Blood Gas
pCO2 (Partial Pressure of Carbon Dioxide)
pCO2 is the partial pressure of carbon dioxide dissolved in blood, reported as part of a blood gas panel. It reflects the respiratory, or ventilatory, component of acid-base balance, since carbon dioxide is cleared from the body by the lungs.
Why it is measured
pCO2 helps distinguish respiratory from metabolic acid-base disturbances and indicates how effectively a patient is ventilating. It is central to assessing respiratory failure, guiding ventilator decisions and monitoring critically unwell patients.
| Typical range | Indicative adult arterial range is approximately 35 to 45 mmHg (4.7 to 6.0 kPa). Venous samples typically read a little higher, around 41 to 51 mmHg (5.5 to 6.8 kPa). Ranges vary by method, sample type and reporting units, so always interpret against the analyser's stated range. |
|---|---|
| Sample | Arterial or venous whole blood, usually collected into a lithium heparin syringe or capillary and analysed promptly without separation. pCO2 is measured directly by the analyser, classically using a Severinghaus-type CO2 electrode or an equivalent sensor. |
| Turnaround | Typically under 1 to 2 minutes from sample insertion at the point of care, with some analysers reporting a full blood gas panel in around 35 to 60 seconds. |
Point of care devices that report it
- Abbott i-STAT (CG4+ and CG8+ cartridges)
- Siemens Healthineers epoc Blood Analysis System
- Radiometer ABL90 FLEX and FLEX PLUS
- Werfen GEM Premier series
- Nova Biomedical Stat Profile Prime series
Questions, answered
What is the difference between pCO2 measured on an arterial versus a venous sample?
Arterial pCO2 reflects gas exchange in the lungs and is the reference standard for assessing ventilation. Venous pCO2 tends to read a few mmHg higher because the blood has already passed through the tissues. Many point-of-care analysers can report either, so the sample type should always be recorded and results interpreted against the correct range. This is general educational guidance, not patient-specific advice.
Why might a point-of-care pCO2 differ from a main laboratory result?
Differences usually arise from the sample type, the delay before analysis, trapped air bubbles, sample temperature and the specific method or calibration of each analyser. Whole-blood gas samples should be analysed quickly and kept free of air, because exposure changes the dissolved gas. When comparing devices, use split samples and document any method differences rather than assuming one value is wrong.
How quickly does a pCO2 sample need to be analysed?
Blood gas samples are time-critical and are generally analysed within minutes of collection, which is a key reason pCO2 is measured at the point of care. Delay, warming or contact with air allows the dissolved carbon dioxide to drift, which can shift the result. Follow local protocols for collection, mixing and analysis timing to keep results reliable.