The danger zone: Staying safe when breath-hold diving

Australia is a nation of great swimmers, and Aussies have consequently developed dozens of competitions around getting ‘in the drink’.  So Virtual Medical Centre would like to remind our sporty readers about what can happen with more dangerous activities such as breath-hold diving.

After one Australian university held a "Dolphin Dive" competition – where the students tried to swim as far as possible underwater – they never expected the participants to have seizures. So neurologists Karl Ng and Kishore Kumar decided to make sure everyone knows the risks.

Two of the three students in the event were experienced swimmers who hyperventilated before they dove. One swam 60 metres and spent 40 seconds underwater, and the other student swam 85 metres and spent 90 seconds underwater. At the end of the dive, both students lost consciousness and spectators saw their limbs jerking.

In medical terms, they suffered a convulsive syncope, where the body jerks when there isn’t enough blood and oxygen carried to the brain.

Dr Ng said this is not epilepsy per se; however, it is a common consequence of temporary lack of oxygen in the brain.

Thankfully, there were many spectators at the event and the students were pulled out of the pool without suffering any long-term consequences – but it’s important to know the risks.

What happened?

When you breath-hold dive, the amount of oxygen (O₂) in the bloodstream falls and the amount of carbon dioxide (CO₂) rises. The CO₂ level triggers the urgent need to breathe and warns you to take a breath, even before your oxygen levels are very low.

However, when you hyperventilate before the dive, you lower your starting CO₂  level. It takes longer for the amount of CO₂  to rise to the level that triggers this natural urge to breathe. Meanwhile, O₂ levels continue to drop to dangerously low levels (called hypoxia).

This means there is a greater chance of losing consciousness underwater and drowning. 

Figure 1: Normal dive

Figure 2: Hyperventilation before dive.

Dr Ng said that in this activity, the chances of long-term consequences are largely related to the injury of drowning, and the danger of the severe hypoxia that can occur in this situation.

Dr Ng said this alone makes breath-hold diving dangerous. "It would be unlikely if such a process occurred on land," he says.

Dr Ng also said that although it is uncommon for people to pass out, it happens the most in people who are really pushing themselves to places they haven’t been before.

Even experienced swimmers who may know what they are doing are susceptible. This group is often less likely to be watched by lifeguards, so when consciousness is lost, nobody is there to help.

No matter how experienced a swimmer is, you can’t swim when you are unconscious!

Dr Ng warns that a person can ‘wake up’ with few short-term consequences, but "if they are unconscious underwater, then they do not have any way of protecting themselves from drowning."

So this practice is discouraged if you have to push yourself ‘to the danger zone’, always remember to have a strong swimmer buddy watching, just in case.

Dr Ng is a consultant neurologist and neurophysiologist, conjoint senior lecturer at Sydney Medical School, University of Sydney, and editorial advisory board member for Virtual Medical Centre.

Dr Kumar was a neurolomuscular fellow at the Department of Neurology and Clinical Neurophysiology, Royal North Shore Hospital, Sydney. Currently, Dr Kumar is on a prestigious overseas fellowship at the Royal Free Hospital NHS, London, England.