Venous Blood Gas (VBG)

Indications

Need for Assessment of pH and pCO2 with an Inability to Obtain an Arterial Blood Gas (ABG) (see Arterial Blood Gas, [[Arterial Blood Gas]])

Clinical Efficacy

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Technique

Parameters

  • General Comments
    • Differences Between Arterial and Venous Values are Due to the Uptake and Buffering of Metabolically-Produced CO2 in the Capillaries and the Addition of Organic Acids Produced by the Tissue Bed Drained by the Vein
  • Venous pH
    • pH from Arterial and Venous Samples Correlate Reasonably Well (Respirology, 2014) [MEDLINE] and (Emerg Med J, 2014) [MEDLINE]: with agreement being highest at normal values
      • Venous pH is Approximately 0.03 Lower than the Arterial pH (95% Confidence Interval: 0.039 to 0.027) (Emerg Med J, 2014) [MEDLINE]
  • Venous pCO2
    • pCO2 from a Venous Sample is Approximately 4.4 mm Hg Higher Than the pCO2 from an Arterial Sample: for this reason, a normal venous pCO2 has a good negative predictive value for a normal arterial pCO2
      • Venous pCO2 is Approximately 4.4 mm Hg Higher than the Arterial pCO2 (95% Confidence Interval: 2.55-6.27) (Eur J Emerg Med, 2014) [MEDLINE]
  • Venous pO2
    • pO2 from Arterial and Venous Samples Do Not Correlate with Each Other
      • Arterial PO2 is Approximately 36.9 mm Hg Higher than the Venous pO2, with Significant Variability (95% Confidence Interval: 27.2-46.6 mm Hg) (Respirology, 2014) [MEDLINE]
  • Venous Bicarbonate
    • Bicarbonate from Arterial and Venous Samples Correlate Reasonably Well
      • Venous Bicarbonate is Approximately 1.03 mmol/L Higher than the Arterial Bicarbonate (95% Confidence Interval 0.56-1.50) (Eur J Emerg Med, 2014) [MEDLINE]: agreement is highest at normal values
  • Venous Carboxyhemoglobin (by Co-Oximetry)
    • Carboxyhemoglobin from Arterial (ABG) and Venous (VBG) Samples Correlate Well (Ann Emerg Med, 1995) [MEDLINE] and (Crit Care Med, 2000) [MEDLINE]
  • Venous Lactate (see Serum Lactate, [[Serum Lactate]])
    • Lactate from Arterial and Venous Samples Correlate Poorly at Abnormal Levels: however, this agreement is closer at normal levels such that, if the venous lactate is normal, the arterial lactate is generally also normal
      • Venous Lactate is Approximately 0.25 mmol/L Higher Than the Arterial Lactate (95% Confidence Interval: 0.15-0.35) (Eur J Emerg Med, 2014) [MEDLINE]

Cautions

  • While Blood Gas Analyzers May Report Potassium Values, These Analyzers Do Not Typically Report if the Sample Has Been Hemolyzed (as Clinical Laboratories Routinely Do): for this reason, use of a VBG sample to assess potassium must be interpreted with caution

References

  • Relationship between venous and arterial carboxyhemoglobin levels in patients with suspected carbon monoxide poisoning.  Ann Emerg Med.  1995;25:481–483 [MEDLINE]
  • Relationship between arterial, mixed venous, and internal jugular carboxyhemoglobin concentrations at low, medium, and high concentrations in a piglet model of carbon monoxide toxicity.  Crit Care Med 2000 Jun;28:1998–2001 [MEDLINE]
  • Agreement between mathematically arterialised venous versus arterial blood gas values in patients undergoing non-invasive ventilation: a cohort study.  Emerg Med J.  2014;31(e1):e46–e49 [MEDLINE]
  • Peripheral venous and arterial blood gas analysis in adults: are they comparable? A systematic review and meta-analysis. Respirology.  2014;19:168–175 [MEDLINE]
  • The role of venous blood gas in the emergency department: a systematic review and meta-analysis.  Eur J Emerg Med.  2014 Apr;21:81–88 [MEDLINE]