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Water is an excellent conductor of temperature. Wetness of body surface will contribute to loss of core temperature through conduction more than any other mechanism. Water will conduct heat away from an object 25 times faster than air. A human body will lose heat in 26.5° C (80° F) water at the same rate as it does in 5.5° C (42° F) air.
Horses rarely suffer from hypothermia under normal conditions. It is when a large percentage of their body surface is in direct contact with a conduction agent like water that the loss of heat will exceed heat production by the body. The rate of body temperature loss in a horse immersed in water or mud is not known. Horses under general anesthesia for more than 45 minutes lost a mean body temperature of 0.8±0.6° C (1.44±1.08° F) per hour (range 0-2.4° C, 0-4.32° F). Wetness of body surface strongly contributed to loss of body temperature during general anesthesia.
Risk factors affecting the onset and severity of hypothermia include:
- Dehydration. Most horses suffering from acute hypothermia also suffer from dehydration.
- Poor body condition. Horses in poor body condition are more susceptible to hypothermia due to a reduced layer of insulating subcutaneous fat.
- Age. The ability to generate heat decreases with age in all mammals studied.
- Body surface/volume (mass) ratio. The larger the body surface relative to body mass, the more core temperature will be lost across the body surface. This explains in part why donkeys and small horses (foals, ponies) are more susceptible to hypothermia than larger horses.
- Drugs. General anesthetics (anesthetic-induced vasodilation), sedatives will aggravate hypothermia through their action on the brain.
- Rough handling. Rough handling of a profoundly hypothermic patient after the rescue can trigger ventricular fibrillation and cardiac arrest.
A further decrease in core temperature, or continued cooling, after the rescue is known as the “afterdrop effect,” and is a phenomenon of conductive heat loss. The afterdrop can occur during shivering, which stimulates a shift of blood to the musculature, or during surface warming, which will stimulate peripheral vasodilation
Restoring Body Temperature
The main focus of rewarming the hypothermic horse should be to restore core temperature. Peripheral warming without core rewarming can be counterproductive. A horse with a rewarmed and dry coat can be suffering from more severe hypothermia than a horse with a wet and “cold” feeling coat.
To implement core rewarming, contact a large-animal veterinarian as soon as possible. No treatment is more effective in hypothermia than core rewarming, which is achieved most effectively through the administration of warm fluids intravenously and through a warm enema.
A large-animal veterinarian should initiate administration of warm intravenous fluids to the hypothermic horse as soon as possible, even before the horse is rescued. Intravenous fluids are important not only to restore core normal core temperature, but just as important to rehydrate the horse. Intravenous fluids should be warmed before administration, which can be achieved by placing the fluid bags in a water bath.
Ideally, fluids should be administered at a temperature of 40° C (104° F), but no less than 33° C (91.4° F). Fluid temperatures at or above 47° C (116.6° F) can cause thermal damage to red blood cells. Warm the fluids by placing the IV fluid bag (5-6 liter bag) in a warm water bath. Do not microwave.
Surface (convective) rewarming can be used as an adjunct to core rewarming through the use of forced hot air, heat lamps, heating pads, blankets and protecting the victim from the wind. If possible, take the horse to a small barn/building and heat it with a forced-air heater. The warm air will help dry the coat without touching it, and the horse will inhale warm air to help in core rewarming. Vigorous rubbing should never be used for surface rewarming because it can damage frozen cells.
Careful handling of a hypothermic horse is extremely important to help prevent cell damage and heart dysfunction (arrhythmia/heart attack).