1Topic 2.13 Thermal AVMO 0019

  1. 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.
  2. 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
  3. Thermal Control Effectors?
    • -Vascular
    • -Shivering
    • -Sweating
    • -Hormonal
    • -Behaviour
  4. Wandering Reference Point?
    • • Circadian variation
    • • Peak in afternoon
    • • Nadir early morning
    • • Ingestion of food
    • • Menstrual cycle
    • • Illness/Fever – action of pyrogens
  5. Heat Balance in the Body?
  6. Heat gain Effectors?
    • • Vascular:
    • • Vasoconstriction and vasodilatation in skin
    • • Shivering
    • • Sweating
    • • Piloerection
    • • Basal metabolic rate (BMR)
    • • Behavioural
  7. Vascular Response to Cold?
    • • Vasoconstriction
    • • “Core” contracts
    • • Small gradient between skin and air
    • • Less heat lost
  8. Vascular Response to Heat?
    • • Vasodilatation
    • • “Core” expands outwards
    • • Large gradient between skin and air
    • • More heat lost
  9. 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.
  10. 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
  11. 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
  12. Radiation
    • • Heat emitted as electromagnetic radiation
    • • Can take place in a vacuum – no air required
    • • Accounts for the Greenhouse Effect in cockpits
  13. 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
  14. Evaporation
    • Conversion of liquid to a vapour requir-es
  15. 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
  16. Measuring the Thermal Environment
    • • Air temperature
    • • Humidity
    • • Radiant Heat
    • • Air movement
  17. Determining humidity?
  18. Indices of Thermal Stress?
  19. 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
  20. 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
  21. 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
  22. Dehydration?
    • • Headache
    • • Rapid pulse
    • • - urine, darker, and may sting
    • • Thirst is late and unreliable sign
    • • - skin elasticity (turgor)
  23. 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
  24. Effects of Heat Stress?
    • • Decreased performance
    • • Decreased G tolerance
    • • Increased fatigue
    • • Heat related disorders
  25. Heat Disorders?
    • • Sunburn
    • • Heat syncope
    • • Heat exhaustion
    • • increased temp
    • • water depletion
    • • salt loss
  26. 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?
  27. 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
  28. 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”
  29. 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
  30. Disorders due to Cold Exposure?
    • Effects of Cold
    • • Decreased:
    • •performance
    • •touch sense
    • •muscle control
    • • Risk factor for DCI
    • • Risk factor for hypoxia
    • • Survival implications
  31. Cold related disorders?
    • •Frost nip
    • •Frost bite
    • •Trench foot
    • •Hypothermia
    • •Post-immersion collapse
  32. Freezing Injury: Frostnip?
    • • Superficial freezing of skin, usually peripheries
    • • Reversible
    • • Symptoms
    • • White
    • • Tingling, burning +/- numbness
    • • Resolves on rewarming
  33. 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
  34. 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.
  35. 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
  36. 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
  37. 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
  38. Cold Water Immersion?
    • • Several physiological responses not occurring on exposure to cold air alone
    • • Immediate
    • • Short-term
    • • Long-term
    • • Post-Rescue Collapse
  39. Cold Water Immersion Effects 0-3 mins
    • • “Cold Shock Response”
    • • Deep uncontrolled inspiratory gasp, followed by hyperventilation
  40. Cold Water Immersion Short-term Effects 3-15 mins?
    • • Erratic and uncoordinated breathing
    • • Death occurring in competent swimmers within short distances from land
  41. Cold Water Immersion Long-term Effects >15 mins?
    • • Hypothermia
    • • Water highly conductive
    • • Increased convective heat loss through swimming
  42. Constant-wear Immersion Suit?
    • Impermeable membrane
    • Air trapped in clothing (layered cotton or wool)
  43. 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%.
  44. 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
  45. 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
Author
david_hughm
ID
328893
Card Set
1Topic 2.13 Thermal AVMO 0019
Description
1Topic 2.13 Thermal AVMO 0019.txt
Updated