[Base excess] vs [strong ion difference]. Which is more helpful?

Abstract

Blood [base excess] ([BE]) is defined as the change in [strong acid] or [strong base] needed to restore pH to normal at normal PCO2. Some believe that [BE] is unhelpful because [BE] may be elevated with a "normal" [strong ion difference] ([SID]), where a strong ion is one that is always dissociated in physiological solution, and where [SID] = [strong cations]-[strong anions]. Using a computer simulation, the hypothesis was tested that [SID] = [SID Excess] ([SIDEx]), where [SIDEx] is the change in [SID] needed to restore pH to normal at normal PCO2. The most current version of the plasma [SID] ([SID]p) equation was used as a template, and an [SIDEx] formula, of the Siggaard-Andersen form, derived: [SIDEx]p = [HCO3-]p -24.72 + (pHp - 7.4) x (1.159 x [alb]p + 0.423 x [Pi]p). [SID] was compared to [SIDEx] over the physiologic range of plasma buffering, and it was found that [SIDEx] varied by approximately 15 mM at any given [SID], thereby faulting the hypothesis. It is concluded that [SID] can be "normal" with an elevated [SIDEx], the latter being an expression of the [BE] concept, and a more helpful quantity in physiology. The "metabolic" component of a given acid-base disturbance is usually estimated as whole blood [base excess] ([BE]WB), where [BE]WB is defined as the change in [strong acid] or [strong base] needed to restore plasma pH (pHp) to 7.4 at PCO2 of 40 Torr. However, the [BE] approach has been criticized as "inadequate for interpretation of complex acid-base derangements such as those seen in critically ill patients." The proposed alternative is the strong ion difference (SID) method, where a strong ion is one that is always dissociated in solution, and where [SID] = [strong cations] - [strong anions]. On the one hand, it does not seem possible, by the definitions of these entities, to change [SID] without also changing [BE]. On the other hand, a selected group of critically ill patients with hypoproteinemia has been reported in whom [SID] was "normal" (i.e. approximately 40 mEq.l-1) but [BE]WB clearly increased. The idea was that hypoproteinemia caused the alkalosis, due to a deficiency of plasma weak acid buffer, necessitating increased [HCO3-]p to maintain electrical neutrality. How could [SID] be "normal," but [BE] increased? The purpose of the current exercise was to address this question. An [SID excess] ([SIDEx]) formula was developed, conceptually identical to Siggaard-Andersen's [BE], and [SID] was compared to [SIDEx] over the physiological range of plasma [albumin] ([alb]p), plasma [phosphate] ([Pi]p), and plasma pH (pHp).