Hypothermia

Moderate hypothermia (33-36°C) has been shown to be neuroprotective, that is, to reduce secondary brain injury and infarction size and to improve neurological outcome in animal models with both focal and global ischaemia. In humans hypothermia has been shown to reduce cerebral blood flow, arteriojugular venous oxygen difference, and cerebral metabolic rate of oxygen.129 Much excitement has followed early reports of ICP reduction130 and improvement of outcome in head-injured patients.130,131 However, a large randomised multicentre trial has failed to demonstrate an improvement in outcome,132 but there have been many criticisms of this study and the effects of hypothermia on the outcome of head-injured patients will be re-examined in a further large trial. For subarachnoid haemorrhage a trial to prove the efficacy of intraoperative hypothermia (IHAST) is currently under way. Cooling can be achieved by surface cooling or with various exchanger devices that are inserted into a large vein. It is to be remembered that brain temperature will be ~0-5°C higher than core temperature. Rewarming is a very important issue and no recommendations on how rapidly patients should be rewarmed are available. However, in animals it has been shown that rapid rewarming can reverse the beneficial effects of hypothermia.133 Hypothermia has relevant side effects: ventricular ectopy and fibrillation limit the extent of hypothermia, but this is known to occur only at temperatures below 30°C. A recent study found a significantly higher rate of pneumonia, electrolyte abnormalities, thrombocytopenia, and increased amylase serum in hypothermic patients when compared to normothermic patients.134 Other side effects of hypothermia (which have been shown in animal studies) are clotting abnormalities and coagulopathy. In the human, the enzymatic reactions of the coagulation cascade were shown to be strongly inhibited by hypothermia. Although an elevation of serum amylase and lipase levels is frequently observed, the association between hypothermia and pancreatitis is still poorly understood.

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