TISSUE OXYGEN AND CARBON-DIOXIDE STORES AND BREATH-HOLD DIVING IN HUMANS

Alveolar gas exchange was studied in 11 submerged subjects during and after 75-s breath holds with or without a transient increase of ambien t pressure to 3 ATA (20 msw). During surface breath holds (SBH), cardi ac index fell to 73% of eupneic control but was partially restored at depth to 88% of control during breath-hold dives (BHD). O-2 uptake fel l to 84% of control during SBH and was restored to control level durin g BHD. The turnover of O-2 stores was much slower during SBH than duri ng the ensuing recovery. Carbon dioxide store dynamics were markedly s lowed after BHD. We conclude that SBH and BHD are associated with larg e shifts in tissue O-2 and CO2 stores and that much of these shifts ca n be explained by primary circulatory events. The changes in turnover rate for tissue O-2 and CO2 stores could not be explained by the cardi ac index changes alone but were compatible with peripheralization of v enous blood volume and preferential peripheral vasoconstriction induce d by apnea with elevated intrathoracic pressure during SBH. The transi ent compression during BHD reversed these central and peripheral circu latory changes by counteracting the increase in intrathoracic pressure .