HIGHLIGHTED TOPIC The Physiology and Pathophysiology of the Hyperbaric and Diving Environments Predictors of increased PaCO2 during immersed prone exercise at 4.7 ATA

Cherry AD, Forkner IF, Frederick HJ, Natoli MJ, Schinazi EA, Longphre JP, Conard JL, White WD, Freiberger JJ, Stolp BW, Pollock NW, Doar PO, Boso AE, Alford EL, Walker AJ, Ma AC, Rhodes MA, Moon RE. Predictors of increased PaCO2 during immersed prone exercise at 4.7 ATA. J Appl Physiol 106: 316–325, 2009. First published September 11, 2008; doi:10.1152/japplphysiol.00885.2007.— During diving, arterial PCO2 (PaCO2) 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 (Ptr) on PaCO2 during immersed prone exercise in subjects breathing oxygen-nitrogen mixes at 4.7 ATA. Twentyfive 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 Ptr 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, PaCO2 (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 Ptr on PaCO2 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 PaCO2 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 Ptr, inspired PO2, and O2 consumption during short-term exercise were not significant.