Positive-pressure oxygen breathing and pulmonary atelectasis during immersion

The effect on vital capacity of 1 h of oxygen breathing with and without continuous positive pressure was studied in five subjects during immersion. Vital capacity (VC) was used as an index of pulmonary atelectasis. The negative-pressure breathing induced by head-out immersion will cause airway closure and thus atelectasis if the closed-off regions contain oxygen. Without positive-pressure breathing, VC was reduced by 42%, and the subjects complained of tightness in the chest and had attacks of coughing. When 1.5 kPa positive pressure was used, three subjects had no decrease in VC, and in the remaining two subjects the decrease was halved. The latter two subjects were also studied during continuous positive-pressure oxygen breathing at 2.5 kPa. The decrease in VC was only one-fifth of the decrease in the control immersion condition without positive pressure (0 kPa), and they had no subjective feelings of discomfort. The results are relevant for the design of closed-circuit oxygen breathing apparatus.