Acid-base analysis: a critique of the Stewart and bicarbonate-centered approaches

Abstract

When approaching the analysis of disorders of acid-base balance, physical chemists, physiologists, and clinicians, tend to focus on different aspects of the relevant phenomenology. The physical chemist focuses on a quantitative understanding of proton hydration and aqueous proton transfer reactions that alter the acidity of a given solution. The physiologist focuses on molecular, cellular, and whole organ transport processes that modulate the acidity of a given body fluid compartment. The clinician emphasizes the diagnosis, clinical causes, and most appropriate treatment of acid-base disturbances. Historically, two different conceptual frameworks have evolved among clinicians and physiologists for interpreting acid-base phenomena. The traditional or bicarbonate-centered framework relies quantitatively on the Henderson-Hasselbalch equation, whereas the Stewart or strong ion approach utilizes either the original Stewart equation or its simplified version derived by Constable. In this review, the concepts underlying the bicarbonate-centered and Stewart formulations are analyzed in detail, emphasizing the differences in how each approach characterizes acid-base phenomenology at the molecular level, tissue level, and in the clinical realm. A quantitative comparison of the equations that are currently used in the literature to calculate H(+) concentration ([H(+)]) is included to clear up some of the misconceptions that currently exist in this area. Our analysis demonstrates that while the principle of electroneutrality plays a central role in the strong ion formulation, electroneutrality mechanistically does not dictate a specific [H(+)], and the strong ion and bicarbonate-centered approaches are quantitatively identical even in the presence of nonbicarbonate buffers. Finally, our analysis indicates that the bicarbonate-centered approach utilizing the Henderson-Hasselbalch equation is a mechanistic formulation that reflects the underlying acid-base phenomenology.

Clinical Utility of Stewart's Method in Diagnosis and Management of Acid-Base Disorders

“…This was followed by publication of a number of studies comparing the utility of the traditional and Stewart's approaches as a diagnostic and prognostic tool. It is interesting that until recently, Stewart's method has been largely ignored by nephrologists and renal physiologists, with only a rare article appearing in renal journals (7,8). This, however, should change as Stewart's method is increasingly used in clinical settings where nephrologists work side by side with intensivists and anesthesiologists who routinely use this method.…”

“…A recent article discussed the mathematical and biochemical basis of Stewart's method in detail and challenged the basic concept that Stewart has created a new paradigm (8), a paradigm that is compared by some to Copernicus's discovering that Earth rotates around the sun and not vice versa (9)! In addition, it challenges the claim that Stewart's approach provides a mechanistic explanation for acid-base disorders (8).…”

Independence of Variables in Stewart’s Model of the Acid-Base Chemistry of the Blood Plasma

“…al, 2007, Kurtz et al, 2008. One of the important improvements proposed by Stewart was the representation of three 'independent variables' (SID, Atot and pCO2), values of which enable unique solution of all Stewart's equations describing acid-base chemistry of plasma.…”

Acute Kidney Injury Following Cardiac Surgery: Prevention, Diagnosis, and Management

“…Observational and randomized studies have failed to show a mortality or morbidity benefit when sodium bicarbonate is administered to correct acidosis. (Forsythe and Schmidt 2000;Kraut and Kurtz 2001;Kurtz, Kraut et al 2008) The proposed rationale for the lack of benefit is that, while bicarbonate may increase extracellular pH, it exacerbates intracellular acidosis by the generation of carbon dioxide in the buffering process. Consequently, the practice of many critical care physicians is to administer sodium bicarbonate only in the presence of profound acidosis (pH <7.1) and associated hemodynamic instability.…”