Voluntary apnea can be achieved by closing the vocal cords, simultaneously keeping the mouth closed and blocking the nasal vestibule, or constantly activating expiratory muscles, not allowing any inspiration. Complications
Under normal conditions, humans cannot store much oxygen in the body. Prolonged apnea leads to severe lack of oxygen in the blood circulation, leading to dysfunction of organ systems. Permanent brain damage can occur after as little as three minutes and death will inevitably ensue after a few more minutes unless ventilation is restored. However, under special circumstances such as hypothermia, hyperbaric oxygenation, apneic oxygenation (see below), or extracorporeal membrane oxygenation, much longer periods of apnea may be tolerated without severe detrimental consequences. Untrained humans usually cannot sustain voluntary apnea for more than one or two minutes, since the urge to breathe becomes unbearable. The reason for the time limit of voluntary apnea is that the rate of breathing and the volume of each breath are tightly regulated to maintain constant values of CO2 tension and pH of the blood more than oxygen levels. In apnea, CO2 is not removed through the lungs and accumulates in the blood. The consequent rise in CO2 tension and drop in pH result in stimulation of the respiratory centre in the brain which eventually cannot be overcome voluntarily. The accumulation of carbon dioxide in the lungs will eventually irritate and trigger impulses from the respiratory center part of the brain and the phrenic nerve. Rising levels of carbon dioxide signal the body to breathe and resume unconscious respiration forcibly.
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.