Heat exhaustion is a moderate heat-related illness characterized by peripheral vasodilation, relative circulatory insufficiency, and mild thermoregulatory dysfunction resulting from water and/or salt depletion during heat exposure. Core temperature ranges from 100.4°F to 104°F (38°C–40°C), with no altered mental status — the key distinguishing feature from heat stroke. [1-2] Mortality is negligible with early recognition and treatment, but untreated heat exhaustion can progress to life-threatening heat stroke. [2-3]
The following algorithm from the AAFP outlines the clinical approach to heat-related illness management:
1. History
- Duration, intensity, and type of heat exposure (exertional vs. passive/environmental)
- Activity level: exercise, outdoor labor, military training, sporting event
- Fluid intake: volume, type (water vs. electrolyte solution), timing
- Onset and progression of symptoms: fatigue, weakness, headache, nausea, dizziness, thirst, inability to continue activity
- Timing relative to activity: during exertion, immediately after, or delayed onset over days (subacute "summer flu" presentation in unacclimatized individuals) [4]
- Acclimatization status: recent travel to hotter climate, new job, early-season training
- Clothing/equipment: heavy gear, PPE, uniforms
- Recent illness (febrile illness, GI illness with dehydration)
- Medication and substance use (see Medications section)
- Prior episodes of heat-related illness [2-3]
2. Alarm Features
These features suggest progression to heat stroke or an alternative dangerous diagnosis and require immediate aggressive intervention:
- Altered mental status: confusion, delirium, combativeness, slurred speech, agitation — the cardinal feature distinguishing heat stroke from heat exhaustion [1]
- Core temperature ≥104°F (40°C) [3][5]
- Seizures or coma — suggest heat stroke or severe exercise-associated hyponatremia [1][4]
- Loss of consciousness during exercise (consider cardiac arrhythmia) [2]
- Persistent or worsening symptoms despite 15–30 minutes of cooling and rehydration [1-2]
- Signs of end-organ damage: oliguria/anuria, dark or cola-colored urine (rhabdomyolysis), respiratory distress, hypotension
- Coagulopathy or DIC signs (petechiae, bleeding) [1]
- Sphincter incontinence [1]
Clinical pearl: Heat exhaustion exists on a continuum with heat stroke. When the diagnosis is unclear, initiate immediate cooling measures — do not delay treatment to differentiate. [1-2]
3. Medications
Medications that increase risk (by mechanism): [1-2][6]
- Impair sweating: anticholinergics (oxybutynin, benztropine), antihistamines (diphenhydramine), topiramate
- Disrupt hypothalamic thermoregulation: antipsychotics (haloperidol, quetiapine), SSRIs, lithium, phenothiazines
- Increase metabolic heat production: amphetamines, thyroid agonists, cocaine, MDMA, stimulant supplements
- Cause dehydration/hypovolemia: diuretics, laxatives, alcohol, ACE inhibitors/ARBs
- Impair cardiovascular compensation: beta-blockers, calcium channel blockers
- Reduce heat awareness: benzodiazepines, opioids, alcohol
The following table from the AAFP provides a comprehensive list:
Contraindicated medications in acute management
- Antipyretics (acetaminophen, NSAIDs) are ineffective — heat exhaustion is not mediated by pyrogens and these agents do not lower core temperature in heat illness [4]
- Dantrolene has no proven benefit in heat-related illness (unlike malignant hyperthermia) [1]
- Diuretics should be avoided as they worsen hypovolemia [3][5]
Treatment medications
- IV normal saline for moderate-to-severe cases with significant volume depletion
- Ondansetron for persistent nausea/vomiting to facilitate oral rehydration
- No specific pharmacologic therapy exists for heat exhaustion — management is supportive [1][3]
4. Diet
- Acute: oral rehydration with electrolyte-containing fluids (sports drinks, oral salt solution: ¼–½ tsp salt per liter of water) [4]
- Water alone is adequate for activities <1 hour; carbohydrate-electrolyte beverages recommended for activities ≥1 hour to avoid hyponatremia [2]
- Plain water plus salty snacks is equally effective and often more palatable [4]
- Avoid alcohol and caffeine in excess during recovery
- Long-term prevention: drink-to-thirst approach during activity; replace 125%–150% of volume lost [2-3]
