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

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Abstract

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.

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Acknowledgments

This work is dedicated to deceased Professor Roland Schmid who was our “chemical advisor” since the beginning of our acid-base scientific work. The authors would like to thank Philip D. Watson for providing his computer program “Acid-Basics II”, with which the acid-base simulations were confirmed.

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Authors and Affiliations

  1. Department of Clinical Pharmacology, Medical University of Vienna, Waehringer Guertel 18-20, 1090, Vienna, Austria

    Daniel Doberer

  2. Department of Respiratory and Critical Care Medicine, Otto Wagner Spital, Vienna, Austria

    Georg-Christian Funk

  3. Institute of Applied Synthetic Chemistry, Vienna University of Technology, Vienna, Austria

    Karl Kirchner

  4. Intensive Care Unit 13H1, Department of Medicine III, Medical University of Vienna, Vienna, Austria

    Bruno Schneeweiss

Authors
  1. Daniel Doberer
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  2. Georg-Christian Funk
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  3. Karl Kirchner
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  4. Bruno Schneeweiss
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Corresponding author

Correspondence to Daniel Doberer.

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Doberer, D., Funk, GC., Kirchner, K. et al. A critique of Stewart’s approach: the chemical mechanism of dilutional acidosis. Intensive Care Med 35, 2173–2180 (2009). https://doi.org/10.1007/s00134-009-1528-y

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  • DOI: https://doi.org/10.1007/s00134-009-1528-y

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