Effect of repeated plasma exchange on steady state kinetics of digoxin and digitoxin

The effect of repeated plasma exchanges on the steady state kinetics of digoxin (3 patients) and digitoxin (4 patients) was investigated in 7 patients. Plasma exchange was performed 3 times a week for 4 weeks up to 12 exchanges using a hollow fiber membrane. In each exchange, 4000 ml plasma were filtered within 1 to 2 h and replaced by an albumin containing (20 g/l) physiological electrolyte solution. Digoxin and digitoxin concentrations in blood and filtered plasma were measured by radioimmunoassay. The effects due to the amount eliminated by plasma exchange were distinguished from the effects due to hypoalbuminemia. The eliminative effect was confined to the plasma compartment. It resulted in a marginal decrease in the elimination half-life from 1.6 to 1.59 days for digoxin and 4.3 to 4.2 days for digitoxin. Theoretically, it can be calculated that the hypoalbuminemia caused an increase in the volume of distribution from 451 to 497 l (digoxin) and 35 to 50 l (digitoxin) and a further decrease in the elimination half-life from 4.2 to 4.1 days in the case of digitoxin (not digoxin). If given within 2 h prior to plasma exchange, 13 to 50% of the digitoxin dose (not digoxin) was eliminated. Alteration of digoxin and digitoxin dosage during repeated plasma exchanges is not recommended, but drugs should be given after, not before plasma exchange.