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if LIBRARY
* .‘ Michigan State
. University

This is to certify that the
thesis entitled

THE CARDIOVASCULAR EVENTS AND ACID-BASE STATUS
OF OVERFED PONIES DURING THE ONSET OF ACUTE LAMINITIS

presented by

Jack Robert Harkema

has been accepted towards fulfillment
of the requirements for

M. S . degree in Physiology

 

Major professor

Date November 17, 1977

 

0-7639

 

k f
300‘ 1' ’51”

Whiz zénu

 

THE CARDIOVASCULAR EVENTS
AND ACID-BASE STATUS OF
OVERFED PONIES DURING THE

ONSET OF ACUTE LAMINITIS

By

Jack Robert Harkema

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

MASTER OF SCIENCE

Department of Physiology
1977

ABSTRACT
THE CARDIOVASCULAR EVENTS
AND ACID—BASE STATUS OF
OVERFED PONIES DURING THE
ONSET OF ACUTE LAMINITIS
By

Jack Robert Harkema

Twelve Shetland ponies were fed a high starch ration.
Seven ponies which had a transitory metabolic acidosis deve-
loped laminitis 56 i 3.5 hours after overfeeding. These
ponies also developed a persistent hypokalemia, hyperther-
mia, and increased heart rate 24 hours before the onset of
lameness. Serum sodium, serum chloride, hematocrit, plasma
volume and blood volume were unchanged. At the onset of
clinical signs of laminitis, cardiac output and blood
pressure increased but total peripheral resistance was
unchanged. None of the measured or calculated parameters
predicted the onset of laminitis. Hypertension appeared to
be a response to rather than a cause of lameness.

Three of the remaining ponies apparently died of
Shock 29.3 i 2.7 hours after overfeeding. All three had
severe metabolic acidosis, decreased cardiac output, sys—
temic arterial pressure and plasma volume, and increased
hematocrit, total peripheral resistance and pulmonary vas-

cular resistance.

ACKNOWLEDGMENTS

The author expresses his sincere gratitude to Dr.

N. E. Robinson for his guidance and assistance throughout
this study and for his aid in the preparation of this thesis.

I am also very grateful to Richard Pleus, Roberta
Milar, and Carl Palazzolo for their technical assistance.

My committee members, Dr. J. B. Scott and Dr. R. P.
Pittman, are also to be acknowledged.

A special thanks goes to my wife, Laurie, for typing
the rough drafts of this thesis and especially for her
support during the course of the study.

This study was supported in part by grants from the

Morris Animal Foundation and Michigan Horse Show Association.

ii

 

‘1'. *f-H‘P.‘ ' -

TABLE OF CONTENTS

Page
LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . v
LITERATURE REVIEW 1
Introduction 1
Clinical Signs 2
Anatomy of Hoof Laminae 2
Empirical Causes . 5
Hypotheses on the Etiology of Laminitis 6
Local Blood Flow and Laminitis . 9
Overfeeding in Ruminants and Horses 9
Experimental Induction of Acute Laminitis . 1”
Cardiovascular Events of Overfed and Laminitis Ani—
mals 1“
Biochemical and HematologiCal Constituents after
Overfeeding . . . . l6
Shock and Laminitis 17
Experimental Rationale 18
MATERIALS AND METHODS . . . . . . . . . . . . . . . . . 21
Experimental Animals . . . . . . . . . . . . . . . 21
Surgical Procedure . . . . . . . . . . . . . . . . 21
Experimental Protocol . . . . . . . . . . . . . . . 23
Measurement of Blood Pressure . . . . . . . . . . . 23
Measurement of Cardiac Function . . . . 2“
Measurement of Blood Volume and Composition . . . . 25
Measurement of Acid- Base Status . . . . . . . . . . 27
Method of Electrolyte Determination . . . . . . . . 28
Statistical Analysis of Data . . . . . . . . . . . 28
TECHNIQUES . . . . . . . . . . . . . . . . . . . . . . 30
Cardiac Output Determination . . . . . . . . . . . 30
Plasma Volume Determination . . . . . . . . . . . . 32
RESULTS . . . . . . . . . . . . . . . . . . . . . . . . 3“
Clinical Signs of Carbohydrate Overload . . . 3“

Effect of Carbohydrate Overload on Acid- Base Status Au
Effect of Carbohydrate Overload on the Systemic
Circulation . . . . . . . . . . . . . . . . . . 44

iii

 

Effect of Carbohydrate Overload

Circulation

Effect of Carbohydrate Overload

Heart Function

Effect of Carbohydrate Overload

Volume and Composition

Effect of Carbohydrate Overload

lytes
DISCUSSION
SUMMARY . . .

LIST OF REFERENCES

iv

01’]
on
on

on

the Pulmonary
the variables of
Body Fluid

oserum Electro—

Page

53
62
62
71
79
89
90

LIST OF FIGURES

Figure Page

1. Frontolateral view of the equine foot indicating
projected cut-out area and histologic cross sec-
tion of the laminar region . . . . . . . . . . . . A

2. Changes in body temperature in laminitis and shock
ponies prior to the disease and death respectively 36

3. Measurements of systemic cardiovascular variables
in laminitis ponies before and after administra-
tion of high starch ration . . . . . . . . . . . . 39

4. Measurements of cardiac variables in laminitis
ponies before and after administration of high
starch ration . . . . . . . . . . . . . . . . . . Al

5. Measurements of acid—base status in laminitis pon-
ies before and after administration of high starch
ration . . . . . . . . . . . . . . . . . . . . . . H3

6. Measurements of acid-base status in shock ponies
before death . . . . . . . . . . . . ... . . . . . U6

7. Measurements of acid-base status in laminitis pon-
ies before and during clinical signs of the
disease . . . . . . . . . . . . . . . . . . . . . A8

8. Measurements of acid-base status before and after
peak base deficit in laminitis ponies . . . . . . 50

9. Measurements of systemic cardiovascular variables
in shock ponies before death . . . . . . . . . . . 52

10. Measurements of systemic cardiovascular variables
in laminitis ponies before and during the clinical
signs of the disease . . . . . . . . . . . . . . . 55

ll. Measurements of systemic cardiovascular variables
before and after the peak base deficit in lamini-
tis ponies . . . . . . . . . . . . . . . . . . . . 57

12. Measurements of pulmonary vascular variables in
shock ponies before death . . . . . . . . . . . . 59

V

Figure

13.

14.

15.

17.

l8.

19.

20.

Measurements of pulmonary vascular variables in
laminitis ponies before and during the clinical
Signs of the disease . . .

Measurements of cardiac variables in shock ponies
before death

Measurements of cardiac variables in laminitis
ponies before and during the clinical signs of the
disease .

Measurements of cardiac variables before and after
peak base deficit in laminitis ponies

Measurements of body fluids in Shock ponies before
death . . . . . . . . . . . . . . . . . . .

Measurements of body fluids in laminitis ponies
before and during the clinical signs of the
disease . . . . . . . . . . . . . . . .

Measurements of body fluids before and after drOp
in packed cell volume in laminitis ponies

Serum electrolytes measurements in laminitis pon-
ies before and during the clinical signs of the
disease . . . . . . . . . . . . . . . . . .

vi

Page

61

6A

66

68

7O

73

75

77

LITERATURE REVIEW

Introduction

 

Laminitis is defined as inflammation of the ungulate
foot. Although the so-called cardinal signs of inflammation
(redness, heat, swelling, and pain) are often present, the
cytologic and histologic signs of inflammation are incon-
clusive in this disease. Commonly known as "founder,"
laminitis has been recognized for centuries as a problem in
cattle and horses, yet its etiology is still unknown. Equine
laminitis was described by Zenophon, Aristoteles, Apsyrtes
and Hierocles (cited by Akerblom, 1934),39 They recorded
the clinical signs and hypothesized ontflmeetiology of the
disease. Laminitis has also been reported in pigs (Nilsson,
196A; Maclean, 1968)33 and experimentally in lambs (Morrow
et_el., 1973).35 It is characterized by congestion of
blood vessels in the laminae of the hoof (Adams, 1971!).2
Laminitis may be acute or chronic and may affect two feet or
all four. In horses the forefeet are the most susceptible,
while in cattle laminitis is more common and more severe in
the hind hooves. The chronic stage of the disease often
results in hoof changes. Common sequelae are rotation and

osteitis of the third phalanx.

Clinical Signs

 

When the forefeet of the horse are affected with lami-
nitis, the horse assumes a characteristic attitude (Adams,
1974).2 The forefeet are placed forward and the hindfeet
are carried under the body to shift the horse's weight to
the heels of the feet. The foundered animal is reluctant to
move in this acute stage and may Show anxiety, trembling,
and elevated temperature. Increased respiration and an ele—
vated heart rate are also seen. Locally, there is a digital
pulse with a very warm coronary band.

In the chronic stage there is often rotation of the
third phalanx. This rotation has been suggested as being
the result of the pull of the deep flexor tendon with corres—
ponding deterioration of the laminae (Coffman et_el., 1970).11
In severe rotation the anterior point of the third phalanx
may be pushed through the sole of the foot. Complications
such as sole ulceration, a most common hoof problem in
dairy cows, may be predisposed by severe, concurrent lamini-
tis (Chew, 1972),8 In the chronic stage, the horse has a
tendency to walk on its heels, and subsequently the toe of
the hoof becomes long and curled upward. Inflammation in
the coronary band may also cause ring formation on the wall

of the hoof.

Anatomy of Hoof Laminae

 

The laminae of the hoof are folds of epidermis and

dermis which form a matrix in the laminar corium between

the wall of the hoof and the third phalanx (Figure 1).12

Figure l.

Frontolateral View of the equine foot indicating
projected cut—out area and histologic cross
section of the laminar region. LC = laminar
corium; = first phalanx, second phalanx;
PI = third phalanx, = cartilage, = primary

fiimal lamina, 2 = secondary dermal lamina; 3 =
primary epidermal lamina; A = secondary epider—
mal lamina; w = wall of hoof.

JKMemae, 77

lC

 

 

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FIGURE

1

The dermal laminae contain a close-mesh network of blood
vessels and nerves. Blood flow through the vessels supplies
needed nutrients for the growth of the hoof and the keratin-
‘izing process of the epithelial cells. The laminae are
defined as primary dermal laminae, secondary dermal laminae,
primary epidermal laminae, and secondary epidermal laminae
(Figure 1).12 These latter laminae are the sites for the

beginning of keratinization.

