| Hypocapnia | |
|---|---|
 | |
| Carbon dioxide |
Hypocapnia (from the Greek wordsὑπό meaningbelow normal andκαπνόςkapnós meaningsmoke), also known ashypocarbia, sometimes incorrectly calledacapnia, is a state of reducedcarbon dioxide in theblood.[1] Hypocapnia usually results from deep or rapid breathing, known ashyperventilation.
Hypocapnia is the opposite ofhypercapnia.
Short term hypocapnia does not usually have any adverse effects.[2] It is sometimes used as lifesaving treatment for conditions such as neonatal pulmonary-artery hypertension and for people with severe intracranial hypertension. If the state of hypocapnia persists or is prolonged, adverse outcomes may occur.[2]
Even when marked, hypocapnia is normally well tolerated. Symptoms include tingling sensation (usually in the limbs), abnormal heartbeat, painful muscle cramps, and seizures. Acute hypocapnia causes hypocapnic alkalosis, which causes cerebralvasoconstriction leading tocerebral hypoxia, and this can cause transient dizziness, fainting, andanxiety.[3] A lowpartial pressure of carbon dioxide in the blood also causesalkalosis (because CO2 is acidic in solution), leading to lowered plasma calcium ions (Hypocalcaemia), causing increased nerve and muscle excitability. This explains the other common symptoms of hyperventilation—pins and needles, muscle cramps andtetany in the extremities, especially hands and feet.[citation needed]
Because thebrain stem regulates breathing by monitoring the level of blood CO2 instead of O2, hypocapnia can suppress breathing to the point of blackout from cerebral hypoxia, as exhibited inshallow water blackout.
Hypocapnia also results in bronchoconstriction[3] in order to decrease ventilation. This mechanism is meant to counteract hyperventilation, and decrease the amount of oxygen coming into the lungs. The body's "goal" is to have a relatively even ratio of the partial pressure of oxygen to the partial pressure of carbon dioxide. Chemoreceptors in the body sense a change in partial pressures and pH (hydrogen ion concentration) in the blood. Chemoreceptors are responsible for signaling vasoconstriction, vasodilation, bronchoconstriction, and bronchodilation.[citation needed]
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The main physiologic causes of hypocapnia are related to hyperventilation. Hypocapnia is sometimes induced in the treatment ofmedical emergencies such asintracranialhypertension[3] andhyperkalemia.
Self-induced hypocapnia through hyperventilation is the basis for the dangerous schoolyardfainting game.[4][5] Deliberate hyperventilation has been used by underwaterbreath-hold divers for the purpose of extending dive time as it effectively reduces respiratory drive due to low CO2 levels. This allows one to break their standard limit of breath holding, at an increased risk ofshallow water blackout (which is a significant cause of drownings). This risk is caused due to air hunger being reduced (due to low blood carbon dioxide levels) but oxygen levels not being 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.[citation needed]
Monitoring the level of carbon dioxide in neonatal infants to ensure that the level is not too high (hypercapnia) or too low is important for improving outcomes for neonates in intensive care.[6] Carbon dioxide can be monitored by taking a blood sample (arterial blood gas), through the breath (exhalation), and it can be measured continuously through the skin by using aminimally invasive transcutaneous device. The most effective and safest approach for measuring carbon dioxide in newborn infants is not clear.[7]