Hypocapnia

Hypocapnia is an abnormally low partial pressure of carbon dioxide in arterial blood, called PaCO₂. In adults, it is commonly defined as a PaCO₂ below about 35 mm Hg, although reference ranges can vary slightly. Hypocapnia most often occurs when the lungs remove carbon dioxide faster than the body's tissues are producing it .¹

The usual mechanism is hyperventilation, in which alveolar ventilation is excessive relative to metabolic carbon dioxide production. Increasing the depth or rate of breathing can therefore lower PaCO₂. This may occur in response to conditions such as hypoxemia, metabolic acidosis, pain, anxiety, fever, or pulmonary disease, and it can also result from excessive mechanical ventilation. In metabolic acidosis, the resulting hypocapnia can be an appropriate respiratory compensation rather than a primary respiratory disorder .¹

Because carbon dioxide is part of the body's bicarbonate buffer system, a primary fall in PaCO₂ raises blood pH and produces respiratory alkalosis. Hypocapnia and respiratory alkalosis are therefore closely related but are not identical terms. Hypocapnia describes the reduced arterial carbon dioxide level, whereas respiratory alkalosis describes the acid-base disturbance that occurs when the fall in PaCO₂ is primary .¹

Carbon dioxide also helps regulate cerebral blood flow. Lower PaCO₂ causes constriction of cerebral blood vessels and can reduce cerebral perfusion. With sufficiently marked acute hypocapnia, this effect can contribute to symptoms such as lightheadedness and, in some circumstances, syncope .^1,2