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]
