Lecture #192: Emergency Treatment of Hyperthermia

What are the session objectives for emergency treatment of hyperthermia?

Summarize heat-loss mechanisms, define acclimation to hotter climates, distinguish each level of heat-related illness, describe appropriate therapy for each level, and identify pitfalls of heat-related illness therapy.

What are the four mechanisms of heat loss?

Radiation, evaporation, convection, and conduction.

What is radiation heat loss?

Radiation is transfer of heat by electromagnetic waves from a warmer object to a colder object.

What is evaporation heat loss?

Evaporation is heat loss by vaporization of water through sweat and respiration.

What is convection heat loss?

Convection is heat transfer by air or liquid moving across the surface of an object.

What is conduction heat loss?

Conduction is heat exchange between two surfaces in direct contact, such as ice packs touching the skin.

What are the primary cooling methods used for hyperthermic patients?

Evaporation, convection, and conduction are the primary methods used to cool hyperthermic patients.

How does sweating help with heat loss?

Sweating increases evaporative heat loss through increased cholinergic stimulation.

Why is sweating less effective in high humidity?

Evaporation is limited in higher humidity, so sweat does not evaporate efficiently and cooling is reduced.

How does cutaneous vasodilation help with heat loss?

Decreased sympathetic tone causes cutaneous vasodilation, moving heat from the core to the skin surface for dissipation.

Why is vasodilation less effective at high ambient temperatures?

At higher ambient temperatures, the body can no longer radiate heat effectively, so vasodilation becomes less useful.

Why is water more effective than air for conductive cooling?

Conduction of heat into water is much more effective than conduction into air.

Why are fans alone insufficient in some hot environments?

Convection becomes less effective above about 32.2°C and 35% humidity, so fans alone may not cool patients adequately.

What hemodynamic response occurs with heat stress?

Cutaneous vasodilation decreases preload and stroke volume, so heart rate must increase to maintain cardiac output.

Who may fail to compensate hemodynamically during heat stress?

Older adults, patients with dysrhythmias, and patients taking negative chronotropic medications may not mount enough tachycardia, causing cardiac output to fall.

What complications can occur if cardiac output falls during heat stress?

Reduced cardiac output can cause myocardial ischemia in patients with fixed coronary lesions, dysrhythmias, or other end-organ decompensation.

When does sweating begin as body temperature rises?

Sweating begins as body temperature rises above 37°C.

How does sweating differ in elderly patients?

Elderly patients may not begin sweating until body temperature is about 1.5°C higher than in younger patients.

How should water loss be estimated by weight loss?

Each pound of water weight loss is about 453 mL, so replace approximately 500 mL per pound lost.

Which medications interfere with heat removal?

Anticholinergics, diuretics, phenothiazines, beta-blockers, calcium channel blockers, sympathomimetics, and ethanol can interfere with heat removal.

How do anticholinergic agents worsen heat illness?

Anticholinergic agents impair sweating, reducing evaporative heat loss.

How do diuretics worsen heat illness?

Diuretics cause volume depletion, which decreases cardiac output and impairs heat dissipation.

How do phenothiazines worsen heat illness?

Phenothiazines have anticholinergic properties and may deplete central dopamine stores, interfering with hypothalamic thermoregulation.

How do beta-blockers and calcium channel blockers worsen heat illness?

They interfere with the appropriate cardiovascular response to heat, limiting the compensatory increase in heart rate and cardiac output.

How do sympathomimetics worsen heat illness?

Sympathomimetics cause cutaneous vasoconstriction, increased muscle activity, and inhibition of sweating.

How does ethanol worsen heat illness?

Ethanol inhibits antidiuretic hormone secretion, causing dehydration, and also blunts heat-avoidance behavior.

What physiologic changes occur with heat acclimation?

Acclimation lowers the hypothalamic thermal set point, triggers sweating at lower core temperatures, increases sweat production up to 3 L/hr, increases aldosterone and plasma volume, conserves sodium, and causes cutaneous vasodilation at lower core temperatures.

How does aldosterone help during heat acclimation?

Increased aldosterone expands plasma volume and conserves sodium, making sweat less salty and sweating more efficient.

What is heat edema?

Heat edema is swelling, usually of the feet and ankles, caused by cutaneous vasodilation plus increased aldosterone leading to venous pooling and increased vascular permeability.

How is heat edema treated?

Heat edema is treated with elevation.

What is prickly heat?

Prickly heat, also called lichen tropicus, miliaria rubra, or heat rash, is inflammation over clothed areas caused by blocked sweat glands.

How is prickly heat treated?

Treatment includes light, clean, loose-fitting clothing, antihistamines, topical calamine, chlorhexidine, or low-potency topical steroids.

What are heat cramps?

Heat cramps are painful involuntary muscle contractions occurring during or shortly after physical exertion in a hot environment.

What causes heat cramps?

The exact cause is unknown, but they are thought to involve muscle fatigue, dehydration, and fluid and electrolyte disturbances.

How are heat cramps treated?

Treatment includes cooling the patient, replacing fluids and electrolytes, and slow stretching of involved muscles.

What therapies are unproven for heat cramps?

Diazepam and magnesium are unproven for heat cramps.

