Acid-base regulation during hibernation. In: Rahn H, Prakash O, editors. Acid-Base Regulation and Body Temperature. Dordrecht: Springer Netherlands; 1985. pp. 33-53. [DOI: 10.1007/978-94-009-5004-7_3]

Review article : Acid-base status, hypothermia and cardiac surgery

The optimal method of pH management during hypothermia is controversial. A review of current experimental and laboratory investigations comparing different pH schemes is presented. The biochemical and metabolic consequences of keeping pH constant (pH-stat) as compared to allowing the pH to increase with hypothermia (alpha-stat) are discussed. It is concluded that, on the basis of experimental and clinical studies, the alpha-stat pH management scheme may be preferable to that of keeping pH constant throughout a wide temperature range in humans.

[Why 37 degrees C? Evolutionary fundamentals of thermoregulation]

Homeothermy is the result of an evolutionary process during which every increase in oxygen supply led to a consecutive increase in metabolic rate and, thus, to a new dependence on favorable ambient conditions. In response to the food scarcity of winter months, some inhabitants of temperate zones developed an ability to hibernate which is characterized by a fully thermocontrolled reduction in body temperature down to near zero values. Hibernation thus illustrates that in homeotherms, not only the body shell is poikilothermic, but also the core temperature is more variable than often assumed. However, in contrast to clinical hypothermia, natural torpidity does not consist of a cold-induced reduction in metabolic rate, but of an endogenous metabolic reduction with subsequent lowering of body temperature. As a factor of metabolic suppression, the pH has been suspected which, in hibernators, is kept constant at 7.4 by relative hypoventilation (pH-stat) which differs from its passive shift in the poikilothermic body shell (alpha-stat). In clinical hypothermia, temperature governs the metabolic rate in that, depending on the state of thermoregulation, either a cold defense reaction with an increased metabolic rate (accidental hypothermia) or a cold-induced reduction in metabolic rate (induced hypothermia) occurs. However, as can be learned from hibernators, the lower limit of hypothermia tolerance seems to be due to a uniform minimal metabolic rate rather than to the species-specific body temperature at which this metabolic limit is reached, depending on body size and basal metabolic rate. Accordingly, in judging the sequelae of hypothermia, the degree of cooling should be given less emphasis than the resulting effects on metabolic rate.

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.

Time-course of blood acid-base state during arousal from hibernation in the European hamster

1. Arterial blood was sampled at 15 min-intervals in European hamsters Cricetus cricetus fitted with indwelling catheters, from deep hibernation to full arousal. Temperature-corrected pH and PCO2, respectively pH* and P*CO2, were directly measured at 37 degrees C. 2. Deep hibernation corresponded to a respiratory acidosis: pH* = 7.01 +/- 0.01 (mean +/- SE), P*CO2 = 160 +/- 4 Torr (n = 9 animals). 3. Three periods could be distinguished in the arousal: (i) a period of hyperventilation (28 +/- 5 min), in which P*CO2 was reduced to 79 +/- 4 Torr, while cheek pouch temperature increased only by 0.9 +/- 0.2 degrees C; (ii) a period of metabolic acidification by lactate accumulation (84 +/- 6 min), corresponding to the period of peak thermogenesis; (iii) a progressive return to euthermic conditions (104 +/- 10 min), by simultaneous respiratory and metabolic alkalinization. 4. Over 60% of the blood CO2 stores accumulated at the beginning of the hibernation bout were released by hyperventilation during the first period, prior to the full development of thermogenesis. This is in agreement with the hypothesis of an inhibitory role of the respiratory acidosis in hibernation.