Empirical Causes

 

Although the pathogenesis of laminitis is still uncer-
tain, the empirical causes of the disease have been described.
It may result as a complication of endometritis or severe
systemic infection. The post-parturient laminitis arises
from infection caused by retention of fetal membranes or
from other uterine infections. Concussion to the feet
from excessive work on hard surfaces causes what is known
as road founder, a traumatic type of laminitis. Water foun-
der results from ingestion of large amounts of cold water
by an overheated horse. Many miscellaneous causes of lami-
nitis have also been recorded, e.g. viral respiratory
disease and administration of exogenous corticosteroids.2

The most common causes of laminitis in cattle and
horses are associated with digestive disturbances. In
horses, eating large quantities of grain may give rise to
laminitis (grain founder). This type of disease is usually
associated with gastroenteritis, and the grains most common—

ly involved are wheat, corn, and barley.2 Experimental

induction of equine laminitis has been accomplished with a
high starch diet (Adams, 1974; Carner e§;el., 1975; Robinson
e§_el,, l976).l’ 17’ “3 It has been prOposed that Changes
occurring inthe hoof during laminitis may be sequelae of
acute gastrointestinal crisis, and the release of vasoactive
toxins (e.g. histamine, lactic acid).

Since the etiology is not definite, numerous hypothe-
ses for the pathogenesis of laminitis have been proposed.
Road founder appears to be simply mechanical damage of the
laminae. However, many other types of laminitis are the
result of disease in areas remote from the hoof. Therefore
toxins, vasoactive agents, bacterial endotoxins, and lower-
ing of certain amino acids have been considered as possible

causes of this disease.

Hypotheses on the Etiology of Laminitis

Presently there are two schools of thoughtcnlthe eti—
ologyciflaminitis. The first holds that acute laminitis is
a primary circulatory abnormality of the laminar corium
which leads to defects in keratin synthesis with subsequent
separation of the hoof wall from the third phalanx. The
second theory supports a primary alteration in keratin syn-
thesis which leads to loss of structural integrity, mechani-
cal damage to the laminar corium and circulatory changes.

The latter theory is supported by morphological stu—

39 and by studies of the biosyn-

dies in early acute laminitis
thesis of keratin in the hoof (Larsson et al., 1956).28

Obel (l9ll8)39 showed that in the early stages of acute

 

e— - n “...—z --— ~__.¢ ;__,_,- - '~‘.h _ .-~;A._.___._._..

laminitis, histOpathological changes occur only in the epi-
dermis and initially no change can be seen in the capillaries
or connective tissue. Consequently, Larsson (1956)28
theorized that the main epidermal changes in laminitis
consist of a disappearance of the "onychogenic substance"
(or keratin precursor) in the deep layer of the keratoge—
nous zone in the lamellar region and in the matrix of the
wall, the sole, and the frog. Through autoradiographic
investigations it appeared that methionine plays an impor-
tant role during keratinization in the stratum germinativum.
Cystine, another important factor, was seen to be incorpo-
rated hithis area when the keratin precursor had reached

the keratogenous zone. Larsson showed that the richness in
—S-S— groups, which characterized the definite keratin, is
due not only to oxidation of -SH groups, but also to a large
extent due to direct incorporation of cystine from the blood
stream. Radioactive cystine and methionine studies revealed
a marked decrease in arteriovenous difference in laminitis
cases. This along with decreased concentration of reducing
substances in the inner layers of the keratogenous zone

led Larsson to conclude that there is a blocking of the
metabolism of sulphur—containing metabolites in acute lami-
nitis. The principle objection to this conclusion is that
in the experiment no measurement of blood flow was made and
theoretically changes in blood flow could cause changes in
cystine and methionine uptake. Therefore it still remains

to be determined if the decrease in uptake of these necessary

amino acids is due to a change in blood flow to the laminae
or a blocking of metabolism.

The other hypothesis suggests that acute laminitis is
the result of a primary circulatory abnormality. Coffman
et_§1. (1970)12 using an angiographic technique have shown
a redistribution of blood flow in acute laminitis, blood
flow decreasing in the hoof and increasing in the adjacent
tissues. In this study the normal foot was characterized by
numerous fine vessels in the corium of the coronary band
and a symmetrical netelike vascular pattern inthe corium of
the hoof. In the laminitis foot there was poor filling of
the terminal arch, larger and fewer primary branches and
irregular vascular patterns in the corium of the hoof.
Coffman suggested that the decreased arterial blood flow to
the laminar corium initiates acute alimentary laminitis.
However, Robinson and his coworkers (1976)u3, by directly
measuring digital blood flow in a group of normal and lami-
nitis ponies, demonstrated that animals with clinical signs
of laminitis have increased digital blood flow. This in-
crease in blood flow is in apparent conflict with Coffman's
angiographic studies. This paradox could be reconciled if,
during laminitis, there was a marked increase in blood
flow through numerous arteriovenous anastomoses at the level
of the coronary band or through capillary beds proximal to
the laminae. Therefore even though there is increased flow
to the hoof, blood could still be shunted away from the
laminae. Further local redistribution studies need to be

done to resolve the problem.

Local Blood Flow and Laminitis

 

Since local blood flow could be changed in response to
naturally occurring vasoactive agents, it has been suggested
that these agents are liberated in the gut or locally in the
hoof and are factors in the develOpment of laminitis. His-
tamine has been suggested as one of these vasoactive agents
in both cattle and horses (Akerblom, 193A, 1939; Chavance,
1946; Nilsspn, l963).7’ 39 Increased blood levels of hista-
mine in equine laminitis have been reported by Akerblom
(193A, 1939) and in subacute and chronic cases of laminitis
in cattle by Nilsson (1963).39 However, in crosscirculation
bioassay eXperiments in ponies (Robinson et:§1., personal
communication), no evidence of vasoactive agents could be
found.

Digital vascular responses to the local perfusion of
acetylcholine, epinephrine, histamine, and seratonin in
normal and laminitis ponies have also been studied (Robinson,

1976).“2’ “3

In these experiments the digital vascular
responses were the same in both laminitis and normal ponies.
Therefore, the increased digital blood flow which was obser-
ved in the laminitis ponies was not attributed to altered

responsiveness to these vasoactive agents.

Overfeeding in Ruminants and Horses

 

Although little has been reported concerning overeat—
ing in horses or ponies, there has been extensive study of
food engorgement in ruminants. Excellent descriptions and

15, 32, 37, 38

reviews of the subject have been published.

 

— T‘l'!‘ 49““ "Afiam: ..~_L..‘..,,..__ ..'_ ...-.1 4...; ...-....

10

The foodstuffs which usually cause the common clinical
signs of overfeeding when consumed in excess are the ferment-
able carbohydrates: barley, maize, oats and wheat, apples,
pears, swedes, etc.37 The clinical signs of the overfed
animal include anorexia, dullness, staggery gait, labored
respiration, and variable changes in heart rate and tempera-
ture.32

Death may occur within 2A hours or the animals may

32 In nonfatal

linger for six to seven days before dying.
cases recovery is uneventful, with gradual restoration of
normal function of the rumen.

Major changes in the blood constituents and urine have
been reported as characteristic of this syndrome. The lac-
tate concentration in the blood is increased markedly.
DunlOp and Hammond (1965)16 have reported that both isomers
of lactate are present with a predominance of the D-isomer.
There is a decrease in the alkali reserve and decreased
blood pH.16 One of the first signs of a change in metabo—
lism is a fall in the pH of urine which is latter followed
by anuria.l6’ 32

Associated with ingestion of large quantities of solu-
ble carbohydrates is a lowering of the pH of the rumen con—
tents, together with an increase in concentration of lactic
acid and volatile fatty acids (Broberg, 1960; Ryan, 196A;
Scarisbrick, 195A).32 When the pH falls to pH 5, the con-
centration of volatile fatty acid tends to decrease, but

lactic acid concentration continues to rise, and the pH may

fall below pH A in severe cases (Ryan, 196M.“O With this

-1..-‘

11

fall in pH various changes occur in the rumen microorganisms.
A loss of protozoa occurs together with a loss of a number of
bacterial species (Hungate e331, 1952; Krogh, 1960) .23" 52 The
latter is replaced by massive numbers of lactobaccilli and
Streptococcus bovis (Krogh, 1961). In other words, the pre—
dominantly gram-negative ruminal flora is replaced by gram-
positive bacteria. The rumen wall may become necrotic and
motility decreases.27

The alterations in the bacterial population of the
rumen appear to be the result of a selectiweproliferationcm‘
organisms capable of fermenting carbohydrates rapidly (Hun-
gate et_el,, 196A).25 In addition, environmental changes in
the rumen make it unsuitable for the survival of other organ—
isms.32

Gutierrez et_el, (1959) reported that the protozoa are
important factors in reducing excess fermentation in the
rumen.Ll0 They engorge themselves with granules of carbohy-
drate and starch, thus delaying fermentation. However,
when large quantities of carbohydrate are introduced, this
system is overwhelmed, and once fermentation has reduced the
pH to 5, the protozoa die.

Dunlop and Hammond (1965)16 reported that a high pro-
portion of the less easily metabolized D(—) lactate is pro-
duced by rapid fermentation of carbohydrate in the rumen.
Using the washed—out rumen of the sheep, the rate of absorp-
tion of lactic acid was found to increase in parallel with
the concentration in the rumen, and it doubled when the pH

AC

was lowered from 5.2 to A.O. It is not until the rumen

12

contents fall below pH 5 that a significant amount of free
lactic acid is absorbed. Some lactate may be absorbed in
the small intestine whenever there is an accumulation of
lactic acid in the rumen. This absorption is probably im-
portant when high concentrations of lactic acid are formed
in the rumen.32
The increase in the lactate concentration in the blood
of these overfed animals has been attributed to at least
three factors: 1) absorption of lactic acid or its salts
from the alimentary tract, 2) an increase in anaerobic
metabolism, either as a result of less efficient transport
of oxygen by the circulation or due to tissue hypoxia caused
by a histotoxic factor, and 3) an increased gluconeogenesis

32

due to adrenocorticoid release. Because of this increase

in blood lactate in overfed ruminants, this metabolic dis-

32, AO An

order has been labelled as lactic acidosis.
increase in blood lactate has also been reported in overfed
horses (Garner, 1975).18 However no investigation into the
mechanism of production in these species has been made.

