VERY HIGH-PRESSURES ARE REQUIRED TO CAUSE STRESS FAILURE OF PULMONARYCAPILLARIES IN THOROUGHBRED RACEHORSES

Thoroughbred horses develop extremely high pulmonary vascular pressure s during galloping, all horses in training develop exercise-induced pu lmonary hemorrhage, and we have shown that this is caused by stress fa ilure of pulmonary capillaries. It is known that the capillary transmu ral pressure (Ptm) necessary for stress failure is higher in dogs than in rabbits. The present study was designed to determine this value in horses. The lungs from 15 Thoroughbred horses were perfused with auto logous blood at Ptm values (midlung) of 25, 50, 75, 100 and 150 mmHg, and then perfusion fixed, and samples (dorsal and ventral, from caudal region) were examined by electron microscopy. Few disruptions of capi llary endothelium were observed at Ptm less than or equal to 75 mmHg, and 5.3 +/- 2.2 and 4.3 +/- 0.7 breaks/mm endothelium mere found at 10 0 and 150 mmHg Ptm, respectively. Blood-gas barrier thickness did not change with Ptm. At low Ptm, interstitial thickness was greater than p reviously found in rabbits but not in dogs. We conclude that the Ptm r equired to cause stress failure of pulmonary capillaries is between 75 and 100 mmHg and is greater in Thoroughbred horses than in both rabbi ts and dogs.