Heat-Related Illnesses

By Marven Ewen, MD
Medical Director

It’s that time of year again when we start seeing heat-related illness. The human body normally maintains a temperature of 98.6F through a complex system of homeostasis regulated by the hypothalamus. Normally at rest, the skin temperature will be about 94F. When exposed to conditions of high ambient temperature, the skin temperature will rise. When it gets to 97F, the body cools itself by sweating. Sweating helps to cool us because heat is lost as the moisture on the skin evaporates. Normally this system works very well; however, there are exceptions.


All the sweating needed to keep cool will quickly dehydrate an individual if they are not hydrating adequately. Also, many people are treated with diuretics for hypertension, which can make them more prone to dehydration. Some people mistakenly drink sodas, alcohol or caffeinated products instead of water. This quickens dehydration. When the patient becomes dehydrated, they can no longer produce enough sweat to cool themselves and shock from fluid loss can ensue. Another problem arises in the case of high humidity – when it is both hot and humid, sweating is no longer effective because the sweat will not evaporate in these conditions, so it has no benefit.


The body has a second mechanism to reduce heat, and that is to vasodilate peripheral blood vessels. During exercise as the body temperature warms due to increased metabolism, the body needs to shed this excess heat. This is done by shunting of the blood to the periphery and causes the skin to appear flushed and warm. Heat can then radiate outwards. The problem is that when the environmental temperature is higher than body temperature, this actually doesn’t work and can result in more heat being absorbed from the environment.


This basic understanding of the body’s attempt to maintain homeostasis helps us predict when there might be an increased risk for heat illness. Specifically hot humid weather, exertion in hot weather, and inadequate hydration or fluid loss through diuretics such as medications, caffeine or alcohol.


There are three levels of heat illness severity. Heat cramps, heat exhaustion, and heat stroke. This is somewhat similar to thinking of increasing stages of shock. First level is the mildest: the muscles start to cramp. This patient needs oral hydration and rest and preferably should move to a cool place.


Heat exhaustion is the second level. The body no longer can keep the peripheral blood vessels dilated because of falling blood pressure due to fluid deficit, so in order to support blood pressure, the blood is shunted to the core. Sweating continues but the skin may feel cool, and pulse is rapid. Blood pressure becomes position dependant, which means it is lowered when standing. Patient will likely complain of a headache, dizziness, and muscle cramps. Immediate treatment is by oral hydration and rest in a cool place.


Finally there is heat stroke: temperature no longer controlled and is 104 or higher. The skin may be dry, hot, red, and the patient is obtunded, nauseated, or vomiting. There is rapid respiration, rapid pulse, and headache. This is a life threatening stage and requires rapid treatment with IV hydration, cooling, and emergency medical support or the patient will die.


Prevent heat-related illness by hydrating before exercising in heat and continue hydrating. The clarity of urine indicates level of hydration. Urine should be clear before even starting to exercise. You need to consume about 8 oz of water every 15 minutes while exercising in the heat. If you wait until you are thirsty, it is too late – you are already dehydrated. Continue to hydrate afterwards also as needed. Avoid excessive exercise at the hottest times of the day. Cover up with light bright clothing and a light colored hat to avoid radiant heat exposure from the sun and decrease environmental conduction. Avoid caffeine or alcohol in the heat. Get out of the heat if you are getting cramps, headache, or starting to feel light-headed.

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