Thermal Flashcards
Mechanisms of heat transfer
Temperature gradient dependent
- conduction: transfer of heat energy between molecules in direct contact eg solid bodies or solid to fluid
- convection: energy is transferred by the mass motion of molecules. Free or force
- radiation: heat emitted as electromagnetic radiation (greenhouse effect)
Vapour pressure (humidity) gradient dependent - evaporation : sweating is the only form of cooling once ambient temp >37 - adaptive mechanism. Rate depends on humidity gradient
What is latent heat
The heat energy required by a change of phase of water eg latent heat of vaporisation
What control temperature of human body
Hypothalamus report set point Circadian variation - peak in afternoon and early morning Increase with ingestion of food Hormones - thyroid -menstrual cycle/pregnancy - Steroid administration Fever - action of pyrogens
How does the thermoneutral zone get maintained
10% of BMR to maintain body temperature
- inefficiency of muscle activity
- catabolism
- Na+/K+ ATPase
Effectors of thermoregulation
Brain - behaviour - most powerful Endocrine - Hormonal control of temp set point and BMR Skin - vasculature: most important organ — blood vessel tone, shunts, counter-current mechanisms - Sweat glands - pilo-erection Muscles: shivering, activity Fat
How does vasculature response to regulate temperature
Peripheral vaso constriction
- counter-current exchange
- core contracts
Extreme cold = stagnant hypoxia
Sources of heat in aircraft
Cockpit thermal load depends on
- environmental factors: ambient air temperature
- aircraft factors: greenhouse effect, avionics, engines, cockpit insulation, ECS
Aircrew factors
Sortie factors: aerodynamic friction, altitude (low level worst) duration/repetition, phase of flight (taxing worst)
Cockpit temp frequently exceed 50 degrees
Effects of heat stress in aviation
Mission degradation Decreased performance Decrease G tolerance Increased fatigue Heat-related disorders - Heat exposure - Heat stress - Heat stroke
Symptoms of dehydration
Headache Dry mucous membranes Reduced urine, darker and may sting Rapid pulse Thirst is late and unreliable sign Reduced skin electricity (turgor) Low blood pressure Collapse
What are the heat disorders and symptoms
Sunburn
- redness +/- pain, swelling of skin +/- blisters if severe
Heat rash
- maculopapular rash +/- itch
- usually in clothed areas of the body
- may be related to obstruction to sweat ducts
Heat cramps
- heavy sweating with activity, painful muscle spasms usually legs. Temp normal
Heat syncope
- mild overheating with fainting, reduced vascular tone +/- volume depletion - faint
Heat exhaustion
- Heavy sweating, weakness, pulse threads, skin - cold,, pale and clammy. Temp may be normal or elevated. Fainting and vomiting. Precursor to heatstroke.
Heat stroke
- elevated core body temp >39
- confusion, agitation, delirium, seizures, coma.
