VBG versus ABG

Reviewed and revised 7 January 2016

OVERVIEW

• Venous blood gases (VBG) are widely used in the emergency setting in preference to arterial blood gases (ABG) as a result of research published since 2001
• The weight of data suggests that venous pH has sufficient agreement with arterial pH for it to be an acceptable alternative in clinical practice for most patients
• Nevertheless acceptance of this strategy has been limited by some specialties and maybe inappropriate in some settings; for instance there is no data to confirm that this level of agreement is maintained in shock states or mixed acid-base disturbances
• Clinically acceptable limits of agreement for blood gas parameters remains poorly defined

ARTERIAL BLOOD GAS PROS AND CONS

Advantages

• gold standard test for determining the arterial metabolic milieu (pH, PaCO2, HCO3)
• can determine PaO2

Disadvantages

• pH, PCO2 (if normocapnic), HCO3 and base excess from a VBG are usually adequate for clinical decision making
• SpO2 is usually sufficient for clinical decision making unless pulse oximetry is unreliable for other reasons (e.g. shock state, poor pick up)
• painful (should be performed with local anaesthetic in conscious patients)
• increased risk of bleeding and hematoma
• risk of pseudo aneurysm and AV fistula
• infection
• nerve injury
• digital ischemia
• injury to staff
• delays in care
• serial exams may be needed
• venous sampling may better represent the tissue milieu

CORRELATION BETWEEN VBG AND ABG

pH

• Good correlation
• pooled mean difference: +0.035 pH units

pCO2

• good correlation in normocapnia
• non-correlative in severe shock
• 100% sensitive in detecting arterial hypercarbia in COPD exacerbation using cutoff of PaCO2 45 mmHg and laboratory based testing (McCanny et al, 2012), i.e. if VBG PCO2 is normal then hypercapnia ruled out (PaCO2 will be normal), though this conflicts with the meta-analysis by Byrne et al 2014 (see below)
• correlation dissociates in hypercapnia – values correlate poorly with PaCO2 >45mmHg
• Mean difference pCO2 +5.7 mmHg (wide range in 95%CIs among different studies, on the order of +/-20 mmHg)
• A more recent meta-analysis by Byrne et al, 2014 found that the 95% prediction interval of the bias for venous PCO₂  was −10.7 mm Hg to +2.4 mm Hg. They note that in some cases the PvCO2 was lower than the PaCO2. The meta-analysis had considerable heterogeneity between studies which limits the reliability of its conclusions.

HCO3

• Good correlation
• Mean difference −1.41 mmol/L (−5.8 to +5.3 mmol/L 95%CI)

Lactate

• Dissociation above 2 mmol/L
• Mean difference 0.08 (-0.27 – 0.42 95%CI)

Base excess

• Good correlation
• Mean difference 0.089 mmol/L (–0.974 to +0.552 95%CI)

PO2

• PO₂ values compare poorly
• arterial PO₂ is typically 36.9 mm Hg greater than the venous with significant variability (95% confidence interval from 27.2 to 46.6 mm Hg) (Byrne et al, 2014)
• See also: Central venous oxygen saturation (ScvO2) monitoring, mixed venous oxygen saturation (SvO2) and SvO2 vs ScvO2

DIABETIC KETOACIDOSIS

VBG can be used to guide management in preference to ABG (Ma et al, 2003)

• VBG correlated with ABG well
• Mean difference in pH -0.015 ± 0.006 units [95% CI]
• ABG pH changed treatment or disposition in 2.5% cases compared to VBG pH

WHEN IS ABG NECESSARY?

ABG may be necessary:

• to accurately determine PaCO2 in severe shock
• to accurately determine PaCO2 if hypercapnic (i.e. PaCO2 >45 mmHg)
• to accurately determine arterial lactate >2mM (rarely necessary)

In general, ABGs rarely need to be performed unless an arterial line is in place (for arterial blood pressure monitoring and ease of blood sampling)

References and Links

Journal articles

• 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 Jan 3. PMID: 24383789.
• Kelly AM. Review article: Can venous blood gas analysis replace arterial in emergency medical care? Emerg Med Australas. 2010 Dec;22(6):493-8. PMID: 21143397.
• 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
• 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
• 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
• 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
• 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. PMID16858095

FOAM and web resources

• ALIEM — Paucis Verbis Card: VBG versus ABG (2012)
• FET — Venous and Arterial Blood Gas Analysis in the ED by Anne-MArie Kelly (2012)
• FET — Can a Venous Blood Gas Substitue for an Arterial Blood Gas by Jason Chu (2013)