The dangers of hyperventilation before breath-hold diving

A recent article in the Medical Journal of Australia1 illustrated the dangers of breath-hold diving by reporting the events of a university medical school swimming competition named the "Dolphin Dive". The goal of the competition was to swim as far as possible underwater in a swimming pool without taking a breath. Three students participated in the event and two students had seizures. In both cases, the students were experienced swimmers who hyperventilated prior to the dive. One student swam about 60 meters and spent 40 seconds underwater, the other student swam about 85 metres and spent 90 seconds underwater. At the end of the dive, both students lost consciousness and were briefly observed to have jerking activity of the limbs. Many people were observing the race so the students were rapidly retrieved from the pool; they regained consciousness and did not experience any long-term consequences. These students most likely suffered convulsive syncope.

During a breath-hold dive, levels of oxygen (O₂) in the bloodstream fall and carbon dioxide (CO₂) levels rise. The rising CO₂ levels trigger the urge to breath before the O₂ levels fall to dangerously low levels (Figure 1). However, if a person hyperventilates prior to the breath-hold, the CO₂ levels will fall (hypocapnia), delaying the urge to breathe, making the person susceptible to low O₂ levels (hypoxia) and loss of consciousness underwater (Figure 2). Reduced O₂ supply to the brain can also cause the brief jerking activity resembling seizures that was observed in the students.
 

Figure 1: Normal dive

Figure 2: Dive with hypocapnia

People who practice breath-hold diving are often experienced swimmers who are not closely watched by lifeguards. Loss of consciousness underwater can lead to drowning.² Anecdotal observations suggest that underwater breath-holding is a relatively common practice in Australia. The practice is particularly dangerous if coupled with prior hyperventilation, and hyperventilation prior to breath-holding should be discouraged.

References

1. Kumar KR, Ng K. Don’t hold your breath: anoxic convulsions from coupled hyperventilation-underwater breath-holding. Med J Aust. 2010; 192: 663-4.
2. Craig AB Jr. Summary of 58 cases of loss of consciousness during underwater swimming and diving. Med Sci Sports. 1976; 8: 171-5.  

Article kindly written by:

Kishore R Kumar; Department of Neurology and Clinical Neurophysiology, Royal North Shore Hospital; and
Karl Ng; Consultant Neurologist and Neurophysiologist, Department of Neurology and Clinical Neurophysiology, Royal North Shore Hospital; Sydney North Neurology and Neurophysiology, Level 2, 511 Pacific Hwy, Crows Nest, NSW 206, Australia; Conjoint Senior Lecturer, Sydney Medical School, University of Sydney; and Editorial Advisory Board member for Virtual Medical Centre.