Predictors of increased PaCO2 during immersed prone exercise at 4.7 ATA.

During diving, arterial Pco₂ (Pa_co2) 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 Po₂, and externally applied transrespiratory pressure (P_tr) on Pa_co2 during immersed prone exercise in subjects breathing oxygen-nitrogen mixes at 4.7 ATA. Twenty-five subjects were studied at rest and during 6 mm 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 cmH₂O; breathing gas Po₂ 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_co2 (Ton) 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 Po₂ or P_tr on Paco₂ or minute ventilation (VE). VE (1/mm) 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 < 0.00 1) during breathing with resistance. We conclude that the major contributors to increased Pa_co2 during exercise at 4.7 ATA are increased depth and external respiratory resistance. HCVR and maximal O₂ consumption were also weakly predictive. The effects of P_tr, inspired Po₂, and O₂ consumption during short-term exercise were not significant. [ABSTRACT FROM AUTHOR]