Blood gases and hypothermia: Some theoretical and practical considerations

Abstract

There is much controversy about the optimal bloodgas management of hypothermic patients, whether the hypothennia is caused by accidents or induced before operations.

The surgeons and anestesiologists have aquired more clinical experience the last years when operating patients in hypothermia. The comparative physiology has given increased information about the blood gas strategy of heterothermic endotherms and poikilothermic ectotherms during lowering of their core temperature.

There are two types of strategies which have been used in clinical medicine the last years in the blood gas management of patients in hypothermia: pH-stat method and a-stat method.

In the pH-stat method, the arterial carbon dioxide tension (pCO₂(a)) is maintained at 5.3 kPa (40 mmHg) and the pH is maintained at 7.40 when measured at the actual temperature. It is then necessary to add CO₂ to the inspired gas.

In the α-method, the arterial carbon dioxide tension and the pH are maintained at 5.3 kPa and 7.40 when measured at +37°C. When a patient is cooled down, the pH-value will increase and the pCO₂-value and the pCO₂-value will decrease with lowering of the temperature if measured at the patients temperature.

Both the pH-stat and α-stat strategies have theoretical disadvantages.

For the optimal myocardial function the α-stat method is the method of choice. The pH-stat method may result in loss of autoregulation in the brain (coupling of the cerebral blood flow with the metabolic rate in the brain). By increasing the cerebral blood flow beyond the metabolic requirements, the pH-stat method may lead to cerebral microembolisation and intracranial hypertension. In Norway the α-stat strategy is the preferred method.

Key Words:

• α-stat
• heterothermic endotherms
• hibernation
• in vitro
• in vivo
• pH-stat
• poikilothermic ectotherms