Predictors of increased PaCO2 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 P CO 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 P O 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 P O 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 P O 2 or P tr on Pa CO 2 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 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 P O 2 , and O 2 consumption during short-term exercise were not significant. transrespiratory pressure; respiratory resistance; hyperoxia; carbon dioxide response; diving Address for reprint requests and other correspondence: R. E. Moon, Dept. of Anesthesiology, Box 3094, Duke Univ. Medical Center, Durham, NC 27710 (e-mail: moon0002{at}mc.duke.edu )