Pregnancy-induced pulmonary hypertension in cows susceptible to high mountain disease.

Observations in several species suggest that pulmonary vascular reactivity may be reduced during pregnancy. We tested this hypothesis in two groups of unanesthetized cows, one "susceptible" and one "resistant" to high mountain or brisket disease. At the altitude of residence (1,524 m), mean pulmonary arterial pressure was elevated during pregnancy by 18% and total pulmonary vascular resistance by 32% in susceptible but not in resistant cows. During acute exposure to simulated altitudes of 2,120--4,550 m, pulmonary arterial pressure was increased by 16% and total pulmonary resistance by 28% during pregnancy in susceptible cows. The pulmonary pressor response to a 5 microgram/kg bolus of prostaglandin FIalpha was not different during pregnancy in either group. Resistant cows hyperventilated while pregnant, raising arterial partial pressure of oxygen (PaO2) by 6 Torr both at 1,524 m and, on the average, by 7 Torr at altitudes of 2,120--4,550 m. Susceptible cows increased their PaO2 less than did the resistant cows during pregnancy. The results indicated that pregnancy was associated with a greater rise in pulmonary arterial pressure and total pulmonary vascular resistance during acute hypoxia and failed to elicit as great a ventilatory response in susceptible than in resistant cows.