Pulmonary vasoconstriction in steers at high altitude

Each of ten steers taken for 9 weeks to 12,700 ft. (Mt. Evans, Colorado) showed a marked increase in pulmonary artery (PA) pressure. Three animals had PA pressures above 90 mm Hg and one developed right heart failure. The bovine species is remarkable for the severe pulmonary hypertension which develops during chronic hypoxia rather than for an excessive PA pressure response to acute hypoxia. The rate at which the pulmonary hypertension developed at 12,700 ft. was extremely rapid compared to that at 10,000 ft. Therefore, not only the duration of the hypoxic stimulus but also its severity determine the response. The severity of the stimulus was augmented by the absence of a sustained increase in ventilation at high altitude. The pressure rise with acute hypoxia during the control period at low altitude and the dramatic fall in PA pressure when oxygen was administered at high altitude provided evidence for hypoxia-induced pulmonary vasoconstriction as an important mechanism in bovine pulmonary hypertension.