Rewarming

Induced hypothermia is employed to varying degrees during cardiopulmonary bypass to reduce oxygen consumption, thus allowing lower hematocrit, lower pump flow, and lower mean arterial pressures to ensure adequate oxygen delivery. Hypothermia also affords a margin of safety should perfusion fail. The extent of hypothermia varys with the technique of myocardial protection employed—cold crystalloid cardioplegia, warm blood cardioplegia, or cross-clamp/fibrillation.

In order to avoid the problems associated with hypothermia, recent interest has focused on 'warm bypass' where the patient is kept normothermic throughout. However, the technique is not widely established and moderate hypothermia of 30 to 32 °C is most commonly used.

Perioperative rewarming, like cooling, is effected by the heat exchanger in the bypass circuit and involves warming the blood above 37 °C, avoiding excessive temperatures that may damage blood components and also have adverse neurological effects. The patient is separated from the bypass at normothermia (measured centrally), but because of uneven rewarming rates different tissues will not be uniformly warm. Further heat is lost during the completion of surgery, so that by the time of arrival in the intensive care unit (ICU) the patient tends to be cold peripherally and centrally, with vasoconstriction. Over the next 2 to 5 h the patient rewarms with progressive vasodilatation, requiring fluid loading to maintain blood pressure and perfusion. Shivering increases oxygen consumption dramatically (by 300-600 per cent) and must be avoided to prevent oxygen demand outstripping oxygen supply with consequent tissue acidosis.

Many techniques are used to aid rewarming, including vasodilators to improve peripheral distribution of heat, prevention of surface loss with blankets, and addition of heat with convective air devices.

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