Ventilatory Responses to Hypercapnia and Hypoxia at Normothermia and Moderate Hypothermia During Constant-Depth Halothane Anesthesia

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Abstract
Ventilatory responses of dogs to hypercapnia and hypoxia were studied at 37[degree] C, 32[degree]C, and 28[degree] C. during constant-depth halothane anesthesia. Minute ventilation fell from 4. 9 liters/minute at 37[degree] C, to 2. 5 liters/min. at 28[degree] C, and returned to 3. 7 liters/minute at 37[degree] C. Resting PacO2 decreased from 47 mm. of Hg to 39 mm of Hg and return to 49 mm of Hg. At 37[degree] C, minute ventilation increased by 380 ml. /mm of Hg increase in PaCO2. At 32[degree] C. this decreased to 160 ml. per mm of Hg and at 28[degree] C. to 100 ml. /mm of Hg. Ventilatory responses returned to 260 ml. per mm of Hg at 32[degree] C, and 380 ml per mm of Hg at 37[degree] C. At 37[degree] C., ventilation increased from approximately 4 liters/minute at a PaO2 of 400 mm. of Hg to 15 liters/ minute at 30-40 min. of Hg. At 28[degree]C., ventilation increased by only 1-2 liters/minute for a similar change in PaO2. These data suggest that the anesthesiologist maintain even greater vigilance over his patient during hypothermia since the usual clinical clues are obscured should hypercapnia or hypoxia develop.