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
