The anion gap of patients receiving bicarbonate maintenance hemodialysis.

To compare the effect of maintenance bicarbonate hemodialysis on the serum anion gap (AG) and the serum HCO3, the authors retrospectively evaluated the data of 28 patients using the first-of-the-month blood chemistries. Data were available for at least 9 of the previous 12 months in each case. For the entire group, the average AG was 15.8 +/- 0.1 mEq/L and the HCO3 21.7 +/- 0.2 mEq/L. Comparing these values with the data compiled recently by three different groups of investigators in undialyzed patients with severe chronic renal failure indicates that the AG of the dialyzed patients was similar to that of untreated patients, but the HCO3 levels of dialyzed patients averaged 2 to 3 mEq/L higher. Separate prospective evaluation of the acute effect of two consecutive bicarbonate dialyses in 31 patients revealed that the average acute (immediate postdialysis) decrease (from predialysis values) in AG was only about 40% of that of the increase in HCO3. The authors conclude that bicarbonate dialysis is more effective in improving serum HCO3 than AG.

The effects of administration of lithium salts and magnesium sulfate on the serum anion gap.

Whether or not an increased serum level of an unmeasured cation will reduce the anion gap (AG) depends on concomitant changes (or lack thereof) in serum unmeasured anions. In the present retrospective study, we sought to determine the effect of lithium carbonate or citrate and magnesium sulfate on the AG. Two groups of psychiatric patients whose average serum lithium levels were 0.6 and 1.0 mEq/L were studied. The AG in each group (10.2 +/- 0.3 [SE] and 9.0 +/- 0.4 mEq/L, respectively) was significantly (P less than 0.05) lower than that of a control group (11.1 +/- 0.3 mEq/L). Separately, we collected 87 pairs of AG and serum magnesium data of patients with toxemia of pregnancy. These included those of 15 patients evaluated both before and during magnesium sulfate infusion. Despite an average serum magnesium level of 4.1 +/- 0.2 mEq/L, the AG of the latter subjects tended to increase slightly from 10.8 +/- 0.5 to 11.7 +/- 0.7 mEq/L during magnesium infusion, instead of decreasing. The other data (72 determinations in the 55 patients without preinfusion values) revealed a mean AG of 11.0 +/- 0.3 mEq/L, coexisting with an elevated serum magnesium of 4.1 +/- 0.1 mEq/L. None of the above-mentioned three mean AG values differed significantly from that of the control group. We conclude that hypermagnesemia resulting from the administration of magnesium sulfate does not reduce the AG, probably because of a concomitant and proportional increase in serum sulfate. The present data reemphasize the need to assess the anion gap as the result of concomitant changes in both unmeasured anions and cations.

Metabolic alkalosis induced by regional citrate hemodialysis.

Two patients are described in whom regional citrate dialysis (RCD) and induced metabolic alkalosis with marked increases in serum HCO3, which were sustained during the period of repeated treatments. As currently employed, RCD of necessity delivers large amounts of potential bicarbonate (several hundred mEq) to the patient and may cause severe metabolic alkalosis. Studies should be carried out to determine an effective method to avoid this complication whenever repeated RCD is necessary or is used in patients with pre-existing alkalosis.