Hyperthermia and Heat Related Illness

What Is Hyperthermia?

Hyperthermia is a dangerous elevation in body temperature due to a failure of heat dissipation, not an intentional increase in set-point like in fever. It represents a collapse of thermoregulatory control, where endogenous and exogenous heat production overwhelm the body's cooling mechanisms.

• Threshold: Core temperature > 40°C (104°F)
• Key feature: Altered mental status (delirium, agitation, coma)

Hyperthermia vs. Fever

How the Body Normally Regulates Heat

The body keeps internal temperature stable near 37°C via:

What Causes Hyperthermia?

Heat load exceeds the body’s ability to cool itself due to:

• Environmental heat (classic heatstroke)
• Endogenous heat generation (exertion, stimulant use)
• Impaired thermoregulation (medications, dehydration, age, CNS injury)
• Social/environmental limitations (lack of AC, shelter, water access)

Spectrum of Illness

Environmental (Classic) Heatstroke

Triggers

• High ambient temperatures, often during heat waves
• High humidity prevents sweat evaporation
• Poor housing, no air conditioning, and limited access to hydration

At-Risk Groups

• Elderly, infants, chronically ill, socially isolated
• Urban populations (especially in redlined areas)

Exertional Heatstroke

Triggers

• Physical exertion in hot/humid environments
• Common in:
• Soldiers, athletes, laborers, firefighters, and inmates
• Outdoor workers under pressure (e.g., delivery, agriculture)

Key Insight

• Sweating is often preserved in exertional cases → do not rely on dry skin as a diagnostic clue
• Frequently presents abruptly with collapse, altered mentation, and rhabdomyolysis

Toxin- and Drug-Induced Hyperthermia

Neurogenic Hyperthermia

Triggers

• Hypothalamic injury (stroke, trauma, hemorrhage, tumor)
• Brainstem injury, spinal cord transection

Mechanism

• Loss of autonomic thermoregulation
• Often resistant to cooling and unresponsive to antipyretics

Clinical Clues:

• Persistent high temperature without obvious cause
• No infection, toxidrome, or environmental exposure

Iatrogenic Hyperthermia

• Over-warming in perioperative, neonatal, or prehospital settings
• Overuse of: Bair Hugger™ blankets, radiant warmers, warming IV fluids, heated ambient air

Combined/Structural Risks

Hyperthermia risk is magnified by systems level vulnerability such as structural, social, and environmental factors:

• Urban heat islands
• Inadequate housing or HVAC
• Workplace conditions without heat protection (e.g., rest breaks, shade, cooling PPE)
• Disability, mental illness, poverty, and language barriers

Mimics

Summary

Hyperthermia causes death through multi-system cellular injury, vascular dysfunction, and immune collapse. Core body temperatures exceeding 40–41°C disrupt protein function, denature enzymes, compromise vascular barriers, and initiate a self-reinforcing inflammatory cascade. Skip to the end of the section for summary.

Three Phases of Heatstroke Injury

CNS Injury

Hyperthermia injures the CNS through multiple converging mechanisms: direct thermal toxicity, vascular dysfunction, and excitotoxicity. As core temperature rises, the blood-brain barrier (BBB) becomes permeable due to endothelial protein denaturation. Plasma proteins, inflammatory mediators, and neurotoxic metabolites enter the brain parenchyma, causing vasogenic edema and astrocyte swelling. Simultaneously, impaired cerebral autoregulation—driven by hypotension and microvascular injury—reduces perfusion, predisposing to cytotoxic edema and ischemia.

Heat stress also triggers excessive glutamate release and calcium influx, fueling excitotoxic neuronal death. Microthrombi in cerebral vessels, a product of coagulopathy and endothelial activation, further compound focal ischemic injury. The cerebellum, particularly Purkinje cells, is exquisitely sensitive—explaining frequent ataxia, dysarthria, and persistent motor deficits in survivors. Imaging and pathology commonly reveal cerebellar atrophy, white matter demyelination, and Purkinje cell dropout.

