Heat Stroke and Heat Exhaustion

• Failure of thermoregulation and heat dissipation mechanisms: The hypothalamic temperature set point is normally maintained at 37°C through a balance between heat production (metabolic activity, muscle contraction) and heat loss (radiation, convection, evaporation, conduction). In heat stress, the anterior hypothalamus triggers sympathetic activation leading to cutaneous vasodilation and increased sweating to maximize evaporative cooling. When environmental temperatures exceed 35°C or humidity prevents effective evaporation, or when metabolic heat production is excessive (as in exertional heat stroke), these mechanisms become inadequate. Sweating-induced volume depletion reduces cardiac preload, leading to compensatory tachycardia. The combination of peripheral vasodilation and volume loss reduces effective circulating volume, which triggers the renin-angiotensin-aldosterone system (RAAS) and sympathetic nervous system activation. Paradoxically, as core temperature rises above 40-41°C, continued sweating becomes insufficient because the body's cooling capacity is exceeded, and continued fluid loss depletes the intravascular compartment further.

• Direct thermal injury to cellular and tissue structures: Progressive hyperthermia causes denaturation of cellular proteins, particularly those critical for maintaining cellular integrity and function. Heat shock proteins (HSP70, HSP90) become depleted as they attempt to refold damaged proteins, and their failure to maintain protein homeostasis leads to apoptosis and necrosis. Cell membrane lipid bilayers become more fluid and lose selective permeability at temperatures >42°C, allowing uncontrolled ion flux. Mitochondrial dysfunction occurs as heat damages oxidative phosphorylation machinery, reducing ATP production at the very moment when cells require maximal energy for maintaining integrity. The intestinal epithelial barrier becomes compromised, allowing translocation of endotoxins and bacteria from the gut lumen into the bloodstream. This endotoxemia activates the innate immune system via toll-like receptors (TLRs), initiating a systemic inflammatory cascade.

• Systemic inflammation, coagulopathy, and multi-organ dysfunction: Heat stroke triggers a sterile inflammatory response comparable to sepsis, mediated by excessive release of pro-inflammatory cytokines (TNF-α, IL-1β, IL-6, IL-8) and activation of complement cascades. Hyperthermia directly activates the extrinsic and intrinsic coagulation pathways, while simultaneously depleting anticoagulant factors (protein C, protein S, antithrombin III) and impairing fibrinolysis. This creates a prothrombotic state culminating in disseminated intravascular coagulation (DIC), characterized by simultaneous microthrombi formation and consumption coagulopathy. Endothelial cell damage increases vascular permeability, contributing to hypovolemia and shock. The combination of hypovolemia, reduced cardiac output, systemic inflammation, and microangiopathic hemolysis (from RBC passage through fibrin strands) precipitates acute kidney injury. Acute hepatic injury occurs from direct heat injury to hepatocytes and from splanchnic hypoperfusion. Rhabdomyolysis develops from direct thermal injury to skeletal muscle and from muscle breakdown secondary to seizures or extreme exertion; released myoglobin is filtered at the glomerulus and precipitates in renal tubules, directly toxic to tubular epithelium and exacerbating acute kidney injury. Cerebral edema results from direct thermal injury to the blood-brain barrier and from excessive fluid administration during resuscitation. Acute respiratory distress syndrome (ARDS) develops from endothelial injury and inflammatory mediator release.

• Rhabdomyolysis-induced acute kidney injury: Skeletal muscle damage results in the release of intracellular contents including myoglobin, potassium, phosphate, and uric acid. Myoglobin is a small protein (17 kDa) freely filtered at the glomerulus. In acidic urine (as occurs with dehydration and hypoxia), myoglobin precipitates within the renal tubules, forming casts that obstruct urine flow and directly injure tubular epithelial cells through oxidative stress. The combination of tubular obstruction, direct cellular toxicity, and hypoperfusion from hypovolemia leads to acute tubular necrosis. Hyperkalemia from muscle breakdown can precipitate cardiac arrhythmias, and hyperuricemia from purine metabolism may contribute to further tubular damage.

