Carbon dioxide (CO2) is a colourless gas produced as a waste product of cellular metabolism when cells use oxygen to generate energy from glucose. It is transported in the blood (mainly as bicarbonate) to the lungs, where it is exhaled.
In the context of cardiac and critical care, monitoring carbon dioxide levels is important for several reasons. The level of CO2 in arterial blood (PaCO2, measured by blood gas analysis) reflects the adequacy of ventilation. A rising PaCO2 indicates that the lungs are not removing CO2 efficiently (hypoventilation), which leads to respiratory acidosis. A falling PaCO2 indicates hyperventilation, causing respiratory alkalosis.
During and after cardiac arrest, CO2 management is critical. End-tidal CO2 (ETCO2), the concentration of CO2 in exhaled breath, is monitored during CPR as a real-time indicator of the effectiveness of chest compressions and cardiac output: rising ETCO2 during resuscitation suggests improving circulation, and a sudden rise often indicates return of spontaneous circulation (ROSC).
In ventilated survivors of cardiac arrest in critical care, tight control of PaCO2 is important because both elevated CO2 (causing vasodilation and raised intracranial pressure) and very low CO2 (causing cerebral vasoconstriction and reduced brain blood flow) can worsen hypoxic brain injury. Current guidelines recommend targeting a normal PaCO2 (4.5 to 6.0 kPa) in the post-arrest period.
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