Extension of pulmonary O2 tolerance in man at 2 ATA by intermittent O2 exposure.

To study extension of O2 tolerance by interruption of hyperoxic exposure, as compared to previous studies of continuous oxygen exposure, five healthy volunteers were exposed to oxygen at 2 ATA on an intermittent schedule of 20 min breathing O2, alternating with 5 min on a normoxic N2-O2 mixture. The cycle was repeated until symptoms or signs of O2 toxicity caused cessation of the experiment. Tracheal irritation and burning on inspiration occurred after 6-9 "oxygen hours" of exposure and progressed to severe tracheobronchial burning sensation, chest pain, and dyspnea after 11-15 h of O2. Average duration of exposure was 13.7 O2 h, inducing a mean vital capacity decrease of 10.3%. The decrease began soon after onset of symptoms. With intermittent O2 administration, nearly a doubling of the average duration of actual oxygen breathing was required to induce marked vital capacity change (greater than 10%) as compared to the previous studies of continuous O2 exposure. The increased duration of tolerable O2 exposure in man resembles the extension of O2 tolerance known to occur in animals exposed to intermittent hyperoxia.