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Three general concepts may serve to detect hypoxia in a patient: l.The oxygen-supply to the tissue can be monitored by parameters of the oxygen-concentration in the blood (e.g. arterial and transcutaneous pO2 or oxygen-saturation).These parameters are easy to follow but do not detect hypoxia in all conditions (e.g. decreased perfusion). 2.Cellular activity: EGG and CTG are routinely used to detect hypoxia and data linking EEG with cerebral hypoxia are beginning to accumulate (1) . 3.Metabolie Products: Lactate, a product of anaerobic glycolysis, is a'well accepted parameter of hypoxia ( 2 ) . Hypoxanthine, a product of anaerobic ATP-catabolism is thought to be accumulated in hypoxia because of its increased production and because its breakdown is also oxygen dependent. In severe hypoxia in newborn (4) and in animal experiments plasma hypoxanthine was elevated. We confirmed these data but failed to show a relationsship between the severity of the hypoxia in newborn and their plasma hypoxanthine-concentration (3) . As other mechanisms: might be involved in the hypoxanthineproduction of the newborn we turned to the less complex Situation of trying to induce pure hypoxia in healthy adults by exposing them to a simulated altitude of 6000 m in a low pressure chamber. Respirationand EGGdata were collected and venous blood was obtained. Standard methods were used for all determinations except for hypoxanthine which was measured by a method combining xanthine-oxidase with an NADH-producing system ( 6 ) . In 14 experiments we have found that exposureto an alveolar p 2 (A°2^ °f ^6 to 33 mmHg during 12 to 56 min. produced a significant increase in lactate but none in hypoxanthine ( 5 ) . Hypoxia was increased in further 4 volounteers by asking muscle-exercise of 20 Watts during 4 minutes at ground level and at 6000 m. Exposition to minimal p 2 lasted from 20 to 30 min. and the Ρ^Ρ2 ^hed 31 to 37 mmHg and was lowered to 29 to 33 mmHg by exercise. Table l shows that hypoxanthine T a b l e T : C h a n g e s of l a c t a t e , h y p o x a n t h i n e and f s not ^ b l o o d gases due to e x e r c i s e at d i f f e r e n t PA09 .^ff .^ , y M 2 neither altitude. A significant rise in the lactate concentration is produced by work at 6000 m. Absence of a lactate eleva-

Comparison of hypoxanthine and lactate as indicators of hypoxia in man