- Skin hot and dry as lost ability to sweat
- tachycardia and hyperventilating
Symptoms of heat exhaustion
Paleness Tiredness Weakness Dizziness Headache Fainting Muscle cramps Heavy sweating Nausea or vomiting
Symptoms of heat stroke
Extremely high body temperature Red, hot, dry skin - no sweating Repaid, strong pulse Throbbing headache Dizziness Nausea Confusion Unconsciousness
Heat stroke management
Medical emergency - ABCDE - aim to get temp <38 in <20mins Remove heat stress - shade, remove clothing, A/C Don’t forget ABCs Cool but avoid shivering - immersion cooling is the best - Taco is the next best - ice over sheet with cool water Rehydration Evacuate as priority 1 - will need ICU Downgrade MEC
Complications of heat stroke
ALI - non-cardiac pulmonary oedema Rhabdomyolysis AKI - renal failure DIC CNS injury - oedema, Haemorrhage Death
Prevention of heat disorders
Hydration Dress for the weather Minimise heat stress/exposure Cooling vests Acclimatise if possible
How to prevent heat disorders in aircrew
Pre-flight
- air-con- accommodation, OPS, vehicles
- Shared OLAs
- Limit stand-by/taxi times
- Appropriate work/rest cycles - think techno
- By proxy walk around
In-flight
- environmental conditioning system - doesn’t start while taxing
- ALSE & personal conditioning system
Effects of the cold on aircrew
Decreased performance, touch sense, muscle control (shivering)
Increase risk of DCI, hypoxia due to increase metabolic rate and gas solubilities
Survival implications - reduced will to survive
Cold related disorders
Localised
- freezing injuries: frost nip, frost bite
- non freezing injury: chilblains, trench foot
Systemic
- hypothermia
- post-immersion collapse
Describe frostnip and symptoms
Superficial freezing of skin Usually peripheries Reversible Symptoms - white appearance - tingling, burning, numb
Describe frostbite and symptoms
Deep freezing of tissues Area white, waxy, hard and numb No pain while frozen Thawing often results in severe pain Permanent damage results
First aid for freezing injuries
ABCDE
- treat hypothermia first - keep casualty dry and warm
Field rewarming rarely practical
For the injury
- avoid partial thawing/refreezing
- Avoid dry, direct heat source - moist heat
- Do not rub or massage
Describe Chillblain and symptoms
Exposure to cold with red, swollen skin on the peripheries (aural ulcers)
Cause is unknown
- ?association with cryoproteins, connective tissue disease
Resolves in days
Tx with steroid
Describe Trench foot and symptoms
Prolonged exposure to wet, cold conditions above freezing Tissue hypoxia and damage due to reduced blood flow Symptoms - body part red, blue, pale - often pulseless - progress to swollen, red and painful - may result in gangrene Potentially reversible
Levels of hypothermia
Mild 32-35
Moderate 28-32
Severe <28
Signs and symptoms of hypothermia
Secondary to effect on temperature-dependent nerve conduction and metabolism - neurological - cardiovascular - muscular in coordination Loss of shivering below 32 Hot and cold flushes Slow pulse and breathing Altered level of consciousness - listlessness, confusion and LOC
First aide for hypothermia
Remove thermal stress - seek shelter from cold and wind - get dry - insulate Rewarming slowly - passive eg sleeping bag with warm body - active (external and/or internal) if possible Warm sweet drinks if conscious Seek aid ASAP DO not - rewarming rapidly eg hot baths, fires - give alcohol
Cold water immersion - immediate effects 0-3mins
Cold shock response
Leads to deep uncontrolled inspiratory gasp > hyperventilation
Tachycardia
Diving reflex
- cold stimulation of face
Bradycardia + apnoea
Autonomic conflict > arrhythmia ? Drowning
Cold water immersion - short term effect
3-30mins Swim failure Erratic and uncoordinated breathing Upright posture Reduced strength and coordination Death occurring in competent swimmers
Cold water immersion - long term effects
> 30mins
Hypothermia
<33 degrees
Water highly conductive 26x heat loss vs air
Increased convective heat loss through swimming
Cardiac arrest ~24 degrees core temp
Plus - motion sickness, non-freezing cold injury
What variables effect cold water survival time
Age Gender Stature and weight % body fat Air temperature Water temperature Wind speed Sea state Successful boarding of a life raft Level of fatigue of survivors Clothing worn/aircrew assembly
Calm water survival time for aircrew
Flying suit and long Johns
- 14 degree - 1 hour
- 6 degree - 30min
Options
- quick-don immersion suits
- constant-wear immersion suit
Post rescue collapse
Immersion provides hydrostatic support
- central redistribution of blood volume
- Diuresis due to perceived fluid overload
- exacerbated by peripheral vasoconstriction
Hypothermia
- bradycardia
- increase blood viscosity
Cardiac arrest swimming to rescuer due to bradycardia + viscosity + physical activity
Vertical winching from water
- peripheral pooling and blunting baroreceptors reflex results in collapse
- Double-strop winching technique
Relative hypovolaemia after rewarming - avsodilation
The most easily recognisable early symptom of dehydration is likely to be A thirst B headache C dark urine D collapse
A thirst
