Management of Heat Stress to Extend Operational Effectiveness for Firefighters Navy and Marine Corps

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Management of Heat Stress to Extend Operational
Effectiveness for Firefighters Navy and Marine
Corps Public Health Center ConferenceMarch 19,
2008Tom M. McLellan, Ph.D.
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Defence RD Canada
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DRDC Toronto
Mission Statement To ensure that Canadian
Defence and National Security capabilities
exploit the full potential of Human Effectiveness
ST.
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Heat Stress/Heat Strain
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Convective Heat Transfer
Solar Radiation
Evaporation From Skin
Evaporation From Airways
40C
Compensable Heat Stress
Work Against Gravity
Metabolic Heat Production
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Solar Radiation
NBC Shell
Evaporation From Airways
Evaporation From Skin
40C
Convective Heat Transfer
Uncompensable Heat Stress
Work Against Gravity
Metabolic Heat Production
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Protective Clothing and Uncompensable Heat Stress

• Exposure Time is affected by 3 factors
• Initial Core Temperature
• Hydration, heat acclimation, menstrual cycle
  phase
• Final Core Temperature
• Aerobic fitness, hydration, state of
  encapsulation
• Rate of Change in Core Temperature
• Clothing characteristics, environment, body
  composition, fluid replacement, rate of heat
  production (work and rest schedules) and cooling

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Rate of Change in Core Temperature

• What is the purpose of work and rest schedules
  while encapsulated?
• To slow the rate of heat production and thereby
  extend tolerance time
• To promote heat loss and lower core temperature
• To increase total work output
• HOWEVER

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250
hot, dry (40C, 15 RH) (WBGT 26.5ºC)
200
150
hot, humid (40C, 65 RH) (WBGT 36ºC)
Tolerance Time (min)
100
50
rest
light
moderate
heavy
0
0
50
100
150
200
250
300
350
400
450
Metabolic Rate (Wm-2)
10
250
hot, dry (40C, 15 RH) (WBGT 26.5ºC)
200
150
Tolerance Time (min)
100
UHS
CHS
50
rest
light
moderate
heavy
0
0
50
100
150
200
250
300
350
400
450
Metabolic Rate (Wm-2
)
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250
200
150
hot, humid (40C, 65 RH) (WBGT 36ºC)
Tolerance Time (min)
100
UHS
50
rest
light
moderate
heavy
0
0
50
100
150
200
250
300
350
400
450
Metabolic Rate (Wm-2
)
12
250
Im/Clo
200
150
Im/Clo
Tolerance Time (min)
100
50
rest
light
moderate
heavy
0
0
50
100
150
200
250
300
350
400
450
Metabolic Rate (Wm-2)
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Rate of Change in Core Temperature

• What is the purpose of work (while encapsulated)
  and rest (non-encapsulated) schedules?
• To slow the rate of heat production and thereby
  extend tolerance time
• To AGGRESSIVELY promote heat loss WITH COOLING
  and lower core temperature
• To increase total work output (PRODUCTIVITY)

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How Do You Generate Valid Work and Rest
Guidelines?

• STEP 1 Conduct laboratory trials in different
  environmental conditions and different metabolic
  rates with a representative subject population
• STEP 2 Conduct thermal manikin evaluations to
  obtain thermal resistance and water vapour
  permeability coefficients
• STEP 3 Use an accepted empirical model to predict
  the heat strain with the laboratory conditions
  tested
• STEP 4 Compare the predicted versus the
  physiological data for acceptability to generate
  INTERNAL VALIDITY
• STEP 5 Use the model to extend the predictions to
  environmental conditions EXTERNAL to the lab
  trials

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Work and Rest Guidelines for the Toronto Fire
ServiceSTEP 1
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Methods

• Thirty-seven subjects (33 men and 4 women) whose
  aerobic fitness and body composition were
  representative of the members of the Toronto Fire
  Service were selected from over 70 volunteers.
• Subjects were divided into 4 exercise groups
  defined as heavy, moderate, light and very light.
• Familiarization session and 3 experimental trials
  (25C, 30C, and 35C and 50 R.H.). Conditions
  were chosen to be representative of Toronto
  summer months.

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Dressing Procedures
HR, Tre, and Tsk
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Firefighting Protective Ensemble

• NFPA firefighting turnout gear
• Bunker pants and jacket
• Fire retardant gloves
• Nomex flash hood,
• Helmet, and
• Self-contained breathing apparatus and modified
  respirator (SCBA).
• Total weight 21.8kg

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Protocol Timeline for Heat-stress Trials 25C,
30C and 35, 50 R.H.
Exercise Phase
Recovery Phase
Repeat x3
Time (min)
0
20
23
27
30
60
57
53
50

30
0
10

Sitting
Group Workrate (H, M, L, or VL)
Group Workrate (H, M, L, or VL)
Exercise Intensity
Standing
Standing
2.5 kmh-1, 0 elevation
2.5 kmh-1, 0 elevation
Simulated Bottle Change
Water Administration (5 mLkg-1)
Weights
Nude
Dressed
Metabolic Measurement
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Exposure Time

• Tre 39.0ºC during work and 40.0ºC during rest
• HR 95 max
• Nausea
• Dizziness
• Exhaustion
• 4 hours

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Exposure Time
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THERMAL MANIKIN TESTINGSTEP 2
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MODEL PREDICTIONS and LABORATORY COMPARISONS FOR
INTERNAL VALIDITYSTEPS 3 and 4
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MODEL PREDICTIONS OF EXTERNAL CONDITIONSSTEP 5
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• Phase 2
• Hydration and Cooling Strategies

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Rectal Temperature 35C Recovery
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Heart Rate 35C Recovery

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Methods

• Fifteen males from the original subject pool
  performed a familiarization session and 6
  experimental sessions at 35ºC and 50 relative
  humidity

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Methods

• Hydration Trials
• Subjects received 0, 1/3, 2/3 or full fluid
  replacement throughout the heat stress exposure.
  Full fluid replacement was estimated from sweat
  rates (1.1 L/h) calculated during the
  familiarization trial where subjects received 5
  ml/kg fluid replacement every 30 minutes.
• Cooling Trials
• Subjects received either no cooling, a fan with
  fine spray or mist (mister) or forearm submersion
  in 18ºC water during a 20-min rest period. All
  cooling trials received full fluid replacement.

