Principles to exploit

We can prevent heat loss, warm the patient or warm materials which come into contact with the patient.

Prevention of heat loss

This occurs by conduction, convection, evaporation or radiation (see 'heat'). All can be reduced by insulating the patient i.e. covering them. In order to undertake surgery, it is necessary to access some areas of the body and the resulting heat loss depends very much on the nature of this exposure. For some areas (e.g ENT) almost the entire body can be covered and even the exposed areas loose little heat. By contrast, during a major laparotomy, a large area of warm, moist tissue is exposed and loses due to evaporation, radiation and convection will be large. Usually we do little to prevent this loss and concentrate on measure to warm the patient. The exception are the burns surgeons who make the sacrifice to work under arduous conditions with high levels of humidity and atmospheric temperature and then add overhead radiant heaters, all of which is very effective in reducing heat loss.

Active warming

This can be broken down in the same way as we consider heat loss.

Conduction

Warming mattresses have long been used. They may use a flow of warmed water or an electric heating element, each of which has its advantages and disadvantages. However, this approach has lost favour. It is only possible to deliver energy to skin in direct contact with the mattress. This must limit the maximum temperature which can be applied and the combination of limited temperature over a limited area means this these devices are not the most effective. Nonetheless for some type of surgery, where other type of warming are impossible, this approach remains an option.

Intra-cavity warming

Although not used routinely, the instillation of warm fluids into the bladder or abdominal cavity is a very effective way of delivering heat. As a result, it is sometimes considered in treating severe hypothermia.

Convection

Arguable as to whether this should be classified as 'conduction' as the air movement is forced, rather than due to convective movement. Nonetheless, the use of forced air warmers is usually classed as 'convective' (CHECK THIS STATEMENT).

Forced air warmers use a remote heater to warm air which is then pumped into the patients environment, usually via a porous balloon-like blanket. As this air is trapped under any overlying draped or covering, it creates a micro-environment which warms all of the accessible, superficial tissue. Just as with conductive heating, the maximum air temperature must be limited to prevent burns, but despite this, this approach has become the most favoured in recent years.

Radiation

Radiant heater can be extremely effective when large areas of tissue are going to be exposed and particularly when the tissue will be moist. Unfortunately, they tend to be even more effective in warming the surgical team and as a result are not popular. You will usually only find this type of heater installed in burns theatres because these patients are subject to such prodigious heat loss that the discomfort must be accepted.

Fluid warming

Remember that the term fluids included both gases and liquids and so we can include not only devices which warm liquids (blood warmers, CVVH and by-pass machines), also devices which warm respiratory and insufflation gases.

Liquids

Conduction

This is the most commonly used approach, with the fluid to be instilled passing over a warmed surface. It is simple and effective, but has its limits. We could increase the amount of energy delivered by increasing the temperature of the warming surface, but this becomes a problem when dealing with blood products. Alternatively, we could increase the surface area over which heat exchange can occur, but this will inevitable increase the volume of fluid in the system. This dead-space must b filled, but will not be delivered to the patient. ELABORATE - simple PLATE warmers / ways of increasing SA -> L1 ->

Radiant

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Gases

Respiratory gases are seldom actively heated, but they can account for considerable energy loss. Where the gas comes from a compressed source, it must have been dried to a very high degree; were this not the gas, then the temperature drop as it decompresses would result in any water freezing, jamming regulators and valves and leading to microbiological contamination of pipelines. When patients breath this gas, their respiratory tracts must provide the energy to warm and humidify it. BADLY DEVELOPED APPROACH THIS FROM SHV. GASES LOW SHC SO LITTLE PINT IN ACTIV WARMING. LIQUIDS HIGH SHS, SO WHEN HUMIFICATION REQUIRED, WARMING BECOMES RELVANT. SO ACTIVE HUMIDIFYERS AND PASSIVE HMES