Do venous blood gas (VBG) and arterial blood gas (ABG) show the same bicarbonate levels?

From the Guidelines

Venous blood gas (VBG) and arterial blood gas (ABG) typically show similar bicarbonate levels, with only minor differences that are usually not clinically significant. The bicarbonate concentration in venous blood is generally 1-2 mEq/L higher than in arterial blood, as bicarbonate is distributed throughout the body's extracellular fluid and is not significantly consumed or produced during the transit of blood from arterial to venous circulation 1. Some key points to consider include:

• The small difference in bicarbonate levels between VBG and ABG results from the higher CO2 content in venous blood, which slightly shifts the equilibrium toward more bicarbonate formation.
• This consistency makes VBG measurements of bicarbonate a reliable substitute for ABG measurements in many clinical scenarios, allowing clinicians to avoid the more painful and technically challenging arterial puncture when only acid-base status assessment is needed.
• However, when precise evaluation of respiratory status is required, ABG remains the gold standard as it provides accurate measurements of PaO2 and PaCO2, which differ significantly between arterial and venous samples. The physiology of carbon dioxide, as described in the BTS guideline for oxygen use in adults in healthcare and emergency settings 1, highlights the importance of understanding the differences between arterial and venous blood gas measurements in clinical practice.

From the Research

Bicarbonate Levels in Venous and Arterial Blood Gas

• The study 2 found that the weighted mean arterio-venous difference for bicarbonate was -1.41 mmol/L, with 95% limits of agreement of the order of ±5 mmol/L, suggesting that venous pH, bicarbonate, and base excess have sufficient agreement to be clinically interchangeable for arterial values in patients who are not in shock.
• Another study 3 reported that the values of bicarbonate on arterial and venous samples showed close agreement, with an average difference between the samples of 1.20 mmol/L (95% limits of agreement being -2.73 to +5.13 mmol/L), indicating that venous bicarbonate estimation may be an acceptable substitute for arterial measurement.
• However, a study 4 found significant differences in acid-base state between venous and arterial blood during cardiopulmonary resuscitation, with mixed venous blood demonstrating striking decreases in pH and increases in PCO2, suggesting that arterial blood gases may not accurately reflect the acid-base state in this emergency setting.
• A study 5 found significant correlations in pH, PCO2, BE, and HCO3 between arterial, venous, and capillary blood gases, suggesting that capillary and venous blood gas measurements may be useful alternatives to arterial samples for patients who do not require regular continuous blood pressure recordings and close monitoring of PaO2.

Comparison of Bicarbonate Levels

• The studies 2, 3 suggest that bicarbonate levels in venous and arterial blood gas may be similar, with acceptable limits of agreement, in certain clinical settings.
• However, the study 4 highlights the importance of considering the specific clinical context, such as cardiopulmonary resuscitation, where significant differences in acid-base state between venous and arterial blood may occur.
• The study 5 found significant correlations in bicarbonate levels between arterial, venous, and capillary blood gases, but noted that capillary and venous blood gas measurements may not be suitable for determining PO2 in certain clinical settings, such as hypotension.