in EELV was due to the effects of airﬂow limitation on the
ventilatory response to exercise. By moving the TBFVL along the volume axis to utilize

higher lung volumes, greater ﬂows could be achieved before being limited by the MEFV

19
curve. It is possible that this adjustment of EELV by a ﬂow-limited patient could

account for the increased ﬂows reported by Pollmann and Homicke (1987) in horses with

COPD.

Clinical exercise testing

Exercise testing is currently used in human athletes for ﬁtness evaluation. It also
enables simultaneous evaluation of the cardiovascular and respiratory systems. The
severity of a disease and its response to therapy can be monitored by measuring aerobic
capacity, anaerobic threshold, ventilatory response, and hypoxemia. The scope of this
review is, of necessity, limited to the ventilatory aspects of clinical exercise testing.

The exercise testing protocol may be performed by incremental increases in work
load to reach a maximum work rate (WR) in a relatively short period of time or by
maintaining a constant WR for a prolonged time period. The constant WR allows the
investigator to measure physiological responses of speciﬁc organ systems to the stresses
of exercise but is not as easily applied to the clinical setting. Wassermam et al. (1987)
states three main reasons for the use of the incremental exercise testing protocol. First,
it does not require a sudden large cardio—respiratory stress because the testing begins at
relatively low WRs. Second, the test requires a relatively short period of time during
which the subject is stressed at high WRs. Finally, a relationship may be determined or
estimated between maximal oxygen uptake and WR for each subject.

Equipment used may be as simple as a stairway to provide exercise and equipment
for data collection during intermittent stops. A treadmill or cycle ergonometer provides

a more controlled exercise stress and allows continuous monitoring of data. The severity

20

of the exercise stress is increased incrementally until the patient reaches his/ her aerobic
capacity.

Many different parameters may be measured to evaluate maximal oxygen uptake
(VOme), anaerobic threshold, and ventilatory response to incremental exercise. The
electrocardiogram (ECG) can be used to monitor the myocardial oxygen availability-
requirement balance.

At any level of training the body has an upper limit for oxygen utilization, which
is determined by cardiac output, oxygen extraction by the tissues, or by having reached
a ventilatory limit. Wasserman et al. (1987) state that VOZmax can be determined by
incremental exercise testing when a plateau occurs in oxygen uptake despite further
increases in WR. Thus VOZmax is a reproducible measurement of aerobic capacity,
which is deﬁned by Weber et al. (1988) as a change in oxygen uptake of <1 ml/min/kg
sustained for a minimum of thirty seconds into the next stage of incremental treadmill
work, but preferably for a full stage or two minutes. By plotting oxygen uptake against
external work performed, information concerning the coupling of internal and external
respiration can be ascertained.

The anaerobic threshold is deﬁned by Wasserman et al. (1987) as the level of
exercise V02 above which aerobic energy production is not adequate and is supplemented
by anaerobic mechanisms. The result is an increased blood lactate and lactate/pyruvate
ratio. This can also be monitored by measuring the rate of increase in VC02 relative to
V02 during the incremental exercise test. Weber et al. (1988) have suggested use of the
anaerobic threshhold as a means of evaluating whether patients can be encouraged to

reach additional increments of exercise stress to achieve VOZmax.

21

The ventilatory response to incremental exercise testing can also be measured and
is of primary interest to this author. Oxygen delivery during exercise cannot be
maintained without increases in cardiac output and minute ventilation (V E). During
aerobic work VE rises in proportion to both V02 and VCOZ. During anaerobic work
buffering of the lactic acid produces more C02, which must be removed by ventilation.

Weber et al. (1988) has stated that in normal human subjects the maximal VB at
VOZWx is less than ﬁfty percent of the maximal voluntary ventilation (MVV). The
MVV is usually measured over a 12- or 15-second time period and is motivation and
effort dependent. The difference between the MW and maximum VE during exercise
is used as a measure of the ventilatory or breathing reserve (Weber et al., 1988). The
increase in VB is accomplished by increases in tidal volume and breathing frequency.

With the advent of high-speed treadmills, clinical exercise testing has recently
become a practical technique for use in horses. While this technique has not yet been
utilized to determine ﬁtness of athletes, studies have been undertaken to establish normal
values of some physiological parameters (Evans and Rose, 1983; Evans and Rose, 1988;
Rose et al., 1990).

The exercise testing used a protocol of incrementally increased work load
performed on a six degree-inclined motorized treadmill. Oxygen uptake (V 02), HR, VB,

and cardiac output were measured. Work rate was calculated using the formula:

WR (watts) = velmity (m/min) x bgﬁy weight (kg) X sin (treadmill angle)
6.12

22
Evans and Rose (1983, 1988) reported linear relationships between V02 and WR,

HR, VB, and cardiac output in detrained Standardbred and Thoroughbred horses. The
VOZmax increased signiﬁcantly with training. There was a linear relationship between
the percentage of VOZmx and the percentage of I-IR1mm but no relationship between
VOZmax and I-IRmax was established.

Lumsden et al. utilized a variation of this clinical exercise testing protocol to
estimate the relationship between HR and treadmill speed for individual horses. This
rapid incremental exercise test (RIET) was performed on a three-degree-inclined
treadmill. Speed was increased at sixty-second intervals and HR was recorded in the
last 15 seconds of each exercise period. Maximum HR and calculated 75 % of HRmax
for each individual horse were then used to calculate a treadmill speed for each animal
that produced a standardized WR to evaluate TBFVLs.

The continued development of more sensitive techniques to evaluate respiratory
function in the horse is necessary. Bayly and Slocombe (1991) state that poor athletic
performance in horses is often linked to abnormal respiratory function. Many affected
animals are asymptomatic at rest. Because of this, accurate clinical evaluation becomes
impossible unless some form of clinical exercise testing is utilized. Evaluation of
changes in Cdyn, functional residual capacity, oxygen utilization, and small airway
function at exercise would greatly improve the assessment of the pulmonary function in
the equine athlete. Concerning the development of this clinical exercise testing, Bayly

and Slocombe (1991) have written that:

 

23

Such developments are essential if the pathogenesis of speciﬁc entities
such as exercise-induced pulmonary hemorrhage, and non-speciﬁc

subclinical problems like bronchiolitis, are to be better understood.

My studies, reported in this thesis, are aimed at the development of clinically
useful tests of pulmonary function. In the next chapter I will report on exercise testing
of normal horses and horses with ovalbumin-induced allergic lung disease. All of the
horses with induced airway obstruction developed severe exercise-induced pulmonary
hemorrhage. Because of this, and because EIPH is such a common occurrence in

exercising horses, a review of EIPH follows.

Exercise-induced pulmonary hemorrhage

Epistaxis after exercise has been observed in horses for many years. It has
commonly been referred to as "bleeding, " "broken blood vessels," or "epistaxis" (Pascoe
et al. , 1981). The origin of this hemorrhage was generally considered to be the nasal
cavity until Mahaffey (1962) suggested a pulmonary source based on necropsy
examinations. Since that time, endoscopy has been used to support the theory that the
hemorrhage originates in the caudal part of the lung (Pascoe et al. , 1981; Hillidge et al. ,
1984, 1985; Raphel and Soma, 1982). On the basis of these observations Pascoe et al.
(1981) suggested the term "exercise-induced pulmonary hemorrhage" to more accurately
describe the clinical condition in the racing horse.

The frequency of EIPH based on the appearance of blood at the nostril after

exercise is quite low, but it has been reported to be much higher when diagnosed by

24
endoscopy. In Thoroughbreds EIPH prevalence has been reported to be between 44

percent and 75 percent (Pascoe et al., 1981; Raphel and Soma, 1982). Hillidge et al.
(1984, 1985) have reported similar rates in other racing breeds.

