Arterial CO₂

Arterial CO₂ usually refers to the partial pressure of carbon dioxide in arterial blood, abbreviated PaCO₂. It reflects the partial pressure of dissolved carbon dioxide and is commonly measured as part of an arterial blood gas test. In healthy adults at sea level, PaCO₂ is typically about 35 to 45 mm Hg, although normal values can be affected by factors such as altitude .^1,2

PaCO₂ is closely related to alveolar ventilation, the amount of fresh air participating in gas exchange each minute. Carbon dioxide produced by metabolism is carried in the blood to the lungs and eliminated during breathing. At a given rate of carbon dioxide production, PaCO₂ varies inversely with alveolar ventilation. Reduced alveolar ventilation tends to raise PaCO₂, while increased alveolar ventilation tends to lower it .¹

Arterial CO₂ also plays an important role in acid-base balance. Dissolved carbon dioxide participates in the carbonic acid-bicarbonate buffer system. A primary rise in PaCO₂ tends to lower blood pH and can produce respiratory acidosis, whereas a primary fall tends to raise blood pH and can produce respiratory alkalosis .^1,2

PaCO₂ should not be confused with total carbon dioxide measured in serum chemistry tests. PaCO₂ is a partial pressure used mainly to assess ventilation and the respiratory component of acid-base balance. Serum total CO₂ measures carbon dioxide in several chemical forms and consists predominantly of bicarbonate .^1,3