The “Shallow Waters” Syndrome
The “Shallow Waters” Syndrome in technical language, the “breath hold anoxic syncope of emersion”— is a sudden blackout that may happen to a free diver during the ascent, close to the surface, at the end of an extended free dive (not with scuba diving equipment). If there are not proper safety measures in place, it may lead to a drowning accident that often ends up in a fatality.
The cause behind such incidents is the resulting hypoxia produced when breathing O2 concentrations below 10-12% at 1 ATA, which is incompatible with human life —in normal air, O2 concentration is 21%—. Thus, during the course of the diving, the O2 concentration of the air stuck inside the lungs of the diver decreases as the oxygen is progressively transferred to the blood through the alveola. However, the diver does not feel any symptom since the declining of the O2 concentration is compensated by the increased absolute pressure at such depth. For instance, at 10m depth there is 2 ATA x 10% O2 = 0.2 ATA, equivalent to breathing an O2 concentration of 20% at the surface.
As the diver goes to the surface, the absolute pressure drops accordingly. Close to the surface, say at 3m depth, there is 1.3 ATA x 10% O2 = 0.13 ATA, equivalent to breathing an O2 concentration of 13%, which is too low to provide enough brain oxygenation, allowing the blackout to suddenly occur.
PREVENTIVE SAFETY MEASURES
The “shallow waters” syndrome can be prevented by:
• Limiting the hyperventilation before the diving up to a maximum of 4 ventilations, in order to avoid an excessive drop of the CO2 concentration in the diver blood, what can delay excessively the time between breathings.
• Diving always with a buddy, so that they can mutually supervise the ascents reacting in time if a blackout eventually happens.