Digoxin bioavailability during quinidine administration

Digoxin serum concentration rises in the presence of quinidine. To determine whether quinidine alters digoxin bioavailability, six subjects received 1.0 rag of digoxin intravenously alone and by mouth on alternate weeks during steady-state oral quinidine administration. The area under the digoxin concentration: tune curves (AUC) and the amount of digoxin excreted in the urine (X_u^χ) were determined for the 96 hr after each of the four experiments. Values for digoxin bioavailability relative to the corresponding intravenous study in the absence and presence of quinidine were (±S.D.) 73.5 ± 8.6% and 79.5 ± 22.6% (P > 0.05),for serum and 69.8 ± 6.8% and 70.2 ± 10.5% (P > 0.05), for urine. There was no difference in the steady-state quinidine serum concentration during the 4 clays after intravenous and oral digoxin. We conclude that quinidine does not alter digoxin bioavailability and therefore that altered absorption does not explain the rise in digoxin serum concentration in the presence of quinidine.