Copyright ©The Author(s) 2015. Published by Baishideng Publishing Group Inc. All rights reserved.
World J Crit Care Med. Feb 4, 2015; 4(1): 62-70
Published online Feb 4, 2015. doi: 10.5492/wjccm.v4.i1.62
Has Stewart approach improved our ability to diagnose acid-base disorders in critically ill patients?
Fabio D Masevicius, Arnaldo Dubin
Fabio D Masevicius, Arnaldo Dubin, Servicio de Terapia Intensiva, Sanatorio Otamendi y Miroli, Buenos Aires C1115AAB, Argentina
Arnaldo Dubin, Cátedra de Farmacología Aplicada, Facultad de Ciencias Médicas, Universidad Nacional de La Plata, 1900 La Plata, Argentina
Author contributions: Both authors contributed to this work.
Conflict-of-interest: The authors have no conflict of interest to disclose.
Open-Access: This article is an open-access article which was selected by an in-house editor and fully peer-reviewed by external reviewers. It is distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited and the use is non-commercial. See:
Correspondence to: Arnaldo Dubin, MD, PhD, Servicio de Terapia Intensiva, Sanatorio Otamendi y Miroli, Azcuénaga 870, C1115AAB Ciudad Autónoma de Buenos Aires, Argentina.
Telephone: +54-91-150102431
Received: September 25, 2014
Peer-review started: September 28, 2014
First decision: December 17, 2014
Revised: December 29, 2014
Accepted: January 15, 2015
Article in press: Janurary 19, 2015
Published online: February 4, 2015

The Stewart approach-the application of basic physical-chemical principles of aqueous solutions to blood-is an appealing method for analyzing acid-base disorders. These principles mainly dictate that pH is determined by three independent variables, which change primarily and independently of one other. In blood plasma in vivo these variables are: (1) the PCO2; (2) the strong ion difference (SID)-the difference between the sums of all the strong (i.e., fully dissociated, chemically nonreacting) cations and all the strong anions; and (3) the nonvolatile weak acids (Atot). Accordingly, the pH and the bicarbonate levels (dependent variables) are only altered when one or more of the independent variables change. Moreover, the source of H+ is the dissociation of water to maintain electroneutrality when the independent variables are modified. The basic principles of the Stewart approach in blood, however, have been challenged in different ways. First, the presumed independent variables are actually interdependent as occurs in situations such as: (1) the Hamburger effect (a chloride shift when CO2 is added to venous blood from the tissues); (2) the loss of Donnan equilibrium (a chloride shift from the interstitium to the intravascular compartment to balance the decrease of Atot secondary to capillary leak; and (3) the compensatory response to a primary disturbance in either independent variable. Second, the concept of water dissociation in response to changes in SID is controversial and lacks experimental evidence. In addition, the Stewart approach is not better than the conventional method for understanding acid-base disorders such as hyperchloremic metabolic acidosis secondary to a chloride-rich-fluid load. Finally, several attempts were performed to demonstrate the clinical superiority of the Stewart approach. These studies, however, have severe methodological drawbacks. In contrast, the largest study on this issue indicated the interchangeability of the Stewart and conventional methods. Although the introduction of the Stewart approach was a new insight into acid-base physiology, the method has not significantly improved our ability to understand, diagnose, and treat acid-base alterations in critically ill patients.

Keywords: Acid-base metabolism, Stewart approach, Base excess, Bicarbonate, Anion gap, Strong ion difference, Strong ion gap

Core tip: In this article, we comprehensively reviewed the evidence that has been used to argue for the superiority of the Stewart approach over the traditional method for the analysis of acid-base metabolism in critically ill patients. The basic principles of the Stewart approach have severe weaknesses. In addition, the contribution of this method to the understanding of mechanisms is minor; furthermore, from a clinical standpoint, the Stewart approach has no advantage for diagnostic or prognostic purposes compared to the analysis based on bicarbonate, base excess, and albumin-corrected anion gap.