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  3. Advanced Anaesthesia for Surgical or Prolonged Procedures (EU21)
  4. Monitoring and maintaining body temperature during surgical or prolonged procedures

Monitoring and maintaining body temperature during surgical or prolonged procedures

Hypothermia

Hypothermia is a fall in body temperature of more than 2˚C. It has major effects on different body systems ( for example very cold animals may have a cardiac arrest ). Hypothermia also alters numerous physiological processes, such as cell membrane transport systems, enzyme activity etc and this could significantly influence your research data. 


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Depression of metabolic rate and enzyme activity can also greatly prolong recovery, especially when injectable anaesthetics have been used.

Small mammals have a higher surface area to body weight ratio than larger species such as the dog, and so lose heat more rapidly, for example significant falls in body temperature (5-10˚C) can occur in mice within 15 minutes of induction of anaesthesia. Most anaesthetics depress thermoregulation, and this effect, coupled with use of cold fluids, shaving and preparation of the surgical site and use of cold anaesthetic gases can rapidly result in severe hypothermia. 

Monitoring temperature

The first step in avoiding these problems is to monitor the animal’s body temperature. We strongly recommend that you purchase an electronic thermometer that can provide a continuous display of a wide range of body temperatures. The rectum is often the most convenient site for placing a temperature probe, but deep body or core temperature will often be underestimated. If the probe is positioned in the middle of a mass of faeces, its response time to the changes in temperature will be slow. For these reasons, it may be preferable to use a probe placed in the oesophagus, but it must be located in the lower part of the oesophagus to avoid the cooling effects of respiratory gases in the upper airway. Probe sizes of most veterinary clinical thermometers are usually suitable for placement in the rectum of all commonly used species, but smaller probe sizes need to be used in mice.

It is also useful to place a temperature probe between the animal and any heating devices that are being used to maintain body temperature. This will detect any over-heating problems that might occur, and enable measures to be taken to correct these and prevent superficial burns.

Maintaining body temperature

The next step is to take measures to reduce any fall in body temperature. Most animals will require some additional heating and insulation to minimize heat loss. Effective insulation can be provided either by wrapping the animal in cotton wool, followed by an outer wrapping of aluminium foil, or by using the bubble packing which frequently forms part of the packaging of laboratory equipment, or other insulating materials. After wrapping the animal in an insulating layer of material, a window can be cut to expose the operative field. When insulating small rodents, ensure that the tail is included in the wrapping, since heat loss from this part can be considerable. These simple measures will help reduce heat loss, but supplemental heating is essential for small animals, even for brief periods of anaesthesia.

Supplemental heating can be provided by heat lamps and heating blankets, but care must be taken not to burn the animal. A thermometer placed next to the animal, or between the animal and a heating pad, will show whether excessive heat is being applied. The probe temperature should not exceed 40 °C. It is possible to cause hyperthermia by over-enthusiastic or uncontrolled heating, and this can result in superficial burns or even the death of the animal. To avoid such problems, and provide effective, well-controlled warming, it is preferable to use a thermostatically controlled heating blanket, regulated by the animal’s body temperature using a rectal probe (eg Kent Scientific). If such a unit is not available, a simple heating pad or lamp can be used which, provided the animal’s rectal temperature is monitored, can be switched on and off manually as required. It is important to switch on heating pads and lamps before they are required, to allow their temperature to stabilize and to prevent a period of inadequate heating when the pad or lamp is warming up. Thermostatically controlled pads should be set up with the probe in contact with the blanket, so that they reach body temperature before the animal is anaesthetised.

Heating blankets may use electrical elements, have circulating warm water, or employ warm air. Warm water heating systems may not provide sufficient warmth for small animals because of the fall in water temperature between the water reservoir and the blanket, but are effective in larger species. Forced air warming systems, such as the Bair Hugger, provide excellent maintenance of body temperature, but the design of the blankets makes them best suited for use in larger (>3–4 kg) animals.

Next Article : Endotracheal intubation

Updated on 17th May 2020

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