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



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.


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



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)


  1. 1.
    Stewart PA (1978) Independent and dependent variables of acid-base control. Respir Physiol 33:9–26CrossRefPubMedGoogle Scholar
  2. 2.
    Figge J, Mydosh T, Fencl V (1992) Serum proteins and acid-base equilibria: a follow-up. J Lab Clin Med 120:713–719PubMedGoogle Scholar
  3. 3.
    Watson PD (1999) Modeling the effects of proteins on pH in plasma. J Appl Physiol 86:1421–1427PubMedGoogle Scholar
  4. 4.
    Constable PD (1997) A simplified strong ion model for acid-base equilibria: application to horse plasma. J Appl Physiol 83:297–311PubMedGoogle Scholar
  5. 5.
    Siggaard-Andersen O, Fogh-Andersen N (1995) Base excess or buffer base (strong ion difference) as measure of a non-respiratory acid-base disturbance. Acta Anaesthesiol Scand Suppl 107:123–128CrossRefPubMedGoogle Scholar
  6. 6.
    Haskins SC, Hopper K, Rezende ML (2006) The acid-base impact of free water removal from, and addition to, plasma. J Lab Clin Med 147:114–120CrossRefPubMedGoogle Scholar
  7. 7.
    Doberer D, Funk GC, Schneeweiss B (2003) Dilutional acidosis: an endless story of confusion. Crit Care Med 31:337–338CrossRefPubMedGoogle Scholar
  8. 8.
    Kurtz I, Kraut J, Ornekian V, Nguyen MK (2008) Acid-base analysis: a critique of the Stewart and bicarbonate-centered approaches. Am J Physiol Renal Physiol 294:F1009–F1031Google Scholar
  9. 9.
    Corey HE (2003) Stewart and beyond: new models of acid-base balance. Kidney Int 64:777–787CrossRefPubMedGoogle Scholar
  10. 10.
    Dubin A, Menises MM, Masevicius FD, Moseinco MC, Kutscherauer DO, Ventrice E, Laffaire E, Estenssoro E (2007) Comparision of the three different methods of evaluation of metabolic acid-base disorders. Crit Care Med 35:1264–1270CrossRefPubMedGoogle Scholar
  11. 11.
    Story DA (2004) Bench-to-bedside review: a brief history of clinical acid-base. Crit Care 8:253–258CrossRefPubMedGoogle Scholar
  12. 12.
    Sirker AA, Rhodes A, Grounds RM, Bennett ED (2002) Acid-base physiology: the ‘traditional’ and the ‘modern’ approaches. Anaesthesia 57:348–356CrossRefPubMedGoogle Scholar
  13. 13.
    Kaplan L (2007) Acid-base balance analysis: a little of target. Crit Care Med 35:1418–1419CrossRefPubMedGoogle Scholar
  14. 14.
    Constable PD (2003) Hyperchloremic acidosis: the classic example of strong ion acidosis. Anesth Analg 96:919–922CrossRefPubMedGoogle Scholar
  15. 15.
    Kellum JA, Elbers PWG (2009) Stewart’s textbook of acid-base, 2nd edn., AmsterdamGoogle Scholar
  16. 16.
    Stewart PA (1981) How to understand acid-base. A quantitative acid-base primer for biology and medicine. Elsevier, New YorkGoogle Scholar
  17. 17.
    Stewart PA (1983) Modern quantitative acid-base chemistry. Can J Physiol Pharmacol 61:1444–1461PubMedGoogle Scholar
  18. 18.
    Mathes DD, Morell RC, Rohr MS (1997) Dilutional acidosis: is it a real clinical entity? Anesthesiology 86:501–503CrossRefPubMedGoogle Scholar
  19. 19.
    Prough DS, White RT (2000) Acidosis associated with perioperative saline administration: dilution or delusion? Anesthesiology 93:1167–1169CrossRefPubMedGoogle Scholar
  20. 20.
    Prough DS, Bidani A (1999) Hyperchloremic metabolic acidosis is a predictable consequence of intraoperative infusion of 0.9% saline. Anesthesiology 90:1247–1249CrossRefPubMedGoogle Scholar
  21. 21.
    Kellum JA (2002) Saline-induced hyperchloremic metabolic acidosis. Crit Care Med 30:259–261CrossRefPubMedGoogle Scholar
  22. 22.
    Peters JP, Van Slyke DD (1946) Quantitative clinical chemistry: interpretations. Williams & Wilkins, BaltimoreGoogle Scholar
