Perspective

Heat-related injury spans a wide spectrum from the unpleasant and transient conditions of heat cramps or prickly heat to the life-threatening multisystem organ failure of heat stroke. The most important risk factors for heat illness are listed in Box 130.1. Mechanisms of acclimatization are listed in Table 130.1.

Table 130.1 Acclimatization Mechanisms

Cardiac	Increased cardiac performance: higher cardiac output
Vascular	Increases in: Plasma volume Renal flow Shunting of blood away from the visceral circulation

Endocrine An enhanced renin-angiotensin-aldosterone system and improved salt retention by the kidneys and sweat glands result in better fluid retention Renal Increased glomerular filtration rate Sweat glands The increased volume of sweat produced and the more dilute sweat reduce salt loss and indirectly diminish dehydration Muscle Improved ability to resist rhabdomyolysis from exertion Cellular Upregulated transcription of heat shock proteins

Epidemiology

Roughly 688 people die yearly of heat-related causes, Arizona has the highest incidence at 1.7 per 100,000, followed by Nevada (0.8 per 100,000) and Missouri (0.6 per 100,000).¹ An estimated half of the deaths are weather related, and 5% result from being enclosed in motor vehicles, boiler rooms, or kitchens; in the remaining cases, the causes are unspecified.² Mortality from heat stroke with appropriate treatment is reported to be between 0% and 28%.^3–5

Pathophysiology of Heat Stroke

Severe heat illness is thought to result from three main mechanisms: physiologic alterations in response to hyperthermia, direct cytotoxic effects of heat, and inflammatory and coagulation responses of the host, which interact to create a spiraling deterioration resulting in circulatory collapse and multisystem organ failure.

Failure of thermoregulation occurs when fluid and electrolyte replacement is inadequate; it results in diminished cardiac output, hypotension and acute renal failure, and shunting of blood away from the splanchnic circulation, which leads to intestinal ischemia. Increased gut permeability and the entrance of endotoxins stimulate additional inflammatory mediators.

Heat itself has a direct cytotoxic effect and causes cell death over a period of 45 minutes to 8 hours at body temperatures of 41.6° C to 42° C⁶; below these temperatures, cell death can still occur through apoptosis.⁷

Heat induces higher production of numerous inflammatory cytokines that contribute to the neuronal injury and hypotension seen in patients with heat stroke, and it results in coagulation and endothelial cell injury, which can lead to disseminated intravascular coagulation and greater vascular permeability.

Figure 130.1 diagrams the current understanding of the pathophysiology of heat stroke.

Minor Heat-Related Syndromes

Heat cramps Severe muscle spasms during or after vigorous exercise

Heat Edema

Heat edema is a self-limited illness characterized by mild swelling of the hands and feet as a result of increased interstitial fluid after heat exposure.

Prickly Heat

Clogging of the sweat glands leads to an erythematous, maculopapular, and pruritic rash commonly called prickly heat.

Heat Syncope

Heat syncope results from heat-induced volume depletion and peripheral vasodilation, which often occur in people with decreased vasomotor tone, such as the elderly, and in people poorly acclimated to the heat. It is important to regard heat syncope as a diagnosis of exclusion and to evaluate for other plausible causes of syncope. In addition, the emergency physician (EP) should remember to evaluate for any injuries sustained during a fall as a result of syncope. The patient should have a rapid response to hydration. Electrolytes should be evaluated in patients with signs of volume depletion.

Heat Cramps

Heat cramps are involuntary muscle spasms, most commonly of the calves, thighs, or shoulders, that occur either during or after vigorous physical activity. Cellular hyponatremia and hypokalemia are thought to cause this problem. Rhabdomyolysis is a very rare complication that can result from prolonged muscle spasm.