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.

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).

Figure 30.1 Kimberly-Clark Patient Warming System. A. back pad with warm water microcirculation channels. B. warm water delivery and control unit.

Images courtesy of Kimberley-Clark Healthcare.

Carbon fibre and polymer devices

Carbon fibre heating mattresses consist of electrically conductive bundles of this material that criss-cross the device in much the same way as the wire element in electric blankets. When an electric current is passed through the device the resistance of the material causes the mattress to heat up. However, the biggest problem with these is that it is difficult for such systems to deliver uniform heating characteristics, with the consequent risk of burns to a patient. This is because the area of heating surface may be inadequate and the hardness of the bundles means that these require some form of pressure relief material on top which attenuates the warming performance.

In contrast to carbon fibre, carbon polymer theoretically benefits from a higher thermal transfer capacity, more uniform heating characteristics and better elasticity (see below).

The heat generated in the polymer is caused by excitation of the carbon atoms within the polymer due to the passage of an electric current. This is produced by a low-voltage source applied across the edges of the sheet. The polymer increases the electrical resistance by holding the pattern of the carbon particles. The variation in temperature across a sheet the size of an operating table is less than 1°C, thereby delivering heat in a uniform manner over a large surface area. The properties of the polymer also allow a viscoelastic foam to be placed under the warming surface which provides pressure relief superior to a standard operating table mattress or gel pad. The moulding of the foam improves the efficiency of the mattress as the heat is transferred through conduction rather than convection. It also prevents one of the problems with other warming mattresses in that there is less pressure to occlude the skin circulation, which in turn reduces the incidence of thermal damage and pressure sores.

A full-length mattress takes approximately 65W at full power (i.e. during warming up phase). The power needed to maintain temperature varies depending on patient characteristics and ambient conditions. A thermistor on the rear face of the polymer sheet is connected to a microprocessor control unit that regulates the power to the mattress to maintain the selected temperature. This can usually be set at between 37 and 40°C.

The working components are encapsulated in a latex-free cover, with welded seams, which means that the mattress can be cleaned in the same way as an operating table (Fig. 30.2).

Figure 30.2 Inditherm carbon polymer warming mattress.

Image courtesy of Inditherm Medical.

The logistical advantages of carbon technologies include that the warming area can be maintained largely irrespective of the surgical access required and the operating theatre is quieter due to the absence of circulating air and complaints from the surgical staff that they are too warm. On the other hand, in circumstances where there is reduced patient contact with the mattress (e.g. lithotomy position) equivalence with the variety of shaped (’specialist’) forced-air warming blankets has yet to be established. Recent advances incorporating carbon polymer into blankets may serve to overcome problems of inadequate mattress contact area.

Forced-air warming blankets

These have revolutionized patient warming. Broadly speaking a large volume of air is blown over a 450–1400 W electrical element which warms it to 35–46°C. This is then passed through a ‘quilted’ blanket that covers the patient (Fig. 30.3). The power consumption is around 850 W for the lower powered devices and up to 1500 W for the more powerful ones (i.e. a factor of 10–20 greater than the carbon polymer mattresses). There is a significant variability in the performance of the different types of forced air heating devices (Table 30.1).

Figure 30.3 Bair Hugger forced air warming blanket.

Image courtesy of Arizant UK.

Various different blankets have been developed in order to maximize the surface area covered during different surgical procedures and exposures; including now forced warm air mattresses for positioning underneath the patient. With improving technology, the heating devices themselves can be much smaller and so it has been possible to develop special jackets with portable heaters that can be used to keep patients warm throughout the perioperative period.

There are both disposable and reusable products. What is gained in terms of reduced consumables with the latter may be partly lost by the environmental and financial costs of laundering.

Of the single-use types, there are two versions which differ in that one is a closed system whereas the other forces air out through small holes on the side of the blanket facing the patient. There is the unproven possibility that the latter may introduce pathogens into the surgical field. They both have filters on the air intake; although these are not small enough to exclude all pathogens that may also exist on and within the warming unit, there is nothing in the NICE literature review to suggest an increase in infection rates.

Radiant heaters

Radiant heaters are electric heaters that generate heat using infrared radiation in the same way that the sun heats the Earth (Fig. 30.4). The infrared spectrum has a wavelength of 0.7–10 µm. Non-industrial heaters use the medium part of the spectrum (approx. 1.5–5.6 µm), typically utilizing the range 2–4 µm. Radiant heat transfer, unlike conduction and convection, requires no intermediate conductor or convector, as infrared energy, like light, passes directly from the source to the receiver. The rate of heat transfer depends on the emissivity of the source, the absorptivity of the receiver, the difference between their absolute temperatures (raised to the fourth power), and the distance between them.

Figure 30.4 An infrared radiant warmer for infants.

The element on a typical neonatal unit typically consumes 600 W power. They can either generate heat to a set air temperature or, via a feedback mechanism, to a set skin temperature. The danger with the latter is that if the sensor falls off it will read air temperature, which will run the risk of overheating of the patient.