- The American College of Sports Medicine recommends 20–30 mmol/L (460–690 mg/L) sodium in rehydration beverages [2]
5. Review of Systems
- Constitutional: fatigue, weakness, malaise, inability to continue activity
- Neurologic: headache, dizziness, lightheadedness, irritability (but NOT confusion, seizures, or coma — these suggest heat stroke)
- GI: nausea, vomiting, anorexia, diarrhea
- Cardiovascular: palpitations, chest pain, presyncope/syncope
- Musculoskeletal: muscle cramps, myalgias, weakness (dark urine → rhabdomyolysis)
- Respiratory: dyspnea, tachypnea
- Genitourinary: decreased urine output, dark urine [1-2][7]
6. Collateral History and Family History
- Witnesses to the event: duration of exposure, activity level, collapse circumstances, mental status at scene
- EMS report: field temperature, cooling measures initiated, response to treatment
- Coaches/supervisors: acclimatization protocol, rest breaks, hydration availability
- Sickle cell trait carrier status — exercise collapse associated with sickle cell trait can mimic heat exhaustion but requires different management [2]
- Family history of malignant hyperthermia (rare but relevant in exertional presentations)
- Social context: housing conditions (air conditioning access), occupation, homelessness, social isolation (especially in elderly with classic heat illness) [2][7]
7. Risk Factors
Modifiable: [2-3]
- Lack of heat acclimatization (most important modifiable factor)
- Deconditioning/poor aerobic fitness
- Inadequate hydration
- Excessive clothing or protective equipment
- Recent illness (especially febrile or GI illness)
- Medication/substance use (see above)
- Obesity (elevated BMI)
- Behavioral factors: ignoring symptoms, inadequate rest breaks
Non-modifiable: [2][6-7]
- Extremes of age (elderly and young children)
- Chronic medical conditions: cardiovascular disease, diabetes, renal disease, pulmonary disease
- Prior heat-related illness (risk factor for recurrence)
- Pregnancy
Environmental: [1]
- High ambient temperature and humidity
- Direct sunlight, radiant heat sources
- Urban heat islands
- Heat waves (especially early-season before acclimatization)
8. Differential Diagnosis
Cannot-miss diagnoses: [1-2]
- Heat stroke — core temp ≥104°F + altered mental status; requires immediate ice-water immersion
- Severe exercise-associated hyponatremia — similar symptoms but serum Na <125 mEq/L; fluid restriction required (giving IV fluids worsens this condition)
- Exercise collapse associated with sickle cell trait — "conscious collapse" with flaccid lower extremities, dark urine; requires aggressive supportive care
- Cardiac arrhythmia/sudden cardiac death — especially if collapse during (not after) exercise
Other important differentials: [1][4]
- Sepsis — fever, tachycardia, hypotension; may be indistinguishable without history
- Hypoglycemia — rapid bedside glucose rules this out
- Exertional rhabdomyolysis — may coexist; dark urine, elevated CK
- Toxicologic emergency — sympathomimetics, anticholinergic toxidrome, serotonin syndrome, NMS
- Endocrinologic emergency — thyroid storm, adrenal crisis
- Ischemic stroke — focal neurologic deficits help distinguish
Clinical pearl: Heat exhaustion developing over several days in unacclimatized individuals is often misdiagnosed as "summer flu" due to overlapping symptoms of weakness, fatigue, headache, nausea, vomiting, and diarrhea. [4]
9. Past Medical History
- Prior heat-related illness or heat stroke (strong risk factor for recurrence) [3][5]
- Cardiovascular disease, hypertension
- Diabetes mellitus
- Chronic kidney disease
- Pulmonary disease (COPD, asthma)
- Obesity
- Dementia or psychiatric illness (impaired behavioral thermoregulation)
- Sickle cell trait
- Skin conditions (burns, extensive scarring — impair sweating)
- Recent sunburn (impairs sweating for ~7 days) [5]
- Surgical history: prior skin grafts, anhidrosis
10. Physical Exam
Vital signs
- Core temperature: 38°C–40°C (100.4°F–104°F); may be normal if pre-cooled [2]
- Tachycardia and tachypnea are common
- Blood pressure: may show orthostatic hypotension from volume depletion
- Rectal temperature is the gold standard for core temperature measurement [2]
Focused exam
