Effect of CO2 on the ventilatory sensitivity to rising body temperature during exercise

Hayashi K, Honda Y, Miyakawa N, Fujii N, Ichinose M, Koga S, Kondo N, Nishiyasu T. Effect of CO2 on the ventilatory sensitivity to rising body temperature during exercise. J Appl Physiol 110: 1334-1341, 2011. First published March 10, 2011; doi: 10.1152/japplphysiol.00010.2010.-We examined the degree to which ventilatory sensitivity to rising body temperature (the slope of the regression line relating ventilation and body temperature) is altered by restoration of arterial PCO2 to the eucapnic level during prolonged exercise in the heat. Thirteen subjects exercised for similar to 60 min on a cycle ergometer at 50% of peak O-2 uptake with and without inhalation of CO2-enriched air. Subjects began breathing CO2-enriched air at the point that end-tidal PCO2 started to decline. Esophageal temperature (T-es), minute ventilation ((V) over dot E), tidal volume (V-T), respiratory frequency (f(R)), respiratory gases, middle cerebral artery blood velocity, and arterial blood pressure were recorded continuously. When V. E, VT, fR, and ventilatory equivalents for O-2 uptake ((V) over dot E/(V) over dot O-2) and CO2 output ((V) over dot E/(V) over dot CO2) were plotted against changes in Tes from the start of the CO2-enriched air inhalation (Delta T-es), the slopes of the regression lines relating (V) over dot E, VT, (V) over dot E/(V) over dot O-2, and (V) over dot E/(V) over dot CO2 to Delta T-es (ventilatory sensitivity to rising body temperature) were significantly greater when subjects breathed CO2 enriched air than when they breathed room air ((V) over dot E: 19.8 +/- 10.3 vs. 8.9 +/- 6.7 l . min(-1) . degrees C (-1), VT: 18 +/- 120 vs. -81 +/- 92 ml/degrees C; (V) over dot E/(V) over dot O-2: 7.4 +/- 5.5 vs. 2.6 +/- 2.3 units/degrees C, and (V) over dot E/(V) over dot CO2: 7.6 +/- 6.6 vs. 3.4 +/- 2.8 units/degrees C). The increase in (V) over dot E was accompanied by increases in V-T and f(R). These results suggest that restoration of arterial PCO2 to nearly eucapnic levels increases ventilatory sensitivity to rising body temperature by around threefold.