Back to the Basics: An Alternative Approach to ABG Interpretation

Medical education has promoted the rapid interpretation of the arterial blood gas (ABG) as a crucial skill in the management of critically ill patients. The first step of many algorithms focuses on using log-linear estimates to determine the internal consistency of ABG values. However, because of modern computing advances, these estimates are no longer necessary. We demonstrate an alternative method for ascertaining internal consistency of ABG values based on direct calculation derived directly from Henderson-Hasselbach and demonstrate its efficiency and accuracy. Deidentified ABG values were collected from electronic medical records for the month of September 2022. Estimated hydrogen concentration was calculated by log linear estimation and by the proposed method of direct calculation. Time of computation, a measure of efficiency, was also recorded. Of the 1008 ABGs used for analysis, 185 (18.4%) had estimated hydrogen concentrations that were greater than a 5% threshold according to the guidelines method, suggesting potential internal inconsistency. However, only 13 (1.3%) had estimated hydrogen concentrations greater than 5% by direct calculation. Analysis was also performed at a 10% threshold. On further analysis, 4 ABGs that were identified by the guidelines method as being internally inconsistent, but not identified by the direct calculation method, were found to have pH <7.0, outside the range provided for the current guidelines for log-linear estimation, and were thus incorrectly identified by the guidelines method as being internally inconsistent. The time of computation for direct calculation was approximately 8-fold faster, although the total time of calculation for this dataset was minimal. This study demonstrates a direct calculation method as an alternative approach to traditional guidelines to assess internal ABG value consistency, the first step of ABG interpretation. We demonstrate that direct calculation is more accurate, identifying less potentially internally inconsistent values, notably at extremes in pH. In addition, the direct calculation algorithm is 8-fold more efficient than guidelines-based algorithm, although actual processing times were miniscule for the data set regardless of the method used. Furthermore, with modern calculators and computers, the direct calculation method is easier to understand and implement than the guideline based approach. Therefore, based on the results of this study, we propose the use of direct calculation as an alternative to log-linear based estimation for the assessment of internal consistency of ABGs.