Response of antidiuretic hormone to acute exposure to mild and severe hypoxia in man.

Eight men, 19-35 years of age, breathed 20.9% (normal oxygen), 13.9% (mild hypoxia) or 11.1% (severe hypoxia) oxygen in nitrogen gas mixtures during three 20 min periods, which were separated by 1 h recovery periods. The order in which the gas mixtures were breathed was random. The partial pressure of oxygen decreased from a mean of 93.5 during exposure to normal oxygen to 53.9 and 36.7 mmHg during mild and severe hypoxia respectively. There were corresponding decreases in haemoglobin saturation. The partial pressure of carbon dioxide was lower and the pH higher during severe hypoxia than during exposure to normal oxygen. There were no changes in the plasma osmolality or in the concentrations of sodium or potassium in the plasma. There was a tendency for both the renin activity and the concentration of aldosterone in the plasma to decrease progressively as the percentage of oxygen breathed decreased. Unlike severe hypoxia, mild hypoxia suppressed the concentration of antidiuretic hormone (ADH) in the plasma of all subjects by about 59%; during severe hypoxia the reduction was not significant, being only about 33%. These data are consistent with the suggestion that the effect of hypoxia on the release of ADH is dependent on the level of hypoxia.