Association of EIPH with the age of the horse has been shown by Hillidge et al.
(1984, 1985) and Raphel and Soma (1982). They were able to show that frequency of
EIPH increased with increased age of the horses.

The distance a horse is raced has also been related to EIPH. Raphel and
Soma(l982) were able to show a direct relationship between distance raced or breezed
and EIPH in Thoroughbreds. This association was not detected in racing Appaloosa or
Quarter horses, but Hillidge et al. (1984, 1985) suggested that it was the intensity of
exercise, rather than the distance run, which affected the prevalence of EIPH in these
breeds.

The effect of EIPH on performance remains controversial. In the Thoroughbred
horse a relationship between EIPH and ﬁnishing position was not observed by Pascoe et
al. (1981) or Raphel and Soma (1982). However, when endoscopic examinations were
performed by MacNamara et al. (1990) on Standardbred racehorses, the ﬁndings
suggested that both EIPH and COPD adversely affected performance. A combination
of the two conditions more severely affected racing performance than did either condition
by itself. Raphel and Soma (1982) were unable to relate the degree of hemorrhage to
ﬁnishing position. Historically horses that bled from their nostrils have been considered
more severely affected, but Raphel and Soma (1982) suggest that absence of blood at the

nostril may not reﬂect severity of hemorrhage observed by endoscopic examination.

 

 

25
Clinical signs of EIPH in affected horses may range from blood ﬂow out of both

nostrils to no detectable external abnormalities. Frequent swallowing after exercise may
indicate that the horse is swallowing blood that is ﬂowing into the pharynx from the
trachea (Pascoe et al., 1981). O’Callaghan et al. (1987a) have reported that focal
abnormal lung sounds may be auscultated in the dorsocaudal lungﬁelds during
rebreathing, but the ﬁndings are inconsistent.

Radiographic changes associated with EIPH have been documented (Pascoe et al. ,
1987; O’Callaghan et al., 1987b). Abnormal pulmonary opacities were identiﬁed in the
caudal lung lobe. These opacities were large and peripherally located, obliterating the
thoracophrenic angle, but often did not have sharp margins. The intensity of the
opaciﬁcation of the consolidated area varied. Pleural effusion was detected in nine of
thirteen horses. Pascoe et al. (1987) found that these pulmonary radiopacities cleared
in ten days in two horses but required months to resolve in ﬁve others. O’Callaghan et
al. (1987b) coded estimates of radiographic lesion opacity and attempted to correlate this
with hemosiderin deposits and bronchial artery neovascularization. They concluded that

accurate prediction of lung lesions based on radiographic criteria was difﬁcult.

Pathologic changes

The gross lesions associated with EIPH are seen in the dorsocaudal regions of the
lung and in some cases areas of discoloration have been observed over as much as one-
third of the lung surface (O’Callaghan et al., 19870). The lesions are characteristically
stained a light brown-bronze color and in the inﬂated lung have a ﬁne reticular pattern.

These discolored areas of lung are denser than normal, collapse less readily, are slower

26

to inﬂate, and may contain pockets of trapped gas. The subpleural bronchial arteries are
more prominent and appear to be enlarged.

Histopathologic examination of the affected regions of lung shows extensive
hemosiderin deposition. Microangiographs of sections of lungs that were perfused with
latex reveal proliferation of the bronchial arterial circulation in regions with evidence of
pulmonary hemorrhage (O’Callaghan et al. , 1987d). There was also evidence of small
airway disease. The most severely affected bronchioles contained mucous exudate or
mucoid plugs and had grossly thickened walls characteristic of bronchiolitis (O’Callaghan
et al. , 1987c). It was suggested that the small airway disease may have precipitated the
bronchial arterial proliferation. Microradiography and computerized tomography
scanning were used to identify plexuses of markedly hypertrophied bronchial arterial
networks centered around small airways (O’Callaghan et al., 1987f). The authors
concluded that the bronchial vascular lesions are the likely source of hemorrhage in
EIPH.

O’Callaghan et al. (1987g) used ventilation/perfusion scintigraphy to demonstrate
pulmonary arterial perfusion impairment. Ventilation deﬁcits in the same areas also

suggested that there was small airway disease as noted on histopathology.

Pathogenesis
The pathogenesis of EIPH is unknown at this time although many theories have
been presented. As it became known that the origin of bleeding was the lung instead of

the upper airway, emphasis has been on an underlying pulmonary disease that could

27
predispose the lung to injury during exercise (MacNamara et al. , 1990; O’Callaghan et

al., 1987h; Pascoe et al., 1989; Robinson and Derksen, 1981).

Robinson and Derksen (1981) proposed that small airway obstruction may be a
cause of EIPH. This small airway obstruction would result in asynchronous ventilation
of pulmonary lobules. Because all structures of the lung are interconnected, movement
of one area will exert forces on adjacent areas. The areas of lung that inﬂate normally
then exert expansive forces on uninﬂated areas. This interdependence of lung structures
may create local areas of increased distending forces in the resting horse’s lung. If
airway obstruction is incomplete, these additional forces may be able to fully inﬂate the
affected areas but the time required for inﬂation will be prolonged. While this delayed
ﬁlling may not be sufﬁcient to cause signs of impaired gas exchange at rest, during
exercise the partial obstruction may prevent ﬁlling of the alveoli distal to the obstruction
in the time available for inhalation. Thus even a slightly obstructed segment moves
asynchronously with the adjacent lung tissue. This asynchrony precipitates an
accentuation of distending forces that may result in tearing of lung parenchyma with
concomitant pulmonary hemorrhage. Inelastic scars within the lung or pleural adhesions
that do not expand with lung volume may be contributing factors. A second effect that
may occur with asynchronous ventilation of the lung is the transmission of the additional
distending forces into the obstructed alveolus. Intraalveolar pressures would decrease
more than normal during a respiratory cycle. This accentuated decrease in alveolar
pressure and presumed increase in pulmonary capillary pressure during exercise could
elevate capillary transmural pressure. If this increase in capillary transmural pressure

is great enough, rupture of capillary endothelium and intraalveolar hemorrhage would

28

result. If airway obstruction is incomplete then the hemorrhage has a route to the
trachea.

The dorsocaudal distribution of lesions found in EIPH has been attributed to
regional differences in pulmonary mechanics. Milic-Emili et al. (1966) proposed that a
vertical pleural pressure gradient would result in alveoli being more distended in the
dorsal regions of the lungs. Robinson and Derksen (1981) proposed that during rapid
breathing the narrower airways in the base of the lung may delay ﬁlling of that region.
Thus, the dorsal region of the lung is more susceptible to excessive distension, which
may result in tearing of lung parenchyma and subsequent EIPH.

A more recent study by Derksen et al. (1992) used an ovalbumin-induced allergic
lung disease model to create bronchiolitis and hemorrhage in the lung. This hemorrhage
did not appear in the large airways unless the horses were exercised on a treadmill. The
mechanism that caused the hemorrhage to appear in the large airways is unknown.
Derksen et al. (1992) postulated that vascular pressure increases or mechanical forces
exerted on the lung during exercise may have caused an increased amount of hemor-
rhage. It is also possible that strenuous exercise served only to mechanically move blood
from the alveoli to the larger airways. In this case, the term ”exercise-induced
pulmonary hemorrhage" would be inaccurate, as hemorrhagic lung lesions may be
present prior to exercise.

