Review Clinical review: The meaning of acid–base abnormalities in the intensive care unit – effects of fluid administration

Stewart’s quantitative physical chemical approach enables us to understand the acid–base properties of intravenous fluids. In Stewart’s analysis, the three independent acid–base variables are partial CO2 tension, the total concentration of nonvolatile weak acid (ATOT), and the strong ion difference (SID). Raising and lowering ATOT while holding SID constant cause metabolic acidosis and alkalosis, respectively. Lowering and raising plasma SID while clamping ATOT cause metabolic acidosis and alkalosis, respectively. Fluid infusion causes acid–base effects by forcing extracellular SID and ATOT toward the SID and ATOT of the administered fluid. Thus, fluids with vastly differing pH can have the same acid–base effects. The stimulus is strongest when large volumes are administered, as in correction of hypovolaemia, acute normovolaemic haemodilution, and cardiopulmonary bypass. Zero