Blood leaving the lungs is normally fully saturated with oxygen, so hyperventilation of normal air cannot increase the amount of oxygen available, as oxygen in blood is the direct factor. Lowering the CO2 concentration increases the pH of the blood, thus increasing the time before blood becomes acidic enough so the respiratory center becomes stimulated, as described above. While hyperventilation will yield slightly longer breath-holding times, any small time increase is at the expense of possible hypoxia, though it might not be felt as easily. One using this method can suddenly lose consciousness unnoticed—a shallow water blackout—as a result. If a person loses consciousness underwater, there is considerable danger that they will drown. An alert diving partner or nearby lifeguard would be in the best position to rescue such a person. Static apnea blackout occurs at the surface when a motionless diver holds their breath long enough for the circulating oxygen in blood to fall below that required for the brain to maintain consciousness. It involves no pressure changes in the body and is usually performed to enhance breath-hold time. It should never be practiced alone, but under strict safety protocols with a safety guard or equipment beside the diver. Apneic oxygenation
Because the exchange of gases between the blood and airspace of the lungs is independent of the movement of gas to and from the lungs, enough oxygen can be delivered to the circulation even if a person is apneic, even if the diaphragm doesnt move.
Apnea testing is not suitable in patients who are hemodynamically unstable with increasing vasopressor needs, metabolic acidosis, or require high levels of ventilatory support. Apnea testing carries the risk of arrhythmias, worsening hemodynamic instability, or metabolic acidosis beyond the level of recovery and can potentially make the patient unsuitable for organ donation (see above). In this situation a confirmatory test is warranted as it is unsafe to perform the apnea test to the patient. Etymology and pronunciation
The word apnea (or apnoea) uses combining forms of a- + -pnea, from Greek Greek: ἄπνοια, from ἀ-, privative, πνέειν, to breathe. See pronunciation information at dyspnea. See also
Apnea and Work
Apnea-hypopnea index

References
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Nunn, J. F. (1993). Applied Respiratory Physiology (4th ed.). Butterworth-Heinemann. ISBN 0-7506-1336-X. External links

Sleep Apnea, a resource from the Harvard Division of Sleep Medicine on Obstructive Sleep Apnea
apneacalculator.com, information about Apnea and the apnea-calculator for clinical treatment of Obstructive Sleep Apnea
Freediving Courses & Training in the UK, information about learning the sport of Freediving, the club is called Apneists UK
DiveWise.Org Non-profit scientific and educational resource for apnea divers
DAN Breath-Hold Workshop Divers Alert Network 2006 Breath-Hold Diving Workshop PDF