Partitioning standard base excess: a new approach

Abstract

‘Standard’ or ‘extracellular’ base excess (SBE) is a modified calculation using one-third the normal hemoglobin concentration. It is a ‘CO₂-invariant’ expression of meta- bolic acid–base status integrated across interstitial, plasma and erythrocytic compartments (IPE). SBE also integrates conflicting physical chemical influences on metabolic acid–base status. Until recently attempts to quantify individual contributions to SBE, for example the plasma strong ion gap, failed to span the ‘CO_2-stable’ IPE dimension. The first breakthrough was from Anstey, who determined the con- centration of unmeasured charged species referenced to the IPE domain using Wooten’s physical chemical version of the Van Slyke equation. In this issue Drs Wolf and DeLand present a diagnostic tool based on an IPE model which dissects a version of SBE (BEnet) into nine independent (BEind) components, all referenced to the IPE domain. The reported components are excess/deficits of free water, chlo- ride, albumin, unmeasured ions, sodium, potassium, lactate, ‘Ca–Mg’ (a composite divalent cation entity), and phosphate. The model also reports individualised volumes of plasma, erythrocytes and interstitial fluid. The tool is an original contribution, but there are concerns. The impact of assum- ing fixed relationships between arterial and venous acid–base and saturation values in sepsis, anaemia and in differing shock states is unclear. Clinicians are also unlikely to accept that unique, accurate IPE volume determinations can be derived from a single set of blood gas and biochemistry results. Nevertheless, volume determinations aside, the tool is likely to become a valuable addition to the diagnostic armamentarium.