Circulatory response to acute hypobaric hypoxia in conscious dogs.

Hemodynamics were studied in seven conscious dogs during acute hypobaric stress at 14,000 ft simulated altitude. Silastic catheters were chronically implanted in the pulmonary artery, left atrium, and aorta. Pulmonary and central aortic pressures, cardiac output, and pulmonary blood volume were determined under conditions of normoxia and acute hypoxia in a hypobaric chamber maintained at 446 mm Hg pressure (14,000 ft). Altitude resulted in significant increases in heart rate, cardiac output, pulmonary blood volume, and pulmonary artery pressure. Left atrial pressure and calculated systemic vascular resistance decreased during hypobaric hypoxia while stroke volume, stroke work index, arterial pressure and pulmonary vascular resistance remained unchanged. Arterial blood PO2 decreased markedly at altitude, and all animals hyperventilated with resultant systemic hypocarbic alkalosis. The combination of elevated pulmonary arterial pressure and increased pulmonary blood volume may by an etiologic factor in the development of high-altitude pulmonary edema.