Deep Water Blackout - Deep Water Blackout Versus Shallow Water Blackout

Deep Water Blackout Versus Shallow Water Blackout

In some places, notably in US scuba diving open water curricula, a blackout from a deep freedive is sometimes referred to as a shallow water blackout. Where this happens there is usually little or no discussion of the phenomenon of blackouts not involving depressurisation and the cause may be variously attributed to either depressurisation or hypocapnia or both. This problem probably stems from the original identification of the mechanism of latent hypoxia arising in the context of a string of fatal, shallow water accidents with early military, closed-circuit rebreather apparatus prior to the development of effective partial pressure oxygen meters. In the very different context of dynamic apnoea sports careful consideration of terms is needed to avoid potentially dangerous confusion between two phenomena having different characteristics, mechanisms and prevention. The application of the term shallow water blackout to deep dives and its subsequent association with extreme sports has tended to mislead many practitioners of static apneoa and dynamic apnoea distance diving into thinking that it does not apply to them even though true shallow water blackout kills many swimmers every year, often in shallow swimming pools.

Some consider deep water blackout to be a special condition or subset of shallow water blackout, which is more accurate and may be acceptable. However, in the interests of accuracy, clarity, helpfulness and safety this article treats the two as separate phenomena with the following characteristics:

• Deep water blackout occurs as the surface is approached following a breathe-hold dive of over ten metres and typically involves deep, free-divers practicing dynamic apnoea depth diving usually at sea. The immediate cause of deep water blackout is the rapid drop in the partial pressure of oxygen in the lungs on ascent.

• Shallow water blackout only occurs where all phases of the dive have taken place in shallow water where depressurisation is not a factor and typically involves dynamic apnoea distance swimmers, usually in a swimming pool. The primary mechanism for shallow water blackout is hypocapnia brought about by hyperventilation prior to the dive.

This confusion is exacerbated by the fact that in the case of deep water blackout hypocapnia may be involved even if ascent is the actual precipitator.