Intensive Care Medicine

, 35:2173 | Cite as

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

  • Daniel Doberer
  • Georg-Christian Funk
  • Karl Kirchner
  • Bruno Schneeweiss
Physiological and Technical Notes

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.

Keywords

Volume expansion Infusion solutions Stewart’s approach Strong ion difference Bicarbonate Acid-base 

Notes

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.

Supplementary material

134_2009_1528_MOESM1_ESM.doc (162 kb)
Supplementary material (DOC 162 kb)
134_2009_1528_MOESM2_ESM.doc (213 kb)
Supplementary tables (DOC 213 kb)

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Copyright information

© Springer-Verlag 2009

Authors and Affiliations

  • Daniel Doberer
    • 1
  • Georg-Christian Funk
    • 2
  • Karl Kirchner
    • 3
  • Bruno Schneeweiss
    • 4
  1. 1.Department of Clinical PharmacologyMedical University of ViennaViennaAustria
  2. 2.Department of Respiratory and Critical Care MedicineOtto Wagner SpitalViennaAustria
  3. 3.Institute of Applied Synthetic ChemistryVienna University of TechnologyViennaAustria
  4. 4.Intensive Care Unit 13H1, Department of Medicine IIIMedical University of ViennaViennaAustria

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