Alveolar-arterial oxygen gradient in the dog at altitude

¹ Department of Physiology, Dartmouth Medical School, Hanover, New Hampshire

The effect of a sojourn of 5 days at an altitude of 14,300 feet (White Mountain, Cal.) on the alveolar-arterial oxygen gradient (A-aDO₂) in the anesthetized dog was studied at three levels of oxygenation. Two dogs breathed 11% O₂, air and 100% O₂ at sea level, and air, 40%, and 100% O₂ at altitude, with ventilation kept constant by means of a respiration pump. The alveolar samples were analyzed for O₂ and CO₂ pressures with the Scholander apparatus and the arterial O₂ pressures were measured in an in vitro polarograph. Control studies showed that the pump itself and hyperventilation had no effect on the A-aDO₂. The average values of A-aDO₂, 8.6, 23.4 and 168.5 mm Hg, at sea level (the three values refer to the respective three levels of oxygenation; the last figure corresponds to a shunt of about 12% of cardiac output) agreed well with previous data. The corresponding average values of A-aDO₂ at altitude were: 3.3, 17.0 and 58.0 mm Hg (corresponding to a shunt of about 4%). The A-aDO₂ breathing air was lower at altitude than when breathing an equivalent oxygen tension at sea level. The most dramatic effect consisted in a threefold decrease of the A-aDO₂ with 100% O₂ at altitude, indicating a decrease in true veno-arterial shunt flow.