Comparison of acid-base models for prediction of hospital mortality after trauma

Abstract

This study determines whether mortality after major trauma is predicted by the strong ion gap (SIG) and whether recent refinements in the calculation of SIG improve its predictive value. The design was an observational, retrospective review of trauma patients admitted on a single service at a level 1 facility. The setting was an urban level 1 trauma facility. An unselected cohort of patients sustaining blunt and/or penetrating injury requiring intensive care unit care was chosen. There were no interventions. Age, injury mechanism, survival, arterial blood gases, hemoglobin, albumin, electrolytes, lactate, standard base deficit, strong ion difference (SID), buffer base, and SIG were analyzed. Patients were grouped into survivors and nonsurvivors according to in-hospital survival truncated to 28 days. Multivariate logistic regression was used for further analysis of univariate predictors of mortality, and receiver-operator characteristic curves were generated for mortality. Both nonsurvivors (n = 26) and survivors (n = 52) were similar with respect to age (31.9 +/- 11.5 vs. 33.5 +/- 11.6 years) and injury mechanism (blunt 61% vs. 58%) Nonsurvivors were more likely to have multicavity injury (54% vs. 26%; P < 0.01) than survivors. Nonsurvivor and survivor pH (7.36 +/- 0.15 vs. 7.38 +/- 0.09), HCO3(-) (20.4 +/- 3.9 vs. 21.7 +/- 2.5 mEq/L; P = 0.11), albumin (3.6 +/- 0.7 vs. 3.5 +/- 0.5 gm/dL), lactate (2.9 +/- 2.5 vs. 2.3 +/- 1.3 mmol/L; P = 0.24), and phosphate (3.1 +/- 0.9 vs. 3.4 +/- 0.8 mEq/L; P = 0.26) were similar. Forty-two percent of nonsurvivors had normal lactate levels, whereas 33% of survivors had lactic acidosis. However, the apparent SID (41.0 +/- 4.2 vs. 36.7 +/- 5.5 mEq/L; P < 0.001), effective SID (32.7 +/- 4.2 vs. 35.4 +/- 4.9 mEq/L; P = 0.019), and SIG (8.3 +/- 4.4 vs. 1.3 +/- 3.6 mEq/L; P < 0.001) were all significantly different between nonsurvivors and survivors. Only one (2%) survivor had an SIG greater than 5 mEq/L, and only two (7%) nonsurvivors had an SIG less than 5 mEq/L. Admission pH, HCO3-, and lactate were poor predictors of hospital mortality after trauma. An elevated SIG presaged mortality after injury and should be assessed on admission.