Hypocapnia, also sometimes known as acapnia, is a state in which the level of carbon dioxide in the blood is lower than normal. This can result 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—pins and needles, stiff muscles and tetany in the extremities, especially hands and feet.

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

Because the body relies on the level of blood CO₂ to trigger the urge to breathe, hypocapnia can suppress breathing to the point of blackout (cerebral hypoxia). Self-induced hypocapnia through hyperventilation is the basis for the deadly schoolyard choking game. It is also commonly 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.

See also

• Hypercapnia, increased level of carbon dioxide

Reference

• Laffey, J.G. & Kavanagh, B.P. (2002). Hypocapnia. New England Journal of Medicine, 347:43-53.

Pulmonology

Hipokapnio