Venous blood gas

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Background

The venous blood gas (VBG) is a multi-component serum assessment of pH, blood gas tensions (PvO2 and PvCO2), bicarbonate (HCO3), and the base excess.

  • can be drawn from an IV catheter along with other bloodwork, unlike an arterial blood gas (ABG),
  • does not accurately reflect (PaO2)
  • arterial values of pH, CO2, and HCO3 can be extrapolated with varying levels of accuracy [1] [2][3]

Components

pH

  • measurement of acidity/alkalinity: normal range 7.37-7.45
  • pH > 7.45 = alkalosis
  • pH< 7.35 = acidosis
  • correlates well with arterial pH [4]

PvO2

  • measures the partial pressure of oxygen dissolved in the serum; that is, the dissolved oxygen not carried by hemoglobin
  • significantly lower than and nonindicative of arterial oxygen pressure (PaO2)

PvCO2

  • The amount of carbon dioxide dissolved in arterial blood. The normal range is 35 to 45 mm Hg.
  • PvCO2 >45 = primary respiratory acidosis
  • PvCO2 <35 = primary respiratory alkalosis
  • correlates with controversial accuracy with PaCO2: see discussion below

HCO3

  • calculated from PvCO2 and pH with Henderson-Hasselbalch equation
  • correlates well with serum HCO3 [5]

Base Excess

  • an estimation of the metabolic component of acid/base status: how much "extra base" there is in the system
  • the theoretical number of H+ needed to bring pH to 7.40 at pCO2=40
  • reference range -2 to +2
    • positive in metabolic alkalosis, negative in metabolic acidosis
  • can be expressed as "base deficit", with the same but opposite values
    • e.g. base excess=-8 in a case of lactic acidosis; base deficit=+8

Comparison with ABG

Disadvantages

  • no information about PaO2
  • poor correlation with PaCO2
    • one study in COPDers shows PvCO2 exceeds PaCO2 by 5.4, but only unreliably so with 95%i nterval -8.8 to +20.5 mmHg[6]
      • sensitivity for elevated arterial hypercapnea was 100% in this study with PvCO2>45; authors suggest that PvCO2 can be used to screen for hypercapnea
    • metaanalysis shows 95% prediction interval PvCO2 of -10.7 mm Hg to +2.4 mm Hg [7]

Advantages

  • convenience
    • no extra, painful radial artery access to draw ABG
    • can be easily obtained with IV access
  • pH very reliable
  • some studies do show
  • in the era of continuous pulse ox, meaningful information about oxygenation is always available
  • the PvCO2 is internally consistent
    • can trend CO2 response to treatment; just don't know exact starting point

External Links

http://www.clinicalcorrelations.org/?p=5608 https://lifeinthefastlane.com/ccc/vbg-versus-abg/

References

  1. Byrne AL, Bennett M, Chatterji R, Symons R, Pace NL, Thomas PS. Peripheral venous and arterial blood gas analysis in adults: are they comparable? A systematic review and meta-analysis. Respirology. 2014 Feb;19(2):168-75. doi: 10.1111/resp.12225. Epub 2014 Jan 3. Review. PubMed PMID: 24383789.
  2. Malatesha G, Singh NK, Bharija A, Rehani B, Goel A. Comparison of arterial and venous pH, bicarbonate, Pco2 and Po2 in initial emergency department assessment. Emergency Medicine Journal : EMJ. 2007;24(8):569-571. doi:10.1136/emj.2007.046979.
  3. 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.
  4. 1: Brandenburg MA, Dire DJ. Comparison of arterial and venous blood gas values in the initial emergency department evaluation of patients with diabetic ketoacidosis. Ann Emerg Med. 1998 Apr;31(4):459-65. PubMed PMID: 9546014.
  5. Nargis W, Rahman AS, Ahamed BU, Hossain MZ. Comparison of values of traditionally measured venous bicarbonate with calculated arterial bicarbonate in intensive care unit patients of a hospital in a third-world country. Nigerian Medical Journal : Journal of the Nigeria Medical Association. 2014;55(4):285-288. doi:10.4103/0300-1652.137186.
  6. Kelly AM, Kyle E, McAlpine R. Venous pCO(2) and pH can be used to screen for significant hypercarbia in emergency patients with acute respiratory disease. J Emerg Med. 2002 Jan;22(1):15-9. PubMed PMID: 11809551.
  7. Byrne AL, Bennett M, Chatterji R, Symons R, Pace NL, Thomas PS. Peripheral venous and arterial blood gas analysis in adults: are they comparable? A systematic review and meta-analysis. Respirology. 2014 Feb;19(2):168-75. doi: 10.1111/resp.12225. Epub 2014 Jan 3. Review. PubMed PMID: 24383789.