Exertional Heat Illness

1. Exertional Heat Illness John W. Gardner, MD, DrPH COL(ret), MC, FS, US Army Uniformed Services University of the Health Sciences Bethesda, MD

2. Less than 20% of energy expended during exercise is converted to mechanical energy The remainder is released as HEAT

3. Heat Dissipation • Heat must be dissipated or the body temperature rises • Rise in body temperature stimulates thermoregulatory mechanisms (in proportion to amount of rise) • Heat is dissipated primarily at the SKIN (and some through respiration)

4. Body Cooling Mechanisms • Conduction • Convection • Radiation • Evaporation

5. Body Cooling Mechanisms • Efficiency of body cooling depends upon the differential between skin and environmental temperatures • When there is no gradient between skin and environmental temperatures, the only mechanism for heat dissipation is through evaporation • In high humidity, evaporation is also ineffective (dripping sweat does not cool, but simply induces further dehydration)

6. Hydration Requirements • Maximal sweating is 2-3 liters/hour • GI water absorption during exercise is limited to about 1.5 liters/hour • Maximal sweat rates cannot be maintained indefinitely, as dehydration always progresses even when drinking maximally

7. Estimated Distribution of Cardiac Output

8. Exertional Heat Illness • The combination of dehydration, circulatory demands, and metabolic processes induce tissue injury & organ dysfunction • The heart must work harder to meet circulatory demands • Redistribution of blood flow may compromise vital organs: • bloody diarrhea in marathoners? • acute renal failure? • encephalopathy? • Acidosis and electrolyte imbalance may disrupt other metabolic processes or induce organ dysfunction • High temperature may alter metabolic rates and induce organ dysfunction • Inflammatory processes initiated? (release of endotoxin through gut compromise?)

9. THE SPECTRUM OF EXERTIONAL HEAT ILLNESS Heat Exhaustion Hyperthermia Dehydration Nephropathy Cell Lysis Encephalopathy Shock Heat Injury Renal Failure Rhabdomyolysis Heatstroke Moderate Severe

10. Severe exertional heat illness: can occur in cool weather can occur without high body temperature Mental status change may reflect severe illness Vital signs & Laboratory values must be closely monitored early rapid cooling essential Dehydration & Acidosis early & aggressive IV therapy Sickle Cell Trait Patients have higher risk of death Key Points

11. The WBGT Indextakes into account air temperature, humidity, radiant heat, and air movement. Wet-Bulb Globe Temperature Index W = Aspirated Wet-Bulb Temperature G = Matte Black Globe Temperature D = Dry-Bulb Temperature WBGT Index = 0.7*W + 0.2*G + 0.1*D

12. PARRIS ISLAND MARINE CORPS RECRUIT TRAINING DEPOT, SC

13. PARRIS ISLAND MARINE CORPS RECRUIT TRAINING DEPOT, SC

14. PARRIS ISLAND MARINE CORPS RECRUIT TRAINING DEPOT, SC

15. Acclimatization • Thermoregulatory mechanisms initiate at lower levels of elevated temperature • Sweating begins sooner and in higher volume • Sweat has much lower sodium content • Blood volume and cardiac capacity expand, with more efficient redistribution of blood flow • In well-conditioned individuals most of acclimatization is accomplished in 3-5 days

16. PARRIS ISLAND MARINE CORPS RECRUIT TRAINING DEPOT, SC June 3, 1991

17. PARRIS ISLAND MARINE CORPS RECRUIT TRAINING DEPOT, SC

18. PARRIS ISLAND MARINE CORPS RECRUIT TRAINING DEPOT, SC

19. PARRIS ISLAND MARINE CORPS RECRUIT TRAINING DEPOT, SC

20. Military and ACSM Flag Conditions

21. Body Mass Index (BMI)on Arrival by Case and Control Status,Male Marine Recruits, MCRD-PI, 1988-1992 BMI CATEGORY OR (95% CI) CASES CONTROLS <22 kg/m2 22-<26 kg/m2 26+ kg/m2 Total 62 156 172 390 449 659 340 1448 1.0 1.7 (1.3-2.4) 3.6 (2.5-5.0)

22. 1.5 mile PFT1 Run Time by Case and Control Status,Male Marine Recruits, MCRD-PI, 1988-1992 Run Time CATEGORY OR (95% CI) CASES CONTROLS <10 minutes 10-<12 minutes 12+ minutes Total 28 156 193 377 204 884 329 1417 1.0 1.5 (0.9-2.4) 5.6 (3.4-9.1)

23. Odds Ratios Combining PFT1 Run Timeand BMI Category for Exertional Heat Illness,Male Marine Recruits, MCRD-PI, 1988-1992 1.5 Mile PFT1 Run Time BMI CATEGORY <10 minutes 10-<12 minutes 12+ minutes 1.0 1.6 3.7 1.5 2.0 3.3 3.5 8.5 8.8 <22 kg/m2 22-<26 kg/m2 26+ kg/m2

