Chawla, G., 2008
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.
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.
Stewart’s proposal provided several sources of acid-base controversy. The underlying science and rationale were less a source of criticism than were:
- The complexity of the chemistry and mathematics.
- Calculating small differences between large numbers with consequent lack of accuracy.
- [SID] only reflects plasma – unlike SBE which reflects the whole body and hemoglobin’s influence.
- Lack of a clinical correlation to validate benefit.
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–], [CO32-], [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 [ATOT], and net Strong Ion Difference [SID]. The influence of these three variables can be predicted through six simultaneous equations:
- [H+] x [OH–] = K ‘w (water dissociation equilibrium)
- [H+] x [A–] = KA x [HA] (weak acid)
- [HA] + [A–] = [ATOT] (conservation of mass for “A”)
- [H+] x [HCO3–] = KC x PCO2 (bicarbonate ion formation equilibrium)
- [H+] x [CO32-] = K3 x [HCO3–] (carbonate ion formation equilibrium)
- [SID] + [H+] – [HCO3–] – [A–] -[CO32-] – [OH–] = 0 (electrical neutrality)