Hypoxic pulmonary hypertension in the pony.

Hemodynamic measurements were made in 6 ponies at low altitude (Madison, WI, altitude, 250 m) and after 1, 2, 4, and 6 weeks at high altitude (Climax, CO, altitude, 3,400 m). The salient findings were that ponies maintain an increased heart rate and cardiac output and develop significant pulmonary hypertension at high altitude. The average control resting mean pulmonary artery pressure (BPpul) was 25.1 mm of Hg at 250 m; this value increased to 56.3 mm of Hg after 6 weeks at 3,400 m. An additional finding was that the pulmonary vascular response to acute hypoxia seemed to increase with time at high altitude.