The necrosis of the rumen wall and the presence of
toxic factors in the digesta have been suggested by Dunlop
and Hammond (1965)16 as being responsible for the continua-
tion of the disease after the initial 2A hours. Histamine
and other amines have been implicated as possible toxic
factors produced in the rumen of engorged aminals.32 Uni—

dentified toxic factors have also been studied in rumen

fluid taken from overfed ruminants. One of these factors

 

13

was nondialyzable and heat stable, which suggests that it
may be an endotoxin.32

Laminitis has been reported as another complication
occurring after overfeeding in ruminants.37 Morrow et_el.
(1973)35 found that signs of laminitis could be created in
sheep when an 85% syrup of D—L lactic acid was injected into
the rumen of these animals. Since equine laminitis commonly
follows overeating of soluble carboyhydrates, it is possible
that the same pathogenesis following engorgement could be
occurring in the horse as in the ruminant. However, since
the horse does not have a rumen, the changes in microbial
populations may possibly occur in the cecum or large colon
since most of the microbial digestion takes place in this
area of the equine alimentary tract.3 Therefore, following
overeating in the pony, the cecum and large colon concentra—
tion of lactate could be elevated as in the rumen of overfed
cattle and sheep.

However, Hintz et_el. (1971), using ponies with cecal
and colonic fistulae, estimated that 65—75 per cent of the
soluble carbohydrates were digested and absorbed pre—cecally.3
The remainder and the insoluble and fibrous plant material
are digested by bacteria fermentation in the large intes—
tine (Elsden et_§l., 19A6). In addition, Alexander (1972)
has reported that the horse's stomach has a significant

concentration of lactic acid.3

Thus, it may be that changes
in carbohydrate digestion after engorgement could occur pre—

cecally in the horse.

lA

Experimental Induction of Acute Laminitis

 

Alimentary laminitis has been induced in the horse
with relative success (Obel, l9A8; Larsson e§_el., 1956;
Coffman e£_e1., 1970; Garner, 1975; Robinson e§_§l., 1976).12’
17’ 28’ 39’ ”3 Ponies in which the disease had been induced
by a dietary method often died of circulatory collapse
prior to development of clinical signs of laminitis (Coffman,

12, A3

1970; Adams, 1972; Robinson et a1, 1975).l’ In 1975,

Garner e2_§1. experimentally inducted equine laminitis with
a ration of 85% cornstarch and 15% wood cellulose flour.l7
The mean time lapse between administration of the gruel and
the onset of lameness was A0 hours, with the time lapse
ranging from 32 to A8 hours. Robinson e£_el. and Adams

report only relative success in inducing laminitis in ponies

with this dietary ration.

Cardiovascular Events of Overfed and Laminitis Animals
Although clinical characterization of the cardiovascu-
lar system at the time of acute laminitis has been extensive-
ly reported, very little exists in the literature concerning
cardiovascular changes after overfeeding and prior to lami—
nitis.
At the time of acute laminitis a hypertensive state
has been reported in horses and ponies.l7’ 18’ 39’ “3 How-
ever, cattle with acute laminitis have been reported to

39

have hypotension. No reason for this difference in species
has been suggested. However, it is known that the onset of

bovine laminitis seems to be less abrupt and the systemic

15

signs less pronounced than in the case of horses. Further—
more, cattle with laminitis seldom adopt the characteristic
posture of laminitis horses. Moreover, cattle often arch
the back, and horses practically never do this.

Overfed ruminants have been reported to exhibit a
shock-like condition,implicating a decrease in blood pres—

32 As

sure and possible death within 2A hours of engorgement.
was previously mentioned, horses and ponies that were exper-
imentally overfed have been reported to die of cardiovascu—
lar collapse. However, Garner et_el. (1975)17 reported

that experimentally overfed horses which developed laminitis
had increasing arterial systolic and diastolic pressures
from 16 to 56 hours after engorgement. Moderate arterial
hypotension has been reported by Garner (1975)18 during the
early onset phasecfi‘engorgement in horses, but in the final
prodromal phase of induced laminitis, arterial blood pres—
sure is significantly increased.

This progressive arterial hypertension during the on—
set of acute laminitis has been deemed the result of a con—
commitant increase in cardiac output. However, this specu—
lation did not take into account any increase in total
peripheral resistance that may also contribute to the
increase in blood pressure. In addition, preliminary studies
in the lab of Dr. N. E. Robinson show no evidence of a pro—
gressive increase in arterial blood pressure prior to acute

laminitis.

16

Heart rate has been reported to be elevated at the
time of acute laminitis in ponies, horses and cattle. Steady
increases in heart rate 16 to 56 hours after overfeeding in

horses has been reported by Garner et a1. (1975).17

Biochemical and Hematological Constituents after Overfeeding
Increased hematocrit values have been reported in
experimentally overfed ruminants (Dougherty eg;el., l975).l)4

Maclean (1970) stated that laminitis—affected barley beef
animals exhibited increased packed cell volumes.3u In
addition, Morrow (1973)35 reported that hematocrit increased
in lambs when 0.50% lactic acid was injected into their
rumens. Horses that were overfed to induce laminitis showed
an increase in hematocrit 2A to A0 hours after overfeeding
(Garner et_el,, 1975).17 This was correlated with increases
in total protein. However, Nilsson (1963)39 has reported

no statistically different packed cell volumes in acute
laminitis cattle compared to normal cattle.

Slightly lower than normal serum pH was observed in
laminitis cattle. Experimentally engorged ruminants Show
decreases in blood pH to values as low as 7.1 to 7.0.39
Lactic acidosis has also been implicated in overfed horses
prior to acute laminitis (Garner, 1975).18

Blood levels of calcium, phosphorous, magnesium,
sodium and potassium in laminitis cattle showed very little
deviation from normal levels (Nilsson, 1963; Maclean,

l970).3u’ 39 When laminitis was experimentally induced in

lambs,significant decreases in potassium occurred two days

 

17

after the time of induction. Although serum electrolyte
measurements have been made during the onset of equine lami-

nitis, no numerical data has been reported.

'Shock and Laminitis

 

Because laminitis follows overeating and a variety of
septic conditions, endotoxins and endotoxic shock are fre—
quently mentioned as part of the pathogenesis of the condi-
tion. Shock has been defined by Wiggers as a lack of
"effective circulating blood volume." Although there are
numerous precipitating causes of shock, the resulting
clinical manifestations are, in general, quite similar.
Usually, the cause of inadequate tissue perfusion in circu-
lation is inadequate cardiac output. The arterial pressure
usually falls at the same time as cardiac output, unless
nervous reflexes keep the pressure from falling. Therefore
any factors that can reduce cardiac output can also cause
shock (e.g. factors decreasing venous return or decreasing
myocardial contractility).

A shock—like syndrome has been produced in ruminants
and dogs when an unidentified toxic substance from the rumi—
nal fluid and blood plasma of overfed sheep was administered
intravenously (Dougherty and Cello, l9A8).l3 After the
injection of the toxic substance, blood pressure of the dogs
and goats was depressed. In addition, rumen motility in
sheep was inhibited and respiration was stimulated in all

the intact animals. Mullenax et a1.38

_—

injected toxic

materials from rumen bacteria and rumen fluid subcutaneously

 

18

and intraruminally in sheep and cattle. Immediate decreases
in arterial blood pressures were followed by increases with-
in one minute to above pretreatment levels. A decrease in
rumen motility was also observed.

Following endotoxin administration, various species
including rabbit, monkey, cat and dog, develop hypotension
(Kuida et_el., 1958, 1961).“8 Arterial pressure and cardiac
output have been reported to decrease during progressive
shock produced by gram-negative E. coli bacteria, but the
calculated peripheral resistance remains unchanged.

Burrows (1971)6 created E. coli endotoxemia in con-
scious ponies by intravenous injections of endotoxin.
Immediately after administration there was a precipitous
decrease in mean arterial blood pressure. Subsequently,
there was a sharp increase in blood pressure above preinjec-

tion levels and then a gradual decrease until death.

Experimental Rationale

 

Ingestion of excessive amounts of readily-fermentable
carbohydrates by ruminants, horses, and ponies results in
acute indigestion. This overeating may lead to a shock—like
condition and subsequent death, or acute laminitis. In
ruminants, the predominately gram—negative ruminal flora is
replaced by gram—positive bacteria, principally Streptococ-

25’ 27 These produce large

cus bovis and lactobacilli.
quantities of lactic acid which alters the intraruminal en-
vironment and affects the acid-base status of the animal.32

A toxic substance may also develOp in the gut as a result of

19

abrupt change in environment. These may cause sudden
changes in the cardiovascular system of the overfed animal .
Similar changes may be occurring in the overfed horse or
pony even though anatomical differences exist. Furthermore,
systemic cardiovascular changes or alterations in the acid—
base status of these animals may be the cause or causes of
acute laminitis.

If a toxic substance is released into the circulatory
system after overfeeding and prior to laminitis, it may be
reflected in the cardiovascular system during the onset of
the disease. Although Garner e3431. (l975)l7’ 18 studied
the cardiovascular events of overfed horses during the onset
of laminitis, some of their results could not be duplicated
in preliminary studies conducted in the lab of N. E. Robin—
son (Michigan State University). In addition, unanswered
questions develOped from that study. The cause of the pro—
gressive hypertension reported during the onset of laminitis
is still unknown. It may be due to an increased cardiac
output, an increased vascular peripheral resistance, or a
combination of these two variables. The reason for the
increase in hematocrit is also unanswered. A decrease in
plasma volume or an increase in the number of circulating
erythrocytes, or both, could explain this elevation. In
addition, the acid—base status and the serum electrolyte
status after overeating and prior to laminitis have not been

thoroughly monitored and reported in the literature.

 

20

Therefore, the present study was designed to monitor
the cardiovascular events and record the acid-base status of
the overfed pony during the onset of laminitis. This inves-
tigation also allowed for concurrent measurements of circu-
lating bloodxnfihnmxsand serum electrolytes in an effort to

answer the previously proposed questions.

 

-..-.m'A. _ « ". .—

MATERIALS AND METHODS

Experimental Animals

 

Twelve mature Shetland ponies (9 females and 3 males),
weighing 162 to 255 kilograms, were brought indoors and fed
grass, hay, and oats for at least one week prior to induc-

tion of laminitis.

