Temperature control during an operation is yet another of the many tasks that anaesthetists are responsible for. Temperature control has a number of benefits and also affects the patient in recovery so I thought it would be useful to include a blog
Humans are mammals and therefore maintain a very strict control over their body temperature. A rise of only a couple of degrees due to infection can cause rigors. A similar reduction can reduce concentration and cause shivering whilst a further reduction results in immobility and can seriously slow the heart.
Under a general anaesthetic we lose the ability to maintain our core temperature. We cannot shiver. Anaesthetic drugs tend to dilate the peripheral blood vessels allowing more heat to be lost through the skin while further heat is lost from the lungs due to bypassing the nose which is the body’s natural humidifier. If the operation involves an open procedure on the abdomen or the chest further heat is rapidly lost from the wound.
To overcome this the gold standard is convective heating where warm air is blown over the patient. This has to be carefully regulated as an excess of only a few degrees can cause burning in the unconscious patient. We also use humidifiers on the breathing circuit and if we are giving large volumes of fluid or blood we can warm those as well.
Maintaining a normal body temperature is not only important for our metabolism:
- It has been shown that pre warming a patient i.e. warming them before the start of anaesthesia can reduce the incidence of wound infection in breast surgery by up to 50%. Although we have not seen any studies for other types of surgery it is logical to believe the same applies.
- Once the body cools down, platelets in the blood are concentrated in the spleen. Although they are released into the blood again when the patient warms up, during this period of sequestration there will be a risk of excessive bleeding
- Patients who are cold in recovery experience increased pain. This in turn leads to an increase in the use of narcotics such as morphine which can produce confusion and nausea
- A cold patient may shiver which is not only unpleasant but also massively increases their metabolic rates and oxygen consumption. This can be a particular problem for patients who have heart conditions
So in practical terms I warm patients before the start of anaesthesia. We have televisions in the ceilings of the anaesthetic room and this all goes together to provide a more reassuring and comfortable environment at what is a stressful time. The warming consists of a paper tube wrapped around the patient’s body covered by a blanket through which warm air is blown. We also use this during the operation together with airway humidifiers and fluid warmers. We can measure the temperature during the operation although it can be difficult to maintain temperature even with the best equipment in some cases. We can continue warming patients in recovery. If you shiver in recovery this is often due to temperature receptors around the spinal cord which have been reset by the anaesthetic drugs. The best treatment is to apply heat to the back i.e. to turn the patient on their side and apply the convective heat directly to the back