STRESS FAILURE OF PULMONARY CAPILLARIES IN RACEHORSES WITH EXERCISE-INDUCED PULMONARY HEMORRHAGE

Bleeding into the lungs in thoroughbreds is extremely common; there is evidence that it occurs in essentially all horses in training. Howeve r, the mechanism is unknown. We tested the hypothesis that exercise-in duced pulmonary hemorrhage (EIPH) is caused by stress failure of pulmo nary capillaries. Three thoroughbreds with known EIPH were galloped on a treadmill, and after the horses were killed with intravenous barbit urate the lungs were removed, inflated, and fixed for electron microsc opy. Ultrastructural studies showed evidence of stress failure of pulm onary capillaries, including disruptions of the capillary endothelial and alveolar epithelial layers, extensive collections of red blood cel ls in the alveolar wall interstitium, protein-aceous fluid and red blo od cells in the alveolar spaces, interstitial edema, and fluid-filled protrusions of the endothelium into the capillary lumen. The appearanc es were consistent with the ultrastructural changes we have previously described in rabbit lungs at high capillary transmural pressures. Act ual breaks in the endothelium and epithelium were rather difficult to find, and they were frequently associated with platelets and leukocyte s that appeared to be plugging the breaks. The paucity of breaks was a scribed to their reversibility when the pressure was lowered and to th e fact that 60-70 min elapsed between the gallop and the beginning of lung fixation. Capillary wall stress was calculated from pulmonary vas cular pressures measured in a companion study (Jones et al. FASEB J. 6 : A2020,1992) and from measurements of the thickness of the blood-gas barrier and the radius of curvature of the capillaries. The value was as high as 8 X 10(5) dyn/cm2 (8 X 10(4) N/m2), which exceeds the break ing stress of most soft tissues. We conclude that stress failure of pu lmonary capillaries is the mechanism of EIPH.