PHYSIOLOGICAL AND PERCEPTUAL RESPONSES TO HYPERCARBIA DURING WARM-WATER AND COLD-WATER IMMERSION

Thermoregulatory, respiratory, and perceptual responses to acute CO2 e xposure during light exercise (75 W) were assessed in 12 U.S. Navy div ers clad only in swim trunks while immersed to the neck in water at 18 degrees and 34 degrees C. The CO2 exposures consisted of a linear 10- min ramp increase in the inspired fraction of CO2 (FICO2) from 0 to 6% followed by 5 min of breathing 6% CO2. The ability to detect and rate the severity of hypercarbia, as well as subjective changes in thermal comfort, were assessed by comparing subjective ratings given during t he CO2 exposures with those given during immersion trials where the FI CO2 was maintained at 0%. Hypercarbia was recognized earlier and, at a given PETCO2, was perceived to be greater during cold- than during wa rm-water immersions (P < 0.01). The CO2 exposures did not affect the t hermal balance of divers as assessed by changes in heat flux and recta l temperature. However, increased feelings of warmth were reported dur ing both the cold and warm immersions when breathing raised concentrat ions of CO2 (P < 0.01). During the cold immersions, acute exposure to 6% CO2 significantly decreased forearm blood flow (P < 0.05), but did not affect finger blood flow. It is concluded that PETCO2 levels can r each unacceptably high concentrations (>60 mmHg) before hypercarbia ca n be reliably detected by working divers. Furthermore, the ability to detect hypercarbia is poorer when immersed in warm water than when in cold water.