Diving

Question 1

What is the diving reflex

Answer 1

The diving reflex is a relic of human evolution from aquatic species.

Exposure of face: specifically trigeminal nerve distribution to ice cold water triggers a reflex that is designed to allow prolonged submersion under water.

Question 2

What are the cardio-pulmonary changes associated with the diving reflex

Answer 2

1. Apnoea (prevent lungs filling with water)
2. Bradycardia (humans 10 - 25%, other mammals 90%) –> vagus nerve
3. Peripheral vasoconstriction
   - -> blood diverted away from peripheries to maximise heart and brain perfusion

Question 3

Why can we take photographs of babies swimming underwater without them drowning

Answer 3

Because of the diving reflex

Question 4

How is bradycardia of the diving reflex mediated. How can this phenomenon be used in anaesthetic practice

Answer 4

Via the vagus nerve which reduces automaticity in the SA node and slows conduction through the AV node.

Face submersion in ice water could terminate an SVT

Question 5

How have people survived prolonged submersion in icy water for 20-30 minutes. And why are children more likely to survive than adults

Answer 5

1. Diving reflex maximises CBF during submersion
2. Fall in body temperature reduces CMRO2

Children have smaller SA: Weight ratio meaning their body temperature falls faster and reduces CMRO2 faster. This means they are more likely to survive than adults

Question 6

What are the physiological changes that occur with ‘head out of water’ body immersion

Answer 6

Distinct from the diving reflex

1. Venous pooling in the legs does not occur
   - -> Hydrostatic forces of surrounding water opposes gravity. –> 500 ml blood back to circulation –> Stretching of atria and ventricles –> Release of BNP and ANP –> Diuresis
2. Increase work of breathing 60%

Question 7

How does ambient pressure change with depth of submersion. What happens to the air in the lungs of a breath hold diver as he descends

Answer 7

Increase in ambient pressure by 100 kPa every 10 meters depth.

So as breath hold diver descends, lung volumes decrease. If volumes reduce below RV then there is a risk of negative pressure pulmonary oedema

Question 8

What happens to the air within the lungs as a breath hold diver ascends? What are the implications of this

Answer 8

Air re-expands. At surface lung volume will be slightly reduced as more O2 would have been consumed than CO2 produced (RQ ± 0.8).

The fraction of O2 within the lung will be decreased –> Therefore rapid ascent will result in rapid fall alveolar PAO2 potentially resulting in cerebral hypoxia and unconsciousness.

Question 9

What does SCUBA stand for and how does SCUBA diving differ from breath hold diving

Answer 9

Self-Contained Underwater Breathing Apparatus.

Compressed air is delivered at ambient pressure (depending on depth) from the tank to the diver via a demand valve.

Breathing at ambient pressure avoids the problem of compressed lung volumes of breath hold divers but introduces a number of other complications.

Question 10

What are the problems introduced in SCUBA divers by breathing air at ambient pressure deep under water

Answer 10

As ambient pressure increases with depth underwater the following complications exist:

1. Increased gas density
   –> Partial pressure of inspired air increases: higher density of inspired air and increased turbulent gas flow –> increased work of breathing
   Rx: Replace N2 with He (less dense) to counteract this problem.
2. Nitrogen Narcosis
   –> Very high partial pressures of N2 can directly affect the CNS –> Euphoria, incoordination, loss of concentration, coma.
   Rx: Replace N2 with He (Has no effect on CNS at equivalent pressures)
3. O2 toxicity
   –> High PaO2 –> Seizures and loss of consciousness
   Rx: divers must calculate the max dive time based on the depth of their dive.

Question 11

Why should divers be conscientious about calculating dive time based on the depth of their dive

Answer 11

To prevent O2 toxicity –> seizures and loss of consciousness with prolonged exposure to high partial pressures of O2.

Question 12

Why do divers substitute N2 for He in their tanks

Answer 12

If the dives are planning prolonged and deep dives, He is less dense than N2 reducing the work of breathing with dense gases at that depth. Furthermore, He does not have similar CNS toxic effects/narcosis at the depths that N2 does. These two problems are prevented

Question 13

What is decompression sickness

Answer 13

this is a consequence of breathing N2 at high partial pressures.

N2 usually poorly soluble in blood. Very little is dissolved in blood at sea level.

Henry’s law sates that the amount of N2 dissolved in solution is proportional to the partial pressure of N2 above that solution. Therefore additional N2 crosses alveolar capillary barrier, dissolves in blood and deposits in tissues.

During SLOW, STAGED ascent –> this is not a problem as slow steady reduction in N2 occurs

During RAPID ascent, sudden change in ambient pressures causes N2 to come out of solution, with N2 bubble forming in

1. Joints –> pain
2. Pulmonary circulation –> dyspnoea/retrosternal pain
3. Arterial circulation –> gas embolism

Decompression sickness can also be prevented by breathing He - O2 mixtures