Heat-Related Emergencies

Heat production

• Basal metabolism: ordinary cell work makes heat
• Muscle activity (exercise): the biggest heat source in someone who's active
• Shivering: involuntary muscle contractions that make heat in the cold

How the body loses heat

• Radiation: the body gives off infrared heat to its surroundings. It's the largest route of heat loss at rest, with no exertion.
• Conduction: direct contact. Heat moves into a cooler surface, like the ground, water, or a cold object.
• Convection: moving air or water carries heat away. Wind speeds it up, and water does it much better than air.
• Evaporation: sweat evaporating off the skin. It's the most effective route during exercise. High humidity slows evaporation, which limits cooling.
• Respiration: a minor route. Exhaled air carries some heat out.

Thermoregulation center

• Hypothalamus: works like a thermostat. It senses changes in core temperature.
• Responses to high temperature:
  • Cutaneous vasodilation: vessels near the skin open up, so more heat leaves by radiation and conduction
  • Sweating: puts moisture on the skin for evaporative cooling (the most effective route in heat and exercise)

Risk factors for heat illness

• Environmental: high temperature, high humidity (which slows sweat evaporation), direct sun, no breeze
• Exertion: hard activity makes metabolic heat
• Dehydration: impairs sweating, so there's less fluid to evaporate
• Medications:
  • Anticholinergics (the kind that cause dry mouth): reduce sweating
  • Diuretics (water pills): cause fluid loss
  • Antipsychotics: interfere with heat regulation
  • Beta-blockers: reduce exercise tolerance
• Alcohol: impairs judgment, and causes vasodilation (more heat loss at rest) and dehydration
• Obesity: extra insulation, and less surface area for heat loss relative to body mass
• Age extremes: the very young and the elderly regulate heat less well
• Prior heat illness: raises the risk of it happening again, and may permanently lower heat tolerance

Heat cramps

What: painful, involuntary muscle spasms, usually in the legs and abdomen

• When: during or right after exercise in the heat
• Cause: lost salt (sodium) and fluid, plus tired muscles
• Signs: muscle pain and spasms in the legs or abdomen. The patient is usually alert and sweating, with a normal temperature.
• Treatment:
  • Stop the activity and rest somewhere cool
  • Oral electrolyte drink (sports drink, salty broth). They need salt and fluid.
  • Gently stretch the cramping muscles
  • Usually settles on its own in 15 to 20 minutes

Heat syncope (fainting)

• What: a brief loss of consciousness from reduced blood flow to the brain
• Mechanism: vasodilation lets blood pool in the arms and legs, and blood pressure drops
• When: during or shortly after standing in the heat
• Treatment:
  • Lay them flat on their back (supine)
  • Move them somewhere cool
  • Oral fluids once they're fully alert and swallowing normally
  • Usually benign. They recover quickly.

Heat exhaustion

What: the whole body straining against the heat. It's still keeping up, and mental status is normal.

• Signs:
  • Heavy sweating (clothes may be soaked through)
  • Weakness, dizziness, headache
  • Nausea or vomiting
  • Fast pulse (tachycardia)
  • Mental status stays normal. Any confusion, odd behavior, or trouble staying awake means heat stroke. Cool them hard, right there.
  • Cool, pale, clammy skin. That's from evaporating sweat and relative hypovolemia, not vasoconstriction. In the heat, the skin's vessels are actually dilated.
• Treatment:
  • Move them to cool shade or air conditioning
  • Take off extra clothing
  • Cool them actively: cool or cold water, fanning, ice packs if you have them
  • Oral rehydration with cool water or an electrolyte drink, if they're fully alert and swallowing normally
  • Rest
  • Keep watching. They should improve within 30 minutes.
  • If they don't improve, or they get worse, evacuate. If their mental status changes at all, treat it as heat stroke and cool first.

Heat stroke: the emergency

Definition: altered mental status (confusion, odd behavior, combativeness, trouble staying awake, unresponsiveness) in someone who's been in the heat or working hard. It's a medical emergency, and you cool now. If you happen to have a way to measure core temperature, it's usually over 40°C (104°F). You go by the mental status, though, not the number.

• Two types:
  • Classic heat stroke: elderly, sedentary people after long exposure to a hot environment. Often hot, dry skin, because the sweating mechanism has failed.
  • Exertional heat stroke: young, healthy athletes during hard exercise. They may still be sweating despite a high core temp.
• Signs of heat stroke:
  • Altered mental status: confusion, combativeness, irrational behavior, unconsciousness. This is the sign that defines it.
  • Core temp usually over 40°C (104°F), if you happen to have a way to measure it
  • Hot skin, dry or wet depending on the type
  • Fast heart rate and fast breathing (tachycardia, tachypnea)
  • Possible seizures
• Why it's an emergency: a high core temp damages the brain, heart, kidneys, and liver. Organ failure and death can come fast.