Surgical Procedure

 

Twenty-four hours prior to iatrogenic induction of
laminitis, the ponies were anesthetized with sodium thiamyl
(10 mg/Kg of body weight, given intravenously). After
intubation, anesthesia was maintained with halothane and
oxygen. The left jugular vein and carotid artery were ex-
posed and freed of fascia. In most of the ponies the left
carotid artery had been surgically exteriorized to lie sub~
cutaneously, at least two months prior to the experiment.
To allow for direct measurement of arterial blood pressure
and arterial blood sampling, the left carotid artery was
cannulated with a saline filled No. 7 French teflon end-hole
catheter secured with a silk ligature. The proximal end of
the catheter was run subcutaneously, surfaced close to the
dorsum of the neck, and was tied loosely with surgical tape

to the mane of the animal.

21

 

 

 

22

Two catheters were introduced into the jugular vein.

A No. 7 French balloon tipped catheter with a thermistor
located near the tip (Columbus Instruments International
Corporation, Columbus, Ohio) was floated through the right
heart and positioned in the pulmonary artery. The catheter
was used to measure body temperature, pulmonary arterial
pressure, and changes in blood temperature from which car-
diac output was calculated. A second No. 7 French end~hole
catheter was positioned in the right atrium. This catheter
was used to record right atrial pressure and served as the
injection port for the bolus of cold saline used in the
thermodilution determination of cardiac output. A silk
ligature around the vein secured both catheters. The proxi-
mal ends of both catheters were run with the arterial
catheter subcutaneously and secured at the same exposed
dorsal location on the mane.

Venous catheter placements were verified by recorded
pressure measurements and wave forms recorded on a multichan-
neled oscillOSCOpe and recorder (VR-6, Electronics For
Medicine Inc., White Plains, N.Y.).

The surgical site was closed using a simple inter—
rupted suture of #1 Dexon suture material. After recovering
from anesthesia, ponies were placed in a specially designed
restraining box bedded with straw. The ponies were given
free access to water and hay, and allowed to become accus-
tomed to the box overnight, before control values were

taken.

 

23

Experimental Protocol

 

Twenty-four hours after placement of catheters, con-
trol measurements were made of the following variables:
arterial blood pressure (systolic, diastolic, and mean),
right atrial pressure, mean pulmonary arterial pressure,
heart rate, cardiac output, stroke volume, plasma volume,
hematocrit, blood volume, total serum solids, arterial pH,
arterial P002, arterial P02,

Cl-), and serum osmolarity.

serum electrolytes (Na+, K+,

After these control measurements were made, the ponies
were given a special ration (17.6 g/kg of body weight) con-
taining 85% starch and 15% wood cellulose (Garner e£_§l.,
1975). The special ration was diluted in water and adminis—
tered by stomach tube, usually in one dose, but occasionally
in two doses 51 half hour apart.

Every eight hours after the administration of the
ration, measurements of the experimental variables were
repeated until laminitis was diagnosed. The clinical signs
for identification of acute laminitis included warm feet,

a digital pulse, lameness, and the characteristic laminitis
posture. Clinical signs were also recorded over the eXper-
imental periods prior to the disease. Euthanasia was per-
formed with an overdose of barbituates no longer than eight

hours after the disease was diagnosed.

Measurement of Blood Pressure

 

Blood pressure measurements were made directly using

the saline filled indwelling catheters connected to a blood

2A

pressure transducer (P23 Db, Statham Instruments, Hato Rey,
PR.) and recorded by the VR—6 recorder. The catheters were
flushed with saline and blood was aSpirated. When blood
could be aspirated freely and wave forms looked normal, it
was assumed that the catheters were recording accurate blood
pressures.

By electronically dampening the response of the

recording system, mean arterial pressures were recorded.

Measurement of Cardiac Function

 

The thermal dilution technique was used to measure
cardiac output. Ten ml of cold physiological saline (0-2° C)
were injected (manually) into the right atrium through the
teflon end—hole catheter. The blood temperature change in
the pulmonary artery was sensed by the thermistor at the tip
of the No. 7 French balloon catheter. This precalibrated
thermistor catheter was connected to a cardiac output com-
puter (Cardiotherm-500, Columbus Instruments International
Corp., Columbus, Ohio). A thermistor sensing the tempera-
ture of the cold injectate was also connected to the compu-
ter. Five cardiac output determinations were made and an
average of these was recorded for each time period.

Heart rate was determined from the systemic arterial
pulse trace, while stroke volume was calculated from the

measured heart rate and cardiac output.

 

 

25

Measurement of Blood Volume and Composition

 

Plasma volume was measured as the volume of distribu-
tion of Evans blue dye which binds tightly to plasma albumin.
Fifty ml of blood were taken with a heparinized syringe and
centrifuged to obtain plasma. Ten samples of Evans blue were
diluted to known concentrations (.005-.080 mg/ml) with this
plasma. The optical densities of these concentrations were
read on a spectrophotometer (Beckman Instruments Inc., Ful-
lerton, California) set at 620 nm and a standard curve of
optical density due to the dye vs. the concentration was
constructed.

For the determination of the pony's plasma volume,
about 10 ml of blood were drawn from the carotid artery.
The withdrawn blood was transferred to a vacutainer collec-
tion tube with an anticoagulant (EDTA). This blood was
used for determinations of the optical density of dye-free
plasma (blank) and for determination of hematocrit. Using
the right atrial catheter, a predetermined amount of a di»
1uted Evans blue solution was injected and the catheter and
syringe were rinsed by repeated aspirations and re—injec-
tions of blood. The catheter was then rinsed with hepari-
nized saline.

Thirty, forty, fifty, and sixty minutes after comple-
tion of the injection of dye, blood samples were taken from
the artery, for determination of the concentration of Evans
blue. Samples were immediately transferred to vacutainer

collection tubes with EDTA. The samples were centrifuged at

 

26

3000 r.p.m. for 10 minutes to obtain the dyed plasma. The
Optical densities of these samples were read at 620 nm
using the Beckman spectrophotometer.

The dye obtained form the Fisher Scientific Company,
Pittsburg, Pennsylvania, was used in 1% aqueous solution for
nine ponies. A new lot of Evans blue from Eastman Kodak
Company, Rochester, N.Y., was used in .2% aqueous solution,
because of its much higher purity, for the last three ponies.
The amount of injected 1% or .2% solution of Evans blue was
as a rule 20 ml, corresponding to a dose which would give
a SpectrOphotometrically determined Optical density in dyed
plasma within a range of .200-.800. Within this range a
linear relation between the concentration of dye and the
optical density due to the dye was demonstrated for the
Evans blue in the plasma.

The difference between the optical density of the
blank plasma (before Evans blue injection) and the Optical
density of a dyed plasma sample was the optical density
value due to the dye. The concentration of Evans blue in
that plasma sample was derived from the standard curve con-
structed for that pony.

The dye concentrations of the four unknown samples
were plotted against time on semi—log paper and extrapolated
to to, or injection time, and this value was used to calcu-
late the plasma volume when the slope of the curve indicated
clearance from the plasma. However, in most cases clearance
was not clearly indicated,and therefore an average of the

four concentrations was used to calculate the plasma volume.

 

27

This volume was calculated from the quotient %§, where EB
is the amount of injected dye in mg and C the Evans blue
concentration in mg per ml of plasma (Chien & Gregersen,
1962).10

The total blood volume was calculated from the equa—

PV x 100
100 - Hct.’

is the hematocrit. No correction was made for trapped

 

tion, where PV represents plasma volume and Hot.

plasma.
Total solids were measured from the blank plasma sam—

ple using a refractometer.

Measurement of Acid—Base Status

 

The arterial blood pH, P002 and Po2 were analyzed
from a 3 ml heparinized anaerobic sampletxnunifrom the caro-
tid artery. After withdrawal, the syringe was immediately
sealed and stored in ice water. The arterial sample was
analyzed within one hour of withdrawal. Pco2, Po2 and pH
were measured with a blood gas analyzer (Blood Micro System,
Type BM53, Radiometer, Copenhagen, Denmark). The pH elec-
trode of the analyzer was calibrated before every measure-
ment with a low pH standard buffer (pH = 6.8A0) and a high
pH buffer (pH = 7.38A). Two pH measurements were made for
each sample. The blood gas electrodes were calibrated with
a low standard tank (Fo2 = 0.0%, F002 = 3.02%) and a high

standard tank (Fo2 = 21%, Fco = 12.05%). Two Pco2 and Po2

2

measurements were made for each blood sample.

28

Method of Electrolyte Determination

 

Serum electrolytes were measured from serum collected
from a 10 ml arterial blood sample. The serum was stored
frozen until the time of analysis. Sodium and potassium
were measured with a flame photometer (Beckman Instruments
Inc., Fullerton, California). Chloride was analyzed with a
chloridometer-automatic titrator (Buchler Instruments Inc.,
Fort Lee, N.J.). Osmolarity of the serum samples was made
with an osmometer (Advanced Instruments Inc., Newton High-

lands, Massachusetts).

Statistical Analysis Of Data

 

All data was statistically analyzed by analysis of
variance, randomized complete block design, because of known
or anticipated sources of variation between ponies other
than experimental effects (e.g. genetic). If the F value
for the time intervals was significant, then specific com-
parisons between treatment means were made using the Student-
Neuman-Keul's test.

The data from those ponies that died of shock were
analyzed as hours before death defined as zero time. There-
fore control, —2A, ~16, and —8 hour measurements Of all the
variables were analyzed.

Measurements taken from the ponies of the laminitis
group were analyzed in several different ways. The first
analysis involved hours after the administration of the diet
with overeating as zero time. Control (0 — hour), 8, 16,

2A, 32, A0 and A8 hour measurements were therefore

29

statistically analyzed. Since the laminitis ponies deve—
loped the disease at different time intervals (between 32
and 72 hours after overeating), a second analysis was per-
formed using the time 1aminitis was clinically diagnosed as
zero time and analyzing A0, 32, 2A, 16, 8 and 0 (time of
laminitis) hour measurements prior to the development of the
disease.

