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ABGs vs. VBGs

Values for pH, HCO3 and lactate do not differ significantly between the two modalities. The pCO2 in COPD patients does not appear to correlate well between ABG and VBG but it is unclear if this affects patient management. Based on the best available evidence, VBGs can be used instead of ABGs in the majority of patients to guide management.

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 Reliability of venous blood gas sodium, potassium and creatinine

Overall there is a good clinical positive correlation between venous blood gas and laboratory biochemistry measurements of sodium, potassium and creatinine.

Articles of Interest

VBG versus ABG

In general, ABGs rarely need to be performed unless an arterial line is in place (for arterial blood pressure monitoring and ease of blood sampling).

VBG vs ABG in the ED

Venous pH is 0.03 lower than arterial pH (venous pH 7.27 = arterial pH 7.3). Venous PCO2 is 6 mmHg higher than arterial PCO2, but with wide variability; in general, difficult to predict arterial PCO2 from venous PCO2 (although a PCO2 can still be useful, as noted above).

ABG Interpretation

VBGs are better for ruling out an abnormality than ruling in one e.g. PCO2 is higher in a venous sample so a normal value in venous sampling suggests a normal arterial value. You cannot use a VBG to make a diagnosis of Type 2 respiratory failure or assess oxygenation.

Blood Gases: ABG vs. VBG

The difference in the pCO2 measurements between the VBG and ABG is the most contested in the literature. There is a correlation between the arterial and venous pCO2, but the confidence intervals are large with an average difference ranging from 5.7- 8.6mmHg.

Hot Take - ABGs

This post is not to debate the accuracy of the difference between ABGs and VBGs; that is well-rooted in the evidence. This post is to question if we still need ABGs.

More on why the ABG is less useful than we think

... if the VBG has a high CO2 you need to do an ABG to find out exactly what it is, But if the venous CO2 is <45 then happy days.

PulmCrit- How to convert a VBG into an ABG

There are significant differences between specialties with regards to the opinion towards using VBG data. Emergency physicians seem to best understand the utility of VBGs (because they are constantly dealing with sick, undifferentiated patients who mostly don't have arterial catheters). Currently there seems to be greater interest in VBGs in the critical care community, as we are moving away from placing A-lines and towards using end-tidal CO2 to monitor patients.

Take to Work Tips: VBG, ABG, WhichBG?

The benefits of a VBG vs ABG are obvious – decreased pain, complications, and time – so perform a VBG instead of an ABG in the ED when appropriate!

The VEINART Trial: VBG vs ABG

Clinical Take Home Point: In non-hypoxemic patients requiring acid-base evaluation: VBG is less painful for patients compared to ABG. VBG is easier for the healthcare team compared to ABG. VBG provides useful information similar to ABG for physicians in regard to treatment decisions.

VBG vs ABG in Hypercarbia

Trending CO2 and pH to normal values can help guide discontinuation of NIV. BEWARE! VBGs have limited utility in trending CO2 until they reach a low-normal level.

VBGs are (almost) as good as ABGs

While arteriovenous agreement for pH is such that values are clinically interchangeable and agreement for bicarbonate is close, agreement for pCO2 is poor and for BE is unclear. That said, given knowledge of the performance characteristics of VBG analyses, integration of the clinical findings with VBG results is often sufficient to safely guide treatment decision making.

What's in a Blood Gas? VBG vs ABG

In conclusion, the VBG is a useful, less painful, and more easily gathered test than ABG to asses for acidosis and hypercarbia. Multiple studies have shown that pH correlates well between VBG and ABG. As for PvCO2, using a cutoff of 45mmHg is very sensitive in determining whether there is arterial hypercarbia. New studies are coming out showing that VBG can be useful even in shock states.