Derksen et al. (1992) also used the ovalbumin-induced allergic lung disease model
to examine the importance of regional differences in interdependence forces in the lung
to the pathogenesis of EIPH. Blood appeared in the large airways following exercise

after ovalbumin-induced lung disease in the dorsal, middle, and apical regions of the

 

29

lung. These ﬁndings do not support regional differences in interdependence forces as the
explanation for the dorsocaudal distribution of lesions found in EIPH.

Another theory of the pathogenesis of EIPH proposed by O’Callaghan et al.
(1987b) is predicated on the presence of preexisting small airway disease. In this study
the primary lesions were multifocal small airway disease in the dorsocaudal lungﬁelds
and "an intense vascular proliferation resulting in apparent domination of the alveolar
vascular bed by the bronchial arteries. " Other changes found on histopathology such as
hemosiderophage sequestration, ﬁbrosis, and lung parenchyma destruction were thought
to be secondary to previous incidents of hemorrhage and inﬂammation.

The proliferation of bronchial vasculature is proposed to occur as part of the
normal inﬂammatory repair response to focal areas of bronchiolitis. If the horse is
exercised during this inﬂammatory response, a localized hypoxia is produced in the
affected secondary lobules. This stimulates a bronchial arterial angiogenesis, which the
authors suggest may be mediated by activated macrophages. In these areas of hypoxia
there is a reﬂex reduction of local pulmonary arterial ﬂow in order to redistribute blood
ﬂow away from non-ventilated areas and thus maintain a balanced ventilation/perfusion
ratio throughout the lung. Flow to the alveolar capillary bed is then maintained from
bronchial arteries through precapillary anastomoses. The resultant bronchial arterial
proliferation potentially may result in increased anastomotic ﬂow and higher pressures
at the arteriolar—capillary junction of the alveoli. Based on this pathogenetic theorem,
hemorrhage in EIPH is probably of bronchial arterial origin.

Although an etiologic agent was not identiﬁed, O’Callaghan et al. (1987h)

speculated that the most likely cause may be a respiratory virus. Rhinopneumonitis

 

30
(EHV-l) virus was suggested as a possibility because of its high morbidity, low

mortality, worldwide distribution and the potential for horses to be infected repeatedly.

Exercise-induced pulmonary hemorrhage does not exclusively involve airways,
but an interaction between the respiratory and circulatory systems. Indeed the most
commonly used treatment for EIPH, furosemide, reduces plasma volume (Freestone et
al., 1988), right atrial pressure, pulmonary arterial pressure (Goetz and Manohar, 1986;
Olsen et al. , 1992), and mean aortic pressure (Erickson et al. , 1992).

Erickson et al. (1989) found pulmonary artery pressure increases to more than 80
mm Hg in the exercising horse. The reasons for this marked pulmonary hypertension
are not known. Goetz and Manohar (1986) suggest that the increase in pulmonary artery
pressure of exercising ponies may be due to a loss of compliance in the pulmonary
arterial system. Another possibility is that an increase in packed cell volume caused by
splenic contraction results in hyperviscosity of the blood (Davis and Manohar, 1988;
McClay et al. , 1992). This hyperviscosity was ameliorated by splenectomy in ponies
(Davis and Manohar, 1988). Cardiac output was unchanged, but systemic and pulmonary
arterial pressures were signiﬁcantly decreased. Total pulmonary vascular resistance and
total peripheral resistance were also signiﬁcantly decreased, suggesting that an increase
in packed cell volume may contribute substantially to pulmonary and systemic
hypertension of exercise in ponies. A considerable decrease in the exercise capacity of
the splenectomized ponies was also shown.

Packed cell volume increases of 58 to 61% and blood viscosity increases of 2 to
3 times were reported in racing Thoroughbreds by McClay et al. (1992). It was

suggested that postrace changes of this magnitude may impede blood ﬂow and may result

31

in microvascular injury. The authors, however, were unable to differentiate between
horses affected with EIPH and normal horses using changes in hematologic and rheologic
parameters. They postulated that all horses may be predisposed to EIPH and clinical
manifestations of bleeding may be dependent on factors such as duration of racing career,
effort expended in a race, and the number of postrace bronchoscopic examinations.
The high pulmonary vascular pressures in the exercising horse could potentially
be damaging to pulmonary capillaries, but this has not been measured directly. Manohar
(1992) has calculated pulmonary capillary pressures by averaging measured pulmonary

artery pressure and pulmonary arterial wedge pressure in the exercising horse. In horses

 

running on a treadmill at 14 m/s the calculated pulmonary capillary pressure was 79 mm
Hg. At capillary pressures greater than 40 mm Hg, West et al. (1991) was able to
demonstrate disruption of the capillary endothelium and alveolar epithelium in rabbits.
This disruption of pulmonary capillaries was termed stress failure and has also been
reported by West et al. (1992) in two racehorses. The two horses were known bleeders

and were galloped at maximum speed on a treadmill before being euthanatized.

Treatment

Many therapeutic regimens have been proposed to prevent EIPH in performance
horses. Management changes, furosemide, bronchodilators, hesperidin and citrus
bioﬂavonoid, estrogens, and water vapor therapy have been recommended (Sweeney,
1991). Of these furosemide is probably the most widely used. Furosemide appears to
have limited effectiveness for the prevention of EIPH but may be able to diminish the

amount of hemorrhage during a race and may improve performance to pre—EIPH levels

32
(Erickson et al., 1989; Pascoe et at., 1985; Sweeney et al., 1990). Furosemide has also

been shown by Sweeney et al. (1990) to enhance performance in geldings but not in
mares and stallions without a diagnosis of EIPH.

The mechanism of action by which furosemide alters performance is unknown.
Freestone et al. (1988) state that furosemide does not decrease PCV, plasma sodium
concentration, or osmolality. Geor et al. (1992) associated furosemide with increases in
mean RBC hemoglobin concentration and blood viscosity. These factors could
exacerbate exercise-associated hyperviscosity in horses. These ﬁndings do not correlate
with reported decrease in EIPH after administration of furosemide (Erickson et al. , 1989;
Pascoe et al., 1985).

Furosemide has been shown to decrease right atrial and pulmonary arterial
pressures (Goetz and Manohar, 1986; Olsen et al., 1992). The signiﬁcance of this
decrease is questionable. In one study, mean pulmonary artery pressure in the exercising
horse was still 240% of the resting value (Goetz and Manohar, 1986). Olsen et al.
(1992) reported that this reduction in pulmonary artery and right atrial pressures can be
partially reversed by administration of the cyclooxygenase inhibitors phenylbutazone and
ﬂunixin meglumine. This suggests that furosemide may mediate its vascular effects by
inﬂuencing arachadonic acid metabolism.

Erickson et al. (1992) did not demonstrate an effect of furosemide on pulmonary
artery pressure but did show a reduction in mean aortic pressure during exercise. The
lack of effect on pulmonary artery pressure was explained by time of furosemide
administration before exercise. In this study one hour elapsed between furosemide

administration and exercise. In other studies where furosemide affected pulmonary

 

33

arterial pressures but not aortic pressures, the drug was administered 2 to 4 hours before
exercise (Goetz and Manohar, 1986; Olsen et al., 1992). Olsen et al. (1992) suggests
that the regulation of vascular tone by arachadonic acid metabolites may differ between
the pulmonary and systemic circulations.

Furosemide has also been shown to have an effect on airways. Exercise-induced
bronchoconstriction in humans was prevented by treatment with inhaled furosemide
(Bianco et al. , 1988). Broadstone et al. (1970) demonstrated that inhaled furosemide
attenuates airway obstruction in ponies with recurrent obstructive pulmonary disease. It
is possible that the effects of furosemide on airways may be important in the dimunition
of hemorrhage during a race and the improvement of performance to pre-EIPH levels

reported by Sweeney et al. (1990).

