BLOOD-GASES AND HYPOTHERMIA - SOME THEORETICAL AND PRACTICAL CONSIDERATIONS

There is much controversy about the optimal bloodgas management of hyp othermic patients, whether the hypothermia is caused by accidents or i nduced before operations. The surgeons and anestesiologists have aquir ed more clinical experience the last years when operating patients in hypothermia. The comparative physiology has given increased informatio n about the blood gas strategy of heterothermic endotherms and poikilo thermic ectotherms during lowering of their core temperature. There ar e two types of strategies which have been used in clinical medicine th e last years in the blood gas management of patients in hypothermia: p H-stat method and alpha-stat method. In the pH-stat method, the arteri al carbon dioxide tension (pCO(2)(a)) is maintained at 5.3 kPa (40 mmH g) and the pH is maintained at 7.40 when measured at the actual temper ature. It is then necessary to add CO2 to the inspired gas. In the alp ha-method, the arterial carbon dioxide tension and the pH are maintain ed at 5.3 kPa and 7.40 when measured at +37 degrees C. When a patient is cooled down, the pH-value will increase and the pCO(2)-value and th e pO(2)-value will decrease with lowering of the temperature if measur ed at the patients temperature. Both the pH-stat and alpha-stat strate gies have theoretical disadvantages. For the optimal myocardial functi on the alpha-stat method is the method of choice. The pH-stat method m ay result in loss of autoregulation in the brain (coupling of the cere bral blood flow with the metabolic rate in the brain). By increasing t he cerebral blood flow beyond the metabolic requirements, the pH-stat method may lead to cerebral microembolisation and intracranial hyperte nsion. In Norway the alpha-stat strategy is the preferred method.