Jeff Holmes M.D. Heat Emergencies

Mechanism of Heat Transfer 1.Convection 2.Radiation 3.Conduction 4.Evaporation **Physiologically, humans are tropical beings, better suited to losing heat than retaining it **

Convection Heat loss via movement of air or liquid across an object Rate depends upon density (eg. H20 vs. air) and velocity of moving substance Can combine with evaporation for extreme heat loss (wet clothes on a cold, windy day)

Radiation Radiation (55-65% heat loss) –Occurs when ambient temp <98ºF – modified by subcutaneous tissue and clothes –Highly pigmented skin protects from UV, but absorbs 20% more heat

Conduction Transfer of heat via direct contact (surface area dependent) Increases 5x in wet clothing Heat transfer 32X faster in cold water (vs. air) due to greater density Other –Granite: 4X conductivity of water

Evaporation Heat Loss from converting liquid to gas (perspiration, respiration) Default mechanism for thermal regulation if radiation, convection, conduction overwhelmed As humidity approaches 100%, evaporative heat loss minimized

Heat Illnesses Illnesses 2 nd to failed psychologic and physiologic adaptation mechanisms during heat stress Most frequent cause of environmentally related death in US in past decade (more than lightning, tornado, flood, hurricane, cold, or winter related fatalities) 60,000 individuals per year

‘Heat Load’ 1. Exogenous Heat -Radiation - Convection: relatively useless above 90°F and 35% humidity - Conduction - Evaporation: 25% of cooling in cooler settings and virtually 100% at higher environmental temperatures (mechanism impaired by high humidity) 2. Endogenous heat production Basal metabolism: 70 Kcal/hr, 70 kg man would experience 1ºC rise in temp if no mech to dissipate heat Exercise: >1000 Kcal/hr (without dissipation, vigorously exercising man increase 5 – 15Cº/ hr)

Heat Loss Mechanisms Radiation Convection Conduction Evaporation: Sweating!

Evaporation –Most important mode of cooling under extreme heat stress –600 Kcal heat energy lost per 1 L sweat evaporated –Well conditioned athlete can produce up to 3 L sweat per hour –Relative humidity > 85% causes evaporation to be ineffective –Well trained athletes can outsweat GI tract H 2 O absorption

Heat Cramps Heavy workload + heat stressed env. Hardest working muscles most often cramp (calves, abdominal muscles) Dilutional hyponatremia the cause, usually from hypotonic (water only) replacement Tx: Fluid and salt replacement

Heat Exhaustion ‘Hypovolemic Shock’ + salt depletion Dizziness, N/V, malaise, fever, weakness, +/- syncope, +/- hyperpyrexia NO MENTAL STATUS CHANGES One step away from Heat Stroke Not immediately life-threatening

Heat Exhaustion - Tx Volume/electrolyte replacement Generally do not need admission unless associated comorbidities or electrolyte disorders Heat exhaustion has the potential to progress to Heat Stroke

Heat Stroke Overwhelming of heat loss mechanisms Mortality 10-20% Hyperpyrexia + AMS (seizure, focal deficits, coma, hallucinations, hemiplegia) CNS dysfunction universal at temp. > 42ºC (107.6ºF) but no arbitrary core temp for CNS dysfunction Exertional heat stroke vs. classic heat stroke

Classic Heatstroke Elderly, poor (lack adequate A.C.), malnourished, chronic disease, substance addiction Physical effort NOT a primary determinant of excessive heat Develops over a period of days (often ample time for fluid and electrolyte imbalances to develop)

Exertional Heat Stroke Usually young, fit, unaccustomed to heat Sweating but producing heat faster than it can be shed

Heat Illnesses – Mgt Principles 1.Cool patient - Severity of Heat stroke directly related to degree of temperature elevation and duration 2. Correct fluid losses and electrolyte imbalances 3. Treat complications

Eliminate exess heat DO: 1.Place in cool environment 2.Remove clothing 3.Evaporative cooling with room temperature water!! (most effective to reduce mortality; do not use sheets) 4.Ice packs to groin and axillae as cooling adjunct DON’T: 1.Give antipyretics (NOT effective; possible additional liver injury with acetaminophen, aspirin may aggravate bleeding tendencies) 2.Alcohol sponge bath DEBATED: 1.Cool with Immersion (impractical; thoretically retards heat loss via vasoconstriction; gold standard for lowering temperature quickly and successfully done without complications in military)

Correct Fluid Loss and Electrolyte Imbalance 1.Calculate free water deficit 2.In hyponatremic patient, NS at rate cc/hr

Complications *Complications secondary to direct thermal injury and cardiovascular collapse** 1.Shivering/Seizures: Benzodiazepines 2.Hypotension (volume depletion, vasodilation, cardiac failure) -Cautious fluid replacement -Wedge pressure may be used 3.Rhabdomyolysis: -more often in exertional heat stroke 2 nd direct heat damage to muscle -If myoglobin present in urine, mannitol (0.25 g/kg) IV for osmotic diuresis -Alkalinization of urine with NaHCO3 4.Hypoglycemia 5.Acute Renal Failure 6.Acute Liver Failure -May not be evident for hours

Diagnostic Studies *Autopsies show multiple hemorrhages, congestion, cellular degeneration in most organs** To detect end organ sequelae CBC, Chem 7, Coags, Total CK, myoglobin, UA, tox screen, ECG, CXR Elevations in Liver enzymes may not be apparent for hours

Elderly Patients Heat as sole cause of fever is dx of exclusion Strongly consider sepsis

Heat Illness Final Points Forms of heat illness are a spectrum Heat cramps caused by sodium depletion, are benign and self-limited Heat exhaustion and heat syncope usually respond rapidly to cool environment and correction of fluid losses with water and electrolytes as needed Heat stroke serious illness with significant mortality

References Auerbach, PS. Wilderness Emergencies, 2001, 4 th ed. Hermann, L. et. al. Hypothermia and other Cold – related Emergencies, Dec 2003, Vol. 5, No. 12. Ungley, CC: Immersion Foot and immersion hand, Bulletin on War Medicine 4:61, 1943 Tilton, Buck. The Wilderness First Responder: A text for the recognition, treatment and prevention of wilderness emergencies. Globe Pequot Press, Tintinalli, JE. Emergency Medicine: A Comprehensive Study Guide. 2004, 4 th ed.