• Central nervous system dysfunction: At core temperatures >40.5°C, direct thermal injury to the CNS produces altered mental status, seizures, and loss of consciousness through multiple mechanisms. Hyperthermia increases neuronal metabolic rate while impairing mitochondrial ATP production, creating an energy crisis. Excessive release of excitatory neurotransmitters (particularly glutamate) occurs, leading to calcium influx and excitotoxic cell death. Heat denatures cytoskeletal proteins and membrane proteins critical for neuronal function. Cerebral vasodilation in response to hyperthermia increases intracranial pressure, and the blood-brain barrier becomes increasingly permeable, allowing fluid extravasation and cerebral edema. Seizures occur from direct neuronal irritability and further increase metabolic heat production and muscle breakdown.

• Non-exertional (classic, passive) heat stroke: Occurs primarily in elderly patients, very young children, and those with chronic medical conditions during periods of extreme ambient heat. The elderly are at particular risk due to impaired thermoregulation (decreased sweating response, reduced thirst perception), reduced cardiac reserve limiting compensatory tachycardia and increased cardiac output, chronic medical conditions (diabetes mellitus, cardiovascular disease, respiratory disease), and polypharmacy. Socioeconomic factors including poor housing with limited air conditioning, social isolation, and inability to access cooling centers increase risk. Urban heat island effects amplify risk in inner-city populations. Medications significantly increase risk: anticholinergics (antihistamines, tricyclic antidepressants, antipsychotics) impair sweating; sympathomimetics (decongestants, amphetamines, cocaine) increase metabolic heat production; diuretics promote volume depletion; neuroleptics impair thermoregulation; stimulants increase metabolic rate. Underlying conditions increasing risk include hyperthyroidism, infection/fever, malignancy, and autonomic dysfunction.

• Exertional heat stroke: Occurs in young, healthy athletes or individuals performing strenuous physical labor in hot environments. Intensity of exercise generates metabolic heat at rates of 15-20 times resting metabolism; the body can only dissipate approximately 50% of this heat through evaporation, conduction, and convection, leading to net heat accumulation. Risk is highest when ambient temperature approaches or exceeds skin temperature, eliminating radiant heat loss and requiring near-total dependence on evaporative cooling. High humidity impairs evaporative cooling as the air is already saturated. Dehydration substantially reduces sweating capacity and increases core temperature; studies show that fluid restriction during exercise substantially increases heat stroke risk. Prior heat illness increases susceptibility to recurrent episodes, possibly through persistent impairment of heat shock protein responses or autonomic dysfunction. Genetic polymorphisms in heat shock proteins and inflammatory cytokine genes (IL-6, TNF-α) may predispose to exertional heat stroke. Male athletes are disproportionately affected, potentially due to greater participation in intense sports and less frequent thermoregulatory breaks. Sickle cell trait carriers have increased risk of sudden exertional heat stroke, likely due to sickling in the hypoxic, acidotic environment of intense exercise and resultant microvascular occlusion and rhabdomyolysis.

• Medications and intoxicants increasing heat stroke risk: Anticholinergics (diphenhydramine, atropine, tricyclic antidepressants, antipsychotics) directly inhibit sweat gland function via muscarinic receptor blockade. Stimulants (amphetamines, cocaine, methylphenidate) increase metabolic rate and sympathetic activity, generating endogenous heat. Neuroleptics (haloperidol, risperidone, clozapine) impair hypothalamic thermoregulation and can precipitate neuroleptic malignant syndrome (NMS), which is clinically identical to heat stroke. Serotonin reuptake inhibitors and monoamine oxidase inhibitors can precipitate serotonin syndrome with hyperthermia. Phencyclidine (PCP) causes intense muscular rigidity and sympathomimetic effects leading to hyperthermia. MDMA (ecstasy) combines sympathomimetic effects with enhanced physical exertion and impaired thermoregulation. Lithium impairs sweating and may increase risk. Diuretics promote hypovolemia and reduced sweating capacity.

• Environmental and behavioral factors: Ambient temperature, humidity, and radiant heat (from sun or industrial sources) directly determine the thermal gradient for heat dissipation. The wet bulb globe temperature (WBGT) combines dry bulb temperature, wet bulb temperature (reflecting humidity), and globe temperature (reflecting radiant heat) into a single index predictive of heat illness risk. WBGT > 32°C (90°F) substantially increases heat stroke risk during exercise. Lack of acclimatization substantially increases heat illness risk; individuals unaccustomed to hot environments lack the expanded plasma volume, increased sweating response, and earlier onset of sweating that develop with 1-2 weeks of heat exposure. Intensity and duration of exercise directly correlate with heat stroke risk. Tight-fitting or non-breathable clothing, particularly heavy protective gear (military uniforms, football equipment), impairs evaporative cooling. Sleep deprivation impairs thermoregulation. Hypohydration before and during exercise substantially increases risk.

