Arterial PaCO2 during chronic hyperthermia in sheep.

This study was undertaken to determine if the decrease in PaCO2 and the concomitant increase in pHa seen during acute (less than 24 h) hyperthermia in all mammals was modified when the hyperthermia was maintained for periods longer than 1 day. Catheters were placed in the descending aortas of anesthetized ewes (pentobarbital, 30 mg/kg, i.v.). After recovery from the surgery, arterial blood samples were drawn daily during a 7 day control period and an 8 day period of continuous hyperthermia. In all animals PaCO2 decreased and remained low during the entire hyperthermic period. PaCO2 (torr) and Tr (degrees C) were inversely correlated by the equation: PaCO2 = - 6.08 Tr + 267.8 (r = 0.84). There was an initial alkalosis with hyperthermia, however pH tended to decrease after the fifth day of hyperthermia. Calculated bicarbonate decreased during hyperthermia. The evidence suggested that when body temperature was increased in sheep, PaCO2 was maintained at a lower value. The low PaCO2 value was maintained independent of changes in pHa.