Hypothermia is the situation when an animal’s core temperature falls below the range necessary to sustain normal cellular metabolism. If this situation persists, cellular death occurs due to decreased metabolism and/or vascular damage. Hypothermia is most likely to occur when ambient temperatures are very low, when there is a significant wind chill factor, and when an animal’s insulation is compromised by a poor coat, inadequate fat, or simply by being wet. Lying in one position for prolonged periods may also contribute to the development of hypothermia due to compression of the underside fur. Smaller individuals are more vulnerable to hypothermia because they have a greater surface area to body mass ratio. Juvenile, injured, immobile or very old animals are particularly vulnerable. Anaesthetic agents generally compromise the body’s natural mechanisms to maintain body temperature (thermoregulation), so that an animal that is cold before anaesthesia is likely to become colder when anaesthetised. Inadequate circulation, such as in dehydrated or shocked animals, exacerbates the problem. Foothold traps and snares may compromise the circulation in the trapped foot, making it vulnerable to cooling, and in extreme cases, frostbite. The metabolism and excretion of anaesthetic drugs in hypothermic animals is slower than normal, therefore recoveries can be prolonged leading to even greater risk of severe hypothermia developing in cold conditions.
As with hyperthermia, it is far easier to prevent hypothermia than it is to treat the problem. In the face of falling rectal temperatures during anaesthesia it is essential to institute remedial measures before the animal is severely hypothermic. Rectal temperatures must be monitored closely and frequently when conditions indicate that hypothermia is a possibility.
The normal rectal temperature of a tiger lies between 37.8 degrees C. and 39.4 degrees C. (100 – 103 degrees F). Hypothermia is considered to exist if the body temperature drops below 36 degrees C (~ 97 degrees F). Other signs of hypothermia include shivering (only likely to be observed prior to anaesthesia), decreased heart rate and blood pressure, cold extremities, unresponsiveness and abnormally slow recovery from anaesthesia.
- Keep the anaesthetic period as short as possible. Do not give supplementary drugs unless there is a genuine safety issue for people involved. Do not carry out nonessential procedures. Use anaesthetic reversal agents if appropriate, but it is not generally recommended before warming the animal.
The only treatment for hypothermia is to prevent further heat loss and warm the animal. This can be attempted in several ways:
- Place a foam pad underneath the animal.
- Wrap the animal in a space blanket.
- Use other blankets or quilted material over the space blanket.
- Move the animal into the sun if possible.
- Cover the feet with gloves, socks, hats, or whatever is available.
- Put hand warmers or heat packs wrapped in cloth between the tiger’s back legs or on the abdomen. Be careful to avoid thermal injury to the skin.
- If safe to do so, consider hugging the anaesthetised animal to transfer body heat to it.
- If warm water is available, give a warm water enema (40 degrees C maximum) and repeat at intervals until core temperatures rises. Under most field conditions, this measure will not be possible.
For the more experienced field vet other treatment options for severe hypothermia include:
- Instilling warm water (40 degrees C maximum) into the bladder via a urinary catheter. Leave for 5 minutes, drain and repeat a number of times. NB: Catheterisation of males is far easier than females.
- Slow administration of intravenous fluids at a temperature higher than normal (maximum 40 degrees C advised). This is not always very effective.
- Provision of humidified oxygen heated to 42 degrees C via a mask or endotracheal tube.
Many of these actions are appropriate to prevent hypothermia if environmental conditions indicate that the condition is likely to develop.
Under conditions that prevail in the Russian Far East, it cannot be over-emphasized that the best approach to hypothermia is prevention. Space blankets and foam pads should be a standard part of the cool-season capture kit.