• Prodromal symptoms (heat exhaustion): Patients often experience prodromal symptoms for hours before progression to heat stroke. Heat cramps, the mildest form of heat illness, manifest as painful muscle spasms (typically in the legs or abdomen) thought to result from a combination of dehydration, electrolyte depletion (particularly sodium), and intense muscle use. These cramps may persist for days. Patients report generalized malaise, weakness, and fatigue from metabolic derangements and hypovolemia. Headache is nearly universal from hypovolemia and increased intracranial pressure. Dizziness and lightheadedness result from orthostatic changes and reduced cerebral perfusion. Nausea and vomiting occur from splanchnic hypoperfusion and direct thermal injury to the GI tract. Anxiety and irritability reflect catecholamine excess and CNS irritability from hyperthermia. Patients may report myalgias from muscle breakdown.

• Heat exhaustion: Defined by core temperature <40°C with preserved mental status. Patients present with profuse diaphoresis (sweating) reflecting maximal activation of thermoregulatory mechanisms. Dehydration manifests as dry mucous membranes, poor skin turgor, and decreased urine output. Patients may report intense thirst. Orthostasis and presyncope occur from hypovolemia and inadequate cerebral perfusion; orthostatic vital signs show substantial drops in systolic blood pressure (>20 mmHg) or increases in heart rate (>20 bpm) with position changes. Tachycardia is invariable, representing compensatory mechanisms to maintain cardiac output in the setting of reduced preload. Tachypnea occurs from metabolic acidosis and sympathetic activation. Skin may be pale and clammy from peripheral vasoconstriction attempting to preserve core blood flow, or flushed and warm if cutaneous vasodilation predominates. Core temperature ranges from 37.5-40°C.

• Heat stroke with altered mental status: The hallmark distinguishing heat stroke from heat exhaustion is CNS dysfunction. This may manifest as confusion, disorientation, or delirium (most common), progressing to obtundation, coma, or seizures. Some patients present with atypical CNS manifestations including cerebral edema (evidenced by headache disproportionate to fever, seizures, altered mental status refractory to cooling), intracranial hemorrhage (from severe coagulopathy), or transient ischemic attacks. Notably, sweating may be absent in severe heat stroke despite high core temperatures; the absence of diaphoresis in a hyperthermic patient does not exclude heat stroke. Core temperature is ≥40°C but often much higher (40.5-42°C or higher). Patients frequently display erythematous, hot, dry skin, though some (particularly those with exertional heat stroke) may continue to sweat. Hypotension may be present from distributive shock, hypovolemic shock, or cardiogenic shock. Severe tachycardia often >120 bpm reflects both hypovolemia and catecholamine excess. Tachypnea with metabolic acidosis produces a characteristic Kussmaul respiration pattern.

• Exertional vs. non-exertional heat stroke presentations: Exertional heat stroke typically occurs in young, healthy athletes with rapid onset (minutes to hours) during or immediately after intense exercise. Collapse during or shortly after exercise is the classic presentation. Rhabdomyolysis is prominent with severe myoglobinuria, muscle pain, and creatinine kinase levels often >5000 IU/L (normal <200). Acute kidney injury develops more rapidly than in non-exertional cases. Coagulopathy may be less severe initially. Non-exertional heat stroke presents insidiously over hours to days in elderly, chronically ill patients. Gradual deterioration in mental status, difficulty with activities of daily living, and vague complaints precede recognition. Rhabdomyolysis is typically mild or absent. Coagulopathy is often severe, with DIC more common. Acute respiratory distress syndrome is more common as a late complication.

• Physical examination findings in severe heat stroke: Hyperthermia with core temperature ≥40°C (measured rectally, as oral, axillary, or tympanic measurements underestimate core temperature). Altered mental status ranging from confusion to coma; seizures occur in 5-15% of cases. Cerebellar signs including ataxia and nystagmus may be present from direct cerebellar damage or edema. Pupillary abnormalities including dilated pupils unresponsive to light may occur from severe CNS injury. Decerebrate or decorticate posturing indicates severe brainstem injury. Respiratory depression may occur in severely affected patients. Pulmonary edema manifests as crackles on auscultation from ARDS or from overly aggressive fluid resuscitation. Hepatic dysfunction produces hepatomegaly (in a minority of cases). Signs of rhabdomyolysis include severe myalgias, muscle rigidity, dark urine (myoglobinuria), and in severe cases, visible myoglobin in the urine. Bleeding manifestations including petechiae, purpura, or frank bleeding indicate severe coagulopathy and DIC. Acute kidney injury manifests as rising creatinine and oliguria or anuria.

