Underwater Hypoxic Blackout Risk Factors
Underwater Hypoxic Blackout (UHB) may be the leading cause of experienced swimmer drownings, though its impact is yet to be fully understood. We do know what breath-holding risks cause UHB though, and factors that increase its likelihood—no matter a swimmer’s skill level.
Scenarios That Heightenthe Risk of UHB
Low CO2 Levels
Rapid, deep breathing (hyperventilation) lowers carbon dioxide levels, which suppresses the body’s natural breathing reflex. Swimmers may not feel the need to breathe even as oxygen drops, leading to a blackout.
Low O2 Levels
Exhaustion from exercise reduces oxygen levels. Continued swimming or repetitive breath-holding can push O2 to dangerous lows, causing blackout—even as rising CO2 triggers the need to breathe.
Normal O2 and CO2 Levels
In competitive scenarios, a swimmer may start with normal O2 and CO2 levels. But their drive to win, similar to a “runner’s high,” can override their urge to breathe, increasing the risk of a blackout.
Both Low O2 and CO2 Levels
This is a particularly dangerous scenario where both oxygen is critically low and the urge to breathe is diminished, making a hypoxic blackout highly likely.
Who Isat Risk?
Swimmers of all skill levels are at risk of UHB, which is why knowledge and prevention are so important. However, there is an even higher risk for swimmers who are:
- Athletes who train and perform in swimming pools, particularly those who practice hyperventilation.
- Athletes practicing military-style workouts (Underwater CrossFit, etc.)
- Practicing breath-holding for freediving, spearfishing, or fitness.
- Physically exhausted.
- Unobserved while breath-holding.
- Consecutively performing a repeated hyperventilation/breath-holding routine (such as the Wim Hof Method).
- Playing breath-holding games.
- Breath-holding and have unknown and underlying medical causes, such as long Q-T, RyR2, seizures, etc.
- Practicing holding their breath in a pool while floating face down or sitting on the bottom. Since the individual is already in a state of relaxation, SWB becomes nearly impossible for an observer to detect.