An arterial blood gas (ABG) is the traditional method of estimating the systemic carbon dioxide tension and pH, usually for the purpose of assessing ventilation and/or acid-base status. However, the necessary sample of arterial blood can be difficult to obtain due to diminished pulses or patient movement. Diminished pulses may reflect poor peripheral circulation or low blood pressure, while patient movement is frequently caused by the pain associated with arterial puncture.
A venous blood gas (VBG) is an alternative method of estimating systemic carbon dioxide and pH that does not require arterial blood sampling. Performing a VBG rather than an ABG is particularly convenient in the intensive care unit, since most patients have a central venous catheter from which venous blood can be quickly and easily obtained.
The sites from which venous blood can be sampled, measurements that can be performed on venous blood, and correlation of venous measurements with arterial measurements are reviewed here. Other alternatives to ABGs for estimating systemic carbon dioxide and pH are also described, including end-tidal carbon dioxide and transcutaneous carbon dioxide. ABGs are reviewed separately. (See "Arterial blood gases".)
VENOUS BLOOD GASES
Sampling sites — A VBG can be performed using a peripheral venous sample (obtained by venipuncture), a central venous sample (obtained from a central venous catheter), or a mixed venous sample (obtained from the distal port of a pulmonary artery catheter).
Central venous blood gases are preferred because their correlation with arterial blood gases is the most well-established by research and clinical experience. Peripheral venous blood gases are an alternative for patients who do not have central venous access. If a tourniquet is used to facilitate venipuncture, it should be released about one minute before the sample is drawn to avoid changes induced by local ischemia . Mixed venous blood gases are a reasonable alternative for patients whose venous access is a pulmonary artery catheter; however, a pulmonary artery catheter should not be inserted for the sole purpose of venous blood sampling. (See "Pulmonary artery catheterization: Indications and complications".)