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Blog > Preventing Hypothermia

Preventing Hypothermia - A Veterinary Nurse's Role

Hypothermia (a low core body temperature) occurs when the core body temperature drops by more than 1 degree from the normal range. Hypothermia during general anaesthesia occurs for several reasons.

What is a normal temperature range?

There are different normal ranges sited, but the normal core body temperature for healthy dogs, cats and rabbits can be classed as:

Dogs: 37.5-39.2 degrees Celsius
Cats: 37.8 – 39.5 degrees Celsius
Rabbits: 38.3 – 39.4 degrees Celsius
Neonates will have higher normal body temperature ranges

During general anaesthesia, we need to take steps to try to ensure the body temperature remains at or as close to this range as possible and prevent hypothermia from occurring.


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Causes of Hypothermia

    Hypothermia is the most common general anaesthetic complication in dogs and cats. During general anaesthesia, the effects of drugs, inhalation anaesthetics, fresh oxygen and lack of muscular activity all have cooling effects. Additionally, if intravenous fluids are not warmed to body temperature, they will exacerbate the problem further.

    Hypothermia may be mild, moderate, severe or critical depending upon the drop in body temperature.

    · Mild - The temperature drops by less than 1 degree Celsius from the lower end of normal
    · Moderate - Down to 2 degrees below the low end of normal
    · Severe – A drop of 4 degrees
    · Critical - The temperature is more than 4 degrees below the lower end of the normal range


Timescale of Hypothermia

The majority of heat loss occurs during the first hour, resulting from peripheral vasodilation and redistribution of body heat. This is why it is so essential to maintain body temperature in the pre-operative period and after the premedicant is given. Following this, the temperature continues to decline over the next 2-3 hours but at a slower rate. After this the temperature will begin to stabilise.


Size Matters!

Smaller animals will lose heat more rapidly due to surface area losses through cutaneous loss. The skin is the major source of heat loss during anaesthesia, and cutaneous heat loss can be decreased by up to 30% by covering the skin as much as possible. Where possible prolonged surgery should be avoided, due to the increased risk of hypothermia, with increased time under general anaesthetic.

How To Prevent Hypothermia

It is important to remember that preventing hypothermia during general anaesthesia, starts in the pre-operative period and continues during the surgery and post-operatively until the animal can mobilise and maintain their body temperature again.

Considerations to Maintain Normothermia

  • Pre-operative: 
    Ensure warm accommodation (18-21 degrees Celsius). Kennels may be frequently made of stainless steel, which can be cold. Give the patient enough bedding to enable them to be comfortable. Wrap blankets around dogs (like a coat) to help insulate.
    Give the premedicant as close to the surgery as possible to minimise cooling effects prior to surgery, and take a baseline rectal temperature.
    Use a multi-modal anaesthesia protocol so doses of individual medications, which may have vasodilation effects, can be kept to a minimum. 
  • During surgery: 
    Keep your theatre warm (25 degrees Celsius and above).
    Apply socks/bubble wrap to the feet and ears where possible.
    Use other heating aids, such as a heated theatre table or forced air warming unit with dedicated patient blankets (Mistral-Air®), which encourages active and optimal warming. Ensure the systems used are suitable for the theatre unit and have HEPA filters. Take extreme care if using heat mats, heat discs or other devices. Recumbent patients under general anaesthesia are at high risk of contact burns if these devices are too warm or too close to the patient.     Keep surgical clipping and scrubbing to the minimum required to ensure sterility for the procedure. The more a patient is clipped and scrubbed, the further their temperature will fall.
    Ensure intravenous fluids are warmed to body temperature at the point of infusion into the patient. There is no point warming the bag of fluids if they are cold when they reach the patient!
    Implement passive warming techniques by using blankets, foil and bubble wrap to insulate the patient and self-generated heat. Keep inhalation gases and oxygen flow rates as low as possible, using capnography to assess for re-breathing.
    Check the body temperature with a rectal thermometer every 10-15 minutes. If the temperature is beginning to drop, apply additional measures of insulation/heating to help prevent and slow heat loss.
    Be prepared and efficient – the longer a general anaesthetic lasts, the higher the incidence of hypothermia and procedures lasting more than 20 minutes increase the risk.
  • Post-operatively/recovery: Keep the patient warm. Keep them wrapped up and ensure the blankets are clean and dry.
    Keep socks and bubble wrap on the patient whilst under supervision. These should be removed when they are not to ensure they are not ingested. If a bair hugger or Minstral air system is available, use this in the post-operative period.
    Continue to monitor the rectal temperature

Summary

Preventing hypothermia in all anaesthetised patients is an essential role for Veterinary Nurses. Hypothermia has been shown to increase the risk of infections, prolongs the recovery period, causes reduced immunity, and is associated with increased blood loss

A holistic approach should be adopted, beginning when the patient is admitted and finishing when the patient is discharged. Veterinary Nurses can implement passive and active warming techniques alongside careful monitoring of the body temperature with a re-evaluation of the warming methods being reassessed based on the animal's body temperature trend.

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