• Important clinical variants and presentations: Exertional heat stroke with mild CNS dysfunction may present with confusion, ataxia, or irritability rather than obtundation

Initial and confirmatory measurement

• Rectal thermometry: the practical gold standard in the field and emergency department. The National Athletic Trainers' Association position statement on exertional heat illness explicitly states that oral, axillary, temporal-artery, and tympanic readings are invalid in a hot, sweating, vasoconstricted patient; they are unreliable and may read falsely low (the usual trap) or falsely high, and must not be used to confirm or exclude heat stroke.
• Esophageal or bladder probe: continuous core monitoring once the patient is intubated or catheterized; preferred for titrating cooling.
• Threshold: core ≥40°C (104°F) with encephalopathy defines heat stroke. A patient cooled by bystanders before arrival may be normothermic on presentation — a lower recorded temperature does not exclude the diagnosis.

Laboratory and imaging evaluation

• Point-of-care glucose: hypoglycemia is common in exertional cases and is an immediately reversible cause of altered mentation.
• CK and urinalysis: markedly elevated CK indicates rhabdomyolysis; a dipstick positive for blood with no red cells on microscopy is the classic signature of myoglobinuria.
• BMP, lactate, ABG/VBG: hyperkalemia, hyperphosphatemia, hypocalcemia, rising creatinine, and lactic acidosis.
• CBC, PT/INR, aPTT, fibrinogen, D-dimer: falling platelets with falling fibrinogen and rising D-dimer support DIC; the ISTH DIC score is the named tool for scoring overt DIC.
• AST/ALT, bilirubin: these rise over 24–72 hours and hepatic injury is eventually seen in most severe cases; normal enzymes on initial presentation are expected and do not exclude heat stroke. Serial trending is more informative than a single value, and marked early elevation suggests severe injury.
• Head CT, lumbar puncture, TSH, toxicology, ECG: obtained selectively to exclude meningitis/encephalitis, intracranial hemorrhage, thyroid storm, sepsis, neuroleptic malignant syndrome, serotonin syndrome, malignant hyperthermia, and anticholinergic or sympathomimetic toxicity — all of which can reproduce hyperthermia with altered mental status.

Immediate stabilization

• Airway, breathing, circulation: intubate for coma or loss of airway reflexes; check point-of-care glucose; establish large-bore IV access and a continuous rectal or esophageal probe.
• Remove from heat, strip clothing and equipment: eliminates insulation and permits cooling.

First-line therapy — cooling, not drugs

• Cold-water or ice-water immersion: the treatment of choice for exertional heat stroke; produces the fastest cooling rate and the lowest mortality. Continue circulating the water and support the head.
• Evaporative–convective cooling: tepid water mist plus high-flow fans, with ice packs to axillae, groin, and neck. Preferred for classic (non-exertional) heat stroke in frail elderly patients in whom immersion is impractical.
• Cold isotonic crystalloid: an adjunct for cooling and for restoring preload; titrate to perfusion, since overly aggressive volume worsens cerebral and pulmonary edema.
• Stop cooling at approximately 38.5–39°C to prevent overshoot hypothermia.

Escalation

• Benzodiazepines (e.g., lorazepam): suppress shivering, which regenerates heat and defeats cooling, and terminate seizures.
• Endovascular cooling catheters, cold gastric/bladder lavage, or extracorporeal circuits: reserved for refractory hyperthermia.
• Aggressive crystalloid for rhabdomyolysis, with renal replacement therapy per KDIGO indications for refractory hyperkalemia, acidosis, or volume overload; vasopressors and organ support for shock, DIC, and ARDS.

Contraindicated or useless

• Antipyretics (acetaminophen, aspirin, NSAIDs): the hypothalamic set point is normal in heat stroke, so they do not lower temperature and add hepatotoxicity and bleeding risk.
• Dantrolene: no benefit in heat stroke; it treats malignant hyperthermia.
• Alcohol sponging and alpha-agonist–induced cutaneous vasoconstriction: impair heat transfer.