Neurologic dysfunction—delirium, ataxia, slurred speech, confusion—is the earliest and most reliable marker of heatstroke. It often precedes overt hemodynamic collapse and should trigger immediate cooling. Late neurologic findings include seizures, coma and brainstem herniation.

Endothelial Damage and Capillary Leak

Heat denatures endothelial proteins and disrupts tight junctions and the glycocalyx, resulting in systemic capillary leak. Plasma floods interstitial spaces—into lungs (causing noncardiogenic pulmonary edema), the GI tract (leading to mucosal sloughing and hemorrhage), and the skin (blistering, bullae). Tintinalli calls this a “burn without a flame.” The ensuing intravascular depletion causes hemoconcentration, hypotension, and distributive shock, with widespread tissue hypoperfusion.

SIRS/Sepsis Without Infection

Hyperthermia initiates a sterile sepsis—a full-blown systemic inflammatory response without infection. Damaged cells release DAMPs, and cytokines like IL-6 and TNF-α surge. Heat shock proteins, normally protective, are overwhelmed. As vasodilation shunts blood from the gut, enterocytes become ischemic and structurally compromised. Tight junctions fail, allowing endotoxins and gut flora—especially gram-negative lipopolysaccharide—to translocate into the bloodstream. Hepatic clearance is outpaced, leading to systemic endotoxemia. This gut-origin SIRS mimics septic shock: hypotension, coagulopathy, and multiorgan dysfunction—even when cultures are negative. As Epstein and Yanovich emphasize, this mechanism may mimic or coexist with true sepsis, complicating management.

Coagulopathy and DIC

Cytokine-induced endothelial injury upregulates tissue factor, initiating the extrinsic coagulation cascade. Thrombin generation, platelet consumption, and fibrin deposition result in widespread microthrombosis and bleeding—clinically manifesting as petechiae, IV site oozing, or hematuria. Disseminated intravascular coagulation (DIC) often emerges 24–48 hours post-insult, peaking during the so-called hematologic-enzymatic phase. Coags may worsen even after cooling, necessitating serial INR, fibrinogen, and platelet monitoring.

Rhabdomyolysis and Renal Injury

Thermal damage to skeletal muscle causes myocyte necrosis, ATP depletion, and severe rhabdomyolysis, especially in exertional heatstroke. CK levels often exceed 10,000 U/L. Myoglobin released into circulation precipitates in renal tubules—particularly under hypovolemia and acidosis—causing acute tubular necrosis (ATN). Hyperkalemia, hyperphosphatemia, and hypocalcemia are common and can precipitate lethal arrhythmias, including sudden VF arrest—a well-documented terminal event. Renal failure is multifactorial: hypoperfusion, myoglobin toxicity, and cytokine-mediated tubular injury all contribute.

Hepatic Injury and Cardiovascular Collapse

The liver often becomes a delayed casualty—centrilobular necrosis, transaminitis, and coagulopathy typically peak 48–96 hours after the event. Trending INR, bilirubin, and LFTs for at least 72 hours is essential. Hepatic failure is both a marker of severity and a predictor of mortality. Circulatory collapse is multifactorial: third-spacing, evaporative losses, and vasodilation cause hypovolemic-distributive shock, while hyperthermia itself depresses myocardial contractility. Arrhythmias, from hyperkalemia and QT prolongation, are common.

TL;DR

The diagnosis is clinical, urgent, and time-sensitive. Early neurologic changes often precede other organ injury. Labs, imaging, and even temperature measurements may lag behind the true severity.

