by Julie Richardson
What Causes Freediver Blackout?
Simplified, freediver blackout is the result of oxygen starvation at the end of a breath-hold dive, and is more likely to occur during ascent 30 ft below the surface and shallower.
In fact, many blackouts occur within 30 seconds after surfacing.
As a diver descends to depth the increasing water pressure causes an increase in overall oxygen pressure, even as oxygen is being consumed. However, during ascent with water pressure and oxygen pressure decreasing, and oxygen supplies running low at the end of the dive, blackout can come on quickly with little or no warning. The diver will experience the greatest decrease in oxygen pressure at about 30 ft below the surface and shallower. After the diver inhales air at the surface, it takes about 20 heartbeats for the fresh oxygen to make it to the brain. Until this circuit is complete, the diver continues to be in danger of blackout even though he is breathing and may have even signaled to his buddy that he is "okay."
Blackout is insidious. Many victims have described the experience as a beautiful way to die. No suffering, no warning, no clue death was near. It is this feature of freediver blackout that makes it so deadly. DiveWise!