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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. 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 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 (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 Ptr 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 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. [ABSTRACT FROM AUTHOR]

The hydrogen (H2) clearance method was adapted for the measurement of regional cerebral blood flow (rCBF) in anesthetized rats and mice during hyperbaric oxygen (HBO2) exposure. Polarographic platinum electrodes 0.1 mm in diameter were used to record H2 clearance curves from the parietal cortex (PC), substantia nigra (SN), and caudate putamen nucleus (CPN) after inhalation of 2.5% H2 in air. The system for H2 breathing under hyperbaric conditions was designed for remote operation from outside the chamber. The rCBF values (measured every 10 min) were calculated from the H2 clearance curves using the initial slope method. During air breathing control, rCBF values were similar to values reported using other methods. Considering all control rats together, blood flow (ml.100 g-1.min-1) was 89 +/- 3.6 in the SN, 78 +/- 4.7 in the CPN, and 76 +/- 6.7 in the PC. Blood flow (ml.100 g-1.min-1) for air-breathing mice was 108 +/- 11.4 in the SN and 74 +/- 8.8 in the CPN. During HBO2 exposure to 3 atm abs, rCBF in rats fell within 30 min by 26-39% (P < 0.05) and by 27-29% in mice (P < 0.05). HBO2 exposure to 4 atm abs induced maximal rCBF decreases in rats within 60 min by 37% (P < 0.01) in the SN and by 47% (P < 0.01) in the CPN. Breathing CO2 during HBO2 exposure to 4 atm abs reversed the vasoconstriction and led to a rCBF increase of 80-96% in rats. The H2 clearance method seems to be an accurate and sensitive technique for the repeated measurement of local CBF under hyperbaric conditions.