Conclusion

Exercise-induced pulmonary hemorrhage is a complex and still poorly understood
syndrome. The clinical signs and lesions have been described in detail but pathogenesis
remains unclear. Because this syndrome only occurs during strenuous exercise, clinical
exercise testing is an indispensable tool in the elucidation of EIPH. Because O’Callaghan
et al. (1987e) reported small airway disease in horses with EIPH, a sensitive and non-
invasive means of evaluating pulmonary function in the exercising horse would appear
to be mandatory to the understanding of this complex entity.

In this study we evaluated TBVFLs obtained during exercise as a measure of
pulmonary function in normal horses and in horses with ovalbumin-induced airway

obstruction. Derksen et a1. (1992) used ovalbumin to induce allergic lung disease

 

34
characterized by bronchiolitis and EIPH. Therefore, this model was used during my

thesis research.

 

CHAPTER II
TIDAL BREATHING FLOW-VOLUME Loor ANALYSIS AS A TEST or

PULMONARY FUNCTION IN THE EXERCISING HORSE

Materials and methods

Six Standardbred horses between 3 and 11 (mean 5.17 i 1.33 S.E.M) years of
age were used in this experiment. The horses were immunized against equine inﬂuenza
and rhinopneumonitis virus and were on pasture for at least one month before use in the
experiments. A physical examination, including auscultation of the heart and lungs, and
endoscopic examination of the upper airway at rest and while exercising on the treadmill
revealed no abnormalities. Experimental procedures were reviewed and approved by

Michigan State University’s All-University Committee on Animal Use and Care.

Sensitization of horses

Horses were sensitized to ovalbumin by intramuscular injection using 10 mg of
ovalbumin dissolved in 1.5 m1 of phosphate-buffered saline and emulsiﬁed in 1.5 ml of
complete Freund’s adj uvant. This emulsion was divided and injected deep into the left
and right semimembranosus muscles. Two weeks later the protocol was repeated using

incomplete instead of complete Freund’s adjuvant.

35

 

36

Induction of disease

At least two weeks after the last injection, the horses were restrained in stocks
and ﬁtted with a mask and non-rebreathing valve. An ultrasonic nebulizer (Model 65
ultrasonic nebulizer, De Vilbiss, Somerset, PA) was used to administer an aerosol of 20
ml phosphate-buffered saline containing 1 gram ovalbumin to each horse over a 15 -

minute period.

Pulmonary function measurements

Experiments were performed with the horses restrained in stocks without sedation.
Pulmonary function was measured as previously described by Derksen et al.(1982a).
Pleural pressure was measured using an esophageal balloon (10 cm long, 3.5 cm
perimeter, 0.06 cm wall thickness) sealed over the end of a polypropylene catheter. The
balloon was passed through the nares into the mid-thoracic portion of the esophagus and
connected to a pressure transducer (V alidyne Model DP/45-35, Northridge, CA). This
pressure transducer was calibrated before each study using a water manometer. A face
mask was placed over the muzzle and a rubber shroud and tape were used to seal the
mask to the face. A pneumotachograph (No. 5 Fleisch, Grand Rapids, MI) attached to
the mask was connected to a pressure transducer (V alidyne DP/45-22, Northridge, CA)
that produced a signal proportional to airﬂow. This signal was processed by a lung
function computer (Model LS-14, Buxco Electronics, Inc. , Sharon ,CT), which integrated
the signal to give tidal volume. The pneumotachograph transducer system was calibrated
using a 2-liter syringe. Flow, tidal volume, and pleural pressure were used by the

computer to calculate RL, Cdyn, respiratory frequency (f), and VB.

 

37

Rapid incremental exercise test

Maximum HR during exercise was established prior to airway function tests. A
rapid incremental exercise test was performed to estimate the relationship between HR
and treadmill speed for each horse. This exercise test was similar to that reported by
Rose et al. (1990) except that a 3-degree inclined treadmill was used. Horses were
exercised at 4 m/sec for 3 minutes, then at 6 m/sec for 90 seconds. Treadmill speed was
then increased at one-minute intervals to 8, 10, 11, and 12 m/sec. The test was stopped
when the horse could no longer maintain position on the treadmill. Heart rate was
recorded in the last 15 seconds of each exercise period. The speed at which the horse
reached HR1mm was determined. The speed corresponding to 75 % of HRmalx was

calculated.

Tidal breathing ﬂow-volume loops

A ﬁberglass mask covering both mouth and nostrils was used to support a 15.2-
cm diameter pneumotachograph (Merriam Instruments, Grand Rapids, MI). A wire
mesh (Mesh SS Screen, McMaster Carr, Chicago, IL) located between the muzzle and
pneumotachograph helped to prevent contamination of the pneumotachograph with
secretions and served as a ﬂow straightener element. The resistance of the pneumotacho-
graph was 0.04 cm HZO/L/sec up to an airﬂow of 90 L/ sec. A differential pressure
transducer (Model DP45-22, Validyne Sales, Northbridge, CA) measured pressure
changes across the pneumotachograph and produced a signal proportional to ﬂow. These

signals were then passed through an 8 Hz low-pass ﬁlter and fed into the lung function

38

computer. The computer plotted ﬂow versus volume for each individual breath to create
a TBFVL.

Ten representative ﬂow volume loops were selected from each exercise period.
Criteria for selection included adequate loop closure (< 5 % difference between
inspiratory and expiratory volume) and lack of artifact such as snorting and swallowing
(Amis and Kurpershoek, 1986). A computer software package (Buxco LS-l4, Buxco
Electronics, Inc. , Sharon, CT) was used for quantitative analysis of the ﬂow volume

loops. Measured values for individual loops were rate, tidal volume, inspiratory and

 

expiratory time, total inspiratory time, peak inspiratory and expiratory airﬂow rates, and
inspiratory and expiratory ﬂow rates at 50%, 25%, and 12.5% (IFso, IF25, IF125; EFSO,
EF25, EF12_5) of tidal volume. Ratios of the ﬂow rates and times were also calculated.
Heart rate was recorded with a telemetry system (Digital UHF Telemetry System,

M1403A, Hewlitt Packard, Palo Alto, CA).

Experimental protocol

Pulmonary resistance and Cdyn were measured in standing horses only.
Subsequently, horses were exercised on a 3-degree inclined treadmill at 4 m/ sec for 2
minutes, followed by a 2-minute exercise period at a treadmill speed corresponding to
75 % of the individual HRmx. All horses were allowed a l-minute rest period prior to
2 minutes at treadmill speed corresponding to HRmax. Data were collected during the
last minute of each 2-minute exercise period. At least ﬁfteen minutes after the last
exercise period, each horse was evaluated endoscopically for signs of EIPH. These

measurements were taken before, and 5 hours, 1, 3, 5, 7, l4, and 21 days after aerosol

39

ovalbumin administration. The horses were not exercised at speeds corresponding to
HR“m ﬁve hours after ovalbumin administration because of the severity of clinical signs

of respiratory disease.

Statistical analysis
The effect of ovalbumin aerosol challenge on ﬂow-volume loops was analyzed
using a repeat measure analysis of variance. Differences between the means were

evaluated using Tukey’s test (Steele and Torrie, 1960).

Results
Pulmonary function tests

Pulmonary resistance and Cdyn in the ovalbumin-sensitized horses before
(baseline) and after aerosol ovalbumin administration are shown in Table 1. Five hours
after aerosol ovalbumin administration, RL was signiﬁcantly increased. It had returned
to baseline values by day l and remained there for the subsequent measurement periods
(Figure 2). Although Cdym was not signiﬁcantly affected by aerosol ovalbumin
administration, the mean Cdyn was less than baseline value at the one— and two-day
measurement periods. Respiratory frequency was signiﬁcantly elevated one day after
aerosol ovalbumin administration. Tidal volumes were decreased and VB was increased
one day after aerosol ovalbumin administration but the changes were not statistically

signiﬁcant (Figure 3).