Emergencies requiring immediate action

• Rhabdomyolysis with acute kidney injury: thermal myocyte necrosis releases myoglobin, which precipitates as tubular casts and generates oxidative tubular injury. Signaled by tea-colored urine, heme-positive dipstick with no RBCs, and markedly elevated CK with rising creatinine.
• Hyperkalemia: potassium efflux from lysed muscle plus impaired renal excretion. Peaked T waves, widened QRS, or a sine wave on ECG mandate calcium, insulin/dextrose, and dialysis per KDIGO indications.
• Disseminated intravascular coagulation: heat-activated coagulation with consumption of platelets and fibrinogen; heralded by petechiae, oozing from puncture sites, falling fibrinogen, and rising D-dimer.
• Cerebral edema and status epilepticus: blood–brain barrier breakdown plus excitotoxicity; suggested by mental status that fails to improve despite achieving normothermia.
• Arrhythmia and cardiogenic shock: direct myocardial thermal injury layered on high-output failure and electrolyte derangement.

Subacute organ failure

• Acute liver injury: hepatocyte necrosis from heat and splanchnic hypoperfusion; transaminases peak at 24–72 hours and severe cases progress to fulminant failure requiring transplant evaluation.
• ARDS: endothelial injury and cytokine-mediated permeability edema; hypoxemia with bilateral infiltrates and normal filling pressures.
• Compartment syndrome: swelling of injured muscle within fascial planes; pain out of proportion and a tense compartment — a surgical emergency for fasciotomy.
• Persistent cerebellar dysfunction: cerebellar Purkinje cells are uniquely thermosensitive; persistent ataxia and dysarthria are the classic (though uncommon) long-term neurologic sequela, while most survivors recover fully.

Complications of treatment

• Overshoot hypothermia: from failure to stop cooling near 38.5–39°C.
• Shivering thermogenesis: paradoxically raises core temperature and CK; treat with benzodiazepines.
• Aspiration or drowning during immersion: the obtunded patient must have the head supported and airway monitored.
• Iatrogenic pulmonary and cerebral edema: from over-resuscitation with crystalloid.

• The dividing line is CNS dysfunction, not temperature alone: a hyperthermic patient who is confused, seizing, or comatose has heat stroke, whereas an oriented patient with cramps, nausea, and orthostasis has heat exhaustion.
• Rectal temperature is the answer: tympanic, oral, temporal, and axillary readings are unreliable and may read falsely low (the usual trap) or falsely high, and must not be used to confirm or exclude heat stroke — the classic stem says the "temperature was only 38.5°C."
• Single best next step is immediate cold-water immersion, before transport and before labs: NATA and the Wilderness Medical Society frame this as cool first, transport second; survival tracks with time spent above 40°C.
• Antipyretics do nothing: the hypothalamic set point is normal, so acetaminophen and aspirin are the distractor — and both worsen hepatic injury and coagulopathy.
• Dantrolene is the other classic distractor: it belongs to malignant hyperthermia (succinylcholine or volatile anesthetic, ryanodine receptor, masseter rigidity), not heat stroke. Neuroleptic malignant syndrome gives lead-pipe rigidity days after a dopamine antagonist; serotonin syndrome gives clonus and hyperreflexia, worst in the lower extremities.
• Anhidrosis is not required: hot dry skin is typical of classic heat stroke in the elderly, but young athletes with exertional heat stroke are usually still drenched in sweat — the presence of sweating never excludes the diagnosis.
• Dark urine, heme-positive dipstick, no RBCs on microscopy equals myoglobinuria; expect a markedly elevated CK, hyperkalemia, hyperphosphatemia, hypocalcemia, and treat with early aggressive isotonic crystalloid.
• Normal AST/ALT early does not exclude heat stroke: transaminases rise over 24–72 hours, so trend them rather than relying on the initial value.
• Two tested associations: sickle cell trait predisposes to sudden exertional collapse with rhabdomyolysis, and cerebellar ataxia is the signature (though uncommon) persistent neurologic sequela of survived heat stroke.
• Stop cooling around 38.5–39°C to avoid overshoot hypothermia, and give a benzodiazepine for shivering rather than slowing the cooling.