Designing 'balanced' crystalloids.

OBJECTIVE: To present a rationale for the design of balanced resuscitation and renal replacement crystalloids based on Stewart's physical chemical approach to acid-base. DATA SOURCES: Articles and published abstracts on acid-base physiology, crystalloid infusions and renal replacement therapy. SUMMARY OF REVIEW: Although it is uncertain that crystalloid-induced metabolic acidosis causes significant harm, Stewart's approach assists in designing balanced fluids without this side effect. In his analysis, the three independent variables determining acid-base balance are PCO2, the total concentration of non-volatile weak acid (A(TOT)) and the strong ion difference (SID). Raising and lowering A(TOT) while holding SID constant cause a metabolic acidosis and alkalosis respectively. Lowering and raising plasma SID while clamping A(TOT) cause a metabolic acidosis and alkalosis respectively. The SID of a crystalloid is its [HCO3-], or that part of an organic bicarbonate surrogate which is metabolised on infusion. Rapid infusion alters plasma SID towards crystalloid SID, but also lowers A(TOT) by haemodilution. We have shown that the SID of a balanced infusion crystalloid is 24 mEq/L. This generates a fall in plasma SID precisely counteracting the A(TOT) dilutional alkalosis. In contrast, a balanced renal replacement crystalloid must generate a higher plasma SID appropriate for the existing A(TOT), since there is no dilution. If A(TOT) is low, as in hypoalbuminaemia, the balanced dialysis SID falls correspondingly. A further SID reduction is needed to counteract Donnan effects within the filter.
CONCLUSIONS: A crystalloid SID of 24 mEq/L is 'balanced' for rapid intravenous administration. The 'balanced' SID of renal replacement fluids is likely to be significantly higher, although less than the normal plasma SID of 42 mEq/L.