History & Context Clues

Environmental & Social

• Recent exposure to extreme heat or humidity
• Lack of air conditioning, outdoor labor, prolonged physical activity
• Recent heat wave or social vulnerability (e.g., homelessness, poor housing, language barriers)

Drug/Toxin Risk

• Anticholinergics, neuroleptics, amphetamines, MDMA, SSRIs
• Salicylates or polypharmacy in elderly
• Alcohol or recreational drug use preceding collapse

Neurogenic Risk

• History of stroke, TBI, spinal cord injury

Key Clinical Features

Temperature Measurement

• Use a rectal thermometer — other methods (oral, tympanic, axillary) are unreliable and dangerously misleading
• Serial rectal temps are useful to track cooling efficacy
• Don’t let a “normal” oral temp delay treatment if the patient is altered and exposed to heat.

Labs

While not required for diagnosis, initial labs guide management and confirm complications:

• CBC: Leukocytosis, thrombocytopenia
• CMP: Elevated AST/ALT, hyperkalemia, renal injury
• CK: Often >10,000 in exertional cases
• ABG/VBG: Metabolic acidosis ± respiratory alkalosis
• Coags: Elevated INR/PTT, ↓ fibrinogen (early DIC)
• UA: Myoglobinuria (dark brown urine, +blood on dipstick, no RBCs)
• Lactate: Often elevated, especially with shock or ischemia
• Tox screen: Consider acetaminophen, salicylates, and coingestants

Cooling is the only intervention proven to reverse heatstroke pathophysiology.

Cooling Targets

• Goal: 38.3–38.5°C (~101 F) within 30 minutes of recognition
• Stop active cooling at that threshold to avoid rebound hypothermia or shivering

Cooling Modalities

Alternative Cooling Methods (Resource Limited)

• Improvised Immersion cooling: Submerge the patient in any tub, trough, storage bin or tarp filled with cold water — fastest and most effective.
• Wet Sheet + Fan: Wrap in soaked sheets and fan aggressively for evaporative cooling.
• Continuous Water Pouring: Constantly pour cool water over the body while fanning to accelerate evaporation.
• Ice Pack Core Cooling: Apply ice packs or frozen items (eg, vegetables) to neck, axillae, and groin — slower but useful in transit.
• Body Bag Ice Bath: Line a stretcher or tarp with a plastic sheet or body bag, fill with ice and water, and lay patient inside.
• Wind or Cross-Ventilation: Strip and wet the patient, then position in natural airflow or fan manually.
• Running Water Immersion: Lay patient in a shaded stream or under a hose — let moving water carry heat away.
• Mud/Sand Burial: Bury patient in cool wet sand or mud up to the chest — last-resort conduction cooling.

Sedation, Shivering, and Airway Control

• Shivering slows cooling; suppress aggressively
• Use benzodiazepines, dexmedetomidine, or intubation + paralysis

Volume Resuscitation

• Most patients are profoundly hypovolemic from sweating and third-spacing
• Start with 20–30 mL/kg of NS or LR, reassess frequently
• Avoid early vasopressors unless hypotension persists after fluids as may worsen CNS perfusion

Tachyarrhythmias

Cardioversion and defibrillation are not contraindicated during cold water immersion. Per AHA guidelines, it is safe to deliver a shock in wet environments as long as AED pads are firmly applied to the chest and rescuers avoid direct contact with the patient during energy delivery. In fact, a case report (PMID: 36334955) describes successful synchronized cardioversion during active immersion cooling in a heatstroke patient with unstable VT. If the patient requires defibrillation or cardioversion, proceed — do not delay for drying or repositioning if it's clinically indicated.

Post-Resuscitation Monitoring

Cooling is not the end — it's the beginning of delayed systemic fallout. Monitor aggressively for 48–96 hours. Patient should be admitted to the ICU if persistently altered or if there is evidence of end organ malperfusion.

Watch Closely For:

• Rhabdomyolysis → rising CK, worsening renal function
• AKI → oliguria, rising Cr → may need dialysis
• Liver failure → INR, LFTs, bilirubin (peaks ~72 hrs)
• Coagulopathy → serial INR/PTT/fibrinogen
• Arrhythmias → electrolyte correction, continuous telemetry
• Cerebral edema → worsening mental status, seizures
• ARDS or pulmonary edema → rising O₂ needs, crackles

Disposition and Follow-Up

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