Nephrology Book


Venous Blood Gas

Aka: Venous Blood Gas, VBG
  1. Approach
    1. Draw venipuncture without a Tourniquet
    2. Send to lab on ice and run immediately
    3. VBG Oxygen Saturation <75% decreases accuracy (and may be unreliable <50%)
      1. VBG pH will be less than ABG pH
      2. VBG pCO2 will be greater than ABG pCO2
  2. Efficacy: Best data points that correlate well between VBG and ABG
    1. Venous pH correlates well with arterial pH
      1. Correlation
        1. Average weighted difference: 0.03 pH units
        2. Variation range: +/- 0.1 pH Units
      2. Conditions where ABG and VBG are well correlated
        1. Diabetic Ketoacidosis (DKA)
        2. COPD Exacerbation
      3. Conditions where ABG and VBG are less correlated
        1. Mixed acid base disorders
        2. Hypotension (e.g. systolic Blood Pressure 60 mmHg)
    2. Venous bicarbonate has fair correlation with arterial bicarbonate
      1. Serum bicarbonate may also be obtained from serum chemistry panel
      2. Correlation
        1. Average weighted difference: 1.3 mmol
        2. Variation range: +/- 4-5 mmol
  3. Efficacy: Results or conditions in which VBG correlates well with ABG in some studies
    1. Carbon dioxide (pCO2)
      1. Correlation
        1. Some studies (see Zeserson study) have shown close correlation: 4.8 mmHg
        2. Other studies have shown 6 mmHg up to 20 mmHg variation from ABG
      2. Normal pCO2 is reassuring
        1. pCO2 <40 has 100% Negative Predictive Value for significant hypercarbia
  4. Efficacy: Results or conditions in which VBG correlates poorly with ABG
    1. Serum Potassium
      1. Varies by as much as 0.5 mEq/L
    2. Cardiac Arrest
  5. Resources
    1. Internet Book of Critical Care (
  6. References
    1. Kelly and Herbert in Majoewsky (2012) EM:Rap 12(12): 8
    2. Zeserson (2016) CJ Intensive Care Med +PMID: 27283009 [PubMed]

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