Hemodynamics and myocardial function during acute hypoxia in the pony.

Hemodynamics and myocardial contractility were evaluated in 6 unanesthetized ponies during hypocapnic and isocapnic hypoxia and during hypocapnic hypoxia after beta adrenergic blockade with propranolol. Hypocapnic hypoxia, with a mean arterial oxygen pressure (PaO2) of 41.9 mm of Hg, produced a decrease in stroke index and an increase in heart rate, with no change of cardiac index. A moderate increase in left ventricular contractility occurred during hypocapnic hypoxia. Beta adrenergic blockade abolished changes in nearly all indices of left ventricular contractility during hypocapnic hypoxia, suggesting that the antonomic nervous system plays a definite role in the genesis of circulatory changes during acute hypoxia. Isocapnic hypoxia produced a more well-defined increase in left ventricular contractility. Changes in right ventricular contractility were not observed in any of the hypoxia periods when the concurrent effects of a significant increase in afterload was taken into consideration. The tension-time index, measured as a guide to myocardial oxygen consumption, increased during each hypoxia period in both the right and left ventricle, and was not affected by beta adrenergic blockade.