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J Crit Care. 2013 Dec;28(6):1048-54. doi: 10.1016/j.jcrc.2013.06.005. Epub 2013 Jul 30.

Stewart analysis of apparently normal acid-base state in the critically ill.

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Department of Intensive Care Medicine, Jeroen Bosch Hospital, 's-Hertogenbosch, the Netherlands. Electronic address:



This study aimed to describe Stewart parameters in critically ill patients with an apparently normal acid-base state and to determine the incidence of mixed metabolic acid-base disorders in these patients.


We conducted a prospective, observational multicenter study of 312 consecutive Dutch intensive care unit patients with normal pH (7.35 ≤ pH ≤ 7.45) on days 3 to 5. Apparent (SIDa) and effective strong ion difference (SIDe) and strong ion gap (SIG) were calculated from 3 consecutive arterial blood samples. Multivariate linear regression analysis was performed to analyze factors potentially associated with levels of SIDa and SIG.


A total of 137 patients (44%) were identified with an apparently normal acid-base state (normal pH and -2 < base excess < 2 and 35 < PaCO2 < 45 mm Hg). In this group, SIDa values were 36.6 ± 3.6 mEq/L, resulting from hyperchloremia (109 ± 4.6 mEq/L, sodium-chloride difference 30.0 ± 3.6 mEq/L); SIDe values were 33.5 ± 2.3 mEq/L, resulting from hypoalbuminemia (24.0 ± 6.2 g/L); and SIG values were 3.1 ± 3.1 mEq/L. During admission, base excess increased secondary to a decrease in SIG levels and, subsequently, an increase in SIDa levels. Levels of SIDa were associated with positive cation load, chloride load, and admission SIDa (multivariate r(2) = 0.40, P < .001). Levels of SIG were associated with kidney function, sepsis, and SIG levels at intensive care unit admission (multivariate r(2) = 0.28, P < .001).


Intensive care unit patients with an apparently normal acid-base state have an underlying mixed metabolic acid-base disorder characterized by acidifying effects of a low SIDa (caused by hyperchloremia) and high SIG combined with the alkalinizing effect of hypoalbuminemia.


APACHE-II; Acid-base; Acute Physiology and Chronic Health Evaluation II; BE; CVVH(D); Critically ill; Hyperchloremia; Hypoalbuminemia; IQR; RIFLE; Risk, Injury, Failure, Loss, and End-stage Kidney disease; SD; SIDa; SIDe; SIG; SOFA; Sequential Organ Failure Assessment; Stewart analysis; Strong ion gap; apparent strong ion difference; base excess; continuous venovenous hemo(dia)filtration; effective strong ion difference; interquartile range; standard deviation; strong ion gap

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