Do I need an Arterial Blood Gas (ABG)?

Is Venous Blood Gas (VBG) as good as an Arterial Blood Gas (ABG)?

In the emergency settings, we often seek a quick answer to patient’s milieu through blood gas parameters. Most ED physicians draw up the venous blood gas sample to look for the metabolic parameters and the electrolyte status of patients. However, in certain specialties and different settings, the usage of a VBG as equal to that of an ABG sample has been often disapproved.

Although an ABG sample serves as a gold standard to determine the pH, PaCO2, HCO3, lactates, and the PaO2 accurately, it, at times, gives poor indication of the electrolyte status. In addition, it comes with additional pain and risk of injury to the vessel (pseudo aneurysms, hematoma and AV fistula) and surrounding structures (nerve injury). Serial exams will need additional pricks, and also carries a risk of digital ischemia. Moreover, additional work may seem an additional delay in care too.

On the other hand, venous blood sampling is usually easier, less painful and more convenient. VBG gives you a rapid and good assessment of the pH, HCO3, and the base excess. As long as the patient is normocapnic, it gives a good estimate of the PaCO2. In cases where the pulse oximetry is reliable and the patient does not appear to be in shock state, the SpO2 is sufficient for clinical decision making. Additionally, it gives a quick estimate of the electrolytes too.

Take Home Points:

•  If you are concerned about the pH: The Venous pH has a good correlation with the arterial pH within 0.03 – 0.04 units
•  The HCO3 and the base excess has good correlation
•  Venous lactate has a good correlation with the arterial lactates unless < 2mmol/L
• If you are concerned about hypercarbia: VBG serves to be 100% sensitive in detecting hypercarbia with a cut off of PCO2 of 45 mm Hg. A venous PCO2 of more than 45 means you are hypercarbic. If you need to know the accurate CO2 levels then get an ABG
• The Venous lactate and the PCO2 may not correlate in shock states
•  Want to know the PaO2 or to calculate the A – a gradient, get an ABG

Further Readings:

Kelly, A. (2010). Review article: Can venous blood gas analysis replace arterial in emergency medical care. Emergency Medicine Australasia 22, p493–498.

 McCanny, P., Bennett, K., Staunton, P., McMahon, G. (2012).Venous vs arterial blood gases in the assessment of patients presenting with an exacerbation of chronic obstructive pulmonary disease. Am J Emerg Med. 30(6): p896-900.

Middleton P, Kelly AM, Brown J, Robertson M. (2006). Agreement between arterial and central venous values for pH, bicarbonate, base excess, and lactate. Emerg Med J. 23(8): p622-4.