Pulmonary mechanics and atelectasis during immersion in oxygen-breathing subjects

It has been suggested that vital capacity (VC) reduction seen during head-out immersion and oxygen breathing is due to atelectasis formation. In this study VC was reduced in 8 healthy subjects by 8.7% as an effect of immersion per se and by a further 14.3% as an effect of oxygen breathing during 30 min of immersion. Every 2nd min during the exposure, functional residual capacity (FRC), dynamic compliance (C-dyn), and static esophageal pressure at end-expiration (Pes-frc) were measured by body plethysmographic technique. Results were compared with an air-breathing, immersed control situation to evaluate any possible atelectasis formation. The only significant changes during immersion were observed after 30 min of air breathing, where FRC decreased linearly by 8.5% and C-dyn by 13.2%. The main conclusion is that atelectases acting solely as volume restrictors cannot explain the whole VC reduction without other additive or synergistic mechanisms. We suggest that the linear decrements in FRC and C-dyn as an effect of immersion time might be explained by either inspiratory muscle fatigue or increasing thoracic fluid compartment during the first 30 min of head-out immersion.