24. PARRIS ISLAND MARINE CORPS RECRUIT TRAINING DEPOT, SC Percent of Population Producing Heat Illness Cases 35% 65% 18% 17% 47% 18%

25. PARRIS ISLAND MARINE CORPS RECRUIT TRAINING DEPOT, SC Percent of Cases with Neurologic Heat Stroke

26. Does Rectal Temperature ≥106°F Predict Heat Stroke (Delirious or worse)? RectalTemperature Delirious + Total Yes No ≥ 106 °F 35 36 71 < 106 °F 34 363 397 Total 69 399 468 Sensitivity = 35 / 69 = 51% Specificity = 363 / 399 = 91% Predictive Value (+) = 35 / 71 = 49%

27. Risk of Exercise-Related Threatened or Completed Sudden Cardiac DeathRecruits, Parris Island, SC 1979-90 [95% confidence limits] Cases (deaths) Recruits/Cadre Population Incidence 137 1,800 267,500 7 (2) 0 4(4) 5.1 % [2.5-10%] — 0.0015% [0-0.004%] w/ Heat Stroke w/ Other EHI w/o EHI

28. Types of Exercise-related Recruit Deaths (96 military recruit deaths, 1979-90)

29. Percent of Exercise-related Recruit DeathsExposed to Environmental Heat Stress* (N = 96) (N = 20) (N = 25) (N = 45) (N = 6) * Same or Prior Day WBGT > 75°F

30. Non-Specific Symptoms: weakness, thirst, headache, cramps, poor concentration Progressive Orthostatic Symptoms: faintness, dizziness, wobbly, visual symptoms, collapse Clinical Assessment of Heat Illness

31. Exertional Syncope: brief loss of consciousness Orthostatic Hypotension: positive tilt tests sustained hypotension Shock/Cardiac Arrhythmia Metabolic Complications Clinical Assessment of Heat Illness

32. 8 — Normal - alert, oriented, cooperative 7 — Drowsy - lethargic, slow mentation 6— Confused & Appropriate - cooperative 5 — Confused & Inappropriate - disoriented 4 — Delirious - disoriented, agitated 3 — Obtunded - minimal mental response 2 — Light coma - reflex responses 1 — Deep coma - no reflex responses Scale of Encephalopathy in Heat Illness

33. RECOMMENDED LABORATORY TESTS IN EXERTIONAL HEAT ILLNESS • CBC:Hgb, Hct, WBC, Platelet Count • Urinalysis:S.G., pH, evidence of myoglobin • Chemistries:Na, K, Cl, HCO3, Glu, BUN, Creatinine, • CK[CPK], AST[SGOT], Uric Acid, LDH, ALT[SGPT] • If severe:ABG; Ca, Phos; PT, PTT, FSP, Fibrinogen

34. Get a Rectal Temperature Assess Mental Status Immediate Cooling with Ice Water Rapid Rehydration Monitor Vital Signs and Serum Chemistries Limit Duty after Treatment Immediate Management ofHeat Illness Casualties

35. Predictors of HospitalizationParris Island Recruits, 1988-92 Clinical VariableScore Maximum body temperature 106+ F 1 Min systolic blood pressure <100 mmHg 1 Disorientation duration 1-29 minutes 1 30+ minutes 2 Minimum serum potassium < 3.7 mEq/L 1 Maximum serum creatinine 1.8+ mg/dL 1 Maximum serum LDH 400+ U/mL 1 A score of 2 or more may require hospitalization.

36. Schedule Training / Exercises During Cool Hours Consider Accumulative Effects of Heat Exposure Minimize Heavy or Retentive Clothing Minimize Unnecessary Strenuous Exercise (Running ) Tailor exercise to physical and medical condition of participants Prevention of Exertional Heat Illness

37. Maintain Good Hydration Provide Shade, Water and Rest Periods Have Medical Personnel On-Site During Strenuous Exercise Prevention of Exertional Heat Illness

38. Prevention of Exertional Heat Illness Forget : NO PAIN, NO GAIN Remember : TRAIN, NOT PAIN

39. The notion that courage and esprit de corps can somehow defeat the principles of physiology is not only wrong but dangerously wrong. — Sir Roger Bannister (1989)

40. Location with Access to Shade and Water Showers for Wet-Down During Run/Between Events Clothing: T-shirts vs. Full Combat Gear Cover: None vs. Helmet Run: Formation vs. Individual (non-competitive ) Hydration Status Checked by Urine Color Example: Combat Confidence Course Modifications to Reduce Risk for Exertional Heat Illness

