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Learning Outcomes
- • Describe the physiological mechanisms for controlling body
- • temperature
- • Identify sources of thermal stress in the aviation environment.
- • Describe the symptoms and treatment of heat exhaustion, heat stroke and dehydration.
- • Describe the symptoms and treatment for freezing and nonfreezing cold disorders, and hypothermia.
- • Describe the methods of preventing and protecting against thermal disorders and dehydration.
- • Describe the effect that thermal stress has on aircrew performance.
- • Outline the appropriate method of rescue following cold water immersion.
- • Provide advice on strategies to mitigate the effects of thermal stress on aircrew performance and survival.
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Revision: Charles’ Law?
- • Pressure constant, volume varies proportionally to temperature (V > T)
- • Volume constant, pressure varies proportioanlly to temperature (P > T)
- • Minimal physiological significance
- • May affect oxygen equipment Problem of conflicting multi-system demands…
- • Exercise in hot environment
- • Nutrients to exercising muscle
- • Peripheral vasodilatation for heat dissipation
- • Fluid depletion secondary to sweating
- • Peripheral resistance to maintain BP
- • Tissue perfusion in cold environment
- • Maintain tissue perfusion to all tissues,
- • Maintain core body temperature
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Thermal Control Effectors?
- -Vascular
- -Shivering
- -Sweating
- -Hormonal
- -Behaviour
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Wandering Reference Point?
- • Circadian variation
- • Peak in afternoon
- • Nadir early morning
- • Ingestion of food
- • Menstrual cycle
- • Illness/Fever – action of pyrogens
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Heat Balance in the Body?
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Heat gain Effectors?
- • Vascular:
- • Vasoconstriction and vasodilatation in skin
- • Shivering
- • Sweating
- • Piloerection
- • Basal metabolic rate (BMR)
- • Behavioural
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Vascular Response to Cold?
- • Vasoconstriction
- • “Core” contracts
- • Small gradient between skin and air
- • Less heat lost
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Vascular Response to Heat?
- • Vasodilatation
- • “Core” expands outwards
- • Large gradient between skin and air
- • More heat lost
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Shivering?
- • Amplification of physiological tremor
- • Makes use of inefficiency of muscle contraction to generate heat
- • Increases BMR x 2-5 fold
- • Fatiguing. Cannot be sustained.
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Sweating?
- • 3-4 million sweat glands in dermis secreting water and salt
- • Only method of heat loss when air temp >35ºC
- • Must evaporate - dripped sweat is ineffective for cooling
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Conduction
- • Transfer of heat within or between solid bodies or solid-fluid interfaces
- • Due to transfer of energy between adjacent molecules
- • Depends on relative temperature difference
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Radiation
- • Heat emitted as electromagnetic radiation
- • Can take place in a vacuum – no air required
- • Accounts for the Greenhouse Effect in cockpits
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Convection
- • Transfer of heat by molecular mass transfer within a fluid medium
- • Molecules retain heat but move through the medium
- • Driving force in humans is the temperature gradient between the skin and surrounding fluid (air or water)
- • Free Convection vs. Forced Convection
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Evaporation
• Conversion of liquid to a vapour requir-es
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Clothing as an Insulator
- • Impede conductive and convective heat transfer
- • Majority insulation from trapped air
- • Ideally vapour-permeable, but water-proof
- • Multiple layers best
- • Total effective insulation related to surface area
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Measuring the Thermal Environment
- • Air temperature
- • Humidity
- • Radiant Heat
- • Air movement
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Indices of Thermal Stress?
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Sources of Heat In Aircraft?
- • Cockpit thermal load depends on:
- • Greenhouse Effect (radiant heat)
- • Ambient air temperature
- • Altitude (low level worst)
- • Phase of flight (taxiing worst)
- • Efficiency of ECS (eg. air vents only in CT4)
- • Also:
- • Aerodynamic / kinetic energy
- • Avionics
- • Engines
- • Cockpit temperatures frequently exceed 50°C
- Greenhouse Effect
- Aircrew Factors
- • Metabolism
- • 60 watts at rest
- • increased 2-3 times with work
- • Flying Clothing
- • interferes with heat exchange
- • may decrease vapour permeability (immersion suits, NBC!)
- Hot Environments
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Sweating?
- • Only method of heat loss when temp exceeds 35ºC
- • Dripped sweat is ineffective for cooling
- • Will acclimatize over time
- • 3-4 million sweat glands in body
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Fluid Balance & Dehydration?
- • With no sweating you need to drink 1.25 litres/day
- • Maximum intestinal absorption = 1.5 litres/hour
- • Max 2 litres/hour may be lost via sweat
- • Greater ambient temperature, and/or greater workload = greater fluid requirement
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Dehydration?
- • Headache
- • Rapid pulse
- • - urine, darker, and may sting
- • Thirst is late and unreliable sign
- • - skin elasticity (turgor)
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Fluid replacement strategies?
- • Drink 500mls 2 hrs prior to activity
- • Drink to plan, not thirst or opportunity
- • Avoid alcohol, coffee, tea and other diuretics
- • Consume at least 150% of losses
- • Estimate loss by Δbody weight
- • Cool, palatable fluids freely available
- • Consume extra salt with food
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Effects of Heat Stress?