Radiant heaters have the advantage that they provide efficient, uniform and immediate heat, and they are unaffected by air currents, such as those in laminar flow operating theatres. In addition, they do not generate air-currents, which might facilitate the spread of pathogens.

The disadvantages are that they warm the operating staff even more than forced-air warmers, that they may be impractical for patients much larger than 8 kg and that they will only warm exposed surfaces (which can increase evaporative losses). Thus, their use is largely restricted to paediatrics.

Locally applied warm water and pulsating negative pressure

With appropriate methodology it is feasible to warm the whole patient with very localized heat application. One study has shown the device illustrated in Fig. 30.5 to be more effective than forced-air warming used during laparotomy¹⁰.

Figure 30.5 Cylindrical transparent plexiglass cylinder.

Redrawn with permission from Rein EB, Filtvedt M, Walloe L, Raeder JC. Hypothermia during laparotomy can be prevented by locally applied warm water and pulsating negative pressure. Br J Anaesth 2007 Mar;98(3):331–6.

It consists of a custom-built, tube-shaped, transparent Plexiglass chamber, which is sealed to the proximal part of the arm by a neoprene collar. Warm water at 42.58°C is circulated between the cylinder and a thermostat-regulated water bath, which is circulated at 3.5 l min⁻¹. Prior to commencing warming, the chamber is three-quarters filled, leaving an air pocket from which the air could be evacuated to give negative pressure, which is pulsated between 0 and 240 mmHg.

Forced-air/coil warmers

As can be seen from the entry for the Baire Hugger (Fig. 30.6) device in Table 30.2, these are the least effective and consist simply of a coil placed inside the hose of a forced air warming mattress. Their poor performance can be explained by the different thermal capacities of air and water (see above). In this case air, which has a low capacity, is being used to heat fluid which has a high capacity.

Figure 30.6 Dry air fluid warming coil.

Plate warmers

In these devices the fluid passes through a special cartridge that brings it into indirect contact with an electrically heated metal plate. The temperature of the plate is set to 40°C (rather than 37°C) to compensate for the loss of heat over the length of line between the warmer and the patient. Small warmer units are also available that can be placed close to the patient’s infusion site (Fig. 30.7). The water channel of the cartridge is made from plastic with one side bonded to an aluminium-serrated plate. The latter is placed in direct contact with the heater element and is responsible for the transfer of heat to fluid passing through the plastic channels in the cartridge.

Figure 30.7 The Vital Signs enFlow dry plate warmer. The small IV casette is shown inserted into the warming plate.

Image courtesy of GE Healthcare.

Counter-current warmers

These attempts to offset the losses between a warming device and the patient by placing a circulating warm water jacket around the intravenous line along its whole length. Although the warmed water does not come into direct contact with intravenous fluids, bacteria can proliferate in the reservoir and so the possibility of cross-infection remains.

Infrared flow compensated fluid warmers

The Fluido system (Fig. 30.8) is a dry fluid warming system. Fluid channelled through a rigid cassette is heated by four infrared lamps with a maximum output of 1200 W.

Figure 30.8 Fluido dry fluid warmer. A. Stucture. B. Insert. C. External appearance.

Image of Fluido device (C) courtesy of TSCI International, BV.

By calculating fluid flow rate from the temperature change across two set points the device is able to alter the output temperature of the fluid to compensate for the expected heat loss along the fluid administration set to the patient. Flow rates up to 800 ml min⁻¹ are claimed at temperatures of up to 39°C at the patient.

High-flow fluid warmers

Devices designed for infusing fluids at high flow rates require both lower resistance ‘cartridges’ and some form of pressurization.

The ‘Level One’ (Fig. 30.9) system from Smiths Medical uses heated water at 42°C and an aluminium counter current heat exchanger to deliver flow rates up to a claimed 1400 ml min⁻¹. Two rigid housings for the bags of infusate are pressurised to 300 mmHg by an in built compressor to deliver an uninterrupted flow. The maximum effective flow rate for fluids at 10°C is approximately half of that for fluids stored at 20°C. These devices have been inadvertently charged with bags partly containing air and have delivered fatal air emboli into patients. Although newer versions have an air detector, this can be bypassed. There is also an upgrade available for the older machines, but the risk of air embolus remains significant with any system that uses a pressurised infusion bag.

Figure 30.9 Level 1, high flow fluid warming system.

Image courtesy of Smiths Medical, UK.

The Fluido infra red warmer (see above) can also be used as a high flow device. The casette can be fitted with two infusion lines that are inserted into IV fluid bags pressurised by pneumatic chambers fed by a compressor. The AirGuard component (Fig. 30.10) protects against air embolism. If the fluid in the chamber falls below a fixed level a valve will close the supply tubing, to prevent the infusion of air. In addition, an infrared tube sensor continuously monitors the presence of this tubing to ensure that it is fixed correctly into the shut-off valve. The complete system is mounted on a drip stand (Fig. 30.11).

Figure 30.10 (A) The Fluido AirGuard. (B) A schematic of the Fluido AirGuard system.

Images courtesy of TSCI International BV.

Figure 30.11 Fluido High flow system.

Image courtesy of TSCI International BV.