During apnea, there is no movement of the muscles of inhalation,[citation needed] and the volume of the lungs initially remains unchanged.
Apnea may first be diagnosed in childhood, and it is recommended to consult an ENT specialist, allergist or sleep physician to discuss symptoms when noticed; malformation and/or malfunctioning of the upper airways may be observed by an orthodontist.
[2] Apnea can be involuntary—for example, drug-induced (such as by opiate toxicity), mechanically / physiologically induced (for example, by strangulation or choking), or a consequence of neurological disease or trauma.
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.
The lungs start to feel as if they are burning, and the signals the body receives from the brain when CO2 levels are too high include strong, painful, and involuntary contractions or spasms of the diaphragm and the muscles in between the ribs.
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.
Of the eight test subjects in this landmark study, the highest recorded PaCO2 was 250 millimeters of mercury, and the lowest arterial pH was 6.72 after 53 minutes of apnea.
[9] A recommended practice for the clinical diagnosis of brain death formulated by the American Academy of Neurology hinges on the conjunction of three diagnostic criteria: a coma, absence of brainstem reflexes, and apnea (defined as the inability of the patient to breathe unaided: that is, with no life support systems like ventilators).
[10] 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).
[9] The word apnea (or apnoea) uses combining forms of a- + -pnea, from Greek: ἄπνοια, from ἀ-, privative, πνέειν, to breathe.