- • Decreased performance
- • Decreased G tolerance
- • Increased fatigue
- • Heat related disorders
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Heat Disorders?
- • Sunburn
- • Heat syncope
- • Heat exhaustion
- • increased temp
- • water depletion
- • salt loss
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Heat Stroke?
- • Rapid rise in core temperature
- • Symptoms - headache, nausea, confusion, drowsiness, restlessness, sweating stops, delirium, coma, death
- • Airway, Breathing, Circulation, Disability
- • Rectal Temp
- • Treatment - cool down: shade, remove clothing, fanning, tepid spray, cold packs to groin/axilla/neck
- • Avoid shivering!
- • Chilled IV NS
- • Evacuate PRI 1
- • Ice bath with continuous rectal temp?
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To Prevent Heat Disorders?
- • Stay hydrated
- • Minimise exposure
- • Acclimatise if possible
- • Make your work environment comfortable
- • Dress for the weather
- • Adjust work timings, load
- • Cooling vests – ground duties
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Acclimatisation?
- • Increased sweat rate, but increased Na+ reabsorption
- • Sweating = “more, early and dilute”
- • Increased aldosterone secretion
- • Heart rate normalises
- • Increased exercise tolerance
- • Process takes 10-14 days
- • “Acclimation”
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To Prevent Heat Disorders in Aircrew?
- • Pre flight
- • Airconditioning: accom, planning, vehicles
- • Park aircraft in shade
- • Limit stand by times
- Appropriate work/rest cycles
- • “By proxy” walk around?
- • In-flight
- • Environmental (cabin) conditioning system
- • Personal conditioning system
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Disorders due to Cold Exposure?
- Effects of Cold
- • Decreased:
- •performance
- •touch sense
- •muscle control
- • Risk factor for DCI
- • Risk factor for hypoxia
- • Survival implications
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Cold related disorders?
- •Frost nip
- •Frost bite
- •Trench foot
- •Hypothermia
- •Post-immersion collapse
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Freezing Injury: Frostnip?
- • Superficial freezing of skin, usually peripheries
- • Reversible
- • Symptoms
- • White
- • Tingling, burning +/- numbness
- • Resolves on rewarming
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Freezing Injury: Frosbite?
- • Deep freezing of tissues causing permanent damage
- • Area white, waxy, hard and numb
- • No pain while frozen
- • Thawing often results in severe pain
- • Later tissue death
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Non-freezing Injury: Trench Foot?
- • Exposure to wet conditions above freezing
- • Tissue hypoxia and damage due to reduced blood flow
- • Symptoms:
- • Body part pale
- • pulseless,
- • later swollen, red and painful
- • Reversible, so recovery is possible.
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Hypothermia?
- • Cooling of core temperature <35 deg C
- • Signs & Symptoms secondary to temperature dependent nerve conduction and metabolism
- • Neurological
- • Muscular incoordination
- • !Loss of shivering below 32 deg Celsius
- • Hot and cold flushes
- • Slow pulse and breathing
- • Listlessness
- • Confusion
- • Unconsciousness
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Effects of hypothermia?
- Confusion, disorientation
- First Aid for Hypothermia
- • Get sheltered and dry
- • Warm slowly (sleeping bag with warm body)
- • Warm sweet drinks if conscious
- • Seek medical aid ASAP
- • DO NOT:
- • Warm quickly (hot baths, fires)
- • Give alcohol
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Remember Wind Chill?
- • Being wet and/or exposed to air movement greatly increases the risk of hypothermia
- • Conductance of water 25x greater than air, heat loss up to 100x greater
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Cold Water Immersion?
- • Several physiological responses not occurring on exposure to cold air alone
- • Immediate
- • Short-term
- • Long-term
- • Post-Rescue Collapse
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Cold Water Immersion Effects 0-3 mins
- • “Cold Shock Response”
- • Deep uncontrolled inspiratory gasp, followed by hyperventilation
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Cold Water Immersion Short-term Effects 3-15 mins?
- • Erratic and uncoordinated breathing
- • Death occurring in competent swimmers within short distances from land
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Cold Water Immersion Long-term Effects >15 mins?
- • Hypothermia
- • Water highly conductive
- • Increased convective heat loss through swimming
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Constant-wear Immersion Suit?
- Impermeable membrane
- Air trapped in clothing (layered cotton or wool)
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Quick don Immersion suit?
- • Effective insulation:
- • ↓ 33-55%
- • ↓ 75% if immersion suit torn or substantial leak.
- • Hydrostatic compression of air barrier;
- • Leakage, wetting the undergarments.
- • Survival in heavy seas reduced by 50%.
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Post Rescue Collapse
- • Hydrostatic counter-pressure during immersion with central shunt of blood volume (exacerbated by peripheral vasoconstriction)
- • Cold-induced diuresis
- • Peripheral pooling during vertical winching from water: Collapse
- • Double-strop winching technique
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Quiz question
- High cockpit temperatures are most commonly the result of:
- a. Aerodynamic heating
- b. Heat transfer from avionics
- c. Greenhouse effect
- d. Heat transfer from engines
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