Injuries due to exposure - Hyperthermia

In temperate weather, the human body naturally regulates its core temperature to an average of 98.6°F (37°C), but an individual's actual core temperature varies in accordance with daily sleep-wake cycles (and with monthly cycles in the case of women), as well as from individual to individual.

Hyperthermia — which means your body temperature is elevated well above normal — can result when you're in an environment with a temperature much higher than the normal range. The lower limit of hyperthermia is poorly defined. Heat stroke can occur when someone's core temperature exceeds just 104°F (40°C). However, extreme ultramarathon runners have been known to sustain core temperatures as high as 108°F (42°C).

How your core temperature responds to heat is strongly influenced by your state of acclimatization (that is, your adaptation to repeated or sustained high temperatures), the arduousness of any physical work you're doing, and the relative humidity of the environment. The heat stress associated with high relative humidity increases dramatically at higher air temperatures. The body's cooling mechanism relies not on sweating but on the evaporation of sweat — and the higher the relative humidity, the more evaporation is inhibited. To account for the effect of relative humidity, in 1990 U.S. National Weather Service developed a heat index scale — that is, a calibration of the apparent temperature.

Immersion in water represents the highest level of relative humidity. Because immersion prevents evaporative cooling, and because of water's huge capacity for holding heat, water temperatures that exceed 97°F (36°C) are not well tolerated by humans, particularly if they must also exert themselves. Water temperatures that high are not commonly experienced by recreational scuba divers. As a result, hyperthermia typically occurs during surface or land-based activities.

Signs and symptoms: The signs and symptoms of hyperthermia vary, according to how severe the condition is. Heat-stress disorders can be divided into the following five categories:
  • Heat edema: Edema means the accumulation of excess fluid in the body's tissues and cavities; heat causes the blood vessels to dilate (expand), so fluid tends to pool in the arms and legs, resulting in a condition known as peripheral edema.
  • Heat cramps: Heat, especially in combination with exercise, can cause a loss of water and electrolytes in the body, resulting in muscle cramps and spasms, especially in the calves, arms, and abdomen.
  •  Heat syncope: Syncope means fainting, or a temporary loss of consciousness; when heat leads to dilation of the peripheral blood vessels, another effect, besides peripheral edema, can be a drop in blood pressure and a decline in the amount of blood flowing to the brain, which can result in fainting.
  •  Heat exhaustion: Heat exhaustion is caused by dehydration — the loss of water and electrolytes from the body; it is characterized by headache, nausea or vomiting, low blood pressure, dizziness, fatigue and temporary loss of consciousness — though victims' mental status remains normal and their rectal temperature remains below 104°F (40°C).
  •  Heat stroke: A more severe form of heat exhaustion, heat stroke is characterized by a pronounced change in mental status, severe headache, nausea or vomiting, loss of consciousness and often cessation of sweating — and a rectal temperature exceeding 104°F (40°C).

Treatment: Victims should be removed from the overheated environment as soon as signs or symptoms of hyperthermia are apparent. The greater the magnitude of the malady, the more aggressive the efforts to cool the victim must be.
  •  Heat edema is easily resolved with rest and elevation of the extremities.
  • Heat cramps can be managed with ice massage (rubbing an ice cube on the affected area for five minutes at a time), stretching, and oral fluids.
  •  Heat syncope can be managed by placing victims in a resting, supine position (lying on their back, face up) with their extremities mildly elevated, and by monitoring their vital signs (blood pressure, heart rate, temperature and respiration).
  •  Heat exhaustion requires monitoring victims' vital signs and core temperature, administering electrolyte-rich fluids (such as Gatorade), and ensuring rest and cooling. If victims become dizzy or their blood pressure drops when they stand up, intravenous fluids may be required.
  •  Heat stroke requires urgent cooling, monitoring of victims' vital signs and core temperature, intravenous fluids, and rest.

Cooling measures can be as simple as finding a seat in the shade for victims of minor heat maladies, but as drastic as immersion in ice water for victims of heat stroke. Immediate cooling is critical in serious cases. Even though ice-water baths are uncomfortable, they have proven to be safe and effective for heat stroke victims. If a victim's signs and symptoms do not begin to abate after treatment has been instituted, especially if the individual appears to be getting warmer, seek medical aid immediately.

Prevention: For divers, water temperatures high enough to cause hyperthermia are typically not a problem during a dive. More common stressors are exposure to hot surface conditions, particularly when divers are wearing insulated suits (especially drysuits) designed to protect them in cool or cold underwater conditions, as well as the physical work involved in carrying dive equipment on land.

Thus preventive measures for divers include adequate hydration, sufficient shade, and the ability to rest and adjust or remove attire as required. Adequate hydration requires continual awareness of your water intake whenever you're in hot environments — and especially in hot and humid environments. The need for good hydration is increased for divers who experience a diuretic effect (that is, promoting the production of urine) from wearing tight wetsuits and/or from immersion. The concentration of your urine can be influenced by changes in your diet, activity level and thermal status, but passing nearly clear, colorless urine several times per day is generally considered evidence of adequate hydration. If your urine volume is reduced or its color darkens, you should drink more water (or other stimulant-free fluids).

Preventive measures for nondivers are similar: maintaining good hydration, staying in the shade outdoors or in air-conditioned spaces indoors, pacing your activity level, and dressing appropriately (in light colors, for example). In conditions of extreme heat, drinking electrolyte-rich fluids may be required.

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