Use of control steady-state serum digoxin levels for predicting serum digoxin concentration after quinidine administration.

Serum digoxin and/quinidine concentration were measured before and following the institution of quinidine therapy in 11 patients who were receiving maintenance oral digoxin therapy and who required quinidine therapy for suppression of extrasystoles. Steady-state serum digoxin concentrations after quinidine administration were found to correlate well with serum digoxin concentrations before quinidine therapy and with peak serum digoxin concentrations after the first dose of quinidine sulfate, but not with serum quinidine concentrations. Thus steady-state serum digoxin concentration after quinidine (Y) could be estimated from the steady-state serum digoxin concentration before quinidine (X) as expressed by the regression formula Y = 2.6X - 0.6, with Syx2 - 0.21, and r = 0.96 (p less than 0.01). Based on these observations, guidelines for administering digoxin with concomitant quinidine therapy are presented.