Heat-Related Emergencies

Heat Production

• Basal metabolism: Normal cellular function generates heat
• Muscle activity (exercise): Largest source of heat in active individual
• Shivering: Involuntary muscle contractions that generate heat in cold

Heat Loss Mechanisms

• Radiation: Emission of infrared radiation from body to environment. Largest source of heat loss at rest (no exertion).
• Conduction: Direct contact — heat transfers to cooler surface (ground, water, cold object)
• Convection: Moving air or water carries heat away. Wind increases heat loss. Water is much more effective than air.
• Evaporation: Sweating and sweat evaporation. MOST effective during exercise. High humidity impairs evaporation (limits cooling).
• Respiration: Minor route — exhaled air carries heat away

Thermoregulation Center

• Hypothalamus: Acts like thermostat. Detects core temperature changes.
• Responses to high temperature:
  • Cutaneous vasodilation: blood vessels near skin dilate, increasing heat radiation/conduction
  • Sweating: produces moisture for evaporative cooling (most effective in heat/exercise)

Risk Factors for Heat Illness

• Environmental: High temperature, high humidity (impairs sweating/evaporation), direct sun, no breeze
• Exertion: High-intensity activity generates metabolic heat
• Dehydration: Impairs sweating; less fluid for evaporation
• Medications:
  • Anticholinergics (dry mouth meds) — decrease sweating
  • Diuretics (water pills) — cause fluid loss
  • Antipsychotics — impair heat regulation
  • Beta-blockers — impair exercise tolerance
• Alcohol: Impairs judgment; causes vasodilation (more heat loss at rest) and dehydration
• Obesity: Excess insulation; less surface area for heat loss relative to mass
• Age extremes: Very young and elderly have less effective thermoregulation
• Prior heat illness: Increases risk of recurrence; may have permanent loss of heat-tolerance

Heat Cramps

What: Painful involuntary muscle spasms, usually legs and abdomen

• When: During or immediately after exercise in heat
• Cause: Loss of salt (sodium) and fluid combined with muscle fatigue
• Signs: Muscle pain/spasms in legs, abdomen. Patient is usually alert, sweating, normal temp.
• Treatment:
  • Stop activity, rest in cool environment
  • Oral electrolyte solution (sports drink, salty broth) — salt AND fluid
  • Gentle stretching of affected muscles
  • Usually self-resolving within 15-20 minutes

Heat Syncope (Fainting)

• What: Brief loss of consciousness from reduced blood flow to brain
• Mechanism: Vasodilation causes blood pooling in extremities; blood pressure drops
• When: During or shortly after standing in heat
• Treatment:
  • Lie supine (flat on back)
  • Move to cool environment
  • Oral fluids if conscious
  • Usually benign; patient recovers quickly

Heat Exhaustion

What: Systemic response to heat; body is struggling but still functioning. Core temp usually <40°C.

• Signs:
  • Heavy sweating (may see sweat-soaked clothing)
  • Weakness, dizziness, headache
  • Nausea/vomiting
  • Tachycardia (rapid pulse)
  • Cool/pale/clammy skin (from evaporating sweat and relative hypovolemia — not vasoconstriction; in heat the skin vessels are actually dilated)
• Treatment:
  • Move to cool shade or air-conditioned environment
  • Remove excess clothing
  • Active cooling: apply cool/cold water, fan, ice packs if available
  • Oral rehydration: cool water or electrolyte solution if patient can drink
  • Rest
  • Monitor — patient should improve within 30 minutes
  • If no improvement or deterioration, evacuate (may be heat stroke)

Heat Stroke: The Emergency

Definition: Core temperature >40°C (104°F) WITH altered mental status. This is a medical emergency.

• Two types:
  • Classic heat stroke: Elderly, sedentary, prolonged exposure to hot environment. Often HOT DRY SKIN (sweating mechanism has failed).
  • Exertional heat stroke: Young healthy athletes during intense exercise. May STILL BE SWEATING despite high core temp.
• Signs of heat stroke:
  • Core temp >40°C (104°F) if measured
  • Altered mental status: confusion, combative, irrational behavior, unconsciousness
  • Hot skin (dry or wet depending on type)
  • Tachycardia, tachypnea
  • May have seizures
• Why it's an emergency: High core temp damages brain, heart, kidneys, liver. Death and organ failure can occur rapidly.

