Acidosis & VBG’s

We are frequently asked to check the lactate on Venous Blood Gases (VBG’s), by the nursing staff. However, remember to look at the first result (pH) it is the most important.

Acidosis: Unless you have a good reason (e.g. you know its due to DKA) you should be investigating and performing an Arterial Blood Gas (ABG)

Comparing VBG & ABG results

pH

Evidence shows that the variation between VBG and ABG is approximately pH ±0.03, so VBG and ABG can be considered equivalent. (Occasionally if there has been difficulty getting the VBG or delays in testing the pH may be artificially low – BUT never assume it prove it)

pH is a log rhythmic scale so small changes in pH make big differences to the concentration of Hydrogen ion concentration.

pCO2

The correlation of pCObetween VBG and ABG is less predictable than pH. Studies tend to state that pCOon VBG is 1-3kPa higher than ABG. (although as you an see on the graph results can vary in both directions)

The Australians, state that a pCO2 <6.0 on VBG can be used to rule out hypercarbia.

pO2

The correlation between pO2 on VBG and ABG is appalling, and if you feel that knowing pOwill help manage the patient you will need to perform an ABG.

 

Bicarbonate (HCO3)

Bicarbonate (HCO3) like pH correlates very well between VBG and ABG, and can be assumed to be equivalent.

However, as with pH, difficult samples can have an artificially low HCO3

Carbon Monoxide (CO)

VBG is a perfectly good way to measure Carbon Monoxide (CO). But be AWARE that while the patient  has been traveling to and waiting in the ED they will have been breathing off their CO. So Make sure you take a good history and if you have concerns get them to get their house checked – Are you CO Aware?

Hb & Electrolytes

This is not the primary function of the blood gas analyser and the results can be up to 20% off. So treat these results will some caution.

 

References

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