40

Table 1. Pulmonary resistance (RI) and dynamic compliance (Cdyn) before (baseline)

and after (5 hours, and 1, 3, 5, 7, l4, and 21 days) ovalbumin administration (mean :1;

 

 

SEM).
Measurement period Cdyn RL

baseline 1.442 1 0.19 0.637 1 0.10
5 hours 1.053 :t 0.20 1.715 :1; 0.30*
1 day 0.962 t 0.35 0.717 1 0.09
3 days 0.830 i 0.09 0.745 1 0.09
5 days 1.976 :t 0.83 0.600 1 0.09
7 days 1.664 :I: 0.23 0.538 t 0.08
14 days 1.379 t 0.27 0.703 1 0.11
21 days 2.112 i 0.67 0.610 1 0.11

 

* = Signiﬁcant difference (p < 0.05)-

41

 

 

 

 

_,\\\
_,\\\\....

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

_ 9

Figure 2. Pulmonary resistance (Rl) (mean :1: SEM) measured before (baseline) and

after (5 hours, and 1, 3, 5, 7, l4, and 21 days) aerosol ovalbumin administration.

* = Signiﬁcant difference (p < 0.05)-

42

 

b
O

 

 

 

 

 

 

 

O

 

‘ U!

VT (Liters)
0

d

/////////A4

7//////////A—+
////////////l-4 W

 

 

 

 

 

 

 

m

 

 

 

 

O

‘ f (breaths/min) 8
/ E 32 E E I? .
/////////A4
W W

 

cu. ‘
8 '3

O
O

V; (L/min)
3

R)
O

//////A—i

 

i
i t

‘ 6‘“ 6‘9 e39 6‘9 e39 6‘9
O O O O O
'5 «o ’\ Q: .9

 

 

/////°'//A—i
///////m

 

 

 

 

 

 

6
‘70 o
9”.
09
so
°o
I

Time

Figure 3. Tidal volume (VT), respiratory frequency (f), and minute ventilation (V 9
(mean :1: SEM) measured before (baseline) and after (5 hours, and l, 3, 5, 7, 14, and
21 days) aerosol ovalbumin administration. * = Signiﬁcant difference (p < 0.05).

43
How-volume loops

The TBFVL indices before (baseline) and after (5 hours, and l, 3, 5, 7, 14, and
21 days) aerosol ovalbumin administration with horses exercising at spwds corresponding
to 75 % and 100% of HR“, are shown in Tables 2—7. Ovalbumin administration did not
result in marked loop changes in any of the horses at any measurement period (Figure
4). Only four indices describing TBFVL were slightly but signiﬁcantly altered by
ovalbumin administration and these indicies are shown in Table 8. At a speed
corresponding to 75 % I-IRmax the EF12_5/IF12.5 was increased at ﬁve hours and EF25 was
increased one day after aerosol ovalbumin administration. When the horses were
exercised at speeds corresponding to HRmax one day after aerosol ovalbumin administra-
tion, IFSO and VT/TI were signiﬁcantly decreased from prechallenge values. TBFVL
indices were unaffected by aerosol administration of ovalbumin at any of the remaining

exercise periods.

 

44

Table 2. Rate and timing of the respiratory cycle measured at spwds corresponding to
75 % of HR“,x before (baseline) and after (5 hours, and 1, 3, 5, 7, l4, and 21 days)

aerosol ovalbumin administration.

 

 

Measurement periods

baseline 5 hours 1 day 3 day 5 day 7 day 14 day 21 day

f (breaths/ 97.3 98.5 102.7 101.4 103.4 103.4 100.5 102.9
min) j: 9.78 :1; 9.42 :1; 6.58 1; 7.74 :1; 8.01 3; 8.13 :1; 7.25 :1: 7.10
T! (ms) 324.83 336.67 308.50 317 294.83 296.33 308.33 297.83
:1: 40.68 1; 43.46 i 24.92 :1: 31.50 i 24.75 :1: 26.84 :1: 25.28 :1: 24.09

TE (ms) 334.50 322.17 290.00 295.50 306.17 304.83 310.00 303.17
:1: 41.65 :1; 42.83 :1: 19.28 :1; 24.73 :1; 26.62 1; 26.80 :1: 23.78 3; 22.80

TE/TI 1.03 0.96 0.95 :1; 0.95 1.04 1.04 1.02 1.03
1; .02 j; .03 .05 j; .05 i .03 j; .03 j; .04 j; .02

Tr/TTOT 0.49 0.51 0.51 0.52 0.49 0.49 0.50 0.49
i: .01 j; .01 :t .01 j; .01 i .01 i .01 j: .01 i .01

VT (L) 11.68 11.09 11.71 11.70 11.62 11.62 11.50 11.62
:1: .94 j; .70 j; .84 i .78 :t .81 j; .83 j; .84 j: .83

VB (L) 11.63 10.87 11.56 11.61 11.46 11.46 11.42 11.54
:1: .95 j: .70 j; .86 :1: .84 j; .84 j; .85 j; .87 j; .86

VE/VI .99 .98 .98 .99 .99 .98 .99 .99
i .01 j: .00 j: .01 j; .01 j; .01 j; .01 i .01 i .00

 

f = respiratory frequency, T1 = inspiratory time, TE = expiratory time, TTOT = T1 +

TE, VT = tidal volume, VB = expiratory volume, V, = inspiratory volume.

45

Table 3. Airﬂow indices taken from ﬂow-volume loops measured at speeds
corresponding to 75 % of HRmax before (baseline) and after (5 hours, and 1, 3, 5, 7, 14,

and 21 days) aerosol ovalbumin administration.

 

 

Measurement periods

baseline 5 hours 1 day 3 day 5 day 7 day 14 day 21 day
VT/Tr 37.27 34.59 38.56 38.01 39.83 39.77 37.91 39.62

1; 2.70 :1: 2.53 :1; 2.27 :1; 2.77 :1; 1.42 :1; 2.06 :1; 2.21 :1; 2.10
PIF 52.69 47.60 59.22 56.74 57.84 57.88 55.25 55.97
(Us) 1 3.95 j; 3.84 i 3.20 1; 3.38 :1; 2.23 1; 2.44 :1; 2.87 :t 2.66
IF” 43.95 40.56 48.01 47.83 48.78 47.20 44.26 47.03
(Us) 1; 4.78 i 3.81 1; 3.89 :1; 3.97 :t 2.78 1: 4.74 1; 4.04 i 3.92
IFzs 49.96 45.74 58.60 56.15 56.57 56.07 53.09 54.74
(Us) 1; 4.75 j; 4.32 :1: 3.21 3; 3.45 i 2.61 :1; 3.01 :1: 3.47 :1; 2.84
11712., 47.19 41.43 52.27 50.33 51.32 51.25 49.29 49.68
(Us) 3; 2.89 i: 3.41 :1; 2.10 i: 2.46 :1; 1.57 :1; 1.51 1; 2.43 d; 2.31
PEP 52.16 51.36 58.83 57.89 55.49 54.81 53.91 58.24
(Us) :9; 1.86 i 2.66 :1; 1.20 i 1.20 :l: 1.33 :1: 2.00 :1: 1.06 :1: 3.70
BF” 44.40 41.91 49.74 50.57 45.34 44.94 45.44 47.31
(Us) 1; 3.25 i 2.85 :1: 2.03 :1: 2.11 :t 2.77 :t 2.09 i 1.25 :t 2.46
BF” 40.37 41.06 48.34 45.71 42.34 42.65 41.60 40.88
(Us) :1; 3.12 3; 3.05 i; 1.49 :1; 1.90 :1; 2.89 1; 3.16 :1; 3.17 1; 3.40
BF,” 33.17 35.01 41.34 38.81 35.61 37.33 35.44 35.75
(Us) :1; 2.79 1; 2.46 3; 1.74 1; 1.03 :1; 1.39 :1; 1.90 j; 2.67 i 2.64

