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. 2009 May;30(2):41-54.

The Stewart approach--one clinician's perspective

T John Morgan  1
Affiliations

The Stewart approach--one clinician's perspective

T John Morgan. Clin Biochem Rev. 2009 May.

Abstract

Peter Stewart added controversy to an already troubled subject when he entered the clinical acid-base arena. His approach puts water dissociation at the centre of the acid-base status of body fluids. It is based on six simultaneous equations, incorporating the Laws of Mass Action, Mass Conservation, and Electrical Neutrality. Together with Gibbs-Donnan equilibria, these equations explain the diagnostically important PaCO(2)/pH relationship, and improve understanding of the physiologic basis of traditional acid-base approaches. Spin-offs have included new scanning tools for unmeasured ions, in particular the 'strong ion gap' and 'net unmeasured ions'. The most controversial feature is the designation of pH and bicarbonate concentrations as dependent variables, answerable exclusively to three independent variables. These are the strong ion difference (SID), the total concentration of non-volatile weak acid (A(TOT)), and PCO(2). Aspects of this assertion conflict with traditional renal physiology, and with current models of membrane H(+)/base transporters, oxidative phosphorylation, and proton and bicarbonate ionophores. The debate in this area is ongoing. Meanwhile, Stewart-style diagnostic and decision support systems such as the 'Strong Ion Calculator' and the web-site www.acidbase.org are now appearing.

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Figures

Figure 1
Figure 1
Acute PaCO2/pH curves and associated SBE values (see text). The middle curve (SBE = 0 mEq/L) is normal. Metabolic acidosis (primary or compensatory) causes a down-shift, with SBE increasingly negative. Metabolic alkalosis (primary or compensatory) shifts the curve up, with SBE increasingly positive.
Figure 2
Figure 2
The effect of varying SID on pH in separated plasma. PCO2 is held constant at 40 mm Hg. ATOT = 20 mEq/L. pKa= 6.7. SID: strong ion difference, ATOT: Total concentration of non-volatile weak acid.
Figure 3
Figure 3
The effect of varying ATOT on pH in separated plasma. PCO2 is held constant at 40 mm Hg. SID = 42 mEq/L. pKa = 6.7. SID: strong ion difference, ATOT: Total concentration of non-volatile weak acid.
Figure 4
Figure 4
An example of a report from the Strong Ion Calculator from Lloyd P (reference 83). (Permission granted by the Joint Faculty of Intensive Care Medicine).
See this image and copyright information in PMC

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