- Skin: moist, diaphoretic (sweating preserved — unlike classic heat stroke where skin is often dry); may appear flushed or pale; cold and clammy in some cases [2]
- Neurologic: mental status must be intact — GCS 15, oriented, no confusion. Mild irritability or anxiety may be present in severe heat exhaustion but frank confusion/disorientation should prompt concern for heat stroke [1-2]
- Cardiovascular: tachycardia, weak pulse, orthostatic changes
- Musculoskeletal: muscle tenderness, cramps; check for compartment syndrome signs if rhabdomyolysis suspected
- Abdomen: assess for tenderness (heat cramps can cause abdominal pain)
11. Lab Studies
Mild heat exhaustion with rapid response to treatment typically requires no laboratory workup. [1][3]
Labs indicated for severe or delayed-recovery cases: [1-2][8]
- BMP/CMP: electrolytes (hyponatremia, hypokalemia, hypochloremia), BUN/creatinine (AKI), glucose
- Serum sodium: critical to rule out exercise-associated hyponatremia
- Creatine kinase (CK): screen for rhabdomyolysis; CK ≥20,000 U/L is a high-risk feature warranting admission [2]
- Urinalysis: myoglobinuria (dipstick positive for blood without RBCs)
- CBC with platelets: leukocytosis, thrombocytopenia (if DIC concern)
- Hepatic panel (AST, ALT): liver dysfunction is a late complication
- PT/INR, PTT: coagulopathy screening if heat stroke suspected
- Lactate: assess tissue perfusion
- Urine drug screen: if substance use suspected [1]
Expected abnormalities
- Hemoconcentration (elevated hematocrit from dehydration)
- Mild hyponatremia or hypernatremia depending on water vs. salt depletion
- Elevated BUN/creatinine ratio (prerenal azotemia)
- Mildly elevated CK (if exertional component)
12. Imaging
- Imaging is generally not indicated for straightforward heat exhaustion [2]
- Chest X-ray: if respiratory distress, concern for aspiration, or pulmonary edema
- CT head: if altered mental status to rule out stroke or intracranial pathology
- CT abdomen/pelvis: rarely needed; consider if concern for bowel ischemia in severe cases
13. Special Tests
- Rectal temperature is the gold standard for core body temperature — oral, axillary, temporal, and tympanic measurements are unreliable in heat illness and may underestimate core temperature [2-3]
- Point-of-care glucose: immediate bedside test to rule out hypoglycemia
- Point-of-care electrolytes/iSTAT: rapid sodium assessment if hyponatremia suspected
- Wet bulb globe temperature (WBGT): environmental assessment tool combining temperature, humidity, wind speed, and solar radiation — used for activity modification and prevention [5]
- Heat index: less accurate than WBGT but useful if WBGT unavailable [5]
14. ECG
- Indicated if: syncope, chest pain, palpitations, cardiovascular risk factors, or prolonged recovery [2]
- Expected findings: sinus tachycardia (most common)
- Concerning patterns: ST changes (ischemia from demand), QTc prolongation (electrolyte abnormalities), arrhythmias
- Electrolyte derangements (hypokalemia, hyponatremia) can produce ECG changes — correlate with labs
- Rule out cardiac cause of collapse, especially if loss of consciousness occurred during (not after) exercise [2]
15. Assessment
Heat exhaustion is a moderate heat-related illness on the spectrum between mild heat illness (cramps, edema, syncope) and life-threatening heat stroke. [1-2]
Key clinical features
- Nonspecific symptoms: fatigue, weakness, headache, nausea, dizziness, thirst
- Core temperature 38°C–40°C (may be normal)
- Preserved mental status — the critical distinguishing feature from heat stroke
- Caused by fluid and electrolyte losses from sweating, leading to hypovolemia and circulatory insufficiency
Severity stratification
- Mild: ambulatory, tolerating oral fluids, rapid symptom improvement with passive cooling
- Severe: unable to ambulate, persistent vomiting, requiring IV fluids, borderline mental status changes (irritability, anxiety), temperature approaching 40°C — these patients are at risk for progression to heat stroke [1-3]
Complications to consider: [2]
- Progression to heat stroke
- Exertional rhabdomyolysis
- Acute kidney injury
- Electrolyte abnormalities (hyponatremia, hypokalemia)
- Hepatic dysfunction (rare, more common in heat stroke)
16. Treatment Plan
Initial stabilization: [1-3][5]
- Remove from heat — move to cool, shaded environment; remove excess clothing and equipment
