ABG vs. VBG

DO I REALLY NEED TO STICK THIS PERSON AGAIN, OR CAN I DRAW OFF A VBG? (scroll down for an even more abridged version)


STUDY #1: (hypothetical: metabolic derangement)
--retrospective review of ABG and VBG’s in stable DKA patients. 


--in patients with DKA, the difference between arterial and venous pH was 0.02 units (-0.009 to +0.021) and the difference between bicarbonate was -1.88 mEq/L. 


--their conclusion: VBGs are fine in a hemodynamically stable DKA patient who isn’t in respiratory distress. 




STUDY #2: (hypothetical: respiratory failure)
--compared ABGs and VBGs in the setting of acute COPD exacerbations. 


--their conclusion: the pH correlates extremely well.  also, when using a cutoff of 45 mmHg, the venous pCO2 will catch all cases of arterial hypercarbia. 


--however, there is a large 95% CI so the authors conclude by saying that a VBG is excellent at detecting hypercarbia (present/absent) and useful for initiating non-invasive ventilation, but it is not good enough to replace the ABG in assess the degree of hypercarbia.




STUDY #3: (hypothetical: the hypotensive patient)
--looked at the difference between values on an ABG and a VBG in the setting of hypotension. 
--about 190 patients, 70 hypotensive and 120 normotensive. 


--the average ABG-VBG difference in hypotensive vs normotensive groups was minimal for pH, small for CO2/bicarb, and larger for PO2/SO2.


--their conclusion: Hypotensive status is associated with an increase in the amount of difference between VBG and ABG analysis regarding pH, HCO(3), and BE, though the amount of increase does not seem to be clinically important




ABRIDGED VERSION: (ABG vs. VBG):
--study 1: in DKA, the pH was very similar, bicarb pretty close
--study 2: in COPD, the pH was very similar, VBG PCO2 > 45mmHg cutoff is 100% sensitive for arterial hypercarbia, but the VBG PCO2 value itself is not as good
--study 3: in hypotension, the pH was very similar, PCO2/bicarb pretty close, not so good for oxygen status




BOTTOM LINE:
--if you care about the pH: VBG seems to work just fine
--if you care about hypercarbia: hypercarbia on VBG (>45) probably means you're hypercarbic
--if you care about the amount of hypercarbia: probably need to get the ABG
--if you care about the P02 or 02 sat: probably need to get the ABG




Submitted by J. Gullo.




Reference(s): Study #1 (PMID:  16454777), Study #2 (PMID:  21908141), STUDY #3 (PMID:  22091230), picture