Chapter 30 Warming devices
Background
Physical principles
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.
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
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).