  23. 23.
    Shires GT, Holman J (1948) Dilutional acidosis. Ann Intern Med 28:557–559PubMedGoogle Scholar
  24. 24.
    Asano S, Kato E, Yamauchi M et al (1966) The mechanism of acidosis caused by infusion of saline solution. Lancet 1:1245–1246CrossRefPubMedGoogle Scholar
  25. 25.
    Goodkin DA, Raja RM, Saven A (1990) Dilutional acidosis. South Med J 83:354–355CrossRefPubMedGoogle Scholar
  26. 26.
    Jaber BL, Madias NE (1997) Marked dilutional acidosis complicating management of right ventricular myocardial infarction. Am J Kidney Dis 30:561–567CrossRefPubMedGoogle Scholar
  27. 27.
    Garella S, Chang BS, Kahn SI (1975) Dilution acidosis and contraction alkalosis: review of a concept. Kidney Int 8:279–283CrossRefPubMedGoogle Scholar
  28. 28.
    Kellum JA (2000) Determinants of blood pH in health and disease. Crit Care 4:6–14CrossRefPubMedGoogle Scholar
  29. 29.
    Morgan TJ (2005) The meaning of acid-base abnormalities in the intensive care unit: part III—effects of fluid administration. Crit Care 9:204–211CrossRefPubMedGoogle Scholar
  30. 30.
    Kaplan LJ (2005) It’s all in the charge. Crit Care Med 33:680–681CrossRefPubMedGoogle Scholar
  31. 31.
    Alston RP, Cormack L, Collinson C (2004) Metabolic acidosis developing during cardiopulmonary bypass is related to a decrease in strong ion difference. Perfusion 19:145–152CrossRefPubMedGoogle Scholar
  32. 32.
    Fall PJ, Szerlip HM (2005) Lactic acidosis: from sour milk to septic shock. J Intensive Care Med 20:255–271CrossRefPubMedGoogle Scholar
  33. 33.
    Watson PD (2001) Acid-Base-Calculator: AcidBasics II. Accessed 5 December 2001.
  34. 34.
    Butler JN (1998) Ionic equilibrium—solubility and pH calculations. Wiley, New YorkGoogle Scholar
  35. 35.
    Figge J, Rossing TH, Fencl V (1991) The role of serum proteins in acid-base equilibria. J Lab Clin Med 117:453–467PubMedGoogle Scholar
  36. 36.
    Anstey CM (2005) Comparison of three strong ion models used for quantifying the acid-base status of human plasma with special emphasis on the plasma weak acids. J Appl Physiol 98:2119–2125CrossRefPubMedGoogle Scholar
  37. 37.
    Wooten EW (1999) Analytic calculation of physiological acid-base parameters in plasma. J Appl Physiol 86:326–334PubMedGoogle Scholar
  38. 38.
    Lang W, Zander R (2005) Prediction of dilutional acidosis based on the revised classical dilution concept for bicarbonate. J Appl Physiol 98:62–71CrossRefPubMedGoogle Scholar
  39. 39.
    Morgan TJ, Venkatesh B, Hall J (2002) Crystalloid strong ion difference determines metabolic acid-base change during in vitro hemodilution. Crit Care Med 30:157–160CrossRefPubMedGoogle Scholar
  40. 40.
    Zander R (2006) Bicarbonate and dilutional acidosis. In: Fluid management, Biblomed—Medizinische Verlagsgesellschaft, Melsungen, pp 13–14Google Scholar
  41. 41.
    Doberer D, Funk GC, Kneidinger N, Lindner G, Kneussl M, Schneeweiss B (2006) Base excess a universal parameter for quantification of several metabolic and respiratory acid-base disorders. Abstract, ERS Munich, GermanyGoogle Scholar
  42. 42.
    Fencl V, Jabor A, Kazda A, Figge J (2000) Diagnosis of metabolic acid-base disturbances in critically ill patients. Am J Respir Crit Care Med 162:2246–2251PubMedGoogle Scholar
  43. 43.
    Rehm M, Conzen PF, Peter K, Finsterer U (2004) The Stewart model. “Modern” approach to the interpretation of the acid-base metabolism. Anaesthesist 53:347–357CrossRefPubMedGoogle Scholar
  44. 44.
    Atkins PW (1998) Physical chemistry, 6th edn. Oxford University Press, OxfordGoogle Scholar

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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