Significant changes of certain variables were analyzed
to determine their effect on the other variables measured.
Since a transitory peak in base deficit was common for every
laminitis pony, this event was analyzed to determine its
effect on the other variables that were measured. The indi-
vidual peaks in base deficit occurred at different time
intervals prior tothe disease, therefore zero time was
defined as the peak in base deficit, and 2A, 16 and 8 hour
measurements before the peak in base deficit, and 8 hour
measurements after the peak were analyzed. Another event
that was characteristic for the laminitis ponies was a drOp
in PCV 8 to 16 hours prior to the signs of the disease. To
determine if this drop was caused by a rise in plasma
volume or a drOp in cell volume of the blood, zero time was
defined as the drop of PCV and 16 and 8 hour measurements
before the drop and 8 hour measurements after the drop

were analyzed.

 

TECHNIQUES

Cardiac Output Determination

 

The thermal—dilution technique introduced by Fegler
(195A)50 is similar in principle to the indicator dilution
technique. It involves injecting a measured amount of room
temperature or cooler indicator (e.g. physiological saline)
rapidly into the right atrium and measuring the subsequent
change in blood temperature by a thermistor in the pulmonary
artery. Cardiac output is calculated from the formula,

(Tb—T1)Vi

0.0. = A , where Tb represents blood temperature, Ti

represents indicator temperature, Vi represents the volume

 

of injected indicator, and A represents the area under the
dilution curve, which is measured as a temperature change
in the pulmonary artery in 00 x min. The accuracy and
reproducibility of the thermal-dilution technique have been

documented extensively in man, dog, and horse.20’ 23’ 36’

“9’ 50 Needs for complex electronics have been minimized
and immediate results are now available. The injection
material (cold physiological saline or 5% dextrose) is
readily obtainable and safely administered. Furthermore,

the thermal-dilution curve has minimal recirculation errors

since temperature of the injectate is equal to blood

30

31

temperature after one circulation time. This eliminates
elaborate extrapolation.

Several practical problems exist with this technique,
which were noticed in this study. Distinct fluctuation in
pulmonary arterial temperature, related to cardiac and
respiratory cycling, PGSUltSZhla fluctuating thermal base

line.50

These fluctuations may approach the magnitude of
the temperature change that follows "cold" injection.
Although electronic averaging provides a more stable base
line, iced rather than room-temperature injectate was found
to give a sufficient signal-tO—noise ratio. These "cold"
injections were made only when stable base lines were indi—
cated by the unchanged microthermistor reading.

Another problem is the determination of the true tem—
perature of the injectate as it enters the vascular system.
Heat is gained by injectate from the syringe, and catheter
wall. Therefore to minimize this error all injections were
done as rapidly as possible.

Finally, although there is no recirculation of the
thermal indicator, there is a delayed return of the vascular
temperature to base line. This delay is probably due to a
slow washout of "cold" from the myocardium and endothelial

50

surfaces and the catheter. Therefore, there is a variable
time delay before a second injection can be made.
The cardiac output computer used in this study elec—

tronically averaged base—line fluctuations, integrated the

thermal dilution curve, applied the necessary correction

32

factor and displayed the cardiac outputs in liters per
minute within seconds after the injection. 1t allowed mea—

surements to be made rapidly and accurately reproducible.

Plasma Volume Determination

 

The Evans blue dye dilution technique was used instead
of the radioiodinated albumin technique because of its
simplicity and safety when frequent determinations must be
made over a period of days. This technique has some disad—
vantages. Since the volume Of distribution of the adminis—
tered dye is that of plasma albumin (Rowson, 19A3)10, any
escape or metabolism of albumin during the course of the
equilibrium will give an overestimation of plasma volume.
These errors may be reduced by taking measurements closer to
the time of injection; but if samples are taken too soon
after injection of the indicator, the error from inadequate
mixing increases.“l To minimize the above disadvantages
the extrapolation method was used in this experiment.

The occurrence of turbidity and residual dye in the
blood are also problems. In an effort to reduce plasma tur—
bidity, all the samples were refrigerated 2A hours and recen-
trifuged before optical density was measured. Since
frequent determinations of PV were made and Evans blue is
eliminated from the blood at a slow rate, the plasma con-
tained residual dye from the previous determination. It
was assumed that no changes in the content of residual dye
occurred, other than those due to normal elimination. There-

fore correction for this factor could be made by regarding

33

the Optical density of 620 nm in the blank sample as back—
ground absorption (Gibson and Evans, 1937; Hungate et_§l,,
1963).”1

Because this investigation dealt with a pathological
condition (which in some cases lead to shock), errors in
PV measurements due to poor perfusion or poor mixing may
have occurred. Measurement of red cell and plasma volumes
in shock have been criticized because of the failure to
obtain complete time concentration curves, since it has been
shown that the mixing of the injected indicator can be Sig-

nificantly delayed.Ll7

Inadequate mixing would lead to the
erroneous conclusion that volume is reduced when in actua-
lity there may be little change in absolute volume. However,
Noble and Gregersenu8 have reported that sluggish blood flow
did not prevent or delaytfluauniform mixing of the dye when

it was intravenously administered. In addition they showed
that dye did not escape from the circulation more rapidly

in shock than in normal conditions. Therefore they concluded
that this dye method of measuring plasma volume was valid,

A8

even in the presence of shock.

RESULTS

Clinical Signs of Carbohydrate Overload

 

0f the twelve mature Shetland ponies that were overfed,
seven developed acute laminitis, three died of cardiovascu-
lar collapse (shock), one was unaffected, and one was eutha-
nized because Of other complications.

The ponies which developed laminitis (No. 1, 2, 3, 6,
10, 11, and 12) showed clinical signs of the disease at
different time periods within a range of 32 - 72 hours after
administration of the diet (mean of 56.0 i 3.5 hours).
Signs ranged in severity from Obel Grade II (pony moved
willingly at a walking pace, but the gait was characteris-
tic of laminitis) to Obel Grade IV (pony did not move with-
out being forced). Ponies were clinically normal for the
first l6-2A hours after administration of the diet, but had
pasty green-white feces and increased intestinal sounds
during the remaining periods prior to laminitis. Body tem-
perature and pulse rate increased in these ponies and the
conjunctival mucosa was injected at the time laminitis was
apparent. At this time body temperature was 39.1A : 0.31°C
compared to a control value of 38.60 i 0.290 C (Figure 2).

Ponies which died of shock (No. 5, 7, 9) had rapid

pulse rates, respiratory distress, increased rectal

3A

Figure 2.

35

Changes in body temperature in laminitis and
shock ponies prior to the disease and death
respectively (mean : standard error). C =
control measurement before administration of high
starch ration; *= statistically significant
change (P < 0.05) from control value, using ran-
domized complete block analysis of variance and
Student—Newman—Keuls' tests.

36

LAMINITIS

 

40.04
39.81
39.61
394-
39.2:
39.0-
38.81
38.8 .
38.4‘
382‘

Body Temp.
CO

 

 

11
7/

 

A
O 1 I T 1 1 v

c 40 32 24 1‘6 8
HOURS BEFORE LAMINITIS

04

SHOCK

 

405+ *
40.0«

Body Temp. 1‘39-5'i
C ° 39.0-
38.5‘ I,«’

38.01

 

OJ—H 1 I I
C Z4 '6 8
HOURS BEFORE DEATH

 

FIGURE 2

37

temperatures, and extremities cold to the touch prior to
death. Abdominal distension was apparent at least eight
hours before death. Body temperature at this time was A0.30
: 0.300 0 compared to the control of 38.37 : 0.25° C. These
ponies died within 2A-32 hours after the administration of
the diet (mean of 29.3 i 2.7 hours).

In all ponies develOping laminitis there were transi-
tory changes in base deficit, arterial blood pH and P002
during the onset of the disease, and an elevation in cardiac
output and arterial blood pressure at the time of laminitis.
In each pony these events occurred at differing time inter-
vals after diet administration, but at fairly constant time
intervals prior to the clinical signs of laminitis. As a
result, when means were calculated using overeating as zero
time, many of the acid—base and cardiovascular changes
were obscured (Figures 3, A, 5). However, using the time of
laminitis as zero time and taking mean values 8, 16, 2A, 32
and A0 hours before laminitis, characteristic changes became
apparent. For this reason all the results from laminitis
ponies are analyzed using the clinical signs of laminitis as
zero time.

A similar problem occurred in the ponies dying of
shock. The course of shock was also highly variable. There
is one endpoint-~death, and two incontrovertible temporal
reference points--the time of onset and the time of death.
Therefore death was designated as zero time and the mean

values 8, l6, and 2A hours before death were used. Since

Figure 3.

38

Measurements of systemic cardiovascular variables
in laminitis ponies before and after administra-
tion of high starch ration (mean : standard
error). P art. = systemic arterial pressure; C =
control measurement before administration of
ration; -*= statistically significant change

(P < 0.05) from control value, using randomized
complete-block analysis of variance and Student-
Newman—Keuls' tests.

Total
Peripheral
Resistance

mmHg
L/manKg

Cardiac Output
L lmanKq

Part.
mmHo

l5001
I400-
|300«
l200<
l IOO‘
lOOO~

ll

.llOd
.IOO-

.080«
.0704
.060d

1

MOA
llO‘l
l20«
llO-l

IOO-4
90

ff

 

<P‘

Systolic
I\\ Mean
\L I
\ Diae'olic
\\
2\\“W

I6 24 32 4O 48
HOURS

 

{b

O
O:

FIGURE 3

Figure A.

A0

Measurements of cardiac variables in laminitis
ponies before and after administration of high
starch ration (mean : standard error). Pra =
right atrial pressure; Vs = stroke volume. C =
control measurement before administration of
ration; -*= statistically significant change

(P < 0.05) from control value, using randomized
complete—block analysis of variance and Student—
Newman-Keuls’ tests.

Heart Rate

Beats Imin.

Pro
mm.Hg

NIL/Kg

ISO-I
l204
IlO-
IOOd

.501

I,

 

 

FIGURE A

Figure 5.

A2

Measurements of acid—base status in laminitis
ponies before and after administration of high
starch ration (mean : standard error). Pco =
partial pressure of carbon dioxide in arterial
blood. C = control value before administration
of ration; ‘*= statistically significant change
(P < 0.05) from control value, using randomized
complete—block analysis of variance and Student—
Newman—Keuls' tests.

A3

381
34" ”
Pco 32-

mm. Hg 30“

28-4
264 *

24.

I

o q
I0.0-l
8.0-1

Base Deficit 6-0‘
meq/L 4.0.. P,”

2.0..

[I
//

7,42.

