Stewart Approach Is Not Always a Practical Clinical Tool

To the Editor:

Dr. Constable presents a very lucid explanation of the strong ion concept in his recent editorial ⁽¹⁾ and its application to understanding hyperchloremic acidosis. However, I would differ with his conclusion that the conventional Henderson-Hasselbalch approach cannot be used to understand this disorder. The decrease in pH that accompanies infusions of saline can be explained because the dilution in bicarbonate is not usually matched by a proportionate fall in Pco₂, which is regulated by the respiratory center ⁽²⁾. The Stewart approach is elegant, and we have found it very useful under some circumstances. However, it is uncertain whether it will become a practical clinical tool. Data about strong ions, phosphate, and protein that are used with the strong ion equations to calculate pH and bicarbonate are frequently unavailable and must be estimated. In practice, the pH, Pco₂ and bicarbonate are measured with great accuracy, and these values can be used with additional information concerning the conventional anion gap to diagnose most acid-base disorders. The reliability of HCO₃- measurements is routinely checked by comparing values calculated from arterial pH and Pco₂ and the venous CO₂ content. It remains to be seen whether the “revolution” predicted by Dr. Constable is at hand.

Richard M. Effros, MD