Water submersion science

• What happens during water immersion under normal conditions?

  • Diving reflex

  • Skin wrinkling
• What's the mechanism behind skin wrinkling?

  • Water gets into the cells & they expand

  • Sympathetic vasoconstriction due to release of NA & adrenaline

  • But not simply hypotonic swelling

  • Peripheral nerve damaged limbs don't really wrinkle - sympathetic vasoconstriction drives wrinkling
• What happens during the diving reflex?


  • Breath hold

  • ↓ HR
• What triggers the classical diving reflex?

  Cold water immersion on face
• Which nerve carries the temperature information from the face during the diving reflex?

  CN V (trigeminal nerve)
• Describe what happens during the classical diving reflex

  • Increased vagus stimulation(this is an inhibitory signal so...)• Directly stimulates CN X to decrease HR & inhibit breathing

  
• What are the 2 responses of cold water immersion?

  • Initial response

  • Over time response
• What happens during the

  initial responseof the cold water immersion?Sympathetic gasp reflexsudden cooling of periphery = symp activation• Actual gasp• Hyperventilation• Sympathetic response (vasoconstriction, ↑HR, hypertension etc)Reduction in cerebral blood flow• Thought to be hypocapnia related• Blowing off CO₂ → alkalosis• CO₂ is a potent vasodilator → constricts cerebral arteries• So decreases cerebral blood flow → impairs consciousness

  
  

  NOTE: the etc stands for other sympathetic responses, so look into these
• What is the initial response of cold water immersion antagonised by?

  • By the diving reflex (which is the opposite)
• What happens during the

  over time responseof cold water immersion?• Sympathetic gasp reflex (fewer gasps)

  → Hyperventilation

  → Sympathetic response

  • Reduction in cerebral blood flow

  • Peripheral hypothermia

  • Severe hypothermia

  • Risk of drowning, arrhythmia, unconscious
• Why is there a risk of drowning during the over time response of cold water immersion?

  • Consciousness affected → can't move → can't swim

  • Gasp reflex → take in water (aspirate water)

  • Changes in resp & hyperventilation
• Why is there a risk of unconsciousness during over time response to cold water immersion?

  Because the cold stops neurons sending info into the brain
• Name the specific part of the hypothalamus that controls the set point temperature.

  Pre-optic anterior hypothalamus
• Describe the pathophysiology of hypothermia

  • Metabolic rate declines & neural transmission is inhibited• Depressed myocardial contractility• Leftwards shift of O₂ dissociation curve (leading to reduced availability of oxygen)• Vasoconstriction• Ventilation perfusion mismatch• Increased blood viscosity (as it cools)All of the above reduce tissue oxygenation• Sympathetic drive increases HR & RR, heat is produced via shivering•At 30ºCthis process ceases - RR, HR, BP & CO fall• Intravascular vol falls due to cold diuresis & fluid shifts into the extravascular space
• What are the normal compensatory mechanisms when heat is lost?

  • Thermoregulatory cutaneous vasoconstriction

  • Nonshivering thermogenesis

  • Behavioural response (huddling, moving)

  • Muscle shivering
• What are some mechanisms of heat loss during water immersion?

  Convection changed whilst in water• As lots of cold water around you• So rapid decrease in body tempEvaporation is an issue once out of the water• Potent heat loss mechanism• Stop evaporation before trying to warm the patient
• What are the 4 normal mechanisms of heat loss?

  • Evaporation

  • Radiation

  • Conduction

  • Convection
• What is the proposed mechanism for cardiac arrhythmia?

  Autonomic conflict

  
• Just incase you want to know more, but questions not set to this level

  
• What are some cold water treatment approaches to think about?

  • What temperature is important to measure? Core

  • Minimise heat loss (evaporation & convection esp)

  • Heat gain may not be quick (vasoconstriction)

  • Consciousness affected by many of the mechanisms... don't assume poor prognosis

  • What else is going on (EMG conduction changes, alkalosis)

  • Do enzymes and therefore drugs work at low temperature? no, as enzymes have optimal temp & pH → so drugs given once pt is warmed up
• Which mechanisms is consciousness affected by?

  • Alkalosis - neurons can't function as well as low pH

  • Decreased cerebral blood flow

  • Temperature
• What are the 2 techniques used during breath hold diving?

  • CO₂ blow off

  • Rapid initial decent (lung stretch)
• Describe the CO₂ blow off technique

  Drivers of inspiration• Low O₂• High CO₂• Low pH (acidic)

  • Body most sensitive to CO₂ & pH (mostly CO₂)• So making CO₂ low by hyperventilating reduces drive for inspiration• So can dive for longer• Also empties lungs for buoyancy effect so can drop down further
• Describe the rapid initial decent (lung stretch) technique

  • Water pressure increases as you go down in the ocean

  • Going down quick decreases lung stretch mechanism