Effect of the independent acid base variables on anion gap variation in cardiac surgical patients: a Stewart-Figge approach.

PURPOSE: To determine the effect of each of independent acid base variables on the anion gap (AG) value in cardiac surgical patients. METHODS: This retrospective study involved 128 cardiac surgical patients admitted for postoperative care. The variation of AG (AGvar) between the day of admission and the first postoperative day was correlated via a multiple linear regression model with the respective variations of the independent acid base variables, that is, apparent strong ion difference (SIDa), strong ion gap (SIG), carbon dioxide (PCO2), and albumin and phosphate concentrations. RESULTS: The variations of all the above variables contributed significantly to the prediction of AGvar (adjusted R (2) = 0.9999, F = 201890.24, and P < 0.001). According to the standardized coefficients (ß), SIGvar (ß = 0.948, P < 0.001), [Albumin]var (ß = 0.260, P < 0.001), and [Phosphate]var (ß = 0.191, P < 0.001) were the major determinants of AGvar with lesser contributions from SIDa, var (ß = 0.071, P < 0.001) and PCO2, var (ß = -0.067, P < 0.001). CONCLUSIONS: All the independent acid base variables contribute to the prediction of the AG value. However, albumin and phosphate and SIG variations seem to be the most important predictors, while AG appears to be rather stable with changes in PCO2 and SIDa.

The base excess gap is not a valid tool for the quantification of unmeasured ions in cardiac surgical patients: a retrospective observational study.

BACKGROUND: The base excess gap (BE(gap)) method is commonly used for the quantification of unmeasured ions in critically ill patients. However, it has never been validated against the standard quantitative acid-base approach. OBJECTIVE: To compare the BE(gap) as a tool for the prediction of the excess of unmeasured ions with the offset of strong ion gap (SIG) from its reference value. DESIGN: A retrospective observational study. SETTING: Adult ICU in a tertiary hospital. PATIENTS: One hundred and thirty-five cardiac surgical patients admitted for postoperative care. INTERVENTIONS: None. MAIN OUTCOME MEASURES: BE(gap) was calculated as BE(gap) = SBE - BE(si) - BE(wa), where SBE is the standard base excess, BE(si) is the partition due to strong ions ([Na+]-[Cl-]-[lactate-] - 30.5) and BE(wa) is the partition due to weak acids [0.25×{42 - (albumin)}]. The deviation of the observed SIG (SIG(ob)) from its reference value was calculated as deltaSIG = 2.85 - SIG(ob). We used Bland-Altman and concordance correlation analysis to compare BE(gap) with deltaSIG. A bias of ±1 meq l(-1) with limits of agreement of ±2 meq l(-1) and a concordant correlation coefficient of more than 0.9 were considered to indicate a strong agreement. RESULTS: The concordant correlation coefficient between BE(gap) and deltaSIG was 0.702. The mean bias between the two variables was 1.8 meq l(-1), with a lower limit of agreement of -0.9 meq l(-1) and an upper limit of agreement of 4.4 meq l(-1). CONCLUSION: The BE gap method cannot reliably quantify the unmeasured ion excess in cardiac surgical patients. Clinicians should use the full Stewart-Figge model for quantitative acid-base assessments.