Intrapulmonary distribution of alveolar gas exchange during breath-hold diving in humans

Expirograms for CO2 and O2 obtained immediately after 75-s breath holds (BHs) in thermoneutral conditions were studied in 10 subjects. BHs were performed at normal atmospheric pressure in dry condition (dry surface BH), at normal atmospheric pressure submerged (wet surface BH), and during a transient increase of ambient pressure to 3 atmospheres absolute submerged (wet BH dive). Cardiac index was estimated by means of impedance cardiography. Phase III (alveolar plateau) slopes for CO2 expirograms were lowest after wet surface BH. The greater slope observed after dry surface BH was attributed mainly to intrapulmonary perfusion heterogeneity and the greater slope seen after wet BH dive to continuing alveolar CO2 exchange during expiration. Cardiogenic oscillations in phase III (evaluated by spectral analysis) were largest after dry surface BH, much reduced by wet surface BH, and further reduced by wet BH dive. This was attributed to more even distribution of pulmonary perfusion during submersion and compression. Terminal changes of the expirograms (phase IV) revealed a less even interregional pulmonary distribution of CO2 and a more even distribution of O2 after wet BH dive compared with wet surface BH. This difference was attributed to improved apical pulmonary perfusion at depth during the wet BH dives.