2016 May - Peri-anaesthetic burns
Burns associated with actively warming anaesthetised patients have recently been a common factor in a number of complaints dealt with by the Board.
Due to their small body size, altered peripheral perfusion from premedication or anaesthetic drugs, intubation by-passing the warming surfaces within the nose, inhaling cold gases and heat loss from skin surface or open body cavities, small animal patients frequently suffer operative hypothermia (body temperature < 36°C). Veterinarians all know that hypothermia can lead to increased anaesthetic morbidity. So it is good practice to be making efforts to maintain body temperature of anaesthetised veterinary patients.
It is important to note that very slightly excessive thermal support can easily lead to severe burns in anaesthetised veterinary patients - the margin for error is really surprisingly narrow! Older thermostatically controlled electrical heating mats or recirculating warm water mats, hot water bottles or latex gloves filled with hot water, or wheat or rice bags, or radiant heat sources are all very inefficient modalities of thermal support, and have all been associated with calamitous burns in small animal patients. The variability of these devices and methods makes them inherently unreliable, and their use should only be considered if they can be very closely and continuously monitored by experienced staff. In most circumstances their use is no longer the standard of care for thermal support for anaesthetised veterinary patients.
Other factors that contribute to these types of burns include the temperature of the heat source of course, and the duration of contact. The relationship between skin surface temperature and time to cause a 3rd degree skin burn is logarithmic, so, while 45°C warming can burn skin after 90 minutes, at 50°C warming can burn skin after ONLY 10 minutes. Dependent, weight-bearing areas of skin will be more poorly perfused, and so the removal of heat from such an area of skin by the patient’s circulation will be significantly impaired. The increased pressure of weight applies the skin to the hot surface more intimately and so the risk of burns is increased. Dry fur provides some insulation and limits heat transfer, and so hairless areas of the ventral abdomen can sometimes be more at risk. However wet fur or a wet towel covering fur can facilitate heat transfer to the surface of the skin through the fur and exacerbate the “burn risk”, so those monitoring anaesthesia must pay constant attention to fluids such as blood or irrigation fluid to limit the chance of wetting occurring between the patient and the source of thermal support.
Patients that are already hypothermic are genuinely problematic – they require more heat to return to normal temperatures and their skin is often profoundly hypoperfused. The risk of a burn in these patients is proportionately increased.
The risks of burns in anaesthetised patients can be considerably reduced with the newer generation of thermal support systems, such as forced-air warming devices (FAWD) and conductive fabric (resistive polymer) electric warming. These technologies are coupled with modern and reliable electronic smart-controllers and thermostats, which further enhance their safety. Some companies now make FAWD that are specifically designed and suited to the characteristics of our furred patients.
There are a number of other steps that can be taken to prevent hypothermia during anaesthesia, including warming of intravenous fluids, and warming of inspired air, that work synergistically with active heating using FAWD or conductive polymer fabric warming, to lower the risk of hypothermia. They effectively “spread the load” so that the skin does not have to accept the whole load of heat transfer to facilitate maintenance of normothermia.
Pre-anaesthetic warming is a recently developed modality for managing operative hypothermia: 30-60 minutes under the cage version of the forced-air warming blanket can make a significant difference to the rate of heat loss, especially in the first few minutes after induction of our small animal patients.
Even if you are doing ALL the possible thermal support measures for your patients, there are still a few rare patients with extremely sensitive skin who seem to develop burns despite all appropriate precautions. Like many aspects of our profession we can only manage to minimise the risks, and we cannot guarantee that our management will prevent a potential problem.
The Board would like to thank Dr Colin Dunlop for his assistance in providing the resources used to write this article.