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

From the Guidelines

Venous blood gas (VBG) and arterial blood gas (ABG) typically show similar bicarbonate levels but different pH values, with venous pH being 0.03 units lower than arterial pH, as stated in the study 1. When comparing VBG and ABG, it's essential to consider the differences in pH values, which can be significant.

• The pH difference between venous and arterial blood is usually around 0.03 units, with venous blood being more acidic due to increased CO2 content from tissue metabolism 1.
• Bicarbonate concentrations, on the other hand, are generally equivalent between venous and arterial samples, with differences usually less than 2 mEq/L, making VBG bicarbonate measurements a reliable substitute for ABG in most clinical scenarios. The study 1 mentions that venous pH can be used to monitor resolution of acidosis, and it is usually 0.03 units lower than arterial pH.
• This information is crucial in clinical practice, as it allows healthcare providers to make informed decisions about patient care, such as adjusting insulin therapy or fluid replacement, based on venous blood gas results.
• Additionally, the study 1 provides criteria for resolution of diabetic ketoacidosis (DKA), which includes a venous pH of >7.3, highlighting the importance of monitoring pH levels in patients with DKA.

From the Research

Comparison of Venous and Arterial Blood Gas Measurements

• The studies 2, 3, 4, 5 compared venous and arterial blood gas measurements, including pH and bicarbonate levels.
• The mean differences between arterial and venous pH were found to be small, ranging from 0.023 to 0.035 pH units 2, 3, 4.
• The mean differences between arterial and venous bicarbonate levels were also found to be small, ranging from -0.80 to -1.41 mmol/L 2, 3, 4.
• The studies 2, 3, 4 suggested that venous blood gas measurements can be used as an alternative to arterial blood gas measurements for determining acid-base status in certain clinical contexts.

Agreement Between Arterial and Venous Measurements

• The study 2 found that the 95% limits of agreement for pH, Pco(2), and bicarbonate were -0.028 to 0.081, -12.3 to 4.8, and -4.0 to 2.4, respectively.
• The study 3 found that the weighted mean arterio-venous difference in pH was 0.035 pH units, with narrow limits of agreement.
• The study 4 found that the limits of agreement for pH, Pco(2), and bicarbonate were -0.048 to 0.094, -15 to 8.1 mmHg, and -2.27 to 2.9 mmol/L, respectively.
• The study 5 found that the Pearson correlation coefficients between arterial and venous parameters were 0.828,0.877,0.599,0.896, and 0.312 for pH, PCO2, PO2, HCO3, and SO2, respectively.

Clinical Implications

• The studies 2, 3, 4 suggested that venous blood gas measurements can be used to diagnose metabolic acidosis and alkalosis, and to monitor trends in pCO(2) and bicarbonate levels.
• The study 5 found that venous values of pH, PCO2, and HCO3 can be an alternative to their arterial equivalents in the interpretation of the metabolic status in patients with acute exacerbation of COPD.
• The study 6 found that there is a considerable discrepancy among authors about the appropriateness and utilization of VBG as an alternative to ABG, and that the physiological differences between venous and arterial blood values and the significance of sampling procedures should be considered.