 

VT = tidal volume, T, = inspiratory time, PIP = peak inspiratory ﬂow, IFso —

inspiratory ﬂow at 50% of VT, IF25 = inspiratory ﬂow at 25% of VT, IF12.5 '-

inspiratory ﬂow at 12.5 % of VT, PEF = peak inspiratory ﬂow, EFSO = expiratory ﬂow
at 50% of VT, EF25 = expiratory ﬂow at 25 % of VT, EF12.5 = expiratory ﬂow at

12.5% 0f VT'

 

46

Table 4. Ratios of ﬂow indices derived from ﬂow-volume loops measured at speeds
corresponding to 75% of HR,” before (baseline) and after (5 hours, and 1, 3, 5, 7, l4,

and 21 days) aerosol ovalbumin administration.

 

 

Measurement periods
baseline 5 hours 1 day 3 day 5 day 7 day 14 day 21 day
PEF/PIF 1.01 1.10 1.01 1.04 0.97 0.95 0.99 1.04
d; .05 j; .04 j; .06 j: .06 j; .03 j: .04 j: .04 j: .05
13lele 1.06 1.07 1.08 1.10 0.94 1.00 1.07 1.05
:1; .10 j; .09 j; .10 j: .09 j; .06 i .11 j; .08 j; .08
EF25/1F25 0.83 0.92 0.83 0.83 0.75 0.76 0.79 0.75
:1; .03 j; .38 j; .38 j; .05 j: .03 j; .03 j; .03 j; .04
EFnJ/IFIM 0.71 0.85 0.79 0.78 0.69 0.73 0.72 0.72
3; .03 j; .02 j; .04 j; .04 j; .02 j; .02 j; .03 j; .03
PIP/IF” 1.23 1.19 1.27 1.21 1.20 1.28 1.29 1.23
1.06 1.04 :1: .06 j; .03 j; .04 j; .11 :1; .07 :1: .06
PIP/IF” 1.07 1.05 1.01 1.01 1.02 1.04 1.05 1.02
:1: .04 j; .02 i .00 :1: .00 1- .01 j; .02 j: .02 j; .01
PlF/lFlu 1.12 1.16 1.13 1.12 1.13 1.13 1.12 1.13
:1: .03 :1; .03 :1; .02 j: .02 j; .03 j; .03 j: .03 j; .02
[Fm/IF” 0.90 0.90 0.82 0.85 0.86 0.84 0.84 0.86
:1; .06 j; .05 j; .03 :1; .02 j; .03 j; .07 ;t .05 j; .03
IF25/IF125 1.05 1.10 1.12 1.11 1.10 1.09 1.08 1.10
i .05 j; .03 j; .03 :t .02 i .04 j: .04 i- .04 i .03
PEF/EF50 1.21 1.24 1.19 1.15 1.25 1.23 1.19 1.23
gt .07 j: .03 j; .04 :t .03 j: .06 j; .03 :t .04 j; .03
PEF/EFZS 1.34 1.28 1.23 1.29 1.35 1.32 1.34 1.47
3; .07 j; .04 j; .03 j; .06 j; .07 j; .07 i .10 j; .13
PEP/E17125 1.63 1.50 1.44 1.51 1.59 1.48 1.59 1.66
:1; .10 :t; .05 j; .05 j: .04 j; .04 j; .05 j: .12 i .10
EFso/EF” 1.12 1.03 1.03 1.12 1.09 1.07 1.13 1.20
j: .05 i .04 j; .03 j; .05 :t .03 j; .03 j; .08 j; .11
EFzleFlu 1.23 1.18 1.17 1.18 1.19 1.14 1.18 1.15
:1; .05 :t .02 j; .02 j: .03 j: .04 j; .03 j; .03 i .04

 

PEF = peak expiratory ﬂow, PIF = peak inspiratory ﬂow, EFSO =expiratory ﬂow at
50% of VT, EF25 = expiratory ﬂow at 25% of VT, 51:12.5 = expiratory ﬂow at 12.5%
of VT, IF50 = inspiratory ﬂow at 50% of VT, IF25 = inspiratory ﬂow at 25% of VT,
IF125 = inspiratory ﬂow at 12.5% of VT.

47

Table 5. Rate and timing of the respiratory cycle measured at speeds corresponding to

100% of I-IRmax before (baseline) and after (1, 3, 5, 7, 14, and 21 days) aerosol

ovalbumin administration.

 

 

Measurement periods
baseline 1 day 3 day 5 day 7 day 14 day 21 day
f (breaths/min) 117.4 108.8 117.7 112.8 119.2 118.8 120.2
3; 9.25 :1; 7.47 i 10.7 1 12.78 :1: 10.93 :1; 12.37 :1; 11.79
T, (ms) 252.83 284.73 259 274 250.7 255.33 252.5
:1; 28.45 1: 26.19 1: 27.69 3: 37.61 1; 31.25 1; 39.36 :1: 35.08
TE (ms) 287.67 300.73 281 .33 304.5 279.33 286.83 278.83
:1; 22.43 1; 18.04 :1; 31 :1; 39.84 :1: 26.17 1; 31.86 :1; 30.71
TE/Tr 1.17 1.09 1.10 1.13 1.14 1.15 1.12
:t .05 j: .04 j; .04 j; .04 j; .04 j; .04 :t .04
Tl/T-Im- 0.46 0.48 0.48 0.47 0.47 0.47 0.47
:1: .01 j; .01 j; .01 :I: .01 j; .01 i .01 j; .01
VT (L) 12.87 11.86 12.27 13.28 13.17 13.04 12.92
:t 1.72 1; .58 :1; 1.08 :1; 1.7 :1; 1.7 1; 1.63 :1; 1.60
VB (L) 12.83 11.75 12.14 13.18 13.21 12.94 12.88
:1; 1.78 :1: .62 :1; 1.14 :1: 1.72 :1; 1.72 :1: 1.68 :1; 1.65
Viz/VI 0.99 0.99 0.98 0.99 1.00 0.99 0.99
:t .01 j: .01 j; .01 j; 00 :t .01 :1; .01 j; .01

 

f = respiratory frequency, T1 = inspiratory time, TE = expiratory time, Tror = TI +

TE, VT = tidal volume, VB = expiratory volume, V, = inspiratory volume.

48

Table 6. Airﬂow indices taken from ﬂow-volume loops measured at speeds correspond-
ing to 100% of HR“mm before (baseline) and after (1, 3, 5, 7, l4, and 21 days) aerosol

ovalbumin administration.

