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PV card: VBG versus ABG


You obtain a venous blood gas (VBG) on a patient with a COPD exacerbation because you are concerned about hypercarbia. You get a value of 55 mmHg. How correlative is that compared to an arterial blood gas (ABG).

There has been a lot of literature on how well the pH correlates between the ABG and VBG but what about pCO2?

A small study (n=89) from 20121 found that with a cutoff of pCO2 < 45 mmHg, the venous pCO2 is 100% sensitive in ruling out arterial hypercarbia. When the pCO2 was ≥ 45 mmHg, the VBG was less correlative.

Below is a review by Dr. Michelle Reina (EM resident at Univ of Utah) and Dr. Rob Bryant (Intermountain Medical Center in Utah) of the VBG vs ABG correlative data, along with a proposed algorithm on what to do with patients with COPD exacerbation.

What is your practice with an elevated pCO2 value on VBG?

Adapted from 1–5
Go to the ALiEM Cards site for more resources.

Updated 1/31/13 at 2 pm PST:

  • Changed range of pH correlation between VBG and ABG = 0.03-0.04
  • Was typo in abstract of Kelly et al article.2 Stated difference between pHs was 0.4, rather than 0.04 as described in main results text.
McCanny P, Bennett K, Staunton P, McMahon G. Venous vs arterial blood gases in the assessment of patients presenting with an exacerbation of chronic obstructive pulmonary disease. Am J Emerg Med. 2012;30(6):896-900. [PubMed]
Kelly A, McAlpine R, Kyle E. Venous pH can safely replace arterial pH in the initial evaluation of patients in the emergency department. Emerg Med J. 2001;18(5):340-342. [PubMed]
Ma O, Rush M, Godfrey M, Gaddis G. Arterial blood gas results rarely influence emergency physician management of patients with suspected diabetic ketoacidosis. Acad Emerg Med. 2003;10(8):836-841. [PubMed]
Middleton P, Kelly A, Brown J, Robertson M. Agreement between arterial and central venous values for pH, bicarbonate, base excess, and lactate. Emerg Med J. 2006;23(8):622-624. [PubMed]
Koul P, Khan U, Wani A, et al. Comparison and agreement between venous and arterial gas analysis in cardiopulmonary patients in Kashmir valley of the Indian subcontinent. Ann Thorac Med. 2011;6(1):33-37. [PubMed]
Michelle Lin, MD
ALiEM Editor-in-Chief
Academy Endowed Chair of EM Education
Professor of Clinical Emergency Medicine
University of California, San Francisco
Michelle Lin, MD
Michelle Lin, MD

Latest posts by Michelle Lin, MD (see all)

  • This is very useful EBM review, I feel like the dynamic is shifting away from absolutely requiring ABGs only, but I still work with attendings who are absolute believers in ABGs. Hopefully in time the consensus will become more evidence based.

  • Ph difference 0.03-0.4 or 0.03-0.04?

    • Wow, what a great question and thanks for asking. Take a look at the Kelly et al BMJ Online site which we referenced for the 0.4 high end range of correlation. Looks like a TYPO in their abstract! The main results text says the average A-V pH difference = 0.04, but the abstract said 0.4!! Am I reading this right? Changed the PV card. Thanks again.

    • UK Study 2012: pH main difference 0,03
      Herrington WG et al. Are arterial and venous samples clinically equivalent for the estimation of pH, serum
      bicarbonate, and potassium concentration in critically ill patients? Diabet Med 2012; 29: 32.

  • A great comment from the Peer Review Demographics spreadsheet from an emergency physician from France:
    “Very good and very useful. I disagree on the point of venous lactates being comparable to arterial lactates. The comparison holds in the same range as pCO² i.e. for low ranges. For levels above 2mM/l arterial lactates dissociate from venous with low correlation (Bland Altman diagram), a better correlation is obtained with capillary lactates ( capillary tube applied to a small puncture of the earlobe that allows serial samples but requires your lab to be familiar with the method).”

    — Thanks for commenting. Is good to know about lactates being less correlative at arterial levels >2 mmol/L. Perhaps partly because VBG’s are less overall reflective of core circulation in states of shock and severe hypoperfusion?

    • I do think however that you can use VBG as a screening tool. If the lactate is elevated, such as in the case of sepsis, you could also trend VBGs and monitor lactate clearance rather than needing an ABG for the exact number.

  • Another great comment from Peer Review Demographics spreadsheet from emergency physician in Italy:
    “1) I would write more clearly that correlations for Acid-base status are not shown to hold up for states of shock and severe hypoperfusion

    2) Add this reference?
    Kelly AM. Review article: Can venous blood gas analysis replace arterial in emergency medical care.Emerg Med Australas 2010; 22(6): 493-498.

    — Good point about clarity. We put it down shock and severe hypoperfusion as CONs under the VBG section. Thanks for the reference! Will check it out. — ML

  • Something else to think about when you get a high lactate back on ABG or VBG is ethylene glycol poisoning. The biproduct, glycolate, reads as lactate on the blood gas machine and would need to be compared to a serum lactate. If the lactate on the gas is drastically discordant (higher) compared to the serum, this is very suspicious for EG poisoning. Just a fun fact!

  • Andrew Harrill

    So, just so I’m clear: in a hypercarbic patient, the utility of VBG is not sufficeient enough to determine the success of your interventions on your patient? ie the trend to normalization of a patient’s pCO2 after application of NIPPV? Likewise, a VBG can be used to obtain a pH and, if below, 45mmHg, a pCO2 that can be utilized to determine its contribution to the pH status?

    • Michelle Lin, MD

      Interesting questions. This is a tough one since the literature is still early on the pCO2>45 mmHg issue. A one-time venous pCO2>45 might not exactly correlate with an arterial pCO2. So I would use it to calculate compensatory pH numbers with caution. You can do it but realize that it might not be very precise.

      That being said I feel that trending venous pCO2 can be a good general reflection of intervention effectiveness. I definitely balance with the patient’s clinical appearance as well. If patient appears clinically better and venous pCO2 is getting better, I don’t get an ABG. The VBG is more of a confirmatory test. If the clinical picture and VBG are discrepant, I factor in the fact that the venous pCO2 isn’t perfect and seriously consider an ABG. Bottom line – A number in isolation such as pCO2 doesn’t change my clinical practice in isolation.

      Hope that helps. Still a fairly muddy issue where people have personal preferences on how to interpret VBGs based on current evidence. These are just mine.