Clinical and Interpretive

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.

A VBG can be performed using:

●A peripheral venous sample (obtained by venipuncture)

●A central venous sample (obtained from a central venous catheter)

●Mixed venous sample (obtained from the distal port of a pulmonary artery catheter)

Central venous blood gases have been preferred because their correlation with arterial blood gases is the most well-established by research and clinical experience. Peripheral venous blood gases have been studied in critically ill patients as 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.

A VBG measures the venous oxygen tension (PvO₂), carbon dioxide tension (PvCO₂), acidity (pH), oxyhemoglobin saturation (SvO₂), and serum bicarbonate (HCO₃) concentration:

●PvCO₂, venous pH, and venous serum HCO₃ concentration are used to assess ventilation and/or acid-base status

●SvO₂ is used to guide resuscitation during severe sepsis or septic shock, a process called early goal-directed therapy

●PvO₂ has no practical value at this time. It is not useful in assessing oxygenation because oxygen has already been extracted by the tissues by the time the blood reaches the venous circulation.

The inability of a VBG to measure oxygenation is the major drawback compared with an ABG. To overcome this limitation, VBGs are often considered in combination with pulse oximetry.

The usual approach to interpreting a VBG consists of using the venous measurements to estimate the corresponding arterial values, then using these estimated values for clinical decision-making exactly as if an ABG had been performed. The difference between the venous measurements and the arterial measurements depends upon the site of venous sampling.

Although arterial blood gas analysis is more accurate than venous analysis for the assessment of oxygenation, measurement of PCO₂, pH, and HCO₃ are similar with some minor adjustments:

●The central venous pH is usually 0.03 to 0.05 pH units lower than the arterial pH and the PCO₂ is usually 4 to 5 mmHg higher, with little or no increase in HCO₃.  Mixed venous blood (ie, SvO₂ drawn from a pulmonary artery catheter) gives results similar to central venous blood (ie, ScvO₂ drawn from a central venous catheter).

●The peripheral venous pH is approximately 0.02 to 0.04 pH units lower than the arterial pH, the venous serum HCO₃ concentration is approximately 1 to 2 meq/L higher, and the venous PCO₂ is approximately 3 to 8 mmHg higher.

There are no venous to arterial conversions for ScvO₂, SvO₂, or peripheral venous oxyhemoglobin saturation (PvO₂).

Importantly, sufficient variability between arterial and venous blood gas values may exist such that periodic correlation between arterial and venous blood gas values is always prudent.

There is poor correlation between arterial and venous blood gas measurements in patients with hemodynamic instability.