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The Physiology and Pathophysiology of the Hyperbaric and Diving Environments

Predictors of increased Pa_CO2 during immersed prone exercise at 4.7 ATA

Center for Hyperbaric Medicine and Environmental Physiology and Department of Anesthesiology, Duke University Medical Center, Durham, North Carolina

Submitted 18 August 2007 ; accepted in final form 10 September 2008

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 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 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 (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 PO₂ or P_tr on Pa_CO2 or minute ventilation (E). E (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_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.

transrespiratory pressure; respiratory resistance; hyperoxia; carbon dioxide response; diving

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