Recognition

• Patient with high heat exposure + altered mental status + hot skin = HEAT STROKE until proven otherwise
• May be unconscious (severe) or just confused/combative (milder but still heat stroke)
• Don't wait for thermometer confirmation if clinical picture fits

Cooling Methods: Ranked by Effectiveness

1. Cold Water Immersion (MOST EFFECTIVE)

• Fill tub, stream, or large container with cool/cold water
• Immerse patient up to neck (head out of water to manage airway)
• Can reduce core temp 0.1-0.2°C per minute
• Monitor for seizures, vomiting
• Only issue: Not always available in wilderness, but if you're near river/lake, USE IT

2. Ice Packs to High Blood Flow Areas

• Apply to neck, armpits (axillae), groin where major blood vessels near skin
• Ice against skin may cause frostbite if directly applied — wrap in cloth
• Effective but slower than immersion

3. Evaporative Cooling

• Wet the patient's skin with cool water
• Use fan (or wave cloth) to create air movement (convection)
• Evaporation of water carries heat away
• Effective but slower than immersion

Cooling Endpoint

• Stop cooling when core temp reaches 39°C (102.2°F)
• If you continue cooling to lower temps, risk of overshoot (temp drops too much) and hypothermia
• If no thermometer, don't wait for one — cool until mental status clearly improves and/or shivering begins

Supportive Care During Cooling

• Airway: Position patient for airway safety, monitor breathing
• Recovery position: If unconscious and no spine concern, position on side to prevent aspiration if vomiting
• Monitor for complications:
  • Seizures: can occur with heat stroke; don't restrain, protect from injury
  • Vomiting: aspiration risk; position to allow drainage
• Do NOT give oral fluids: Patient may not be able to swallow safely

Evacuation

• All heat stroke = URGENT evacuation even if patient improves with cooling
• Complications can develop hours after cooling
• Hospital evaluation and monitoring essential
• Call for helicopter if available; patient needs advanced care

What is Hyponatremia?

• Definition: Serum sodium <135 mEq/L (normal ~140)
• Cause: Excessive plain water intake without sodium replacement, especially during prolonged endurance activity
• Common in: Marathon runners, ultramarathons, very long hiking days

How It Happens

• Athlete/hiker drinks large quantities of plain water (no electrolytes)
• Fluid intake exceeds both sweat loss and urine output
• Blood sodium becomes diluted
• Water moves into cells (osmotic effect), causing cerebral edema (brain swelling)

Signs & Symptoms

• Nausea, headache, dizziness
• Confusion, disorientation
• Seizures in severe cases
• MIMICS heat exhaustion: Weakness, confusion, feels like patient overheated

Field Treatment

• STOP water intake: Do NOT give more plain water
• Restrict fluid: Limit total fluids
• Sodium supplementation — ALERT patients ONLY: Only if the patient is fully alert and can swallow and protect their own airway, encourage salty foods (nuts, broth, pretzels, beef jerky). Any confusion, vomiting, or seizure means NOTHING by mouth and urgent evacuation.
• Evacuation: Symptomatic hyponatremia requires hospital evaluation. Hospital care may include 3% hypertonic saline — this is a hospital treatment, not a field intervention.

Prevention

• Drink to thirst, not beyond — the primary prevention of exercise-associated hyponatremia is avoiding overdrinking. Sports drinks are hypotonic and do NOT reliably prevent hyponatremia if overconsumed.
• During prolonged exertion over several hours, salty snacks and electrolyte drinks help replace sodium lost in sweat — but they are not protective against drinking too much fluid.

Heat Acclimatization

• What: Adaptation of body to heat exposure over time
• Timeline: Takes 10-14 days of progressively increasing activity in heat
• Adaptations achieved:
  • Earlier sweating and more profuse sweating
  • Decreased sweat sodium (conserves salts)
  • Increased plasma volume (more blood = better cooling efficiency)
  • Better cardiac output
  • Lower core temperature during exercise
• Bottom line: Acclimatized individuals tolerate heat much better. Plan heat exposure gradually if going to hot environment.

WBGT: Wet Bulb Globe Temperature

• What: Most accurate index of heat stress, accounting for temperature, humidity, radiant heat, and wind
• More accurate than air temperature alone because: Humidity greatly affects evaporative cooling ability
• Uses:
  • Sports/military establish activity modification based on WBGT thresholds
  • Typical threshold: suspend strenuous activity if WBGT >32°C (90°F)
  • Work-rest ratios: at high WBGT, require frequent breaks and hydration

Hydration Guidelines

• Drink to thirst, not beyond: The primary rule — avoiding overdrinking is what prevents hyponatremia
• Pre-hydration: Start activity well-hydrated
• For prolonged exertion over several hours: Salty snacks and electrolyte drinks help replace sodium lost in sweat, but do NOT protect against overdrinking
• Avoid excessive hydration: More water than needed increases hyponatremia risk

Clothing & Environmental

• Clothing: Light color (reflects heat), loose fit (air circulation), moisture-wicking material
• Sun protection: Hat, sunscreen to reduce radiant heat absorption
• Activity timing: Avoid peak sun hours (10am-4pm) if possible
• Buddy system: Watch partners for early signs of heat illness (often can't recognize own symptoms)