INTRAPULMONARY DISTRIBUTION OF ALVEOLAR GAS-EXCHANGE DURING BREATH-HOLD DIVING IN HUMANS

Expirograms for CO2 and O-2 obtained immediately after 75-s breath hol ds (BHs) in thermoneutral conditions were studied in 10 subjects. BHs were performed at normal atmospheric pressure in dry condition (dry su rface BH), at normal atmospheric pressure submerged (wet surface BH), and during a transient increase of ambient pressure to 3 atmospheres a bsolute 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 obser ved after dry surface BH was attributed mainly to intrapulmonary perfu sion heterogeneity and the greater slope seen after wet BH dive to con tinuing alveolar CO2 exchange during expiration. Cardiogenic oscillati ons in phase III (evaluated by spectral analysis) were largest after d ry surface BH, much reduced by wet surface BH, and further reduced by wet BH dive. This was attributed to more even distribution of pulmonar y perfusion during submersion and compression. Terminal changes of the expirograms (phase IV) revealed a less even interregional pulmonary d istribution of CO2 and a more even distribution of O-2 after wet BH di ve compared with wet surface BH. This difference was attributed to imp roved apical pulmonary perfusion at depth during the wet BH dives.