Exercise-induced changes in the lung of Shetland ponies: ultrastructure and morphometry

The ultrastructural changes in pulmonary alveoli produced by running two ponies on a high speed treadmill at 7.6 m/sec, 3-degree incline, for 2 min support the hypothesis of pulmonary capillary stress failure as an explanation for exercise-induced pulmonary hemorrhage (EIPH). Light microscopy combined with scanning and transmission electron microscopy confirmed the presence of red blood cells and proteinaceous material in the alveolar lumina and interstitial swelling in approximately one third of the pulmonary alveoli examined. Morphometric analysis revealed that the blood-gas barrier was 30-77% thicker on the thin respiratory surface of the interalveolar septa in the cranial lobe of the two exercised ponies, i.e., 0.62 and 0.46 micron, compared to that of the unexercised control pony (0.35 micron). No change in blood-gas barrier thickness was observed in the caudal lobe, although that is where EIPH lesions have been observed in race-horses. Vascular pressures were low (20 mm Hg) in the pulmonary circulation of the Shetland pony at rest but increased more than three fold to 63 mm Hg during exercise. These preliminary morphological and physiological results indicate that a short burst of near-maximal exercise in a non-athletic equine can lead to structural changes in the blood-gas barrier and leakage of blood from pulmonary capillaries despite pulmonary vascular pressures being significantly lower than previously found in the racehorse.