The American Journal of Physiology

The experiments to be described here were designed primarily to determine the threshold of the normal human respiratory and circulatory systems to anoxemia. In addition, data were also obtained which related to other basic problems in respiration and circulation: the magnitude and variability of response of large numbers of normal men to inhalation of low oxygen mixtures, the existence of tonic activity of the chemoreceptors of the carot,id and aortic bodies in man, the effect upon respiration and circulation of brief exposures to 100 per cent oxygen and the correlation between decreases in arterial oxygen saturations and changes in respiratory minute volume, pulse rate and cardiac output. 14t0tempts have been made previously to measure the slightest decrease in inspired oxygen tension which will produce measurable physiological changes in ma’n. Lutz and Schneider (1) and Ellis (2) stated that inhalation of 18 per cent oxygen is stimulant to human respiration while Rootlhby’s data (3) suggest that the threshold is not reached until 16 per cent oxygen is breathed. Each of these studies yielded valuable information with respect to conditions that might obtain in airplanes but none supplied accurate data regarding thresholds of basal subjects breathing constant low oxygen concentrations. Most of their experiments were conducted with non-basal subjects sitting in a low pressure chamber. Some were rebreathing experiments in which const,ant concentrations were not maintained and respiration was measured in 5 minute units. In others, respiratow minute volume was not measured but was inferred from alveolar CO2 measurements. Since the desired data could be secured only by great attention to detail, our method is presented fully. .