NAPA

What it isN-acetylprocainamide, the active metabolite of procainamide, with class III (potassium-channel) antiarrhythmic activity of its own.

Why it mattersAcetylator phenotype decides how much of it a patient makes, so two patients on the same procainamide dose can carry very different NAPA burdens.

What it changesIt is renally cleared and torsadogenic on accumulation, which is why therapeutic monitoring of procainamide historically meant following both levels.