Anaesthetists have had a considerable interest in maintaining their patients’ body temperature for several decades. Under a general anaesthetic, the ability to shiver is eliminated. The blood vessels under the skin and in the peripheries dilate and these two factors result in a loss of body heat.

A drop in temperature or hypothermia can result in problems both during and after the anaesthetic. Platelets are sequestered or stored in the spleen leaving less in the blood. Platelets are important for normal blood clotting and together with a reduction in the effectiveness of other clotting factors, hypothermia can increase bleeding and make it more difficult to control.

Hypothermia increases the work of the heart which can cause myocardial ischaemia – a condition where the muscle of the heart cannot get enough oxygen. This is particularly important in patients with pre-existing coronary artery disease.

Hypothermia can also have neurological effects resulting in muscle weakness and prolonged recovery although in practical terms this is relatively rare.

Patients who are hypothermic will feel more uncomfortable when they regain consciousness in recovery. They are more likely to shiver and obviously they will feel cold. Shivering is not only unpleasant: it dramatically increases energy consumption and can cause problems in patients with coronary artery disease. It causes hypertension which can increase bleeding and increases cardiovascular risk. Patients with hypothermia also have more pain and require more opiates and other pain relieving drugs. Most importantly, intraoperative hypothermia increases wound infection and reduces wound healing.

Over the last few years, more and more hospitals routinely use body warmers during operations. The commonest what are called convective blankets which blow warm air over the patient. Unfortunately, these are often used only once the operation is underway which can be half an hour after the start of anaesthesia. If the patient has been left with minimal covering during this time their body temperature will have already fallen and it can be difficult if not impossible to regain normal body temperature. Some equipment manufacturers have introduced equipment that will blow warm air through patients’ clothing but in my experience this is only of benefit in cold hospitals to keep patients comfortable before they leave their room because the equipment is disconnected and patients become very cold walking to the operating theatre in a paper gown. Once a patient has become cold they vasoconstrict or shut down the blood vessels in their skin putting a further strain on the heart and making it more difficult for the convective equipment to subsequently work.

So my approach is rather different. First I only work in hospitals with full air conditioning and central heating so that my patients are warm in their rooms. I discourage the use of these paper gowns because most of my patients prefer cotton gowns and dressing gowns. However, in the anaesthetic room I actively warm any patient undergoing a significant operation using a convective warming system for 10 minutes. Patients find this very pleasant and it also gives them a chance to relax which is excellent for their blood pressure. The equipment can be used continuously and is left on from the start of anaesthesia until the end of the operation ensuring that all my patients are kept warm throughout the operation. We can also use convective heating in recovery to ensure each patient is as comfortable as possible, pain is minimised and the risk of wound infection kept as low as possible.

About the author

Dr Aubrey Bristow is a consultant anaesthetist in central London. These articles are his personal views and reflect individual issues of interest to patients. They are not a comprehensive review of the subject nor a substitute for a consultation with your anaesthetist.