Prewarming of surgical patients to prevent hypotension on induction of anaesthesia

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General anaesthesia, whilst very safe, has some undesirable side effects. Two of these, a fall in body temperature and a fall in blood pressure, are usually very mild and require little treatment. Occasionally, more severe changes can result requiring significant intervention by the anaesthetist. Whilst prewarming of patients prior to surgery is sometimes employed to reduce the drop in body temperature seen with anaesthesia, it is not known whether this warming may also help reduce the fall in blood pressure. Warming patients results in dilation of blood vessels, with some of the circulating blood volume then moving into more peripheral tissues. This is similar to the process that happens under anaesthesia, with the relaxation of blood vessels caused by anaesthetic drugs resulting in lowered blood pressure. This is usually mild and easily correctable with medications and intravenous fluid administered by the anaesthetist, though occasionally can be more serious. This study aims to answer the question of whether prewarming patients may reduce the fall in blood pressure seen with the onset of anaesthesia, due to blood vessels already being dilated, and hence reduce the need for intervention by the anaesthetist.