740-
p H 7.38~

7.36- I"r~

7.3+

7.32.

 

 

5
m
.8.
on
N
.5
o
45-4
0

FIGURE 5

 

 

 

AA

only three ponies died of shock, trends rather than signifi-

cant changes were looked for in this data.

Effect of Carbohydrate Overload on Acid-Base Status

 

In both groups of ponies a metabolic acidosis develOped
after overeating. Acidosis was most severe in ponies dying
of shock. Base deficit increased with time and eight hours
before death was 21 i A meq/l (Figure 6). Laminitis ponies
developed a transitory base deficit (2A : 2 hours before
laminitis and 30 i 3 hours after diet administration), and
it was less severe (10 i 2 meq/l) than in the shock group
(Figure 7). When data was analyzed using the peak in base
deficit for each pony as zero time and taking mean values 8
and 16 hours before the peak and 8 hours after the peak,
significant decreases of arterial blood pH and P002 corre-
lated with the base deficit. Base deficit and arterial

blood pH were normal when laminitis was diagnosed (Figure 8).

Effect of Carbohydrate Overload on the Systemic Circulation

 

Cardiac output (C.0.) and mean arterial blood pressure
decreased and total peripheral resistance (TPR) increased
8-16 hours after the diet, in ponies dying of shock (Figure
9). Eight hours before death 0.0. had decreased from a con-
trol value of .083 i .005 l/min./Kg to .0A5 : .00A l/min./Kg,
systemic pressure decreased from 123 i 7 mmHg to 89 i 5
mmHg, TPR increased from 1360 i 100 mmHg/l/min./Kg to 1820
i 180 mmHg/l/min./Kg, and pulse pressure decreased from 39

i A mmHg to 18 i 1 mmHg.

Figure 6.

A5

Measurements of acid-base status in shock ponies
before death (mean : standard error). Pco2 =
partial pressure of carbon dioxide in arterial
blood. C = control value before administration
of ration; ‘*= statistically significant change
(P < 0.05) from control value, using randomized
complete—block analysis of variance and Student—
Newman—Keuls' tests.

Pcoz
rman

Base Deficit
med/L

pH

40-
35.
30-
25-
204

Z4-
13-
12-
1h
10.

11

A6

 

 

i,
\
\
\\
\ 4 ¥
4
t
4
4
I 4
/
I
II
I.\
4
——4}——nr-$. f e .
C 24 IS 8

HOURS BEFORE DEATH

FIGURE 6

Figure 7.

A7

Measurements of acid—base status in laminitis
ponies before and during clinical signs of the
disease (mean : standard error). Pco = partial
pressure of carbon dioxide in arteriaI blood.

C = control measurement before administration of
high starch ration; '*= statistically Signifi—
cant change (P < 0.05) from control value, using
randomized complete—block analysis of variance
and Student—Newman—Keuls' tests.

Pco2 30+

mmHg

742-
7404
7.
736‘
734-
232-
7304

PH

BASES
DEFlClT

meq/L

 

A8

38-
34. ,..
32-

281
26-1
24-

[1
7r

 

384

 

 

 

 

 

I V

C --40 ~32 ‘24 -l6 -8
HOURS BEFORE LAMINITIS

0‘

FIGURE 7

Figure 8.

A9

Measurements of acid-base status before and after
the peak base deficit in laminitis ponies (mean :
standard error). C = control measurement before
administration of the high starch ration; * =
statistically significant change (P < 0.05) from
control value, using randomized complete—block
analysis of variance and Student—Newman—Keuls'
tests.

SO

36-
Peak Bose Deficit

\ l

344

CO
2 30.
mmHg 28‘

 

0.
l24 ‘V‘
Base Deficit ,0.

meq / L 8-

II
’I

 

 

FIGURE 8

Figure 9.

51

Measurements of systemic cardiovascular variables
in shock ponies before death (mean 1 standard
error). TPR = total peripheral resistance; CO =
cardiac output; P art = arterial blood pressure.
C = control measurement before administration of
high starch ration; ‘k= statistically signifi—
cant change (P < 0.05) from control value, using
randomized complete—block analysis of variance
and Student—Newman—Keuls' tests.

52

ZOOOT

l800l
TPR l600« //
mm.i-lg /'

L/min./Kq l4004 f,
IZOO-

I)
’I

.081 1.

co .0 7- \

\
L/min./Kq ‘0 5‘
.0 5- 4
.044

I501 I
140- \

[L
'/

I304 \\
Part '20, L
"‘m' ”9 llOA ?\\\
lOO- \ Systolic
Mean

I Diastolic

 

 

O'L—+_T—4 I I
C l6
HOURS BEFORE DEATH

N
.b
0‘

FIGURE 9

53

Animals develOping laminitis showed no significant
changes in cardiac output, arterial blood pressure or total
peripheral resistance during the onset of the disease (Figure
10). At the time of laminitis both 0.0. and arterial blood
pressure were Significantly increased above control values
(Figure 10). Cardiac output increased from a control of
.070 i .010 l/min./Kg to .100 i .010 l/min./Kg while mean
arterial blood pressure increased from around 100.3 : A.7
mmHg to 130.1 : 7.8 mmHg (Figure 10). Both systolic and
diastolic pressure were also elevated at the time of the
disease. TPR remained unchanged (Figure 10).

The transitory metabolic acidosis occurring in the
laminitis ponies didrkfi2correlate with changes in blood

pressure, C.O., or TPR (Figure 11).

Effect of Carbohydrate Overload on the Pulmonary Circulation

 

The pulmonary vascular resistance (PVR) increased
with time in all the shock ponies, while those that deve-
loped laminitis showed no change in PVR during the onset of
the disease (Figures l2, 13). In the shock group PVR
almost doubled from the control (7A% increase) eight hours
prior to death. Even though the cardiac output decreased
significantly in these ponies, their mean pulmonary arterial
pressure remained unchanged because of the increase in PVR

(Figure 12).

5“

Figure 10. Measurements of systemic cardiovascular varia-
bles in laminitis ponies before and during the
clinical signs of the disease (mean : standard
error). TPR = total peripheral resistance; CO =
cardiac output; P art = arterial blood pressure.
C = control value before administration of high
starch ration; * = statistically significant
change (P < 0.05) from control value, using
randomized complete—block analysis of variance
and Student—Newman—Keuls' tests.

ISOO‘
TPR I400d

l300~
mm. Hg

L/manKq '200‘
noo-

IO 00-

55
d
.llO-q *
.lOO-
CO .090.
lein. / Kg .080 -
.070‘ ‘I...~
.060~

o.
i

7’

"

 

O-l
ISO
1 * Systolic
I40‘
I30- * Mean
l20~ T
Part. ‘\\ it Diastolic
IlOd \
mmHg
IOO-l I‘\\
90 "L I
Ix“? I I
80«
70-

 

 

-4O '32 -24 -l6 -8
HOURS BEFORE LAMINITIS

0.1

FIGURE 10

Figure ll.

56

Measurements of systemic cardiovascular varia—
bles before and after the peak base deficit in
laminitis ponies. TPR = total peripheral resis-
tance; CO = cardiac output; P art = arterial
blood pressure. C = control value before admin-
istration of high starch ration; *= statisti-
cally significant change (P < 0.05) from control
value, using randomized complete—block analysis
of variance and Student—Newman—Keuls' tests.

I500-
l4004
TPR BOO-i

mmHg IZOO-

L/min./Kg ”004
ioooi

CO

L/minJKq .07-
06+

Oq
ISO‘
IZO-ii

P art "0‘

mm. Hg '00‘

 

QCD
flglgno

57

Peak Base Deficit

l

 

\

I\\\L\I/I\I Systolic

\
\ Moan

Y\\‘:T\<f—§\x/f/I’Y/r DiastoIic

 

 

FIGURE 11

Figure 12.

58

Measurements of pulmonary vascular variables

in shock ponies before death (mean 1 standard
error). PVR = pulmonary vascular resistance;
CO = cardiac output; Ppa = pulmonary arterial
pressure. C = control value before administra—
tion of high starch ration; = statistically
significant change (P < 0.05) from control
value, using randomized complete—block analysis
of variance and Student—Newman—Keuls' tests.

 

59

500a
4;
460-

4204
PVR 380-

mqu .
L/minJKg 340- /

300« ,’
/
260- I

41
fl

\
CO 006‘ \.
L/minJKg

Ppa

mm. H9 261 I\\

 

 

oejay
C

2% uh é
HOURS BEFORE DEATH

FIGURE 12

Figure 13.

60

Measurements of pulmonary vascular variables in
laminitis ponies before and during the clinical
signs of the disease (mean 1 standard error).
PVR = pulmonary vascular resistance; CO = car-
diac output; Ppa = pulmonary arterial pressure.
C = control value before administration of high
starch ration; *'= statistically significant
change (P < 0.05) from control value, using
randomized complete-block analysis of variance
and Student-Newman—Keuls‘ tests.

61

400
380-
360~
PVR 340*
320.

mmHg 300.
L/min/Kg 280q

 

 

 

 

 

260+

 

 

I
II

01

.qu

CO 091

L Imin. /Kq 08‘
or ——-

.06~

11
7]

321
3!-
30« .

P pa .. v ‘1
29d

rum-Ho 28- --- +

274
26" L Ji.

 

 

 

 

I

II
I,

 

04.4, . - a

 

c «10 -32 1'4 4? -e o
nouns BEFORE mems

FIGURE 1 3

62

Effect of Carboyhydrate Overload on the Variables of Heart
Function

In the shock group, heart rate increased and stroke
volume decreased after overeating, but right atrial pressure
did not change (Figure in). Heart rate increased 91% and
stroke volume decreased 71% from control eight hours before
death.

There was a transitory decrease in stroke volume in
the laminitis ponies 22 i 2 hours before laminitis (36 i 6
hours after the diet administration) (Figure 15). This
decrease was less severe (37% decrease from control) than in
the ponies that died of shock (71% decrease from control).
Heart rate increased significantly above control 32 i 2 hours
before the signs of the disease and remained elevated through
the time of laminitis (Figure 15). Heart rates in these
ponies were never as great as those in the shock ponies.

At the time of peak base deficit there was a signifi-
cant increase in heart rate and a decrease in stroke volume.
Stroke volume returned to normal eight hours after the peak

in base deficit, while heart rate remained elevated (Figure

16).

