Hyperventilation while swimming can be intentional or accidental, but both types of incidents can kill, even in shallow water. Caused by breathing rapidly or taking too many deep breaths in succession, hyperventilation fundamentally alters your blood chemistry and the way your body sends warning signals to your brain. Rather than enriching yourself with oxygen, hyperventilation can lead to dangerously low levels of oxygen.
Beginning swimmers might be nervous about breathing during swimming, as it can be daunting to swim facing down in the water and to regulate breathing during the stroke. You might react by breathing fast, and inadvertently reducing levels of carbon dioxide in your bloodstream. Anxiety mixed with inexpert technique causes swimmers to overexert themselves during the stroke and quickly use up oxygen supplies. Elite swimmers sometimes deliberately hyperventilate in competitive workouts whether swimming underwater or mindfully limiting breathing during lap swimming. Either way, you use up oxygen but might be unaware of the dire situation.
Sensors in your aorta and carotid arteries alert your brain when carbon dioxide levels rise, and trigger you to take a breath. Hyperventilating sends your cardiovascular system out of balance, diluting carbon dioxide concentration. Diminished concentration levels of carbon dioxide lull your brain into the false security. You run out of oxygen without ever feeling the urge to replenish the supply.
Holding your breath underwater after you hyperventilate can lead to shallow-water blackout, a condition in which you become disoriented and then lose consciousness. Once unconscious, you reflexively gasp for air, taking in water and potentially drowning. Hyperventilation and resulting low air, or hypoxia alters your blood's pH levels, and levels falling below 7.2 can cause fatal heart arrhythmias and death according to Dr. Tom Griffiths and Dr. Walter Griffiths in an online article for Aquatic Safety Research Group.
You can prevent accidental hyperventilation by learning to relax in the water. Practice drills can teach you how to regulate your breathing while swimming. The more you practice, the less anxious you will feel, and the easier you will breathe. Never engage in competitive breath-holding or extensive "no-breather" underwater training, regardless of challenges from teammates or even coaches. The American Red Cross and the YMCA work to raise public awareness about the dangers of hyperventilation while swimming.