Warming devices

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Chapter 30 Warming devices

Chapter contents

Physical principles

Devices used to prevent perioperative hypothermia

Background

Inadvertent perioperative hypothermia (IPH), defined as core body temperature ≤36.0°C, is a common consequence of anaesthesia. It has a number of adverse effects, including greater intraoperative blood loss and consequent blood transfusion,¹ an increased rate of wound infection,² morbid cardiac events³ and pressure sores,⁴ as well as a longer stay in both recovery and hospital.⁵ It also causes subjective discomfort.

Maintaining normothermia perioperatively can reduce the incidence of these adverse effects. There are a number of devices that can be used to this end. They may be devices that attempt to conserve the patient’s own heat (passive) or devices that transfer heat from an external source to the patient (active). The latter may warm the patient externally or via warmed intravenous and irrigation fluids.

In 2008, the United Kingdom’s National Institute for Clinical Excellence (NICE) produced the ‘Management of inadvertent perioperative hypothermia in adults’ guideline.⁶ Its strength is in the fact that it is a clear endorsement of the clinical and cost benefits of perioperative warming, but its weakness is that it does not cover the full range of technology available due to a sparse research base.

The recommendations can be succinctly summarized as:

• all patients having operations lasting more than half-an-hour, or

• at high-risk of perioperative hypothermia

should receive warming. In addition, all fluid infusions of 500 ml or more should be warmed.^7,8 Together, these recommendations, therefore, encompass the majority of operations and most intravenous infusions, and highlight the need for a wide knowledge of the available warming technologies.

Temperature monitoring is covered elsewhere in this book. However, it should not be forgotten that this is an integral part of perioperative thermal management. Unfortunately, there are limitations to all currently available methods of perioperative temperature monitoring and it should be remembered that accuracy in the laboratory does not necessarily imply accuracy in the clinical setting.

Physical principles

These are important in understanding how warming devices work, how heat is lost and gained by the body, how warming devices work and, consequently, the best way to go about maintaining normothermia.

Heat generation

In the human body, the generation of energy is by chemical reaction and its quantity determined by the substrates and products of the reaction. Combustion of glucose and protein produces 4.1 kcal/kg, whereas fat provides 9.3 kcal/kg. Although heat generated in this way depends on the level of activity/metabolism, which is reduced under anaesthesia by 15–40%, most core hypothermia is the result of altered distribution of body heat rather than alterations in the balance of heat production and dissipation.

Heat transfer

Heat transfer can only take place down a temperature gradient. Within the body there is a gradient between core and peripheral ‘compartments’. Peripheral tissues are usually 2– 4°C cooler than the core. There is then the much more variable gradient between the peripheral tissues and the environment. This simplistic model is, however, somewhat modified by the body’s control over heat distribution via the circulation. The importance of this is demonstrated by the fact that even during warming, the peripheral compartment remains about 1°C less than the core temperature.

Conduction

This is defined as the transmission or conveying of energy through a medium without perceptible motion of the medium itself. In terms of heat, this is the transfer of thermal energy through a substance from a higher- to a lower-temperature region. Heat conduction occurs by atomic or molecular interaction. Core cooling occurs through conduction loss to the cooler peripheral tissues. In the same way, temperature loss through the infusion of cold intravenous fluids may, for practical purposes, be considered a conductive loss.

Convection

This is defined as the transfer of heat through a fluid medium (liquid or gas) caused by molecular motion. This is an important route of heat loss, wherein heating of air in contact with the body causes it to expand and rise resulting in the formation of convective currents that carry away heat. These losses are greatly exaggerated wherever external air currents remove this warmed air more rapidly. It is is the dominant source of heat loss in environments such as laminar flow operating theatres.

Radiation

This is defined as the transfer of heat from one surface to another via photons. It is not, therefore, dependent on the temperature of the intervening air. It is dependent upon the emissivity of two surfaces and the difference between the fourth power of their temperatures in degrees Kelvin. A black body absorbs and emits heat perfectly and has an emissivity of one (the opposite is a perfect mirror). Human skin acts more like a black body having an emissivity of 0.95 for infrared light. This is a significant source of perioperative heat loss.

Evaporation

This is the process whereby heat energy is used to change water from a liquid state into a vapour. This phenomenon which causes an observed heat loss, is governed by the latent heat of vaporization of water and consumes 0.58 kcal for each gram of sweat evaporated. Under normal circumstances and in the absence of sweating evaporation contributes only 10% of heat loss, mainly due to losses from the respiratory tract. However, its contribution becomes quite substantial during operations in which there is significant visceral exposure. It has also been suggested that alcoholic skin preparations may decrease the temperature of a 70 kg person by up to 0.7°C.

Thermal capacity

This is defined as the amount of heat energy required to increase the temperature of a unit quantity of a substance by a specific temperature interval and is significant in both the loss of heat and its prevention. All alterations in body temperature are the result of changes in the heat content of the tissues.

The specific heat capacity of body tissue is 0.83 kcal/kg/°C. In terms of perioperative thermal balance, of additional importance are the specific heat capacities of (dry) air, which is 0.24 kcal/kg/°C, and water, which is 1.0 kcal/kg/°C.

Insulation

Within the body this affects the conductive component of heat loss insofar as fat insulates almost three times as well as muscle. It is also the principle behind passive means of preventing IPH.

Devices used to prevent perioperative hypothermia

Passive devices

Although ordinary blankets, bedding and clothes prevent heat loss to some extent, they are not appropriate in the setting of the operating theatre where higher standards of cleanliness are required. The first products specifically designed for this setting were called ‘space’ blankets. These are made from a lightweight non-permeable material incorporating a reflective layer that reduces the patient’s radiant heat losses. The non-permeable element provides insulation from the operating theatre environment and reduces the convective heat losses. Their effectiveness is partly based on the high emissivity of heat from the human body. They also have the advantage that they meet the safety standards of ‘Flammable Fabrics’ Acts. However, for the majority of procedures, insulation alone is insufficient in preventing heat losses during anaesthesia, surgical preparation and subsequent surgery. There is, therefore, the need to provide heat from an external source.

Active devices

Circulating water devices

Initially, prior to the advent of forced-air warming, patients were placed on circulating hot water mattresses in an attempt to counteract heat loss and maintain normothermia. In theory the high specific heat capacity of water in the mattress should be very effective at providing heat. In practice, however, these devices only effectively deliver heat to those areas in direct contact with the mattress, which constitutes a relatively small proportion of the body. Furthermore, those small areas in direct contact are under pressure and so have a compromised blood supply that reduces the amount of heat transfer even further. Additionally, in this situation the relatively high thermal capacity of the water is a disadvantage as it increases the likelihood of thermal damage, which has been described at settings of 39°C.

Newer devices overcome these problems by circulating the water through special garments or pads. They include the Kimberly-Clark Patient Warming System (Figs 30.1A and B), which uses adhesive ‘energy transfer’ pads with micro-channels for circulating water that can be applied to the back, thighs, chest, or any combination of the three, depending on the site of surgery. Another modern system is the Allon circulating-water garment. This conductive heating garment is divided into separate segments for arms and thighs, which allows clinicians to cover different body surfaces depending on the site of surgery. Perhaps unsurprisingly, given the different thermal characteristics of water and air, both the above have both been shown to be more efficient at warming volunteers than forced-air devices (see below).

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