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Strong ions and the analysis of acid-base disturbances (Stewart approach)

Richard H Sterns, MD
Section Editor
Michael Emmett, MD
Deputy Editor
John P Forman, MD, MSc


Most internists use an approach to acid-base disturbances that was popularized by Relman and Schwartz in the 1960s [1,2]. This "traditional" approach accepts the Brønsted-Lowry definitions of acids and bases. The hydrogen ion concentration is expressed in blood as a function of the ratio between the PCO2 and the serum bicarbonate concentration as defined by the following equation:

 [H+]  =  K  x  PCO2/[HCO3-]

where K is the dissociation constant for the reaction and PCO2 is equal to the partial pressure of carbon dioxide.

An analysis of metabolic acidosis using the traditional approach is presented in detail separately. (See "Approach to the adult with metabolic acidosis".)

By comparison, many surgeons, critical care specialists, and anesthesiologists have embraced an approach introduced by Peter Stewart in 1981 [3-7]. This alternative approach, termed "strong ion difference" (SID) or the Stewart approach, will be reviewed briefly in this topic.

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Literature review current through: Nov 2017. | This topic last updated: Aug 29, 2017.
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