Carbon dioxide
Classification and external resources
MeSH D016857

Hypocapnia or hypocapnea (from the greek words υπό + καπνός) also known as hypocarbia, 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.

Hypocapnia is the opposite of hypercapnia.


Even when marked, hypocapnia is normally well tolerated.[1] 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 increased 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.

Because the brain stem regulates breathing by monitoring the level of blood CO2, hypocapnia can suppress breathing to the point of blackout from cerebral hypoxia.


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

Self-induced hypocapnia through hyperventilation is the basis for the dangerous schoolyard fainting game. Deliberate hyperventilation has been used by underwater breath-hold divers for the purpose of extending dive time as it effectively reduces respiratory drive due to low CO2 levels allowing one to break one's standard limit of breath holding at the risk of shallow water blackout (which is a significant cause of drowning) as while air hunger is reduced the oxygen levels are not increased, in fact hypocapnia reduces the oxygen levels available to the brain due to the elevated affinity of oxygen to hemoglobin (Bohr effect) hence highly increasing the chances of blackout.

See also


  1. Laffey JG, Kavanagh BP (2002). "Hypocapnia". N. Engl. J. Med. 347 (1): 43–53. doi:10.1056/NEJMra012457. PMID 12097540.
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