- Position supine with legs elevated
- Cooling measures (simple/passive):
- Ice packs to neck, axillae, groin
- Cold towels or sheets
- Fanning with skin misting
- Forearm immersion in cool water
- Dousing skin with cool water
- Rehydration:
- Oral (mild cases): electrolyte solution or sports drinks; water with salty snacks
- IV (moderate-severe, vomiting, or unable to tolerate PO): normal saline bolus (1–2 L), titrate to clinical response
- Monitor mental status continuously — any deterioration mandates treatment as heat stroke
Severe heat exhaustion: [1][3]
- Active convective cooling (fans) + conductive cooling
- IV fluid resuscitation
- If diagnosis unclear between heat exhaustion and heat stroke → treat as heat stroke with aggressive cooling (ice-water immersion)
There is no role for: [4]
- Antipyretics (acetaminophen, ibuprofen)
- Dantrolene
- Diuretics
17. Disposition
Discharge criteria: [2]
- Complete symptom resolution within a few hours
- Normalization of vital signs (HR, BP, temperature)
- Ability to ambulate without recurrent symptoms
- Tolerating oral fluids
- Normal mental status
- Safe home environment with access to cooling
Admission/observation criteria: [2][8]
- Delayed recovery (symptoms persisting >2–3 hours despite treatment)
- Core temperature ≥104°F or any altered mental status → manage as heat stroke; generally requires hospitalization
- Laboratory abnormalities: AKI, CK ≥20,000 U/L, significant electrolyte derangements, liver dysfunction
- Concern for rhabdomyolysis requiring IV hydration
- Inability to tolerate oral fluids
- Significant comorbidities or elderly patients without safe discharge environment
Specialist consultation triggers
- Nephrology: AKI, severe rhabdomyolysis
- Critical care/ICU: heat stroke, multiorgan dysfunction
- Surgery: concern for compartment syndrome
- Cardiology: arrhythmia, cardiac syncope
18. Follow Up / Return Precautions
Follow-up timing: [2][9]
- Primary care follow-up within 24–48 hours for patients discharged from the ED
- Gradually reintegrate physical activities after 24–48 hours of rest following complete recovery
- For severe heat exhaustion or heat stroke, refrain from physical activity for at least 7 days after medical clearance, then gradually resume over 2–4 weeks [9]
Return precautions — instruct patients to return immediately for:
- Confusion, disorientation, or unusual behavior
- Inability to keep fluids down (persistent vomiting)
- Fever ≥104°F
- Dark or cola-colored urine
- Seizures or loss of consciousness
- Worsening weakness, dizziness, or chest pain
Patient counseling: [1-3][5]
- Acclimatize gradually before returning to heat exposure (1–2 hours/day of heat-exposed exertion over 7–14 days)
- Hydrate to thirst (drink-to-thirst approach); avoid overhydration (risk of hyponatremia)
- Wear lightweight, light-colored, loose-fitting clothing
- Avoid peak heat hours for strenuous activity
- Review medications with primary care provider for heat-sensitizing agents
- Prior heat illness is a risk factor for recurrence — maintain heightened awareness
- Sunburn impairs sweating for approximately 7 days [5]
References
1. Treatment and Prevention of Heat-Related Illness. — Sorensen C, Hess J. The New England Journal of Medicine. 2022.
2. Heat-Related Illnesses. — Gauer RL, McNutt R, Bryan K. American Family Physician. 2026.
3. Wilderness Medical Society Clinical Practice Guidelines for the Prevention and Treatment of Heat Illness: 2024 Update. — Eifling KP, Gaudio FG, Dumke C, et al. Wilderness & Environmental Medicine. 2024.
4. Heat and Cold Illness in Travelers. — Howard D. Backer and Luanne Freer CDC Yellow Book. 2025.
5. Prevention and Treatment of Heat Illness: Guidelines From the Wilderness Medical Society. — Nelson M. American Family Physician. 2025.
6. Occupational Heat-Related Illness. — Spector JT, Sack CS, Bonauto DK. The Journal of the American Medical Association. 2025.
7. Climate Change, Extreme Heat, and Health. — Bell ML, Gasparrini A, Benjamin GC. The New England Journal of Medicine. 2024.
8. ACSM Expert Consensus Statement on Exertional Heat Illness: Recognition, Management, and Return to Activity. — Roberts WO, Armstrong LE, Sawka MN, et al. Current Sports Medicine Reports. 2023.
9. Heat-Related Illnesses. — Gauer R, Meyers BK. American Family Physician. 2019.