Effect of Carbohydrate Overload on Body Fluid Volume and
Composition

 

 

In all three ponies dying of shock, plasma volume was
decreased eight hours before death (Figure 17). Packed cell
volume increased from a normal of 38.0 i 1.5% to “8 i 2.5%
and 5H t 4.0%, sixteen and eight hours before death respec-

tively. Even though plasma volume decreased, blood

Figure 1H.

63

Measurements of cardiac variables in shock
ponies before death (mean i standard error).

Pra = right atrial pressure; Vs = stroke volume;
HR = heart rate. C = control value before
administration of high starch ration; *'=
statistically significant change (P < 0.05) from
control value, using randomized complete-block
analysis of variance and Student—Newman—Keuls'
tests.

ml. qu

P ra
mmHg

Heart Rate
Beats Imin.

l.2-
LOq

IZOq
I l 0-

IOO-
90-
80~
701
60-1

 

 

vi

 

OL—v—‘i. v t

C 24 IS 8
HOURS BEFORE DEATH

FIGURE 1U

Figure 15.

65

Measurements of cardiac variables in laminitis
ponies before and during the clinical signs of
the disease (mean : standard error). Pra =
right atrial pressure; Vs = stroke volume; HR =
heart rate. C = control value before adminis-
tration of high starch ration; *'= statisti—
cally significant change (P < 0.05) from control
value, using randomized complete-block analysis
of variance and Student—Newman—Keuls' tests.

I401
[2.04
Pro IO.0-4
mmHg 80« ,’
6.0-4 II/
‘$0- 1

OJ
l.2«

[1
r]

0..
l20- 4
||O~ ¥ 4 .1.
IOOR
Boats/min. 90~ ,’ ‘
1’
:t

0+4 I 1

c -40 $2 -24 -l6 -8
HOURS BEFORE LAMINITIS

 

 

O-i

FIGURE 15

Figure 16.

67

Measurements of cardiac variables before and
after peak base deficit in laminitis ponies
(mean 1 standard error). Pra = right atrial
pressure; Vs = stroke volume; HR = heart rate.
C = control measurement before administration
of high starch ration; *-= statistically signi—
ficant change (P < 0.05) from control value,
using randomized complete-block analysis of
variance and Student-Newman-Keuls' tests.

Pro

mmHg

HR

Beats /min.

11
’7

o ..
IZO-

IIO-
IOO-
901
80«

q

68

Peak Base Deficit

~L

’7

 

OJ

 

FIGURE 16

Figure 17.

69

Measurements of body fluids in shock ponies
before death (mean 1 standard error). TP =
total protein; PCV = packed cell volume; PV =
plasma volume. C = control measurement before
administration of high starch ration; *'= sta—
tistically significant change (P < 0.05) from
control value using randomized complete-block
analysis of variance and Student-Newman-Keuls'
tests.

TP
ngdL

PV
mL/KO

PCV

8V
NIL/K9

7 0
B47
in. “I
8.0T ’
7.81 1” ‘
7.64

 

A

0-4

50.,

45.

40‘

30-

[A
'7

55-1

so-
45. 4

40-‘ P’a-

35-4

0.
90-
80- /"

70— {4’

60-

 

 

C 24 IS 8

HOURS BEFORE DEATH

FIGURE 17

71

volume remained unchanged because of this increase in
PCV.

There was no significant changes in blood volume,
plasma volume, or PCV during the onset of laminitis (Figure
18).

Even though no significant changes in blood volume,
plasma volume, or PCV were seen when the data was analyzed
in terms of hours before laminitis, in every pony that deve-
loped the disease there was either a transient decrease in
PCV (4.0 i 0.2 percentage points in ponies l, 2, 3, and 6)
or a transient discontinuation of the rise in PCV (ponies
10, 11, and 12) eight to ten hours before the clinical signs
of the disease. The PCV returned to normal or was slightly
elevated at the time of laminitis. Therefore, to determine
the reason for this transitory event, plasma volume and blood
volume measurements were analyzed with zero time defined as
the transitory decrease in PCV for every pony (Figure 19).
Plasma volume and blood volume, at the time of the change in
PCV, decreased slightly though not significantly from con-

trol.

Effect of Carbohydrate Overload on Serum Electrolytes

 

There were no significant changes in potassium,
sodium,or chloride in the three shock ponies 2H, 16, and 8
hours before death.

In the laminitis group, potassium decreased 24, 16,
8, and 0 hours before the disease (Figure 20). At the time

of laminitis potassium was 2.5“ : .1u meg/l compared to a

Figure 18.

72

Measurements of body fluids in laminitis ponies
before and during the clinical signs of the
disease (mean 1 standard error). TP = total
protein; PCV = packed cell volume; BV = blood
volume; PV==packedce11-volume. C = control
measurement before administration of high starch
ration; *'= statistically significant change

(P < 0.05) from control value, using randomized
complete-block analysis of variance and Student-
Newman-Keuls' tests.

TP
qm/dL

PCV

BV
ml/Kq

PV
ml/Kq

as.
BAR
Eb
BO«
734

oi

391
38-

361
354
34-
33«
324

[1
7r

”0-

90+

70«

up

11
7,

01
so-

504

40a

30-

11
ll

 

A

 

é

 

 

A
7 I v I v
'40 '32 '24 “IS
HOURS BEFORE LAMINITIS

I
'8

FIGURE 18

I—._
r
L
I"—

 

o.-

Figure 19.

7“

Measurements of body fluids before and after
drop in packed cell volume in laminitis ponies
(mean : standard error). TP = total protein;
BV = blood volume; PV = plasma volume. C =
control measurement before administration of
high starch ration; it: statistically signifi-
cant change (P < 0.05) from control value,
using randomized complete-block analysis of
variance and Student—Newman-Keuls' tests.

+.O = values significantly different (P < 0.05).

 

 

PCV Drop
8.61
8.44
3.2.
TP 8.0- ‘~~.
qm/dL 734
7.6-
74-
it:
0.
IDO-
x. ¥
8V ‘0‘ ’4
I
L/Kg ,’
.0. I’
4:
01
70+
so-
PV ERR/I
/
501 /
L/Kg ,’
40- 1’
4:
OJ
3.4 i
37-
am I
PCV 5“ ‘7
34- i0
"' 33-
32~
<t
o A, r a . . fl . -
c -24 -l6 -8 0 4-8

HOURS

FIGURE 19

Figure 20.

76

Serum electrolyte measurements in laminitis
ponies before and during the clinical signs of
the disease (mean : standard error). C = con—
trol measurement before administration of high
starch ration; *'= statistically significant
change (P < 0.05) from control value, using
randomized complete—block analysis of variance
and Student—Newman—Keuls‘ tests.

SAi
3.21
3.0‘
2.84
2.6“
2.4+

KO
meq/L

04
l32"
ISO“
l28‘
l26‘

NO‘
meq/L
l24“

l22"

IL

IO4"
IOZ"
mod
98'1
96"
94‘1

Cl'
meq/L

L1

 

77

 

 

\
\
\
\
\
t
\
\
\
\
\
t
T
\\
\\
1
k
K
+T—'$ I I I r W I
C -40 -32 -24 'l6 -8 0

HOURS BEFORE LAMINITIS

FIGURE 20

78

control of 3.36 : .2l meq/l. Sodium and chloride in these
ponies remained unchanged during the onset of the disease
(Figure 20). Because of insufficient amounts of sample a
complete statistical analysis of osmolarity could not be

done.

DISCUSSION

Events in the systemic circulatory system after over-
feeding have been suggested as causes of acute laminitis.
The results from this study suggest that the major cardio-
vascular occurrences are responses to rather than causes of
the disease. The increased arterial blood pressure at the
time of laminitis results entirely from an increased cardiac
output, since total peripheral resistance is unchanged.
This hypertension occurred only at the time of the clinical
signs of the disease and not during the onset of laminitis.
The increased cardiac output was due to an increase in
heart rate.

In one pony with acute laminitis, high volar nerve
blocks were performed by injecting 2% lidocaine in the
medial palmar nerve of all four feet. Within lO—15 minutes
the elevated cardiac output of 23.8 l/min decreased to a
control value of 13.“ l/min and concomitantly the mean ar-
terial pressure decreased from 15“ mmHg to 100 mmHg. This
suggests that the increased cardiac output resulted from
increased sympathetic outflow caused by pain in the feet.

These results are in some conflict with previously
published findings where hypertension was described as a

progressive develOpment following overeating rather than an

79

8O

abrupt develOpment concurrent with clinical signs of lamini-
tis (Garner, 1975).17 However, in more recent studies by
these investigators, arterial pressure increased only in

the final prodromal phase of the disease.18 The conflict

in the time of hypertension in this study and the investiga—
tion of Garner, probably is due to the methods of statisti-
cal analysis. Since animals deveIOp laminitis at varying
intervals after overeating, using overeating as zero time

in determining group means may hide many of the subtle and
sudden changes occurring around laminitis. When clinical
signs of laminitis are used as zero time, the onset of hyper-
tension and other events become more obvious.

Following overfeeding with the high starch ration, the
ponies either developed acute laminitis, died of shock, or
remained normal. The ponies develOping laminitis showed no
decrease in cardiac output prior to the time of the disease.
In contrast, the first event indicating the onset of shock
in the overfed pony was the abrupt decrease in cardiac out—
put 8-16 hours after the diet administration. This
decrease of cardiac output appears to be due to a decrease
in the contractility of the heart, since right atrial pres—
sure remained unchanged and heart rate was acutely ele—
vated.22

No consistent cardiovascular signs of transitory
shock occurred in animals prior to the clinical signs of
laminitis. In the present study, total peripheral resis—
tance was unchanged at the time of laminitis. It should be

noted that it has been previously reported that digital

81

vascular resistance was decreased at a similar time.“3

Since total peripheral resistance is the algebraic sum of
the resistances of parallel systemic vascular beds, it is
reasonable to assume the dilation in the digital vasculature
was counteracted by constriction elsewhere. Such a differ-
ential response of vascular beds has been reported during
intravenous infusion of epinephrine22 and may well occur
during an in vitro sympatho—adrenal discharge.

No changes in the pulmonary vascular resistance (PVR)
occurred in those ponies that develOped laminitis, but the
elevation of PVR in the three shock ponies was much more
pronounced than that of TPR. This correSponds to previous

19, 21, an

shock studies in the dog. In addition, pulmonary

constriction and edema of the lungs of calves in shock
have been reported.32

The ponies' PVR during shock were twice control. The
elevations of pulmonary vascular resistance probably result
from an increase in viscosity (increased hematocrit and
possible intravascular clotting), since very little sympa-
thetic vascular control exists in the lungs.

