Epidemiology of Heat Illness

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Epidemiology of Heat Illness
Robert Carter, PhD, MPH, FACSM MAJOR,
MS Deputy for Medical Science and
Technology Office of the Deputy for Medical
Systems 22 October 2008
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Overview

• What is EHI?
• Epidemiological Study Design
• Observational Studies
• Case Reports
• Epidemiology of EHI
• Military
• Are there long term consequences of EHI?
• Factors predisposing EHI

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Purpose

• Review the epidemiological evidence
• Military Exertional Heat Illness (EHI)
• Epidemiological Approach
• Observational Studies (incidence rates)
• Case Control Studies (risk factors)
• Long term follow-up after EHI

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Definitions

• Minor Heat Illnesses
• Heat cramps intense muscle spasms
• result from fluid and sodium deficits and occurs
  mostly in persons with lack of heat
  acclimatization.
• Heat syncope result from pooling of blood to the
  skin and extremities and occurs mostly in
  dehydrated and inactive persons with lack of heat
  acclimatization.

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Definitions

• Serious Heat Illnesses
• Heat Exhaustion, is a mild-to-moderate illness
  characterized by an inability to sustain cardiac
  output with moderate to high body temperatures.
• Heat Injury, is a moderate-to-severe illness
  characterized by organ injury with high body
  temperatures
• Heat Stroke is a severe illness characterized by
  severe central nervous system dysfunction with
  high body temperatures usually, but not always,
  gt40oC.

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Understanding EHI
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Epidemiology 101

• Observational studies provide unique aspects
• Generate hypothesis regarding medically related
  issues, incidence rates, risk factors, and
  behaviors
• Observational studies susceptible to confounding
  and bias
• Difficult to establish causal links between
  variables

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Epidemiology 101

• Incidence is the rate at which new events (i.e.,
  EHI) occur within a population at risk.
• 14 heat strokes within 1219 individuals
• X cases per 1,000 or 100,000 persons
• Person-time (i.e., person-exposures, AEs),
  address the problem of competing risks
• Prevalence existing cases / population at risk
• Issue in military EHI? EHI Recovery?

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Epidemiology 101

• Case reports are not very helpful in estimating
  the extent of EHI within a population.
• Published case reports or media reporting of high
  profile athletes or Soldiers who develop an
  illness (i.e., heat stroke) can have a profound
  impact on perceptions, medical advice, and future
  behaviors

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Tracking EHI cases
Operational
Inpatient
Outpatient
Data Source (i.e., DMED, TAIHOD)
?

• Potential Issues
• Data Reporting
• Define (MSMR, 2008)
• Under-reporting
• Interview Bias
• Recall Bias

Aid Station
Garrison
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Sources of EHI cases
Fort Campbell
Fort Bragg
Fort Benning
Fort Bliss
Fort Sill
Fort Stewart
Fort Polk
Fort Hood
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EHI Hospitalizations
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Reportable EHI Data

• In 2006, heat stroke hospitalizations incidence
  rates increased 7-8 fold during past 20 years
  (OTSG, 04/2007).
• 220 heat stroke cases, 57 hospitalized, 163
  outpatients and 2 deaths (Heat Injury Prevention
  Policy, 2007) in garrison and operational
  environments.
• In 2007, 1,840 reported heat injuries of which
  259 were heat stroke and 3 heat related deaths 1
  in Iraq, 1 during basic training, and 1 on a land
  navigation course (ALARACT Message 122/2008, May
  2008)
• In 2007, 329 cases of heat stroke and 1853
  incident cases of heat exhaustion active all U.S.
  Military members (MSMR, March 2008)

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Reportable EHI Data
Active Heat Exhaustion Cases
MSMR, 2008
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Reportable EHI Data

• ICD-9-CM 992.0
• Hosp. Record or reportable event record
• One heat injury per service member / 6 mo.
• DMSS limited to data reported from fixed medical
  facilities
• Medical encounters for heat injuries that
  occurred during deployment are not included
  (unless resulted in MEDEVAC outside theater.

Active Duty Heat Stroke
DMSS, 2008
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Other EHI Studies

• Military epidemiological studies
• Focus on specific bases for relatively brief
  periods and with relatively small populations
• 12 of exercise related deaths EHI (Gardner,
  1999).
• Smalley et al. reported that 51 cases (1.3 per
  1,000 persons) of EHI occurred among basic
  trainees at Lackland Air Force Base (LAFB) in
  1999. 7 heat stroke deaths 1956 to 1999 (Mil Med,
  2004).
• 217,000 Marine recruits1982-1991 (Kark, ASEM,
  1996)
• 1454 individuals suffered EHI which is the
  believed to be among the highest incidence rates
  (67 per 1000 persons) among the military services
  (Kark, ASEM, 1996).

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Heat Stroke Hospitalizations
Carter et al., J Sports Rehab 2007
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Heat Stroke Hospitalizations
Carter et al., J Sports Rehab 2007
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Summary EHI Incidence

• Heat stroke hospitalizations incidence rates
  increased significantly during past 20 years.
• Preventable heat stroke and heat related deaths
  continue to occur in both operational and
  training environments.
• Although, EHI are tracked very well in the
  military, sources of error (i.e., bias,
  definitions, data reporting) and large diverse
  populations can have an impact on incidence
  rates.

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Long term consequences EHI?
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Long term consequences EHI?

• Population based study examined association
  between EHI and serious arrhythmias in military
  cadre (1979-1990)
• EHI accounted for more than half the episodes of
  unexplained life threatening arrhythmia (LTA)
  (RR928)
• LTA 3/216 persons w/EHI 4/267,000 w/o EHI
• Suggesting elevated risk of CV injury from EHI.

Kark et al, Circulation, 1997
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Long term consequences EHI?

• Marine Corps members who completed at least 6
  months of military service and suffered EHI
  treated as outpatients (N 872) or inpatients (N
  50) during basic training in 1979-1991
• Military retention rates were slightly lower for
  those who suffered EHI during basic training
• Outpatient EHI cases also had about 40 higher
  subsequent hospitalization rates in military
  hospitals than non-cases during their continued
  military service
• EHI cases had higher rates of subsequent
  hospitalization for EHI, but the number was too
  small (5 cases) to provide stable comparisons.

Phinney et al., MSSE, 2001
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Long term consequences EHI?
Wallace et al., Environ Res, 2007
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Long term consequences EHI?
Wallace et al., Environ Res, 2007
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Summary consequences of EHI

• Military retention rates were slightly lower
  for those who suffered EHI during basic training.
  
• Outpatient EHI cases also had about 40 higher
  subsequent hospitalization rates in military
  hospitals
• HI cases experience an increase in risk of all
  cause deaths.
• Possible biological connection.
• Tissue damage to heart, liver, kidneys, etc.
• Possibility of confounding factors.
• Post-military occupation?
• Post-military life-style behavior?

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Factors serious EHI
Carter et al., GSSI, 2007
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Sources of Epidemiology Data
On Point for MRMC
G3/5/7, G8, OCLL
OSD(HA), DHP, DDRE, OSD (C), DARPA
ASA-(FMC)
ASBREM
OTSG, MEDCOM
TRADOC Requirements
Congress