Predictors of increased Pa CO 2 during immersed prone exercise at 4.7 ATA

During diving, arterial Pco 2 (Pa CO 2 ) levels can increase and contribute to psychomotor impairment and unconsciousness. This study was designed to investigate the effects of the hypercapnic ventilatory response (HCVR), exercise, inspired Po 2 , and externally applied transrespiratory pressure (P tr ) on Pa CO 2 during immersed prone exercise in subjects breathing oxygen-nitrogen mixes at 4.7 ATA. Twenty-five subjects were studied at rest and during 6 min of exercise while dry and submersed at 1 ATA and during exercise submersed at 4.7 ATA. At 4.7 ATA, subsets of the 25 subjects (9–10 for each condition) exercised as P tr was varied between +10, 0, and −10 cmH 2 O; breathing gas Po 2 was 0.7, 1.0, and 1.3 ATA; and inspiratory and expiratory breathing resistances were varied using 14.9-, 11.6-, and 10.2-mm-diameter-aperture disks. During exercise, Pa CO 2 (Torr) increased from 31.5 ± 4.1 (mean ± SD for all subjects) dry to 34.2 ± 4.8 ( P = 0.02) submersed, to 46.1 ± 5.9 ( P < 0.001) at 4.7 ATA during air breathing and to 49.9 ± 5.4 ( P < 0.001 vs. 1 ATA) during breathing with high external resistance. There was no significant effect of inspired Po 2 or P tr on Pa CO 2 or minute ventilation (V̇e). V̇e (l/min) decreased from 89.2 ± 22.9 dry to 76.3 ± 20.5 ( P = 0.02) submersed, to 61.6 ± 13.9 ( P < 0.001) at 4.7 ATA during air breathing and to 49.2 ± 7.3 ( P < 0.001) during breathing with resistance. We conclude that the major contributors to increased Pa CO 2 during exercise at 4.7 ATA are increased depth and external respiratory resistance. HCVR and maximal O 2 consumption were also weakly predictive. The effects of P tr , inspired Po 2 , and O 2 consumption during short-term exercise were not significant.