Exercise-induced pulmonary hemorrhage (EIPH) is diagnosed by the presence of frank blood in the airways following a bout of intense exercise. EIPH affects all racehorses, and has been diagnosed in other athletic species, including humans. EIPH is associated with impaired racing performance and significant pulmonary pathology in the caudodorsal lung, while the cranioventral lung is spared. Lesions include hemosiderin accumulation, interstitial and septal fibrosis, angiogenesis, capillary wall disruption and remodeling of small-caliber (100 - 200 μm diameter) intralobular pulmonary veins. High pressures in the pulmonary circulation of the exercising horse cause capillary stress failure, resulting in the main symptom: hemorrhage. Stress failure alone does not account for all EIPH lesions, and in particular, venous remodeling. Nor does it explain regional predilection of EIPH pathology. EIPH pathogenesis awaits complete explanation at this time. Capillary pressure is determined in part by resistance to flow in the arteries and veins that supply and drain a capillary bed. Decreased arterial, and increased venous resistance are conditions under which capillary pressures will increase, and may approach arterial pressure values. I hypothesize that a combination of increased blood flow during exercise to caudodorsal lung, coupled with exercise-associated alterations in vessel tone provides transient but sufficient hemodynamic stimuli to initiate venous remodeling in this region only. Therefore the impact of ongoing venous remodeling would be to reduce venous wall compliance, thereby increasing venous resistance to flow and increasing capillary pressures in the caudodorsal lung. Capillary wall stress failure, hemorrhage, and EIPH result. The characteristics of small vessels that determine capillary pressure include mechanical and reactivity profiles of small arteries and veins that work to effect passive and active changes in vessel diameter, along with any impact of remodeling on venous wall compliance. These were evaluated in three studies. The important findings are as follows:First, regional differences in mechanical properties of arteries and veins exist in control, unraced horses, which may reflect inhomogeneous blood flow distribution in the equine lung. Racing is associated with increased stiffness of caudodorsal pulmonary veins only, despite the absence of severe EIPH pathology. Second, autonomic control of small, equine pulmonary arteries and veins is not consistent across lung regions, and the reported differences, when extrapolated to in vivo exercising conditions of increased sympathetic input, can account for increased capillary pressure in caudodorsal lung. Finally, although exercise causes significant alterations in mRNA expression in vein walls, the changes do not support initiation of a remodeling response after only 2 weeks of intense exercise. These data contribute to the understanding of EIPH pathogenesis, and highlight the pivotal role of the pulmonary microvasculature, in particular the pulmonary veins, in this significant, ubiquitous disease. Furthermore, regional differences in mechanical properties and reactivity profiles of pulmonary vessels of this caliber are not reported in any other species, and as such, these findings may have farther-reaching applications in the field of pulmonary vascular biology.
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