Cardiac output but not high pulmonary artery pressure varies with FIO2 in exercising horses.

Horses have high mean pulmonary artery pressure (Ppa) both at rest and during exercise (approximately 30 and > or = 80 mmHg, respectively). The mechanisms are unknown. To see if hypoxic pulmonary vasoconstriction (HPV) plays a role, we compared pulmonary artery pressure-flow (Ppa-Q) curves when inspired O2 fraction (FIO2) was 0.16, 0.21, and 0.30, in 5 normal Thoroughbred horses standing quietly and while galloping at 10 and 14 m/sec on a level treadmill. We calculated O2 consumption (VO2) from measurements of respired gas composition and flow, and calculated Q from VO2 and measurements of oxygen content in arterial and mixed venous blood (CaO2 and CVO2). VO2 was 3.8, 74 and 128 ml.min-1.kg-1, at rest and at 10 and 14 m/sec, and did not vary with FIO2 at any speed. At 14 m/sec only, when FIO2 was lowered to 0.16, CaO2 fell (to 14.7 from 20 ml/dl on air), Q increased (to 0.86 from 0.66 L.min-1.kg-1 on air), and stroke volume increased (to 4.1 from 3.2 ml.kg-1 on air). Slopes and intercepts of Ppa-Q curves did not vary with FIO2. We conclude that HPV does not contribute to the high Ppa of exercising horses breathing air near sea level.