Venous blood gas (VBG) interpretation

• Arterial blood gases (ABGs) are commonly used for estimating the acid-base status, oxygenation and carbon dioxide concentration of unwell patients. However, arterial blood can be difficult to obtain due to weak pulses or patient movement. Due to thicker, muscular and innervated walls, arteries are also more painful to puncture than veins. As such, a venous blood gas (VBG) is an alternative method of estimating pH and other variables.
• To learn how to interpret an ABG sample (including acid-base disturbances) click here.

Is a venous blood gas comparable to an arterial blood gas?

• The values on a VBG and ABG are comparable (arterial and venous values are NOT significantly different for practical purposes) except in the cases of O2 and CO2.
• From a 2014 meta-analysis and related papers:
  • VBG analysis compares well with ABG analysis for pH estimations in adults
    • Peripheral venous pH is only pH 0.02 to 0.04 lower than the arterial pH
    • Peripheral venous HCO3 concentration is approximately 1 to 2 meq/L higher than arterial HCO3
  • However, venous and arterial PCO2 are not comparable
    • The 95% prediction interval of the bias for venous PCO2 is unacceptably wide, extending from -10.7 mmHg to +2.4 mmHg
  • Venous and arterial PO2 are also not comparable
    • The arterial PO2 is typically 36.9 mmHg greater than the venous with significant variability (95% confidence interval from 27.2 to 46.6 mmHg).

• Note that much of the utility of VBG and ABG analysis is to gather serial readings to determine response to treatment. An initial VBG allows repeated VBG analysis (limiting patient discomfort) as opposed to requiring repeated ABG analysis to compare with an initial ABG.
• In light of the above if there is no concern about the respiratory system it is reasonable to take a VBG as opposed to an ABG in the first instance when presented with an unwell patient.
• A classic example of this is DKA, where the key variables are pH, lactate and potassium, all of which can be gathered from a VBG
• However where there is concern about a respiratory component to an illness, an initial ABG is necessary.

References