 

 

Measurement periods

baseline 1 day 3 day 5 day 7 day 14 day 21 day

VT/TI 51.16 44.04 48.61 49.16 52.58 52.15 51.81
1; 2.46 :t 2.11 1; 2.32 :1; 1.83 :t 1.86 i 2.62 :1: 2.62

PIF (L/s) 74.77 67.94 70.72 72.80 75.60 74.63 75.19
1; 3.86 :1: 2.19 j; 3.73 :1; 2.67 :1: 3.25 :1; 3.17 1; 3.08

IF“, (L/s) 67.25 58.21 62.31 64.92 65.41 67.05 66.52
:t 3.33 i 3.9 :1; 4.19 :1; 2.54 i 4.47 :1: 3.75 i 2.38

IF” (L/s) 73.41 66.45 69.09 71.33 73.79 72.88 72.64
:1: 3.63 :1: 2.45 :1: 3.77 :1: 2.56 :t 3.06 i 3.07 1; 4.32

“312.5 64.13 58.70 61.16 63.11 65.57 64.37 62.28
(Us) :1; 4.04 :1; 1.16 :1; 3.17 :1; 3.09 d; 3.36 :1: 2.65 :1; 5.88
PEF (L/s) 65.43 58.01 65.34 64.61 69.05 65.91 66.23
1; 5.3 3; 2.01 1; 3.66 1: 3.59 :1; 4.03 :l: 3.27 :1; 2.80

E1350 (L/s) 57.50 53.12 57.75 56.27 60.35 55.91 58.50
i 4.98 3; 1.46 :1: 4.07 i 3.52 :t 4.25 :1: 2.83 :1; 1.84

EF25 (L/s) 48.47 46.78 49.09 47.72 51.79 48.97 48.79
3; 3.68 :1; 1.43 1; 3.27 i 3.69 :1; 4.24 :1: 2.85 :t 3.91

121312., 43.56 41.19 42.01 42.59 47.59 44.03 44.75
(Us) 1; 3.27 3; 1.13 :1; 3.24 1; 2.55 :1; 3.75 :i: 2.41 :1; 1.88

 

 

V,- = tidal volume, T1 = inspiratory time, PIF = peak inspiratory ﬂow, IF” =
inspiratory ﬂow at 50% of VT, IF25 = inspiratory ﬂow at 25% of VT, IF125 =
inspiratory ﬂow at 12.5 % of VT, PEF = peak inspiratory ﬂow, EFSO = expiratory ﬂow
at 50% of VT, EF25 = expiratory ﬂow at 25% of VT, EF12_5 = expiratory ﬂow at

12.5% of VT.

49

Table 7. Ratios of ﬂow indices derived from ﬂow-volume loops measured at speeds
corresponding to 100% of HRmx before (baseline) and after (1, 3, 5, 7, 14, and 21 days)

aerosol ovalbumin administration.

 

 

Measurement periods
baseline 1 day 3 day 5 day 7 day 14 day 21 day
PEF/PIF 0.88 0.87 0.93 0.89 0.92 0.89 0.88
1; .05 i .06 j: .05 j; .04 :1: .04 j; .04 :1: .02
EFso/IFSO 0.85 0.99 0.94 0.87 0.93 0.84 0.88
:1: .05 j: .10 j; .06 i .05 j; .06 i .03 i .03
EF25/1F25 0.66 0.72 0.73 0.67 0.70 0.67 0.68
1: .02 j; .02 j; .06 j; .04 j; .04 j; .03 j; .04
EF12.5/IF12.5 0.68 0.70 0.70 0.68 0.72 0.68 0.75
:1: .01 j; .02 j; .05 :t .02 j: .03 j; .02 j; .07
PIP/11:50 1.12 1.21 1.15 1.12 1.18 1.12 1.13
:1; .02 i .05 j; .03 i .02 j; .06 j; .02 i .04
PIP/IF” 1.02 1.03 1.03 1.02 1.03 1.03 1.04
:1: 0.0 :t .01 j; .01 j: .01 j; .01 :9; 0.0 1; .03
PIE/11312.5 1.17 1.16 1.16 1.16 1.16 1.16 1.26
:l; .03 j; .02 :t .03 :t .03 j: .03 j; .03 j: .11
[Fa/IF” 0.92 0.87 0.90 0.91 0.89 0.92 0.93
1; .02 j; .02 :t .02 j; .01 j: .04 :t .02 j: .05
IF25/IF125 1.15 1.13 1.13 1.14 1.13 1.14 1.20
1: .03 j: .03 j; .03 j; .04 j; .03 i .03 j; .07
PEP/BF” 1.15 1.10 1.15 1.16 1.16 1.19 1.14
3; .04 j; .02 j; .04 j; .03 j; .05 j; .06 j; .04
PEF/EF25 1.36 1.25 1.35 1.39 1.37 1.37 1.40
:1; .07 j; .06 j; .07 j; .06 j; .08 :1: .09 j; .13
PEP/BF!” 1.52 1.42 1.60 1.54 1.49 1.53 1.51
j; .06 j; .06 j: .10 i .03 :1: .08 :t .10 j; .07
EFw/EFzs 1.19 1.14 1.19 1.20 1.19 1.16 1.24
:1; .04 j; .05 j; .05 j: .03 j; .04 i .05 i .09
EF25/EF12_5 1.12 1.14 1.18 1.12 1.10 1.11 1.10
3; .02 j: .01 j: .02 i .04 j; .06 j: .02 j; .08

 

 

PEF = peak expiratory ﬂow, PIF = peak inspiratory ﬂow, EFSO =expiratory ﬂow at
50% of VT, EF25 = expiratory ﬂow at 25% of VT, EF125 = expiratory ﬂow at 12.5%
of VT, IFSO = inspiratory ﬂow at 50% of VT, IF25 = inspiratory ﬂow at 25% of VT,
11:12.5 = inspiratory ﬂow at 12.5% of VT.

50
Table 8. Flow-volume loop variables measured at speeds corresponding to 75 % (75 %

HRH”) and 100% (100% HRmx) of maximum heart rate, which changed signiﬁcantly

after ovalbumin administration (mean i SEM).

-
_—

 

 

Measurement periods
baseline 5 hours 1 day 3 day 5 day 7 day 14 day 21 day
75% HR":
Elﬁn/117,2", 0.707 0.850‘ 0.795 0.783 0.695 0.727 0.717 0.718
1: 0.03 :l; 0.02 i 0.04 i 0.04 :1; 0.02 :1; 0.02 :1: 0.03 3; 0.03
EF25 40.37 41.06 48.34' 45.71 42.34 42.65 41.60 40.88
:1: 3.12 :l: 3.05 3; 1.49 i 1.90 :1; 2.89 j; 3.16 i 3.17 :1; 3.40
100% HRum
V-,-/T, 51.16 — 44.04“ 48.61 49.16 52.58 52.15 51.81
:t; 2.46 3; 2.11 :1; 2.32 1: 1.83 :t 1.86 :1; 2.62 :1: 2.62
1F“, 67.25 — 58.21" 62.31 64.92 65.41 67.05 66.52
i 3.33 3; 3.90 :1: 4.19 :1: 2.54 3; 4.47 d: 3.75 :t 2.38

 

* = Signiﬁcance at p < 0.05; BF,” = expiratory ﬂow at 12.5% of VT, IF,” =
inspiratory ﬂow at 12.5% of VT, EF25 = expiratory ﬂow at 25% of VT, VT = tidal

volume, T, = inspiratory time, IFS0 = inspiratory ﬂow at 50% of VT.

 

 

 

 

 

 

51

c D-
2?:
.2 ..
5' /"\

_. \
:5“ \
g .—
§ t : = : : i :
9!,
g -
tr. _,-

'- \
C
.2 I-
g
g. .—
" l.

