Hypocapnia

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Hypocapnia, sometimes incorrectly called acapnia, is a state of reduced carbon dioxide in the blood. Hypocapnia usually results from deep or rapid breathing, known as hyperventilation.

Even when severe, hypocapnia is normally well tolerated. However, hypocapnia causes cerebral vasoconstriction, leading to cerebral hypoxia and this can cause transient dizziness, visual disturbances, and anxiety. A low partial pressure of carbon dioxide in the blood also causes alkalosis (because CO₂ is acidic in solution), leading to lowered plasma calcium ions and nerve and muscle excitability. This explains the other common symptoms of hyperventilation —pins and needles, muscle cramps and tetany in the extremities, especially hands and feet.

Hypocapnia is sometimes induced in the treatment of the medical emergencies, such as intracranial hypertension and hyperkalaemia.

Because the brain stem monitors the level of blood CO₂ in blood to regulate breathing, hypocapnia can suppress breathing to the point of blackout from cerebral hypoxia. Self-induced hypocapnia through hyperventilation is the basis for the deadly schoolyard choking game. Deliberate hyperventilation has been unwisely used by underwater breath-hold divers to extend dive time but at the risk of shallow water blackout, which is a significant cause of drowning.