A reminder of the risk of burns with external heating devices in anaesthetised patients

Despite previous warnings of the risk of burn injuries with external heating devices in anaesthetised patients, ¹ it appears that burn injuries with these devices continue to occur. The aim of this brief review is to provide another reminder of the hazards associated with external heating devices, and how they can be avoided.

All clinicians are aware that burns result from the total heat energy transferred to tissue, not temperature alone, as temperature only indicates the direction of energy transfer. Even tissue temperatures at 43°C may be associated with a burn injury, given enough time. ²

Conduction heaters (i.e. heated mattresses or blankets) ‘transfer heat through matter (i.e. solids, liquids, or gases) without bulk motion of the matter’. ³ They cause a burn when excessive thermal energy transfer occurs. The temperature differential determines the rate of energy transfer. Engineering and practical restrictions dictate that we measure temperature only in the device controller, sometimes in the heating device itself and usually only at a single point in the patient. Measuring the temperature at these points does not exclude higher temperatures at other points on the device and consequently on the patient. This means that the safety of conduction heaters depends on even heating of the mattress or blanket. If small areas exceed the set temperature, a burn can occur. For example, in one report, three children were burnt by a heating mattress that was operating normally according to the displayed temperature on the controller. ⁴ The problem with conduction heaters is that it is extremely difficult to guarantee even heating unless multiple temperature measurement sensors are incorporated, which does not occur in any standard device. To partially mitigate this hazard, the ‘Hotdog’ Heating Mattress (Models U1XX, U3XX and U5XX, Augustine Temperature Management, Eden Prairie, MN, USA) measures the patient–mattress contact temperature, but only at a single point, and it is mandatory that this point is covered by patient contact. ⁵ Additionally, the Hotdog patient warming mattress has a maximal temperature setting of 39°C. The Hotdog controller (Models WC7X, WC71and WC77, Augustine Temperature Management, Eden Prairie, MN, USA), however, allows a maximal temperature of 43°C in Hotdog blankets. ⁶

Some conductive heating devices cannot guarantee that heating is even; nor do they incorporate temperature monitoring (e.g. the BARRIER® EasyWarm® blankets (Mölnlycke Health Care, Göteborg, Sweden). ⁷ To maintain safety margins these devices need strict practice restrictions, as outlined in the instructions. ⁷ Misuse can result in device temperatures of up to 70°C. ⁷ As such, in my opinion, it would be unwise to use them on children, unconscious patients or patients with distracting pain issues, neuropathies or reduced tissue perfusion.

Convection heaters ‘transfer heat between two bodies by currents of moving gas or fluid.’ ³ Forced air warmers, properly applied, can eliminate convection losses while adding thermal energy. However, they too are vulnerable to uneven heating issues. All devices warn against ‘free hosing’ (i.e. discarding the inflatable perforated blanket and placing the hose directly under the patient’s bed linen or the surgical drapes).^8–10 This practice concentrates the warm air on one point on the patient’s body and can cause a burn. ¹¹ Additionally, ‘hot spots’ can be created by inadvertently hindering full blanket inflation, again potentially resulting in a patient burn.^2,12

The safety of convection warmers is dependent on temperature regulation of the heated air they supply. If the heater is functioning normally, eliminating temperature settings above 39°C should eliminate burns, with the caveat that all devices operate to an accuracy of about ±2°C. This accuracy is reportedly slightly better in the Bair Hugger® (Model 775, 3M™, MN, USA) ⁸ and the Covidien Warmtouch® (Covidien, Mansfield, MA, USA) at ±1.5°C. ¹⁰ Problems may arise when the device is set to temperatures above 39°C. This can be as high as 43°C in the Bair Hugger®, 46°C in the Cocoon® Convective Warming Machine (CWS 4000, Laubscher & Co., AG Hölstein BL, Switzerland) and 47°C with the Covidien Warmtouch®.^8–10 All these maximal settings are close to or within the danger range of >43°C.

Recognising this hazard, some manufacturers set time limits for higher temperature settings. This is ten min in some devices (Cocoon® Convective Warming Machine) but up to 45 min in others (e.g. Covidien Warmtouch®). ⁹ Surprisingly, the Covidien Warmtouch® step-down temperature is still 45°C. ⁹ Unfortunately, clinicians can over-ride the time limit safety feature in all devices.

Most clinicians feel reassured that the over-heating safety cut-off will prevent burns if the convection device malfunctions. However, they should be aware that the ‘Over-temperature’ shut down is set at 56°C with the 3M® Bair Hugger, 53°C with the Cocoon Convective Warming Machine and 49–50°C with the Covidien Warmtouch.^8–10

In summary, it is an easy engineering task to guarantee that the air leaving the hose in a convective warmer will be at the set temperature. In contrast, it is difficult to guarantee perfectly even heating in conduction heaters. A solution, incorporating multiple temperature sensors, to my knowledge, does not occur in any device.

All the problems with convective and conduction heaters are heightened with children. ¹³ A heating device designed for an adult may deliver the same amount of thermal energy to a child. Thus, if a burn occurs, is likely to be larger and deeper on a child.

The redistribution of the thermal energy from any heating device by the patient’s circulation adds another layer of complexity. The blood flowing through the heated surface of the patient’s body has a cooling effect. Hypoperfusion of skin areas by pathology or patient positioning increases the risk of a burn in that area. Laterally positioned patients, with their hip and shoulder pressure points on a heated mattress, are particularly vulnerable.

Based on my personal experience, I would make the following recommendations. They are not part of any professional or other organisation’s guidelines and do not represent consensus statements.

Consider the risk of burns when setting any device to 43°C or above.

It is preferable to prevent hypothermia occurring in the first place than to use devices set to 43°C or higher to heat cold patients.

Be mindful of the ‘time to default temperature’ when setting any device to 43°C or higher.

Do not use a convective heater without an inflatable blanket.

Ensure that convection heater blankets inflate evenly.

Be aware that some conductive heating devices require the patient be positioned so the device temperature sensor is always in contact with the patient.

Use caution with conduction heating mattresses when the patient is positioned in the lateral position.

Use all devices and their blankets strictly as per instructions.