Acute mild hypothermia in awake unrestrained rats induces a mixed acid-base disorder

The interactions between components that contribute to acid-base homeostasis were studied in the first steps of acute hypothermia [body temperature (Tb) 37-31 degrees C] in awake unrestrained rats as an experimental model of accidental hypothermia in mammals. The concurrent changes in blood gases, plasma ions, and plasma protein concentrations in arterial blood were analyzed. Acute decreases in Tb decreased PCO2 and increased pH. The ratio of Na+ concentration to Cl- concentration increased at 35-33 degrees C Tb, leading to an increase in the plasma strong ion difference ([SID]). These increases were transient, and levels returned to baseline at lower Tb (31 degrees C). Lack of change in hematocrit, hemoglobin, plasma osmolality, or plasma protein concentration indicated stability in plasma volume. Therefore, [SID] changes were related to ionic shifts with respect to the extravascular space and not to ionic depletion. A feasible role in this ionic exchange for contracting skeletal muscle during shivering thermogenesis is given. Significant decrease in HCO3- concentration at lower Tb (31 degrees C) was related to an apparent increase in relative ventilation (lung ventilation per unit of CO2 removed). It is concluded that, during the first stages of body cooling, the blood acid-base status of conscious hypothermic rats is affected by PCO2 changes, apparently because of uncoupled changes between ventilation and metabolism, but it is also affected by a transitory metabolic disorder due to ion imbalance.