Strong Ion Difference

Introduction

In 1981 Peter A. Stewart published his book How to understand acid-base – A quantitative acid-base primer for biology and medicine.. Two years later, in 1983, he published a paper also describing his concept of employing Strong Ion Difference as an alternative means of assessing clinical acid-base disturbances.

Now, some thirty-five years later, Stewart’s Textbook of Acid-Base edited by John Kellum and Paul Elbers is available via acid-base.org and via Lulu Marketplace.

Stewart’s approach has been critically reviewed by Morgan. In Addition Rastegar published a valuable summary indicating that SID is of benefit only infrequently. Below is a review of the essentials of SID as well as an outline of the principal sources of criticism.

More Controversy:

Stewart’s proposal provided several sources of acid-base controversy. The underlying science and rationale were less a source of criticism than were:

1. The complexity of the chemistry and mathematics.
2. Calculating small differences between large numbers with consequent lack of accuracy.
3. [SID] only reflects plasma – unlike SBE which reflects the whole body and hemoglobin’s influence.
4. Lack of a clinical correlation to validate benefit.

Traditional Approach:

It is too easy to believe that the concentrations of the hydrogen and bicarbonate ions, [H⁺] and [HCO₃^–], are at the heart of the problem: we discuss them, measure them, and treat them! whatever an acid or a base does must surely be due to the pH, i.e., the concentration of H⁺.

Such thinking is transparently incorrect: in alkaline solutions, like plasma, there are virtually no hydrogen ions present; so, whatever causes the evil behavior of an alkaline solution, the only thing that cannot be responsible is the hydrogen ion. And, clinically, both respiratory and metabolic changes affect the [HCO₃^–]. So, what is responsible for [H⁺] and [HCO₃^–]? Far from being central, or controlling, factors they actually depend on the concentrations of the other ions in solution. This should be obvious and Stewart’s method serves to re-emphasize these relationships.

Stewart’s Dependent Variables:

Stewart listed a total of six ion concentrations as dependent: [H⁺], [OH^–], [HCO₃^–], [CO₃^2-], [HA], [A^–] (weak acids and ions). In-vivo and clinically, therefore, these are not subject to independent alteration. Their concentrations are governed by concentrations of other ions and molecules.

Stewart’s Independent Variables:

There are three variables which are amenable to change in-vivo: partial pressure of carbon dioxide (PCO₂), total weak non-volatile acids [A_TOT], and net Strong Ion Difference [SID]. The influence of these three variables can be predicted through six simultaneous equations:

1. [H⁺] x [OH^–] = K ‘_w (water dissociation equilibrium)
2. [H⁺] x [A^–] = K_A x [HA] (weak acid)
3. [HA] + [A^–] = [A_TOT] (conservation of mass for “A”)
4. [H⁺] x [HCO₃^–] = K_C x PCO₂ (bicarbonate ion formation equilibrium)
5. [H⁺] x [CO₃^2-] = K₃ x [HCO₃^–] (carbonate ion formation equilibrium)
6. [SID] + [H⁺] – [HCO₃^–] – [A^–] -[CO₃^2-] – [OH^–] = 0 (electrical neutrality)