To the Editor: We read with great interest Dr. Doering's article on the digoxin-quinidine interaction in the August 23 issue. Although we agree with most of his findings, we would like to present some additional results from our study1 that suggest a different mechanism of the digoxin-quinidine interaction. In our study serum concentrations of digoxin and quinidine as well as urinary levels of digoxin were measured in 11 volunteers under steady-state conditions (0.375 mg of digoxin per day). The group's mean (±S.D.) serum digoxin concentration was 0.75±0.2 ng per milliliter before and 1.2±0.4 ng per milliliter after (P<0.0001) concomitant administration.
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