Recognition

• Heat exposure + altered mental status + hot skin = heat stroke until proven otherwise
• They may be unconscious (severe) or just confused or combative (milder, but still heat stroke)
• If the picture fits, don't wait for a thermometer to confirm it

Cooling methods, ranked by effectiveness

1. Cold water immersion (most effective)

• Use a tub, a stream, or a large container of cool or cold water
• Immerse them up to the neck, with the head out of the water so you can manage the airway
• It can drop core temp 0.1 to 0.2°C per minute
• Watch for seizures and vomiting
• You won't always have it in the wilderness. If you're near a river or lake, use it.

2. Ice packs to high blood flow areas

• Put them on the neck, armpits (axillae), and groin, where large blood vessels run close to the skin
• Ice straight on skin can cause frostbite, so wrap it in cloth
• Effective, but slower than immersion

3. Evaporative cooling

• Wet their skin with cool water
• Fan them, or wave a cloth, to move air over the skin (convection)
• As the water evaporates, it carries heat away
• Effective, but slower than immersion

Cooling endpoint

• Cool until their mental status clearly improves or they start shivering. Then stop and keep watching them.
• Keep cooling past that and you can overshoot, dropping their temperature so far that they become hypothermic
• If you happen to have a way to measure core temperature, stop at 39°C (102.2°F). Don't wait for a thermometer to start.

Supportive care during cooling

• Airway: position them to keep the airway safe, and watch their breathing
• Recovery position: if they're unconscious and there's no spine concern, roll them onto their side so vomit drains out instead of being inhaled (aspiration)
• Watch for complications:
  • Seizures: they can happen with heat stroke. Don't restrain them; protect them from injury.
  • Vomiting: aspiration risk. Position them so it drains.
• No oral fluids. They may not be able to swallow safely.

Evacuation

• Every heat stroke is an emergent evacuation, even if they improve with cooling
• Complications can show up hours after cooling
• They need hospital evaluation and monitoring
• Call for a helicopter if one's available. They need advanced care.

What is hyponatremia?

• Definition: serum sodium under 135 mEq/L (normal is about 140)
• Cause: drinking too much plain water without replacing sodium, especially during long endurance efforts
• Common in: marathon and ultramarathon runners, and on very long hiking days

How it happens

• An athlete or hiker drinks large amounts of plain water, with no electrolytes
• Intake outpaces both sweat loss and urine output
• Blood sodium gets diluted
• Water moves into cells (an osmotic effect), and the brain swells (cerebral edema)

Signs and symptoms

• Nausea, headache, dizziness
• Confusion, disorientation
• Seizures in severe cases
• Looks like heat exhaustion: weakness, confusion, and it seems like they've overheated

Field treatment

• Stop the water. Do not give more plain water.
• Restrict fluid: limit total fluids
• Sodium, for alert patients only: only if they're fully alert, can swallow, and can protect their own airway, encourage salty food (nuts, broth, pretzels, beef jerky). Any confusion, vomiting, or seizure means nothing by mouth and emergent evacuation.
• Evacuation: symptomatic hyponatremia needs hospital evaluation. Hospital care may include 3% hypertonic saline. You won't be doing that on the trail.

Prevention

• Drink to thirst, not beyond. Avoiding overdrinking is the main way to prevent exercise-associated hyponatremia. Sports drinks are hypotonic, and they do not reliably prevent it if people drink too much of them.
• Over several hours of exertion, salty snacks and electrolyte drinks help replace the sodium lost in sweat. They don't protect against drinking too much fluid.

Heat acclimatization

• What: the body adapting to heat exposure over time
• Timeline: 10 to 14 days of gradually increasing activity in the heat
• What changes:
  • Sweating starts earlier, and there's more of it
  • Less sodium in sweat (the body holds on to its salt)
  • More plasma volume (more blood, better cooling)
  • Better cardiac output
  • Lower core temperature during exercise
• Plan for it. Acclimatized people handle heat much better. If you're heading somewhere hot, build up your heat exposure gradually.

WBGT: wet bulb globe temperature

• What: the most accurate index of heat stress. It accounts for temperature, humidity, radiant heat, and wind.
• Why it beats air temperature alone: humidity has a big effect on how well sweat can cool you
• Uses:
  • Sports and military programs set activity limits by WBGT thresholds
  • A typical threshold: suspend strenuous activity when WBGT is over 32°C (90°F)
  • Work-rest ratios: at high WBGT, build in frequent breaks and hydration

Hydration guidelines

• Drink to thirst, not beyond. This is the main rule. Not overdrinking is what prevents hyponatremia.
• Pre-hydration: start the activity well hydrated
• For exertion over several hours: salty snacks and electrolyte drinks help replace sodium lost in sweat, but they do not protect against overdrinking
• Avoid overhydrating: drinking more water than you need raises the risk of hyponatremia

Clothing and environment

• Clothing: light colors (they reflect heat), a loose fit (so air moves), moisture-wicking fabric
• Sun protection: a hat and sunscreen to cut radiant heat absorption
• Activity timing: avoid peak sun hours (10am to 4pm) when you can, and start early.
• Buddy system: watch your partners for early signs of heat illness. People often can't recognize their own symptoms.