What defines heat stress or heat exhaustion?

Heat stress, also called heat exhaustion, has normal or slightly elevated body temperature below 40°C with normal mental status.

What causes heat stress or heat exhaustion?

Heat stress is caused by water and/or salt depletion.

What symptoms occur in heat stress or heat exhaustion?

Symptoms include headache, nausea, vomiting, malaise, muscle cramps, presyncope, dehydration, tachycardia, orthostatic changes, hemoconcentration, and variable electrolyte findings.

Is sweating intact in heat exhaustion?

Yes. Sweating remains intact despite volume depletion.

How is heat exhaustion treated?

Move the patient to a cool shady area if outside, elevate the legs, and replace water and electrolytes orally or intravenously.

Where is heat exhaustion usually treated?

Heat exhaustion is most often treated successfully outside the hospital.

What defines heat stroke?

Heat stroke is body temperature greater than 40°C with altered mental status.

Why is heat stroke life-threatening?

Heat stroke causes impaired or overwhelmed thermoregulation, direct cell toxicity, protein denaturation, cell necrosis, inflammatory cascade activation, vascular endothelial damage, DIC, and multisystem organ failure.

Is sweating always present in heat stroke?

No. Heat stroke often has no sweating.

What is classic heat stroke?

Classic heat stroke occurs during high ambient temperatures, usually in elderly or very young patients, over a prolonged period, with common volume and electrolyte disturbances.

Who is at increased risk for classic heat stroke?

Patients with psychological, physiologic, or pharmacologic impairment are at increased risk, such as those with Raynaud disease or taking anticholinergics, diuretics, antipsychotics, cocaine, or ethanol.

What is the classic example of classic heat stroke?

An elderly patient who lives alone with no air conditioning.

What is exertional heat injury?

Exertional heat injury usually occurs with athletic activity, often in the first 4 days of athletic practice, and is driven more by increased heat production than high ambient heat.

What is confinement hyperpyrexia?

Confinement hyperpyrexia occurs in enclosed hot spaces, such as children left in cars or stowaways in poorly ventilated vehicles.

Why are infants at high risk for confinement hyperpyrexia?

Infants have immature regulatory mechanisms, cannot remove themselves from the environment, and gain more heat due to increased body surface area relative to volume.

How quickly can non-ventilated cars become dangerously hot?

Non-ventilated cars can reach 60°C, or 140°F, in less than 10 minutes.

What CNS finding is the key warning sign in heat stroke?

Mental status change is the key warning sign and is described as the “canary in the coal mine.”

What CNS effects can occur in heat stroke?

Heat stroke can cause irritability, confusion, bizarre behavior, hallucinations, abnormal plantar responses with upgoing toes, decorticate or decerebrate posturing, hemiplegia, coma, and seizures.

Why are seizures dangerous in heat stroke?

Seizures increase heat production at the exact time when heat production needs to be minimized.

What organ dysfunction can occur in heat stroke?

Heat stroke can cause hypotension, electrolyte disturbances, renal failure from rhabdomyolysis, heart failure, liver failure, ARDS, and DIC.

How should hypotension be treated in heat stroke?

Treat with IV fluids, not oral fluids, because altered mental status makes oral intake unsafe.

Which vasopressors or inotropes are preferred or avoided in refractory heat-stroke hypotension?

Consider inotropes such as dobutamine or low-dose dopamine, but avoid agents with alpha-adrenergic effects such as norepinephrine or higher-dose dopamine because they cause cutaneous vasoconstriction.

What urine finding suggests rhabdomyolysis in heat stroke?

Tea-colored urine suggests rhabdomyolysis, not just concentrated dark urine.

What labs should be considered in heat stroke?

Consider stat glucose, CBC, CMP, ABG, coagulation profile including INR and fibrin split products, CPK, ECG, and CT head or lumbar puncture if evaluating other causes of altered mental status.

What is the overall treatment principle for heat stroke?

Treat with immediate cooling and support of organ dysfunction.

What cooling methods can be used for heat stroke?

Methods include evaporative cooling, immersion, axillary and groin ice packs, body bag with ice, ECMO or cardiopulmonary bypass, cooling blankets, and cold water gastric, bladder, rectal, or peritoneal lavage.

Why is evaporative cooling preferred?

Evaporative cooling is cheap, readily available, and effective.

What are pitfalls of evaporative cooling?

It may cause shivering, which raises body temperature, and it can make monitor leads difficult to keep attached.

How is shivering during evaporative cooling treated?

Shivering can be treated with benzodiazepines.

What is the cooling goal in heat stroke?

The goal is to cool the core to normothermia, about 37°C, while avoiding overshooting.

What role do IV fluids have in cooling?

IV fluids have little cooling effect but are used to restore volume.

Why do antipyretics have no role in heat-related illness?

Acetaminophen, ibuprofen, and other NSAIDs act on pyrogen-mediated hypothalamic set-point changes seen in fever, but heat illness involves an overloaded healthy hypothalamus rather than a raised set point.

Which heat illness patients may be discharged?

Healthy patients with heat edema, heat stress, or heat cramps may be discharged.

What disposition is required for heat stroke?

Heat stroke patients require hospital admission, usually to the ICU.