PaCO2 (arterial carbon dioxide): the breathing side of a blood gas

Every cell produces carbon dioxide as a waste gas. It travels to the lungs in blood and is breathed out. If breathing is too shallow or slow, carbon dioxide accumulates; if breathing is too fast or deep, it is washed out. PaCO2 captures this balance in one number. Lab Tests Online UK describes PCO2 as the partial pressure of carbon dioxide, the amount of carbon dioxide gas dissolved in the blood.

In water, carbon dioxide forms carbonic acid, which is why PCO2 is the respiratory (lung) part of acid-base balance, while bicarbonate is the metabolic (kidney) part. Their ratio sets blood pH.

It is part of an arterial blood gas, requested for breathing difficulty, shortness of breath, hyperventilation, to monitor oxygen therapy, during operations, in COPD to assess long-term oxygen therapy, and in patients on ventilators, with head or neck trauma or prolonged anaesthesia. Capillary heel-prick blood may be used for babies.

Blood gases are usually done in hospital because analysis must be immediate. The test cannot name the cause of an imbalance but points to a respiratory or metabolic problem.

Blood is usually taken from the radial artery at the wrist; an Allen test may first check the hand's circulation. Pressure is held on the site for at least 5 minutes, longer on blood thinners. Typically no preparation is needed, but if you are on oxygen the team may switch it off for 20 to 30 minutes for a room-air sample or record the oxygen amount, as Lab Tests Online UK explains.

A raised PCO2 with a low pH is respiratory acidosis, which Lab Tests Online UK ties to respiratory disease with not enough oxygen in and carbon dioxide out. Examples named are pneumonia, chronic obstructive pulmonary disease and over-sedation from drugs. Over time the kidneys retain bicarbonate to compensate, so in long-standing COPD the pH may be near normal despite a high PCO2.

Symptoms of carbon dioxide retention can include drowsiness, headache and confusion. These are reasons for urgent assessment, and in someone with COPD oxygen must be given at carefully controlled levels as directed by clinicians.

A reduced PCO2 with a raised pH is respiratory alkalosis, caused by over-ventilation; the source gives hyperventilation due to pain or emotional distress from a severe infection as an example. The body also lowers carbon dioxide deliberately to compensate for metabolic acidosis, which is why a low PaCO2 together with a low pH and low bicarbonate points to a metabolic problem such as ketoacidosis or kidney failure.

Treatment depends on the cause: bronchodilators and controlled oxygen in COPD, antibiotics for pneumonia, reversing sedatives, or breathing support such as non-invasive ventilation. The gas is repeated to monitor response. Call 999 for severe breathlessness, blue lips, drowsiness or confusion.