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

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.

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

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.

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.