Heat Illness and Heat Stroke

Heat stroke is the most severe form of heat illness. The brain is what defines it: a core temperature above 40 °C (104 °F) with central nervous system dysfunction after heat exposure or exertion. Bouchama's definition gets used most often. Underneath, it is hyperthermia with a systemic inflammatory response. That response causes multiorgan dysfunction, and encephalopathy is the main piece of it.

• Classic (non-exertional): older adults, people with chronic illness, people who are isolated or lack climate control, and people taking heat-sensitive medications. It often shows up during heat waves.
• Exertional: young, otherwise healthy people working or exercising in heat, such as athletes, laborers, soldiers and first responders. It can also happen in temperate climates. The trigger is heat production exceeding heat loss.
• Mechanism: heat gain beats heat loss. Cutaneous vasodilation pulls blood to the skin and lowers circulating volume. Hyperthermia drives endotoxin and cytokine release, endothelial injury, microthrombosis and disseminated intravascular coagulation (DIC).
• Course: how high and how long the hyperthermia runs drives CNS injury and survival. Without cooling, it slides toward multiorgan failure: acute kidney injury, liver injury, coagulopathy, respiratory failure and rhabdomyolysis.

The Japanese criteria (JAAM) do not need a temperature. Under those criteria, a patient exposed to high environmental temperature has heat stroke if they show a Glasgow Coma Scale (GCS) of 14 or lower, creatinine or total bilirubin of 1.2 mg/dL or higher, or a JAAM DIC score of 4 or higher.

Heat exhaustion and heat cramps do not involve central nervous system dysfunction. Confusion, seizure or coma after heat exposure? That is heat stroke until proven otherwise.

Presentation

• Core temperature above 40 °C with CNS change. In the field, measure rectally. Rectal readings are more accurate than axillary, oral, temporal or aural ones. Once the patient is intubated, use an esophageal probe. Published thresholds run from 40 to 40.6 °C, so the number moves between studies.
• Central nervous system: confusion, irritability, agitation, delirium, ataxia, slurred speech, seizure or coma.
• Skin: hot to the touch and dry or flushed in classic cases. Sweating can be there, especially in exertional heat stroke.
• Exposure: heat, humidity, sun or strenuous exertion in the hours before onset.

Red Flags

• Coma, seizure, rigidity, or GCS 14 or lower
• Systolic BP under 100 mmHg. In the Paris 2003 heat wave cohorts, low systolic pressure predicted death
• Creatinine or total bilirubin of 1.2 mg/dL or higher, or DIC: bleeding, bruising or falling platelets
• Higher temperature: in a 2026 prehospital cohort, survival fell as temperature rose and was lowest at 42.2 °C or above. Those were tympanic readings. No validated cutoff exists for a red flag
• Airway compromise or failing airway protection

This is a clinical diagnosis, and it rests on core temperature and CNS change. Start cooling first. The checks below run alongside it, and none of them may delay cooling.

The JAAM DIC score is a Japanese scoring system. A score of 4 or higher meets the heat stroke definition, and so does a GCS of 14 or lower. No other score changes management on this page.

• Sepsis: infection brings fever and CNS change, and it can coexist with heat stroke. Look for a source. Draw blood cultures when infection is suspected. See Sepsis and Septic Shock
• Toxic or drug-induced hyperthermia: anticholinergic, sympathomimetic, salicylate and serotonergic drugs can raise temperature and change mental status. The medication and overdose history tells them apart
• Malignant hyperthermia: anesthetic agents trigger it, not heat exposure. Onset during anesthesia tells it apart
• Thyroid storm: thyroid derangement can drive hyperthermia and altered mental status. Check TSH and the thyroid history
• Heat exhaustion: no CNS dysfunction. Add confusion, agitation or seizure and the diagnosis becomes heat stroke

Cooling is the treatment. Local protocol decides who may start immersion and which cold packs are stocked.

1. Recognize and call early. This is a time-critical emergency. Start cooling on scene. Cool first, transport second. Local protocol sets how it is done.
2. Move to shade or a cooler space. Remove the patient from hot ground using a barrier, and loosen or remove clothing.
3. Cold-water immersion (CWI) when available: immerse the torso and extremities with the head and airway above water. Use ice water (about 2 °C) or cold water (about 20 °C). Stir or circulate the water and add ice to keep it cold. A stirred immersion cools about 0.13 to 0.35 °C per minute. Rates above 0.15 °C per minute are desirable. Watch the airway. A confused patient in water can drown. Protect the airway per protocol before immersing a patient who cannot protect it. Suspected spinal injury? Follow local protocol for spinal precautions. The ERC 2025 guideline recommends CWI for heat stroke without cardiac arrest. Derkenne 2026 summarizes it the same way. In arrest, start CPR and ALS (see ALS Management).
4. If immersion is not possible: apply ice-filled sheets or towels to the axillae, groin and neck, and change them every 2–4 minutes. Tarp-assisted cooling is the alternative: ice and water on a tarp, folded up at the sides.
5. Airway and breathing: protect the airway, give oxygen, and position on the side if the patient vomits or is unconscious.
6. Glucose check where the protocol allows. Treat hypoglycemia per protocol.
7. Seizure or agitation: protect the patient from injury. Skip physical restraint. It adds heat and muscle injury. ALS gives chemical sedation.
8. Do not give aspirin, NSAIDs or acetaminophen for temperature. None of them lowers core temperature in heat stroke.
9. Reassess and notify: mental status, breathing, pulse and BP. Call for ALS and give the ICU-capable hospital early notice. Keep cooling in transit.

