It has been hypothesized that respiratory 'gain' to hypoxic stimulus is not depressed in hypothermic animals though ventilation and that metabolic O2 demand (VO2) decreases with reduction in body temperature. The present study addressed this hypothesis by quantitative analysis of ventilatory and carotid chemoreceptor responsiveness to hypoxia during induced hypothermia in halothane anesthetized and spontaneously breathing rats. Rectal temperature was lowered from 37 °C (normothermia) to 30 and 25 °C by cooling body surface at comparable anesthetic depth without inducing shivering. Ventilation (VE), VO2, Pao2 and carotid chemoreceptor afferent discharges were measured during hyperoxic and hypoxic gas breathing. Pao2 values at the same Flo2 (range 0.35-0.08) decreased progressively as rectal temperature decreased. Both the VE/VO2- and chemoreceptor discharge-response curves shifted toward a lower Pao2 range with a slight increase in the response slopes during hypothermia. The results indicated that the sensitivity of carotid chemoreceptor and ventilatory responses to hypoxia did not decrease at reduced body temperature. It is concluded that carotid chemoreceptor mediated regulation of ventilation is tightly coupled to changes in Pao2 range in halothane anesthetized rats during induced hypothermia.
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