Comparison of hypoxanthine, lactate, and ECG signs as indicators of hypoxia.

High altitude-induced hypoxemia in 26 young adults was used to compared hypoxia parameters such as blood lactate, hypoxia-induced electrocardiogram signs, and the recently proposed plasma hypoxanthine concentration. At a PaO2 of 30 mm Hg, no elevation in hypoxanthine and small increase in lactate was observed. Minimal leg exercise was added to further increase hypoxic during hypoxemia. A significant increase in lactate and considerable ST-T depression in electrocardiogram indicated hypoxia, but no change in plasma hypoxanthine was found. Work-dependent hyperventilation as a main cause of lactate elevation was excluded. Blockade of hypoxanthine breakdown by allopurinol in vivo increased plasma hypoxanthine levels, but hypoxemia with or without leg exercise failed to consistently increase this level further. Elevations of blood lactate above the normal range were found at a PaO2 of 30 mm Hg and below.