Written by Jason Lesnick
Spoon Feed
This article summarizes data supporting use of venous blood gas (VBG) screening for hypercarbic respiratory failure rather than an arterial blood gas (ABG).
Because patients shouldn’t have to lose a radial pulse for us to learn acid-base
The Things We Do For No Reason™ series aims to help us all practice higher-value care by eliminating low-value practices. This narrative summary covers multiple studies that all compared using VBGs to screen for hypercarbic respiratory failure and comparing those results to ABGs.
Across multiple studies, a PvCO₂ <45 mmHg ruled out hypercarbia with 100% sensitivity and 34–57% specificity. VBG sampling caused significantly less pain (14.3 mm difference, 95%CI 8.1–25.3 mm) and avoided other ABG-related complications (thrombosis, aneurysm formation, hematoma, and needlestick injuries).
Although VBGs can rule out hypercarbia, they cannot quantify it accurately. 8 studies assessed 965 patients and found a weighted mean difference of 6.2 mmHg (PvCO2 is approximately 6.2 mmHg higher than a PaCO2), but the 95% limits of agreement varied widely–––between -17.4 and 23.9 mmHg.
Another fun piece of info from this paper is that a review of 13 studies comparing VBG and ABG values found that one can estimate the ABG’s pH by adding 0.033 to the pH on a VBG.
How will this change my practice?
Replacing ABGs with VBGs whenever possible is beneficial from a patient-centered perspective (VBGs are much less painful) and a cost perspective (often only specific personnel can perform ABGs). I’ve been practicing this way since at least 2019, when I co-wrote a chapter comparing ABGs to VBGs in a book on myths in emergency medicine. I highly recommend minimizing ABG use when VBGs can suffice.
Source
Things We Do for No Reason™: Arterial blood gas testing to screen for hypercarbic respiratory failure. J Hosp Med. 2025 Sep;20(9):1002-1004. doi: 10.1002/jhm.70039. Epub 2025 Apr 4. PMID: 40183604; PMCID: PMC12406756.
In the hypoperfused state or in cardiac arrest, the venous CO2 may dramatically increase at the same time the arterial CO2 decreases.
Great point. Better yet in an unstable/peri-arrest patient, start an art line and draw an ABG from it.
Is the book on myths in emergency medicine still available? It sounds interesting
Available and there’s 2 volumes!