--- module: 104-03 language: en chapter: 104 title: "Exercise, Adaptation, Thermoregulation, Altitude, and Extreme Environments" module_title: "Heat, cold, altitude, diving, and microgravity" source_sha256: 5405f4a36efd71cb6f65560a3c273451e2c9fff4570a8334ccb538ce40e3c2c9 --- # Heat, cold, altitude, diving, and microgravity ## Environmental stress principles ### Exchange of heat, gas, fluid, load ### Stress depends on intensity, duration, equipment ### Exercise raises heat and oxygen demand ### Adaptation extends range, not physical limits ## Heat exchange and acclimation ### Core temperature balances production and exchange #### Radiation, convection, conduction, evaporation ### Radiation and convection add heat when hotter ### Evaporation needs favourable vapour pressure #### Humidity, impermeable clothing, dripping sweat ### Skin and core sensors to hypothalamus, brainstem ### Skin flow competes with muscle, central volume #### Plasma loss raises heart rate ### Forced drinking risks dilutional hyponatraemia ### Heat acclimation with repeated exposure #### Plasma volume expands #### Earlier, dilute sweat via aldosterone #### Days to begin, one to two weeks, decays ## Heat illness ### Heat exhaustion: cannot continue, no severe CNS injury ### Heat stroke: hyperthermia with CNS dysfunction #### Liver, kidney, muscle, coagulation injury #### Sweating does not exclude it ### Exertional in active, classic in vulnerable ### Medicines impair sweating or judgement ### Cool rapidly without delaying care ### Antipyretics fail: set point not raised ## Cold exposure and injury ### Vasoconstriction and behaviour conserve heat ### Shivering and non-shivering thermogenesis ### Water removes heat faster than still air ### Alcohol accelerates heat loss ### Hypothermia slows metabolism and conduction #### Confusion to arrhythmia and arrest #### Irritable myocardium: move with care ### Passive, active external, internal rewarming ### Frostbite: ice, endothelium, thrombosis #### Refreezing after thaw worsens injury #### Rubbing adds mechanical injury ### Non-freezing injury from wet cold ## Altitude physiology ### Pressure falls, oxygen fraction constant ### Hypoxic ventilatory response #### Respiratory alkalosis restrains ventilation #### Renal bicarbonate loss permits hyperventilation ### Maximal aerobic performance falls ### Erythropoietin raises red cells over weeks #### Plasma contraction raises concentration first #### Excess viscosity impedes flow ### Acclimatisation differs from evolutionary adaptation ## Altitude illness ### Acute mountain sickness: headache after ascent #### Ascent rate, sleeping altitude, susceptibility ### Cerebral oedema: ataxia, altered mental state ### Pulmonary oedema: breathlessness, hypoxaemia #### Widespread hypoxic vasoconstriction #### Uneven constriction and high flow ### Descent and oxygen are central ### Acetazolamide aids acclimatisation ### Glucocorticoids ease cerebral symptoms only ## Diving and immersion ### One atmosphere per ten metres of seawater ### Boyle's law: gas compresses and expands #### Unequalised ears, sinuses, mask, lungs #### Breath-hold ascent: barotrauma, gas embolism ### Henry's law: more gas dissolves at depth #### Fast ascent: bubbles, decompression sickness #### Oxygen and urgent recompression assessment ### Nitrogen narcosis impairs cognition ### Oxygen toxicity: seizures, lung injury ### Carbon dioxide retention worsens both ### Immersion shifts blood centrally, diuresis ### Cold immersion: gasping before hypothermia ### Pre-dive hyperventilation risks blackout ## Microgravity ### Fluid shifts toward chest and head #### Diuresis lowers plasma volume ### Deconditioning, bone loss, muscle atrophy ### Orthostatic intolerance on return ### Urinary calcium raises stone risk ### Vestibular reinterpretation, motion sickness ### Radiation risk with dose and duration ### Countermeasures reduce, do not eliminate ## Integrated prevention ### Identify the governing physical exposure ### Match work and equipment to reserve ### Allow graded adaptation ### Recognise early failure before injury