How do Venous Blood Gases (VBG) and Arterial Blood Gases (ABG) compare in assessing partial pressure of carbon dioxide (pCO2)?

From the Guidelines

Venous blood gases (VBGs) can be used as a reasonable substitute for arterial blood gases (ABGs) when assessing pCO2, with some important caveats. Typically, venous pCO2 runs about 4-6 mmHg higher than arterial pCO2 in stable patients, making VBGs a less invasive alternative for monitoring respiratory status. This relationship is most reliable in hemodynamically stable patients with normal perfusion. However, in critically ill patients, especially those with shock, poor perfusion, or severe respiratory distress, the venous-arterial pCO2 difference can become unpredictable and wider. In these situations, ABGs remain the gold standard for accurate assessment of respiratory status, as noted in the British Thoracic Society guidelines for home oxygen use in adults 1. The physiological basis for this difference is that venous blood contains more carbon dioxide as it returns from tissues where CO2 is produced, while arterial blood has already been cleared of some CO2 through pulmonary gas exchange. When interpreting VBGs for pCO2 assessment, clinicians should mentally adjust the value downward by approximately 5 mmHg to estimate the corresponding arterial value, but should obtain an ABG for definitive assessment when precise measurements are needed for critical decision-making. Some studies suggest that capillary blood gases (CBG) sampling can be used in place of ABG sampling for re-measuring PaCO2 and pH at different oxygen flow rates 1, while others highlight the utility of transcutaneous CO2 monitoring (PTCCO2) and end-tidal PCO2 (PETCO2) as surrogates of PaCO2 during diagnostic studies 1. However, the most recent and highest quality study on this topic is from 2015, which emphasizes the importance of ABG sampling for initial assessment and confirmation of the need for long-term oxygen therapy (LTOT) 1.

Key Points

• VBGs can be used as a substitute for ABGs in stable patients
• Venous pCO2 is typically 4-6 mmHg higher than arterial pCO2
• ABGs remain the gold standard in critically ill patients or when precise measurements are needed
• Capillary blood gases and transcutaneous CO2 monitoring can be used as alternatives in certain situations
• The most recent guidelines recommend ABG sampling for initial assessment and confirmation of LTOT 1

Clinical Implications

• Clinicians should be aware of the limitations and potential differences between VBGs and ABGs when assessing pCO2
• The choice of blood gas sampling method should be based on the individual patient's condition and the clinical context
• ABGs should be obtained when precise measurements are needed for critical decision-making, especially in critically ill patients or when adjusting oxygen therapy 1

From the Research

Comparison of Venous and Arterial Blood Gases

• Venous blood gases and arterial blood gases are two different methods of assessing pCO2 levels in patients.
• A study published in 2016 2 compared the agreement between transcutaneous measurement of carbon dioxide (PtCO2) and arterial/venous carbon dioxide tension (PaCO2/PvCO2) in patients with severe hypoxaemic and/or hypercapnic respiratory failure during noninvasive ventilation.
• The results showed that the mean difference between PaCO2 and PvCO2 was 0.46 kPa at baseline and 0.12 kPa after noninvasive ventilation, with a correlation R2 of 0.88 and 0.99, respectively.
• In contrast, the mean difference between PvCO2 and PaCO2 was 0.64 kPa at baseline and 0.80 kPa after noninvasive ventilation, with a correlation R2 of 0.78 and 0.91, respectively.

Agreement between Venous and Arterial Blood Gases

• Another study published in 2019 3 compared the measurement of end-tidal partial pressure of carbon dioxide (PETCO2) with arterial, venous, and transcutaneous PCO2 in patients with respiratory failure.
• The results showed a significant relationship between PETCO2 and PaCO2, between PtcCO2 and PaCO2, and between PvCO2 and PaCO2.
• Bland-Altman analysis revealed a bias of 6.48 mm Hg between PETCO2 and PaCO2, with a precision of 6.01 mm Hg.
• The study suggests that PETCO2 can be used to estimate PaCO2, but with wide limits of agreement.

Clinical Implications

• A study published in 2011 4 evaluated the accuracy of a transcutaneous carbon dioxide device in assessing PaCO2 in patients with acute respiratory disease.
• The results showed a mean difference of -0.13 mm Hg between PaCO2 and PtCO2, with limits of agreement of ±3.8 mm Hg.
• The study suggests that transcutaneous PCO2 monitoring can be a reliable and feasible alternative to repeated blood gas analysis in patients with acute respiratory disease.
• Another study published in 2016 5 compared transcutaneous and capillary measurement of PCO2 in hypercapnic subjects, and found a good agreement between the two methods.
• The study recommends continuous transcutaneous measurement of PCO2 as the method of choice in the diagnosis of nocturnal hypercapnia.