Volume (4.00 Ildiv)
C
.8 '-
é
% r—
2. ..
8.
g, : : : a
5%, ..
C
8 p-
g
8 ..
_r_:_

Volume (4.00 l/div)

Figure 4. Representative ﬂow-volume loops measured before (solid line) and after
(broken line) aerosol ovalbumin administration. The upper loops are measured at 75 %
of HRmx. The broken line is ﬁve hours after ovalbumin administration. The lower
loops are measured at HRH”. The broken line is one day after ovalbumin administra-
tion. *

 

 

 

52
Heart rate

Heart rate was signiﬁcantly elevated at ﬁve hours after aerosol ovalbumin
administration when the horses were running at speeds corresponding to 75 % of HRH“,x

(Figure 5). This elevated rate was equal to the mean baseline HRmax for the horses in

this study.

 

 

 

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) measured at treadmill speeds corresponding to 75 %
( on s and l, 3, 5, 7, l4, and 21 days) aerosol
rfference (p < 0.05).

 

54

Endoscopic examination

Endoscopic evaluation at least ﬁfteen minutes after the last exercise period
revealed the presence of EIPH in all of the horses at 5 hours and at one day after
ovalbumin administration. In four of the six horses EIPH was also observed on day
three. Exercise-induced pulmonary hemorrhage was not observed after exercise at any

of the remaining measurement periods.

Discussion

Administration of ovalbumin aerosol to sensitized horses resulted in airway
obstruction, indicated by an approximately two-fold increase in RL and a smaller decrease
in Cdyn. Similar pulmonary function changes in response to ovalbumin aerosol challenge
to sensitized ponies has been previously reported. In sensitized horses, aerosol
ovalbumin challenge causes acute ﬁbrinopurulent obstructive bronchiolitis, and bronchitis
with pulmonary congestion and hemorrhage. Thus the airway narrowing observed is due
primarily to airway obstruction with exudate, edema ﬂuid, and hemorrhage (Derksen et
al. , 1982b).

In human medicine, MEFVL analysis is a commonly used, noninvasive, and
sensitive test used to detect airway obstruction (Hyatt and Black, 1973). The loop is a
ventilatory function test, whereby a continuous graphic measure of air ﬂow vs. volume
is plotted for a single maximum inspiratory and expiratory effort. The test requires
patient cooperation to perform this maneuver (Bass, 1973). In veterinary medicine the
noncooperative nature of our patients has prevented the clinical use of the MEFVL. In

noncooperative subjects such as dogs (Amis and Kurpershoek, 1986), cats (McKiernan

55
et al.), and human neonates (Godfrey et al., 1983), TBFVLs have been used to evaluate

airway obstruction. Tidal breathing ﬂow-volume loops have lower than maximum ﬂow
rates and greater ﬂow variability because of the voluntary control over breathing patterns.
These factors contribute to the lack of sensitivity of TBFVLs in the detection of airway
obstruction.

Horses exercising maximally generate near-maximum ﬂow rates. Lumbsden et
al. recently reported that TBFVL analysis in exercising horses is a quantitative, speciﬁc,
and repeatable test in the detection of upper airway obstruction. We reasoned that tidal
breathing ﬂow-volume loop analysis in exercising horses may also be used to detect the
presence of intrathoracic airway obstruction. We expected that TBFVL analysis
generated in exercising horses after ovalbumin challenge would be signiﬁcantly different
from TBFVL in the same horses before ovalbumin challenge and that this difference
would be detectable for several days after the RL had returned to baseline values. Figure
4 illustrates that in horses with airway obstruction, exercising at 75 % or 100% of HRmx,
the shape of the TBFVL was not markedly different from normal horses exercising at the
same speeds. Both inspiratory and expiratory ﬂow patterns were not markedly affected
by airway obstruction. Similar conclusions are reached by evaluation of quantitative
indices describing loop shape and size. Although the indices listed in Table 3 were
signiﬁcantly altered by airway obstruction, the changes were small numerically.

What could have caused our failure to detect airway obstruction in exercising
horses with airway obstruction? The increase in RL and decrease in Cd,“ in standing
horses after aerosol ovalbumin administration suggests the presence of detectable airway

obstruction. It is possible that this airway obstruction was alleviated by exercise itself.

 

56

This possibility is unlikely because the mechanism of ovalbumin-induced airway
obstruction in sensitized horses involved physical factors such as exudate, edema, and
hemorrhage, rather than reﬂex mechanisms or smooth muscle contraction (Derksen et al. ,
1982b). It is unlikely that exercise resolved this inﬂammatory process. Indeed,
following exercise in all horses EIPH was observed at the 5 -hour and 1-day measurement
periods. If EIPH occurred during exercise the blood may have exacerbated already
existing airway obstruction.

In humans, supreme athletes employ near-maximum air ﬂows during maximal
exercise (Grimby et al., 1971). In contrast, in sedentary persons airﬂow during
maximum exercise is signiﬁcantly less than maximum achievable air ﬂows. It is possible
that normal horses also use less than maximally achievable air ﬂows during high-intensity
exercise. If this is so, signiﬁcant reduction in maximal air ﬂows may not be detected
unless the obstruction is severe.

In the present study we did not measure EELV. In exercising humans EELV
stays constant or is reduced (Henke et al., 1988). The advantage of this breathing
strategy is that it places the diaphragm in a more optimal part of its length-tension curve,
improving the mechanical advantage of the inspiratory muscles and reducing the elastic
work of inspiratory muscles. When this breathing strategy is employed, subjects impinge
on their maximal expiratory ﬂow-volume curve near end expiration. In contrast, EELV
at maximal exercise is increased in patients with mild-to-moderate airway obstruction
(Babb et al., 1991). In these patients, maximal expiratory ﬂow at low lung volumes is
very low. Therefore, increasing EELV to a level where maximal expiratory ﬂow is

higher increases the mean expiratory ﬂow rates, and thus ventilatory capacity. The cost

 

57
of this breathing strategy is an increased work of breathing. Five hours following the

induction of airway obstruction, HR at the ﬁrst exercise period was signiﬁcantly
increased, and indeed was maximum. This suggests that the work of breathing in these
horses may have been increased. Increase in work of breathing may have been due to
airway obstruction itself, but also may be due to an increase in EELV allowing high air
ﬂow rates in spite of airway obstruction.

Aerosol ovalbumin administration to sensitized horses resulted in a signiﬁcant
increase in f. Similar changes have been reported previously in ponies. Because in
ponies the tachypnea is alleviated by vagal blockade, this ventilatory response to inhaled
antigen is due to stimulation of pulmonary receptors with vagal afferent ﬁbers (Derksen
et al., 1982b).

In conclusion, aerosol ovalbumin administration to sensitized horses caused
moderate airway obstruction. Evaluation of TBFVLs generated in these horses exercising

at 75 and 100% of their HRmax failed to detect this airway obstruction.

 

SUMMARY AND CONCLUSIONS

The purpose of this study was to evaluate the usefulness of TBFVLs in evaluating
pulmonary function in the exercising horse. Maximum expiratory ﬂow-volume loops
have been used as a noninvasive and sensitive test for the evaluation of airway
obstruction in humans. Because the horse is a noncooperative patient, our measurements
were taken during intense exercise to increase airﬂow. This not only served to more
closely approximate airﬂows achieved with MEFVLs, but also simulated the conditions
under which exercise-limiting respiratory dysfunction becomes clinically detectable.

Administration of aerosol ovalbumin to previously sensitized horses resulted in
airway obstruction, which was measured by increased RL and decreased Cdyn. This
airway obstruction was not detected by TBFVL analysis.

We did not measure EELV in these horses during exercise. In healthy, exercising
humans EELV is decreased or remains constant. However in patients with mild-to-
moderate airﬂow limitation, EELV increases. By increasing lung volume, the maximum
expiratory ﬂow would be higher. This allows higher expiratory ﬂow rates and thus an
increase in ventilatory capacity. It is possible that this is why we were unable to detect

airway obstruction with TBFVL analysis.

58

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