All of the following antiarrhythmic drugs prolong the QT interval EXCEPT:

Explanation

## QT Prolongation and Antiarrhythmic Classification ### Class III Antiarrhythmics and QT Prolongation **Key Point:** Class III antiarrhythmics (potassium channel blockers) characteristically prolong the QT interval by delaying repolarization. This is their mechanism of action and a hallmark ECG finding. ### Drugs That Prolong QT | Drug | Class | Mechanism | QT Effect | | --- | --- | --- | --- | | Amiodarone | III (+ I, II, IV) | K^+^ channel blockade | **Prolongs QT** | | Sotalol | III (+ β-blocker) | K^+^ channel blockade | **Prolongs QT** | | Dofetilide | III | Selective K^+^ channel blockade | **Prolongs QT** | | Verapamil | IV | Ca^2+^ channel blockade | **Shortens PR, no QT change** | ### Verapamil's Mechanism **High-Yield:** Verapamil is a Class IV antiarrhythmic (calcium channel blocker) that acts primarily on the AV node by slowing conduction velocity and increasing refractoriness. It does **not** block potassium channels and therefore does **not** prolong the QT interval. Its ECG signature is PR prolongation (AV nodal delay), not QT prolongation. ### Clinical Pearl **Warning:** QT prolongation with Class III drugs carries risk of **torsades de pointes**, especially in hypokalemia, hypomagnesemia, or bradycardia. Verapamil's lack of QT prolongation makes it safer in this regard, though it can cause bradycardia and hypotension via AV nodal effects. ### Summary - **Amiodarone, Sotalol, Dofetilide** → Class III → QT prolongation - **Verapamil** → Class IV → PR prolongation, **no QT change**