Doses vary by protocol; follow local protocols and medical direction. Cooling is what treats this. Medications handle agitation, seizures and complications.

• Continue cooling (Paramedic/RN/CCT): keep immersion or ice sheets going. Stop active cooling near 39 °C core temperature to avoid overshoot. Overshoot triggers shivering. Keep monitoring after 39 °C, because the temperature can climb again.
• Chilled IV fluid (Paramedic/RN/CCT): isotonic crystalloid, such as normal saline or lactated Ringer's. It adds cooling when it is chilled and stocked, but it is not sufficient alone. Bridwell 2026 cites a 30 to 40 mL/kg cooled bolus. That figure comes from the review's own secondary sources, and it conflicts with the same review's advice on judicious fluid. Bolus volume depends on protocol. Give fluid in boluses. Reassess lung sounds and SpO₂ after each one.
• IV or IO access (Paramedic/RN/CCT): two large-bore IVs when possible. IO if IV access fails (IO access).
• Cardiac monitor and 12-lead (Paramedic/RN/CCT): watch for tachyarrhythmia, QT prolongation and ST or T changes. Track core temperature the whole way through cooling.
• Seizure (Paramedic/RN/CCT): treat a seizure of 5 minutes or longer as status epilepticus. Adults: lorazepam 4 mg IV slowly (no faster than 2 mg/min), repeated once if seizures continue or recur after 10 to 15 minutes. No IV access? Midazolam 10 mg IM (adults over 40 kg) is an option. Children are dosed by weight. See Status Epilepticus. Benzodiazepines can depress breathing. Have airway equipment ready and monitor SpO₂ and ETCO₂.
• Agitation and shivering (Paramedic/RN/CCT): chemical sedation, not restraint. Give a benzodiazepine, such as midazolam or lorazepam, titrated to effect per protocol. Ketamine is the alternative per protocol. Shivering adds heat production, so it gets the same treatment.
• Do not give: aspirin, NSAIDs or acetaminophen for temperature control. Dantrolene has no evidence of benefit in heat stroke.
• Hypotension (Paramedic/RN/CCT): cooled fluid first. Reassess after each bolus. Bridwell 2026 advises judicious fluid, because most heat stroke patients are not profoundly fluid-depleted on central venous monitoring. Vasoconstriction slows heat loss, which is why vasopressors come later. Start one only if mean arterial pressure stays below 65 mmHg despite cooling and volume. The MAP target matches sepsis. What differs is the wait while cooling does its job. Follow protocol and medical direction.
• Potassium (Paramedic/RN/CCT): rhabdomyolysis and kidney injury can push potassium up. Look for ECG changes: peaked T waves, a wide QRS or bradycardia. Treat per protocol, with calcium for ECG changes. See Hyperkalemia.
• Airway (Paramedic/RN/CCT): consider intubation for refractory agitation, failing airway protection, respiratory failure or a seizure that does not stop. Use RSI with a non-depolarizing agent such as rocuronium, per protocol. Avoid succinylcholine. Rhabdomyolysis and kidney injury raise the risk of life-threatening hyperkalemia. The succinylcholine label warns of hyperkalemia after rhabdomyolysis and major injury. Bridwell 2026 also advises avoiding it, citing hyperthermia and a rare malignant hyperthermia risk.
• Cardiac arrest (Paramedic/RN/CCT): start CPR and the standard ACLS arrest algorithm. Adult epinephrine: 1 mg IV or IO every 3–5 min (see Epinephrine IV). Whether to cool during arrest depends on protocol. Two 2026 case reports describe ALS during immersion, with electrical devices kept out of the water. Cooling was slow. Do not delay ALS to cool.

• Core temperature: esophageal probe for intubated patients. Keep cooling to about 39 °C, then keep monitoring. Temperature can rebound. Continuous core readings come from a temperature-sensing Foley catheter or an esophageal probe, as Bridwell 2026 notes.
• Kidney monitoring: track urine output through a urinary catheter, plus creatinine and CK. Rhabdomyolysis can injure the kidney. Involve nephrology or the ICU early, per protocol. Bridwell 2026 also advises judicious fluid resuscitation.
• Cooling options: keep up CWI or ice packs. Manufactured surface cooling devices and tepid water with fanning (evaporative cooling) are alternatives. Older and agitated patients often tolerate evaporative cooling better.
• Invasive cooling: the data on intravascular cooling catheters and body-cavity lavage in heat stroke are limited. Neither replaces simpler, faster cooling.
• Diagnostics: glucose, potassium and other electrolytes, creatinine, liver tests, CK, urinalysis, platelets, PT/INR, fibrinogen, blood gas, ECG, and cultures if infection is suspected. Cooling does not wait for results.
• Organ support: for kidney injury, use fluids and renal replacement therapy per ICU protocol (CRRT). Manage DIC and blood product use per ICU protocol (blood products). No prospective study has confirmed a benefit from anticoagulation or blood purification in heat stroke.
• Hemodynamics: cooled fluid comes first. Use a vasopressor per protocol if mean arterial pressure stays below 65 mmHg despite cooling and volume (norepinephrine). No prospective study has confirmed a specific vasoactive drug or fluid target.
• Sedation for intubated patients: propofol or dexmedetomidine are the preferred choices, since they also suppress shivering. Benzodiazepines are the alternative.
• Disposition: admit to the ICU. The risk of multiorgan dysfunction is high in heat stroke.

