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Re: Treatment of hyperthermia

Dear Sir

Much of the controversy about the cooling methods used in hyperthermia arises because of a failure to consider the underlying physiology. There are two distinct mechanisms for the development of hyperthermia, SLOW and FAST, and each requires a different method of cooling.

The SLOW hyperthermia involves exposure to heat with only mild physical activity. In this situation the hyperthermia develops purely because of an inadequacy of the heat loss mechanisms. The body is then in a situation where skin vasodilatation is maximal and there is very little heat being generated by the body. In this case cold on the skin will cause a reduction in the vasodilatation, a reduction in heat loss with a possible stimulus to increased heat production and the core temperature will continue to rise and cause disaster. This mechanism is found in the elderly, those with heart disease, and, classically, among the pilgrims to Mecca. The safest cooling method for this group is bathing with warm/tepid water and fanning.

The FAST hyperthermia involves vigorous exercise in hot conditions with the complication that in some cases the conditions may not be excessively hot but the casualty has been wearing too much body insulation. This is the type found in sport and also, classically, in situations such as military personnel running across mudflats wearing wetsuits. In this mechanism the main problem is the excess generation of heat from the skeletal muscles, and survival will depend mainly on curbing this excess heat production. These muscles are subcutaneous and any vasoconstrictor stimulus from cold on the skin will be overridden by the excessive subcutaneous heat generation. For FAST hyperthermia therefore ice or very cold water will not be harmful but will be beneficial because it will penetrate to the muscles and will reduce their heat generation by direct cooling. Failure to stop the continuing heat generation will be disastrous.

As in any medical situation deciding treatment on a single measurement diagnosis (in this case temperature) can be disastrous cf anaemia or hypothermia, and a history should be taken before treatment is decided. Safety is more important than rate of cooling.

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Re: Cooling heat stroke patients

Dear Editor,

Smith in his recent article in the Br J Sports Med highlights the importance of aggressively cooling heat stroke victims to improve prognosis.[1] The paper extensively reviews the major cooling methods, and the author advocates the immersion in an ice-cold bath as the method of choice. Whereas this conclusion might be true for young patients who may tolerate aggressive treatment with ice water (1–5°C), the more vulnerable patients and those with prior cardiovascular illnesses should not be exposed to unnecessary risks. Therefore, the more conservative technique that has been proven effective, the pouring of large volumes of tepid water (12–16°C) and fanning, should be used instead (especially under field conditions, where ice is not available).[2]

It should also be emphasized that the use of antipyretics in the treatment of heat stroke is contraindicated.[3] In heat stroke the accumulated body heat is not a result of a change in the thermoregulatory set-point, as is the case in fever. Thus, antipyretics are not effective in lowering body temperature. Furthermore, certain antipyretics can cause additional damage; i.e. temperature-induced hepatic dysfunction may worsen from the use of paracetamol (acetaminophen), administration of nonsteroidal anti-inflammatory drugs may reduce potassium excretion, and aspirin (acetylsalicylic acid) may aggravate bleeding diathesis.

References

1. Smith JE. Cooling methods used in the treatment of exertional heat illness. Br J Sports Med 2005; 39:503-7.

2. Hadad E, Rav-Acha m, Heled Y, Epstein Y, Moran DS. Heat stroke: a review of cooling methods. Sports Med 2004; 34:501-11.

3. Heled Y, Rav-Acha m, Shani Y, Epstein Y, Moran DS. The "golden hour" for heatstroke theatment. Mil Med 2004; 169:184-6.