Garner (1975) has reported that plasma volume
decreases concomitantly with increases in cardiac output and
arterial pressure after overfeeding.l8 No evidence of this
type of correlation was seen in any of the overfed ponies
in this experiment. Cardiac output and plasma volume
decreased concomitantly in the shock ponies, while in the

laminitis ponies plasma volume remained unchanged at the

time cardiac output and blood pressure were elevated.

82

Although no changes in the vascular fluids (plasma
volume, blood volume) was apparent in the laminitis group,
there was a significant increase in the packed cell volumes
of the shock group. Through the following calculations it
was determined that there is not only a decrease in plasma
volume, but also an increase in the cellular volume (CV) in
the circulation of the pony in shock:

(Hct ) (PV )
CV 1 1

 

 

control = (l-Hcti)
. (Hct2) (PV2)
CV =
final (l-Hct2)

The calculated control CV was approximately 26.2 ml/Kg,
while "eight hours" prior to death it was 39.8 ml/Kg. The
increased CV implies either an increase in the number of
erythrocytes in the circulation during shock oréniincrease
in mean corpuscular volume. In several animal species
including the horse, the spleen acts as a reservoir of red
cells and can under sympathetic stimulation discharge blood
with a high hematocrit into the circulation.”1 Splenic
contraction is the most likely cause of the increased CV in
the shock ponies.

Overfed ruminants have decreased plasma volume,
increased hematocrit, and no change in blood volume. The
increase in hematocrit and the decrease in plasma volume of
overfed ruminants has been attributed to release of red
blood cells into the circulation and to water movement into
the rumen. It is very possible that the pony in shock,

like the overfed ruminant, is losing plasma volume into the

83

gut and also contracting its spleen, both of which are
reflected in its hematocrit.

There were no significant changes in hematocrit or
plasma volume in the laminitis ponies, however, there was a
transient decrease in hematrocrit 8-16 hours before the
disease in each individual laminitis pony. This must be
explained by a sequestration of erythrocytes rather than a
transitory increase in plasma volume, since PV also was
tending to decrease at this time (Figure 19). This seques—
tration of erythrocytes may occur in the spleen or as a
result of peripheral intravascular coagulation. Some
evidence of coaguIOpathies in laminitis has been observed
(Bell, T. G., personal communication),5 and intravascular
agglutination has been implicated in shock.”7

The metabolic acidosis seen in both groups of overfed
ponies may have several causes. First, hypoxia may increase
blood lactate levels, decreasing blood pH. The increase
in TPR in shock causes decreased tissue perfusion which may
cause stagnant anoxia. Increased lactate/pyruvate ratios

“7

occur in shock in other species. Increases in the latter
are thought to reflect increased anaerobic metabolism,
since the increase in the lactate concentration in the
blood should be accompanied by a prOportional increase in
pyruvate concentration if it is due solely to aerobic meta-
bolism.2u

Even though there was no apparent change in TPR in

the laminitis ponies, this does not mean that tissue hypoxia

8A

was absent. Redistribution of blood flow could create areas
of hypoxia resulting in lactic acidosis.

Another contributor to the acidosis may be increased
fermentation of carbohydrate in the gut. There are numerous
reports in the literature of metabolic disorder in ruminants
associated with ingestion of large quantities of carbohy-

32

drates. However, little has been written about the patho-
genesis of the similar acidosis in overfed ponies or horses.
When cattle and sheep are overfed soluble carbohydrates, the
pH of the rumen contents decreases together with an increase
in ruminal concentration of lactic acid (normally indetecta—
ble) and an increase in concentration of volatile fatty

acid.32 The rumen pH may fall to below pH 4.0 in very

16’ 32 Correlated

severe cases of carbohydrate overfeeding.
with the fall in pH is a marked increase in concentration
of lactate in the blood with the D-isomer predominating.
Although the absorption of lactic acid is a relatively slow
process, once the pH of the rumen contents falls below pH 5
a significant amount of free lactic acid is absorbed.32
Furthermore, it is probable that substantial amounts of
lactic acid or its salts may be absorbed from the
intestine. The presence of D(-) lactate in the blood indi-
cates that sufficient absorption occurs from the alimentary
tract to overload the mechanism for removing this isomer
from the blood. This decreases both the alkali reserve and
pH of the blood.32’ “0

The cecum and large colon are the regions of the

disgestive tract which enable the horse to utilize most of

85

the carbohydrates which require microbial digestion. As

in the ruminant, the principle products of microbial diges-
tion of polysaccharides appear to be volatile fatty acids3
which can be produced in large quantities. Following over-
feeding of ponies, lactate production in the cecum and large
colon may increase, resulting in the increased levels of
blood lactate.18 The stomach may be a further source of
lactic acid, since the stomach of the normal adult horse
produces substantial amounts of lactic acid by bacterial

3

fermentation. This lactic acid which is produced in greater

quantities than in pigs and rabbits3

appears to be absorbed
in the small intestine. When a pony is overfed, an
increased gastric source of lactic acid could result in
elevated levels of lactic acid absorbed into the blood
stream from the small intestine.

Another factor adding to the acidosis may be increased
gluconeogenesis due to stress related adrenocorticoid
release. Increased blood glucose has been reported in

25, 32

overfed ruminants. Adrenocorticoid hormones increase

gluconeogenesis, resulting in glucose accumulation with
reduced pyruvate metabolism due to a lack of NAD.32
The control of blood pH relies on excretion of hydro-
gen ions in the urine or changes in ventilation. The over-
fed ponies appeared to urinate less frequently after the
diet administration (dry bedding noticed). Oliguria has

been reported in overfed ruminants. Ruminant urine is

usually alkaline and the fall to pH 5 found in acidosis

86

represents a substantial increase in excretion of acid. The
Oliguria frequently observed in overfed animals would con-
tribute significantly to the acidosis.

Finally, the increase in blood acidity could be due
to the loss of bicarbonate via the bowel since diarrhea was
frequently seen hithe ponies that develOped laminitis.

All the laminitis ponies exhibited only a transitory
acidosis. A respiratory compensation occurred within eight
hours after their individual peaks in base deficit (Figure
8).

The respiratory compensation of the laminitis group,
whose peak base deficit was 12.0 i 1.8 mg/l, effectively
returned blood pH back to normal at the time of laminitis.
When the base deficit was greater than 1“ mg/l, as in the
shock ponies, the respiratory system apparently was unable
to compensate sufficiently to return the animal to a normal
acid—base status. Acidosis presumably contributed to the
events leading to death in this shock group of ponies.

Acidosis also affected the cardiovascular system by
increasing heart rate and subsequently decreasing stroke
volume. The increased hydrogen ion concentration of the
blood causes decreased contractility of the myocardium,26’
31’ “5 which in turn decreases arterial pressure, causing
reduced baroreceptor stimulation. The decrease in impulse
from the baroreceptor decreases excitation of the cardioin-
hibitory center and reduces vagal activity, increasing heart

rate.30’ “6

87

Although acidosis may increase heart rate during the
onset of the disease, it does not maintain the elevated
heart rate at the time of clinical signs of laminitis,
since the acid-base status is then normal. Other factors
increasing heart rate may be fever and pain. Body tempera-
ture increased at the same time as heart rate and both
remained elevated. Pain from the gut or feet may increase
heart rate by increasing sympathetic outflow to the heart.

Even though both groups of ponies developed acidosis,
neither group develOped concurrent hyperkalemia which usu-
ally results from movement of K+ from the intracellular to
the extracellular compartment. K+ was unchanged in shock
ponies and hypokalemia develOped in the laminitis group
before the disease and continued until the onset of clinical
signs. In the laminitis group, K+ may have been excreted
in the loose, sloppy feces faster than it left the ICF result-
ing in hypokalemia. Shock ponies had greater acidosis and
no diarrhea. Any loss of K+ into the gut was replaced by
K+ displaced from the ICF, resulting in normal K+ levels.

The present study indicates that the systemic cardio-
vascular adjustments seen after overfeeding and prior to
alimentary laminitis are responses to rather than primary
causes of this hoof disease. The characteristic hyperten—
sion of acute laminitis occurred only at the time of the
disease. The results of this study and our preliminary
nerve block eXperiment suggest that pain is a primary factor
in the cardiovascular response (hypertension) at the time

of laminitis.

88

This study, however, does not indicate whether or not
blood flow to thelaminae was changed prior to laminitis.
Even though TPR remained unchanged during the onset of lami-
nitis, this does not exclude the possibility of redistribu—
tion of blood flow in the systemic or local circulation.

If blood flow is shunted away from the laminae, this depri-
vation would initiate breakdown of the tissue.

Finally, this investigation dealt only with the hypo-
thesis that laminitis is a circulatory problem. The disease
may not be the result of changes in blood flow, but the
direct effect of a substance (e.g. endotoxin) or a state
(e.g. acidosis) on the laminae of the hoof. Even though
systemic cardiovascular events did not resemble shock prior
to laminitis, an endotoxin(s) may be causing direct break-
down of the laminar corium without affecting blood flow.
Deleterious agent(s) may be produced at the site of the
laminae or in a remote area of the body before the develOp—
ment of the disease. In addition, the transitory acidosis
may be causing direct laminar breakdown during the onset of

the disease.

SUMMARY

From the results of this investigation, the following

conclusions can be made concerning the development of exper—

imentally induced alimentary laminitis in ponies:

1.

There is no evidence that the major cardiovascular
events prior to laminitis are causes of the
disease. The cardiovascular changes that were
seen appear to be responses to rather than causes
of the disease. I

Hypertension occurs only at the time of laminitis.
There was no evidence of a progressive develOpment
of hypertension. Rather, an abrupt increase in
blood pressure occurred when the clinical signs

of the disease were apparent. The hypertension is
due to an increase in cardiac output.

A transient metabolic acidosis occurs prior to
laminitis. However, at the time of acute lamini-
tis, arterial blood pH is near normal.

Hypokalemia develOps prior to the disease and
continues to the time of acute laminitis. No
changes in arterial sodium and chloride concentra-
tions were apparent after overeating and prior to

laminitis.

89

10.

11.

12.

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