How To Prevent It

Breath-hold divers who hyperventilate before a dive are at risk of drowning. Many drownings unattributed to any other cause result from shallow water blackout and could be avoided if this mechanism was properly understood and the practice eliminated. Shallow water blackout can be avoided by ensuring that carbon dioxide levels in the body are properly calibrated prior to diving and that appropriate safety measures are in place; this can be achieved if divers do the following:

-Take a moment on the edge of the water to relax and allow blood oxygen and carbon dioxide to reach equilibrium.

-Breathe absolutely normally; allow the body to dictate the rate of breathing to make sure the carbon dioxide levels are properly calibrated.

-If excited or anxious about the dive, take extra care to remain calm and breathe naturally; adrenaline also causes hyperventilation without the diver knowing.

-When the urge to breathe comes on near the end of the dive, immediately seek access to air.

-Never dive alone. Dive in buddy pairs, one to observe, one to dive.

-Buddy pairs must both know CPR current practice.

Excessive hypocapnia is readily identifiable as it causes dizziness and tingling of the fingers. Conservative breath-hold divers who hyperventilate but stop doing so before the onset of these symptoms are likely already hypocapnic without knowing it. These extreme symptoms are caused by the increase of blood pH  following the reduction of CO2, which is required to maintain the acidity of the blood. The absence of any symptoms of hypocapnia is not an indication that the diver’s CO2 is properly calibrated and cannot be taken as an indication that it is therefore safe to dive.

Note that the body can actually detect low levels of oxygen but that this is not normal. Persistently elevated levels of carbon dioxide in the blood, hypercapnia (the opposite to hypnocapnia), tend to desensitise the body to CO2, in which case the body may come to rely on the oxygen level in the blood to maintain respiratory drive. This is illustrated in the scenario of type II respiratory failures . However, in a normal healthy person there is no subjective awareness of low oxygen levels.

Shallow water blackout should be considered alongside surface and deep water blackout.