VBG versus ABG

The purpose of performing an arterial blood gas (ABG) is to identify alterations in acid base status or ventilation/oxygenation. Arterial blood gases can be troublesome at times. Patients may have hemodynamic instability (miking it tough to find the pulse), be unable to hold still for sample collection, and the procedure can occasionally be technically difficult. As a result, venous blood gases are often taken as a surrogate for arterial measures given they are easier to perform, less painful, and patients in the ICU often have central venous access.


There are several different areas from which a blood gas can be drawn:

1. Arterial blood gas - usually radial artery, though can be taken from brachial/femoral stab
2. Peripheral venous blood gas - any peripheral vein
3. Central venous gas - from right atrium, usually drawn from central line
4. Mixed venous gas - taken from the distal port of pulmonary arterial catheter

Of these different types of blood gas', the correlation with ABG's are variable:

1. Central venous vs ABG - pH 0.003-0.005 lower, pCO2 4-5 mmHg higher, no change in HCO3
2. Mixed venous vs ABG - similar to above
3. Peripheral venous vs ABG - pCO2 3-8 mmHg higher, HCO3 1-2 meq higher

pO2 values can not be reliably calculated from any form of venous gas.

Of these options central venous gas is preferred given its correlation with the ABG is most consistent and most studied, that being said, many patients dont have central lines.There was a study examining the effects of tourniquets on venous blood gas values and found that their use doesnt alter blood gas variables when used, which is reassuring.

The clinical context needs to be considered. If pateints are hypotensive and in shock, venous values are less reliable and ABG's are preferred. A study from NEJM in 1989 showed that as patients become more unstable, the arterial-venous difference gets larger. The difference between central venous and ABG pH in patients with shock was 0.1, highlighting that both measures need to be evaluated. They also concluded that tissue hypoperfusion is better assessed with central venous gas than ABG. Another NEJM study looked at a similar question in patients during cardiac arrest. The average ABG pH with during CPR was 7.41, while the mixed venous was 7.15! The pCO2 also changed dramatically. ABG may not reveal the true extent of tissue hypoxia compared to the mixed or central venous blood gas.

So, when in doubt, I say get an ABG, but in the critically ill patient/post CPR, a venous gas may show the true extent of the disease.