Peri-anaesthesia hypothermia (body temperature < 36°C) occurs in up to 80% of anaesthetized cats and dogs. This can be due to their small body size, altered peripheral perfusion from premedication or anaesthetic drugs, intubation by-passing the nose, inhaling cold gases and heat loss from skin surface or open body cavities. Prevention of peri-anaesthesia hypothermia has focused on body surface heat conservation and surface warming using electric or warm water under-blankets, but this has been proven to be ineffective and can burn animals.

Heat loss is exponential. Premedication causes mild hypothermia with dogs and cats typically losing 0.5–1°C over 30 to 60 minutes before anaesthesia induction. This initial drop in core body temperature precedes the precipitous, critical heat loss of 1–3°C that occurs in the first 15 to 30 minutes after induction during "clip & prep time." Once animals are draped for surgery, heat loss slows (see Graph 1: table warming only line). The Darvall warm air blanket system can raise body temperature in surgery but heat gain is also exponential. There is typically a 30 to 45 minute "lag" from onset of warming until an increase in body temperature is observed (see Graph 1: magenta line, Graph 2). During this "lag" period skin, subcutaneous tissue, and blood warming occurs. Simultaneous use of heated breathing circuits will enhance the warming response to warm air blanket heating (see Graph 1: yellow line). If the procedure time is less than 45 to 60 minutes, surface warming electric heat pads insulated by a towel, combined with heated breathing circuits, may be as effective as warm air blankets (see Graph 1: red line, pre-warming). Warming hypothermic animals recovering from anaesthesia is a slow, laborious process often taking more than 1 to 2 hours.

Graph 1 Hypothermia in anaesthesia occurs with the most rapid heat loss during "clip & prep" time. Warming hypothermic animals is difficult (blue line). Darvall's heated breathing circuits (HBC) used from the moment of intubation reduce this heat loss (red line). Darvall's warm air blankets (WAB) can warm hypothermic animals in surgery (magenta line), which is faster when combined with a heated circuit (yellow line). Warming premedicated animals with WAB for 30–45 min. before induction can also reduce post-induction hypothermia (red line). Adapted from Dunlop C. Heated smooth wall circuits. In: Proceedings from the World Congress of Veterinary Anesthesiology Meeting; Sept 23–27, 2012; Capetown, South Africa. Abstract.

Warming devices can burn animals. The margin of safety for causing significant thermal injury is surprisingly narrow. Skin can be burnt from devices supplying as little as 46°C surface heat for one hour. Containers of warm water, heated wheat bags, or "on-off" electric heat pads not specifically designed for sedated or anaesthetized animals which can't move away from excessive heat, can cause severe burns. The Veterinary Defence Society Ltd., the UK professional indemnity insurer, has specifically advised against their use. Thermostatically controlled, constant warming devices such as warm water blankets with even heat distribution are safer.

Pre-warming before induction of anaesthesia. Providing thermal support before anaesthesia may seem counter-intuitive, but recent research shows that effectively warming patients from the time of premedication to the time of induction can prevent the initial mild drop in body temperature and then slow the rapid critical heat loss immediately following anaesthesia induction. A warm air cage blanket system is thermostatically controlled and safely delivers a high volume of warm air at a constant 43°C and can effectively pre-warm caged animals before anaesthesia. It takes at least 30 minutes to properly pre-warm cats and small dogs and 45 minutes for larger dogs.

Forced warm air heating for surgery. Ten year safety record for Darvall's warm air blanket system: most surface contact warming devices used in anaesthetised animals, including electric or warm water under blankets and warm air blankets with high surface air flow designed for humans (e.g., Bair Hugger^TM 3M Corp.), are unable to raise the temperature of hypothermic dogs and cats during surgery (Machon et al. 1999; Tan et al. 2004). Darvall Cocoon® warm air blankets are designed with a porous patient contact surface resulting in low surface air flow. Blankets used for cats and dogs during surgery are designed to be positioned around and underneath the animal, enabling the hair coat to trap rising warm air.

In a recent study in dogs 17.3 ± 11 (mean ± SD) kg anaesthetised for surgery, following onset of heating there was an initial 30 to 40 minute "lag" phase, possibly where skin warming occurred. Then there was a consistent increase in the body temperature to near normothermia by the end of the surgical procedure (see Graph 2).

Graph 2 Darvall warm air blankets compared to electric heat mats for treatment of hypothermia in anaesthetized dogs 17.3 ± 1 (mean ± SD) kg. Lau A et al. [Honours Thesis]. University of Sydney; 2008.

Pre-warming caged animals before anaesthesia. Recent research shows that premedicated animals lose 0.5–1°C before anaesthesia, then rapidly lose a further 2–4°C in 15 to 30 minutes following induction. Effective pre-anaesthesia warming combined with warming inspired gas immediately after intubation can prevent the rapid, critical heat loss from occurring during "clip and prep," before animals are draped for surgery.

The Darvall warm air caged blanket system forced safely and effectively warms premedicated animals, delivering a large flow of warm air at constant thermostatically controlled temperatures (43–46°C) without risk of burning. The cage blankets are designed to go over the top of conscious animals. The cage adapter allows the door to open and close properly for patient safety and to prevent damage to the heater hose. The Darvall cage warming system is cost effective because one Vet Cocoon® heater can warm up to 2 cages simultaneously. Blankets can be reused between caged animals prior to anaesthesia because they have not yet been clipped or prepped for surgery. Pre-warming cats and small dogs takes 30 minutes; medium to large dogs need 45 minutes of warming.

Warm Inspired Gas - A Totally New Solution to Hypothermia

Rapid heat loss occurs during "clip and prep" time when the animal is being moved which results in poor surface heat transfer from heat mats. Recently developed heated breathing circuits (Darvall) warm inspired gases from the first breath after intubation, targeting the post-induction rapid heat loss occurring during "clip and prep." Inspired gas is usually cold, about room temperature (21 to 24°C). These heated breathing circuits have a heating element imbedded into the ribbing of the smooth-wall tubing. A sensor molded in the tubing at the Y-piece connector monitors gas temperature and the microprocessor controls heating. From the end of the heated tubing to the end of the endotracheal tube, the warmed gas typically loses 10°C, so in the inspired limb of the anaesthesia circuit, gas is heated to 43–45°C, resulting in 35 to 37°C gas in the animal's airway. Closed-loop feedback is provided by an oesophageal temperature probe which enables the microprocessor to monitor the animal's body temperature and turn off heating if > 37°C.

To be effective, the heated tubing should be used from the moment of intubation. Because animals only breathe 10–15 breaths/min with about 1 second inspiration under anaesthesia, heated breathing circuits alone won't raise body temperature.

Peri-Anaesthesia Thermal Support Selector^TM

Chart for selecting effective warming treatment options by type and length of procedure.

References

1.  Read M, et al. Conductive warming prevents the development of hypothermia in anaesthetized dogs (abstract). Vet Anaesth Analg. 2014;41:A22.