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Phase II Protocol Timeline35C, 50 R.H.
Repeat cycle until End-pt Criteria reached
(39.5C, 95 HR, Exhaustion)
Time (min)
20 23 27 30
50 0 10
20
0
Exercise
4.5 kmh-1, 0 elevation
4.5 kmh-1, 0 elevation
Cooling Strategy
Hand and Forearm Immersion Fan Mister Passive
Rest
Standing
2.5 kmh-1, 0 elevation
Rest Phase
Exercise Phase
Simulated Bottle Change
Weights
Nude
HYDRATION Fluid replacement every 30 minutes
starting prior to start of trial
Metabolic Measurement
Dressed
Blood Pressure Measurement
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• Phase 2
• Hydration Strategies

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Sweat rates and fluid consumption
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Core temperature response
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Significantly different from no fluid.
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Hydration Summary

• Hypohydration increases resting Tre and decreases
  Tre tolerated at exhaustion
• ? heat storage capacity and exposure time
• Fluid replacement increases heat storage capacity
  and exposure time
• cardiovascular stability
• heat sink effect of ingested fluid
• Choice of beverage (E-CHO vs water) to maximize
  volume of ingested fluid

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• Phase 2
• Cooling Strategies

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Role of Arteriovenous Anastomoses (AVAs) in Heat
Loss

• Location
• palmar side of fingers and plantar side of toes,
  palm, sole, ear and nose

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Role of Arteriovenous Anastomoses (AVAs) in Heat
Loss
from SD Livingstone et al Aviat Space Environ Med
60 166-71, 1989.
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Role of Arteriovenous Anastomoses (AVAs) in Heat
Loss

• Control
• during heat stress AVAs appear to be regulated
  centrally and therefore remain dilated until body
  temperature returns to normal levels.

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Role of Arteriovenous Anastomoses (AVAs) in Heat
Loss
from JR House J Def Sci 3 108-114, 1998.
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Cooling Strategies during Rehabilitation Periods
for the Toronto Fire Service
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Methods

• Cooling Trials
• Subjects received either no cooling, a fan with
  fine spray or mist (mister) or hand and forearm
  submersion in 18ºC water during a 20-min rest
  period. All cooling trials received full fluid
  replacement.

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Passive Cooling
Passive Cooling (PC) 35C and 50 R.H.
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Active Cooling Mister
Local Ambient Conditions 35C, 50 R.H. ? 31C,
73 R.H.

• ? in effective air velocity promoted greater
  evaporation and convective heat transfer.
• Flash evaporation ? local temp(4C), further ?
  convective heat transfer
• However, ambient VP ? from 2.8 kPA to 3.1 kPa
  thereby decreasing the effective evaporative
  potential of the environment

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Active Cooling
Hand and Forearm Submersion (FS) Water bath-
17.4 ? 0.04 C
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Heat Gain of Water Bath During 20 Minute Recovery
Periods
A Second recovery period gt first recovery period,
B 0-10 min gt10-20 min
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Rectal Temperature With Cooling
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Exposure Times with Cooling
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Work Times with Cooling
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Importance of Forearm with Hand Cooling

• (G.G. Giesbrecht et al. ASEM 78 561-7, 2007)
• Experimental Protocol
• 3 repeats of 20-min stepping exercise in
  firefighter ensemble at 40ºC and 40 RH followed
  by 20-min rest at 21ºC with most of ensemble
  removed.
• 5 conditions
• CONTROL (Passive rest at 21ºC with no hand or
  forearm immersion.)
• EXPERIMENTAL
• Hands vs Hands and Forearms
• 10ºC vs 20ºC Water Temperature

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Importance of Forearm with Hand Cooling
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Cooling Summary

• Either hand and forearm submersion or the use of
  the mister extends exposure and work times
  significantly. In the present format, cooling
  with forearm submersion is the best option.
• 18ºC water temperature for forearm cooling was
  selected to represent the temperature of the
  available water supply. If cooler temperatures
  are possible greater cooling of personnel would
  also be expected.
• As exposure time is extended subjects may end due
  to reasons other than thermal strain.

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SUMMARY

• Cooling vs Physiological Manipulations
• Magnitude of improvement is far greater with
  cooling (gt 100) vs hydration and heat
  acclimation ( 25)
• Combination of physiological factors (fitness and
  body composition) may approach benefits of some
  cooling systems
• Cooling strategies can convert uncompensable to
  compensable heat stress

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SUMMARY

• Exposure Time is affected by 3 factors
• Initial Core Temperature (Tre, initial)
• Final Core Temperature (Tre, final)
• Rate of Change in Core Temperature

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Rate of Change in Core Temperature

• Work and Rest Guidelines
• Purpose of Work and Rest (encapsulated vs
  non-encapsulated) Schedules
• Development of Guidelines
• Cooling and Hydration Strategies
• Forearm and hand immersion
• Benefits of fluid replacement

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Outcomes
Ontario Fire Marshall Summer 2006
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Methods of Hand and Forearm Immersion
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Thank You ?