Comparison of venous and calculated blood gas values to arterial values in critically ill patients

Background: Arterial blood gas (ABG) values are important in the assessment of critically ill patients. However, arterial puncture may be challenging to perform in these patients. The venous-to-arterial conversion method (v-TAC) is used to convert venous blood gas values to calculated values meant to resemble arterial values. Calculated pH and partial pressure of carbon dioxide (PCO₂) values have shown good agreement with arterial values in stable patients, but performance of the method in patients with severe acid-base disturbances is unknown. We aim to evaluate venous and calculated blood gas value congruency with arterial values in critically ill patients.

Methods: Critically ill adult patients with abnormal arterial pH (<7.35 or >7.45) admitted to an intensive care unit were included in this study. Patients were divided in groups based on arterial pH (alkalemia, moderate acidaemia and severe acidaemia). ABG samples were obtained from arterial catheters and venous samples simultaneously from peripheral or central venous catheters. Venous blood gas values were converted to calculated values using the v-TAC software. Both venous and calculated values of pH, PCO₂ and partial pressure of oxygen (PO₂) were compared to arterial values in scatterplots and using the Spearman's Rank Correlation.

Results: Arterial and venous sample pairs were obtained from 28 patients, and 24 venous values were converted using the v-TAC method. Scatterplots showed similar congruency of venous and calculated pH and PCO₂ values to arterial values. However, one patient in the severe acidaemia group had arterial pH 7.07 and venous pH 7.04, but calculated pH 7.42. Spearman's Rank Correlation rho showed no correlation of venous and calculated PO₂ values compared to arterial values (p = .989 and p = .361).

Conclusion: Venous and calculated pH and PCO₂ values showed similar congruency with arterial values in patients with alkalemia and moderate acidaemia, while the method was unreliable in a patient suspected of severe metabolic acidaemia.