Comparison of acid/base status in conscious and anaesthetized rats during acute hypothermia

Acute hypothermia was surface-induced in unrestrained conscious rats at two different levels, moderate (30 degrees C TB) and severe (20 degrees C TB). Data reflecting the acid/base status were determined. The values obtained for moderate hypothermia were compared with the acid/base pattern observed during hypothermia induced by two different anaesthetics, sodium pentobarbital and urethane, at room temperature. Conscious, hypothermic animals developed an apparent respiratory alkalosis, with an increase in pHa (from 7.476 to 7.546 in moderate hypothermia and from 7.484 to 7.563 in severe hypothermia) correlated with a decrease in arterial bicarbonate levels (from 22.9 to 16.8 mmol l-1 and from 20.7 to 14.9 mmol l-1 respectively). Lactate increased slightly in conscious, severely hypothermic rats (1.02 mmol l-1). This acid/base pattern was clearly different from that seen in sodium pentobarbital (mild respiratory acidosis) and urethane-induced hypothermia (metabolic acidosis). These results suggest that conscious rats follow a pattern closer to that underlying the relative alkalinity shown by many poikilotherms than to that underlying the constant pH shown in hibernating mammals. This latter pattern, nevertheless, approaches that observed during moderate pentobarbital hypothermia and the acid/base pattern during shallow hypothermia in birds. Anaesthesia may interfere with the development of the processes that lead to the acid/base pattern observed in conscious animals.