41. Exertional Heat Illness Outbreaks, Ft. Bragg, 2000-2001 9/22/00 EFMB March (12 miles) 6 hospitalizations 11/4/00 Perimeter Challenge (60 miles) 5 hospitalizations 4/12/01 EFMB March (12 miles) 9 hospitalizations 6/14/01 Army Birthday Run (10 miles) 6 hospitalizations 7/20/01 EIB March (12 miles) 19 hospitalizations 8/9/01 Corps Birthday Run (4 miles) 4 hospitalizations

42. Exertional Heat Illness Ft. Bragg, 2000-2001 • STUPIDITY: • Death related to 6 mile run in new transfer at pace faster than his 2-mile PFT run (coronary heart disease) • Three heat stroke cases related to 8 mile run in new transfers on their first day of arrival • Heat stroke related to chemical gear at Black Flag conditions • Permanent mental disability related to recurrent heat injury when on medical restriction after release from hospital for heat stroke - “commander insisted on 100% field participation” • Numerous heat casualties related to use of ephedra-containing ‘nutritional supplements’

43. Career Implications of Heat Stroke Diagnosis • Mandatory MEB with 3-month P3 medical restriction, followed by 6-12 month P2 medical restriction • Airborne operations restricted • Pilots grounded for a minimum of 3 months, then can request waiver; recurrent episode - waiver not to be recommended • Single episode of heat stroke may preclude flight school entry • Medical restrictions usually make soldiers non-deployable for a prolonged period of time

44. Diagnosis of Heat Stroke • Recommendations: • In the setting of heat exposure or exertion, any of the following (elevated body temperature not required): • persistent (at least 10-20 minutes) disorientation, confusion, or combativeness • delirium or obtundation beyond 3-5 minutes • coma – beyond the three minutes of a simple faint • amnesia beyond 10-15 minutes surrounding the event • elevated CK>700, AST>60, ALT>60, or LDH>400 at 24 hours post-event (particularly if rising after initial values, or if associated with myoglobinuria)

45. Diagnosis of Heat Exhaustion • Recommendations: • In the setting of heat exposure or exertion, all patients not meeting heat stroke criteria who experience exercise-related collapse/illness and require medical intervention (e.g., more than two liters of IV fluids) and/or more than one hour to recover (unable to return to work at light-duty within one hour) • Includes exertional dehydration, cramps, syncope • These patients should all be evaluated by an experienced clinician, preferably in an Emergency Room setting and with laboratory workup

46. Field Management of Exertional Heat Illness • Mild patients - Alert with appropriate behavior, near-normal and rapidly stabilizing vital signs, and able to drink fluids • Care in the field for up to one hour with up to 2 liters of fluids (NS if IV) • Rest in the shade, cooling, rehydration, frequent vital signs and mental status assessment (every 5-10 min) • Upon realization that recovery will require more than this, or if the patient is not improving, then evacuation to an Emergency Room should be quickly arranged • No patient leaves medical care until providing urine

47. Field Management of Exertional Heat Illness • Moderate/Severe patients - Mental status changes, amnesia, syncope, seizure, unable to drink fluids, unstable vital signs, or temp >104 • Care in the field includes rest in the shade, cooling, rehydration, frequent vital signs and mental status assessment (every 5-10 min), while quickly arranging evacuation to an Emergency Room • These patients require immediate evaluation by an experienced clinician, and laboratory tests (CBC, electrolytes, creatinine, liver enzymes, CK, urinalysis) • No patient leaves medical care until providing urine

48. ER Management of Exertional Heat Illness • ACLS procedures as needed, to include aggressive cooling and rehydration • Stop aggressive cooling at 102 to avoid hypothermia • Repeat vital signs and mental status assessments every 5-10 minutes until stable and temp <100 • Lab assessment is usually required, with follow-up the next day in all but very mild patients • All ER and hospitalized patients to be followed-up in the Preventive Medicine clinic for reporting, medical restrictions, MEB referral, and review of need for further medical management or follow-up

49. Disposition of Exertional Heat Illness Cases • Mildly ill patients who appear to be fully recovered in the ER and have no laboratory abnormalities may return to light duty the next day; maximal exercise should be avoided for several days • Patients not fully recovered or who have laboratory abnormalities require next day follow-up by an experienced clinician, with laboratory evaluation • All patients remain on quarters, convalescent leave, or P4 medical restriction until all symptoms have completely resolved and laboratory tests are normal • When fully recovered, the patient may begin exercise at own pace, building slowly up to maximal efforts

50. MEB for Heat Stroke Cases • All heat stroke or rhabdomyolysis cases require MEB • If no complications, MEB will provide P3 restriction for 3 months which limits vigorous exercise to periods no longer than 15 minutes, no maximal efforts, no PFT, and no chemical gear or significant heat exposure • If after 3 months there has been no indication of heat intolerance, the restriction is changed to P2 through the next hot season, which allows normal work but restricts significant heat exposure and maximal exertion • If no heat intolerance, return to full duty after the hot season; if signs of heat intolerance, refer to PEB