What is the diving reflex?

The diving reflex is a relic of human evolution from aquatic species. Exposure of the face, specifically the areas of trigeminal nerve distribution, to ice-cold water triggers a reflex that is designed to allow prolonged submersion underwater. There are three cardiopulmonary changes:

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  Apnoea. Breathing stops, often with a gasp, which prevents the lungs filling with water.

  
  
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  Bradycardia. In humans, the bradycardia is mild (in the region of 10–25%), but in some sea mammals heart rate falls by up to 90%.

  
  
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  Peripheral vasoconstriction. Blood is diverted away from the peripheries to maximise heart and brain perfusion.

In humans, the diving reflex is strongest in neonates and infants – the diving reflex enables photographs of babies ‘swimming’ underwater to be taken without the baby drowning! Perhaps more relevant to anaesthetists is that:

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  The bradycardia is mediated by the vagus nerve. In addition to reducing the frequency of sinoatrial node impulses, conduction through the atrioventricular node is also reduced. Immersing a patient’s face in a bowl of ice-cold water can therefore be used to terminate a supraventricular tachycardia.

  
  
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  People have survived prolonged immersion (20–30 min) in icy water (e.g. falling through ice on a frozen lake) – this is in part due to the diving reflex maximising cerebral blood flow by intense peripheral vasoconstriction. Children are more likely to survive prolonged immersion than adults owing to:

  
  
  
  
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     – A stronger diving reflex;

     
     
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     – A greater surface area-to-bodyweight ratio, resulting in a faster fall in body temperature, which reduces cerebral metabolic rate and protects against cerebral ischaemia.

  
  

Which physiological changes occur during head-out immersion?

The physiology of a body immersed in water differs from normal physiology on land in a number of ways:

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  Venous pooling in the legs does not occur. The effect of gravity is opposed by the external hydrostatic pressure of the surrounding water. The end result is the mobilisation of around 500 mL of blood back into the circulation. In the heart, increased blood volume stretches the atrial and ventricular walls, causing the release of atrial natriuretic peptide and brain natriuretic peptide, which produce a diuresis.

  
  
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  Increased work of breathing. The hydrostatic pressure of the surrounding water also has implications for respiratory mechanics, increasing the work of breathing by 60%.