It's all in the charge

A critique of Stewart's approach: the chemical mechanism of dilutional acidosis

OBJECTIVE

While Stewart's acid-base approach is increasingly used in clinical practice, it has also led to new controversies. Acid-base disorders can be seen from different viewpoints: on the diagnostic/clinical, quantitative/mathematical, or the mechanistic level. In recent years, confusion in the interpretation and terminology of Stewart's approach has arisen from mixing these different levels. This will be demonstrated on the basis of a detailed analysis of the mechanism of "dilutional acidosis." In the classical dilution concept, metabolic acidosis after resuscitation with large volumes is attributed to the dilution of serum bicarbonate. However, Stewart's approach rejects this explanation and offers an alternative one that is based on a decrease in a "strong ion difference." This mechanistic explanation is questionable for principal chemical reasons. The objective of this study is to clarify the chemical mechanism of dilutional acidosis.

METHODS

Experimental data and simulations of various dilution experiments, as well as theoretical and chemical considerations were used.

RESULTS

1. The key to understanding the mechanism of dilutional acidosis lies in the open CO2/HCO3 (-)-buffer system where the buffer base (HCO3(-)) is diluted whereas the buffer acid is not diluted (constant pCO2). 2. The categorization in independent and dependent variables depends on the system regarded. 3. Neither the principle of electroneutrality, nor a change in [SID], nor increased H2O dissociation plays a mechanistic role.

CONCLUSION

Stewart's approach is valid at the mathematical level but does not provide any mechanistic insights. However, the quantification and categorization of acid-base disorders, using Stewart approach, may be helpful in clinical practice.

ELECTRONIC SUPPLEMENTARY MATERIAL

The online version of this article (doi:10.1007/s00134-009-1528-y) contains supplementary material, which is available to authorized users.

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

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.

Das Säure-Basen-Modell nach Stewart

In addition to paCO(2), Stewart's acid base model takes into account the influence of albumin, inorganic phosphate, electrolytes and lactate on acid-base equilibrium. It allows a comprehensive and quantitative analysis of acid-base disorders. Particularly simultaneous and mixed metabolic acid-base disorders, which are common in critically ill patients, can be assessed. Stewart's approach is therefore a valuable tool in addition to the customary acid-base approach based on bicarbonate or base excess. However, some chemical aspects of Stewart's approach remain controversial.