Thursday, January 31, 2013

PV card: VBG versus ABG


Accuracy

Valid for practice

True to literature

Overall quality


Please peer-review rate this blog post by clicking on the stars.

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) published in American Journal of Emergency Medicine in 2012 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?


Feel free to download this card and print on a 4'' x 6'' index card.
[MS Word] [PDF v2]

 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]. View free BMJ Online full-text here. Stated difference between pHs was 0.4, rather than 0.04 as described in main results text.
References
  1. 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 Jul;30(6):896-900. PMID 21908141
  2. Kelly AM, 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 Sep;18(5):340-2. PMID 11559602
  3. Middleton P, Kelly AM, Brown J, Robertson M. Agreement between arterial and central venous values for pH, bicarbonate, base excess, and lactate. Emerg Med J. 2006 Aug;23(8):622-4. PMID 16858095
  4. Koul PA, Khan UH, Wani AA, Eachkoti R, Jan RA, Shah S, Masoodi Z, Qadri SM, Ahmad M, Ahmad A. Comparison and agreement between venous and arterial gas analysis in cardiopulmonary patients in Kashmir valley of the Indian subcontinent. Ann Thorac Med. 2011 Jan;6(1):33-7. PMID 21264169
  5. Ma OJ, Rush MD, Godfrey MM, Gaddis G. Arterial blood gas results rarely influence emergency physician management of patients with suspected diabetic ketoacidosis. Acad Emerg Med. 2003 Aug;10(8):836-41. PMID 12896883

Posted by
Labels: ,

8 comments:

  1. 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.

    ReplyDelete

  2. Ph difference 0.03-0.4 or 0.03-0.04?

    ReplyDelete
    Replies

    1. 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.

      Delete

    2. 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.

      Delete

  3. 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?

    ReplyDelete
    Replies

    1. 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.

      Delete

  4. 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

    ReplyDelete

  5. 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!

    ReplyDelete

Subscribe to: Post Comments (Atom)