Effects of Mild Chronic Hypoxia on the Pulmonary Circulation in Calves with Reactive Pulmonary Hypertension

Abstract

The effects of the mild hypoxia at an altitude of 5280 ft on the pulmonary circulation were examined in normal calves and calves with a preexisting stimulus to maintain a reactive pulmonary vascular bed. In a serial study, 9 calves born at 5280 ft and 9 calves born at sea level underwent left pulmonary artery (LPA) ligation; 3 calves born at 5280 ft and 8 calves born at sea level underwent right pulmonary artery (RPA) ligation within 24 to 48 hours after birth. Progressive pulmonary hypertension and right ventricular heart failure developed in all calves operated on at 5280 ft but not in those with LPA ligation performed at sea level until they were transferred to 5280 ft. In contrast, in some animals with RPA ligation at sea level, progressive pulmonary hypertension was noted. The increased pulmonary blood flow consequent to LPA ligation does not result in progressive pulmonary hypertension at sea level but the addition of the mild hypoxia at 5280 ft appears to provide additional sufficient stimulus to result in progressive pulmonary hypertension. However, the slightly higher pulmonary blood flow subsequent to RPA ligation is capable of producing progressive pulmonary hypertension at sea level. Correlation was found between these results and human patients with congenital unilateral absence of a pulmonary artery, indicating that, in subjects with some stimulus to maintain a reactive pulmonary vascular bed, the mild hypoxia of 5280 ft may exert a significant effect on the pulmonary circulation.