• Destination: an ICU-capable hospital. Patients with multiorgan dysfunction need intensive care. Some need renal replacement therapy.
• Cool first, transport second: start cooling on scene and keep it going in the ambulance. EMS protocols differ from place to place. A 2021 review of 51 US EMS protocols found nine that explicitly recommended cold-water immersion. Six told crews not to delay transport to cool the patient, and the authors called that incorrect. Follow local protocol.
• Prehospital immersion results: one 2026 multisite program found a median immersion time of 13.5 minutes. Temperature fell by an average of 2.0 °C before ED arrival in 72% of patients. Median GCS rose from 8 to 15 at discharge. This study is retrospective.
• In transit: keep cooling with ice packs to the axillae, groin and neck, or with wet sheets and airflow. Watch core temperature, shivering, seizure and airway.
• Handoff: time of onset and time cooling started. Core temperature at first contact and at handoff. GCS trend. Cooling method and duration. Drugs and fluid volumes given. Glucose. Any seizure. Outcome follows how long the severe hyperthermia lasts.
• Equipment: ice, water and immersion equipment or tarps; sheets and fans; thermometer able to read core temperature (rectal probe); suction; airway kit; cardiac monitor with 12-lead; benzodiazepine and ketamine per protocol.

• Pediatrics: children are at risk in classic heat stroke. Bridwell 2026 notes that it affects prepubertal children, including children left in hot vehicles. The cooling is the same, and it starts right away. Children have a high surface-area-to-mass ratio. As ACSM 2023 notes, they may cool faster, so watch core temperature closely and avoid overcooling. Check glucose early. The sources reviewed set no pediatric cooling endpoint. The thresholds on this page were studied in adults. Dose sedatives and anticonvulsants by weight, never with adult fixed doses (Weight-Based Dosing). Compare heart rate, BP and SpO₂ with pediatric vital signs.
• Pregnancy: treat the mother first. The cooling approach does not change. The sources reviewed set no pregnancy-specific heat stroke threshold. Involve obstetric care early. Assess fetal status after maternal stabilization, per local protocol.
• Older Adults: older adults get classic heat stroke most often. Risk factors include age over 80, living in an institution, diuretic use, cardiac disease and cancer.
• Renal Impairment: kidney injury is common, and it is linked to death. No heat-stroke-specific dose adjustment is established. Adjust renally cleared drugs per pharmacy and ICU protocol.
• Hepatic Impairment: liver injury can progress to acute liver failure. Avoid acetaminophen.
• Obesity: in a UK military case series, higher BMI went with slower cooling. Obesity is also a listed risk factor.
• Immunocompromised: infection can cause hyperthermia or come along with it. When infection is suspected, draw cultures and treat per sepsis protocol.
• Patients on Anticoagulants: heat stroke causes DIC. Check PT/INR and platelets on arrival. Anticoagulation has no proven benefit in heat stroke. Anticoagulant status still matters for bleeding after any fall or head injury, per protocol.
• Patients on Beta-Blockers: beta-blockers are a listed heat stroke risk factor. So are calcium channel blockers, anticholinergics, antihistamines, salicylates, thyroid agonists, laxatives, sympathomimetics, tricyclic antidepressants and SSRIs. No specific dose adjustment is established.

• Error: treating it as fever. None of aspirin, NSAIDs or acetaminophen lowers core temperature in heat stroke. Cool instead.
• Error: waiting for transport, IV access or a temperature reading before cooling. Start cooling at recognition. Keep it going in transit.
• Error: overcooling. Near 39 °C, stop active cooling. Overshoot triggers shivering, and shivering adds heat. Keep monitoring past 39 °C.
• Error: restraining an agitated patient. Restraint adds heat and muscle injury. Use chemical sedation instead.
• Error: reflexive vasopressors. Vasoconstriction slows heat loss, so cooled fluid comes first.
• Error: succinylcholine for intubation. Rhabdomyolysis and kidney injury push potassium up. Succinylcholine can then cause life-threatening hyperkalemia. Choose a non-depolarizing agent such as rocuronium, per protocol.
• Error: giving dantrolene. It is the malignant hyperthermia antidote. In heat stroke, it has no proven benefit.
• Error: anchoring on heat and missing a mimic. Before you blame the heat, check glucose, medications and overdose, sepsis and thyroid status.