Pathophysiology

The body maintains homeostasis by efficiently balancing heat gains and losses. Thermoregulatory centers of the CNS, including the hypothalamus and spinal cord, address heat gains by increasing blood flow to the skin, dilating peripheral blood vessels, increasing eccrine gland production, and increasing heart rate and cardiac output.³ Ineffective heat regulation can be caused by numerous factors, resulting in the core temperature being elevated beyond the capabilities of the thermoregulatory compensatory systems.¹ Escalating ambient environmental temperatures and humidity can overwhelm the body’s natural ability to dissipate heat. Increases in internal heat production related to disease processes or hypothermic dysfunction, as well as impaired heat dissipation caused by medications or age, can result in deficient heat regulation.¹

In heat stroke, an extremely elevated core body temperature can result in cerebellar and liver dysfunction. Heat exhaustion, heat syncope, and heat cramps are caused by dehydration and electrolyte depletion associated with heat exposure.^1,4

Clinical Presentation

Heat-related illnesses can develop rapidly or over a period of several days. Physical symptoms may differ in the amount of time they take to manifest. Rapid diagnosis and treatment are imperative. A comprehensive history and physical examination are invaluable. The clinician must determine the following:

Heat stroke is considered a medical emergency in which core body temperature exceeds above 104° F.^5,6 Patients with heat stroke will have CNS abnormalities, which can include hallucinations, confusion, slurred speech, and headache.^1,6 Dehydration, tachycardia, and hypotension can occur. Red, hot, dry skin is a key characteristic of heat stroke. Heat stroke can rapidly deteriorate to hepatocellular damage or multiorgan system failure.^1,5

Symptoms are milder in patients with heat exhaustion. These patients may have generalized fatigue, weakness, profuse sweating, nausea, vomiting, diarrhea, irritability, and potentially hypotension, but no CNS involvement.^1–6 Skin will be pale and flushed, which is significantly different from the red, hot, dry skin of patients experiencing heat stroke.^1,6 Patients with heat exhaustion will have a pulse that is fast and breathing that is rapid and shallow.¹

The presentation of patients with heat syncope will involve vertigo, lightheadedness, and syncope.^1,4–6 Heat cramps will involve pain or spasms in muscles of the abdomen, arms, or legs.^2,5,6 These symptoms are caused by dehydration and electrolyte depletion.

Physical Examination

Rapid identification of heat stroke and heat exhaustion is imperative to prevent complications and untoward outcomes. A complete comprehensive physical examination including past extent and severity of exposure is required.¹ Evaluation of airway, breathing, and circulation (ABCs) is warranted. Physical examination findings related to the neurologic system include inappropriate behavior, impaired judgment, vertigo, delirium, seizures, and other symptoms of CNS dysfunction.^1,2,6 A baseline Glasgow Coma Scale score should be obtained and reassessed throughout treatment.

Cardiovascular findings may include tachycardia, and hypotension may occur because of vasodilation and dehydration. Patients with a heat-related illness could manifest symptoms associated with decreased preload, decreased peripheral vascular resistance, increased stroke volume, and increased cardiac output.¹ It is possible to have normotensive findings in some patients as well because of compensatory mechanisms.³ Musculoskeletal examination may demonstrate muscle tenderness, cramping, or weakness.

Diagnostics

Diagnostics should be based on the patient’s exposure and severity history, the clinical presentation, and past medical history. Testing should be used in conjunction with physical examination findings. Diagnostics should be used to monitor treatment as well as to determine the presence of associated complications. Cardiac monitoring and pulse oximetry may be indicated to obtain baseline and monitor hemodynamic status. Arterial blood gas (ABG) analysis and chest x-ray examination can be beneficial for patients with shallow breathing. Computed tomography may be indicated for patients with altered mental status. A urine sample should be obtained to monitor kidney function. Laboratory tests that may be indicated include a complete blood count (CBC) and differential, coagulation studies and an electrolyte panel, blood urea nitrogen (BUN), and creatinine. The following values should be obtained to assess for progression of disease or complications: BUN, creatinine, sodium, potassium, calcium, lactate dehydrogenase, aspartate aminotransferase, alanine aminotransferase, creatine kinase, and bilirubin. Liver function test (LFT) results can be elevated, in some cases, 12 hours after initial injury.¹ Creatine kinase should be measured if there are concerns surrounding potential rhabdomyolysis.¹

The diagnosis of heat-related illness is based on clinical presentation, history, and physical examination. The differential should include infections, head trauma or CNS injury, epilepsy, thyroid storm, acute cocaine intoxication, malignant hyper­thermia, pheochromocytoma, anticholinergic poisoning, serotonin syndrome, drug-associated toxicity, and environmental exposure.¹