4430 Antiarrhythmics Practice Exam

Potassium channel blockers work by inhibiting the outward flow of K+ during phase 3 of the cardiac action potential. This slows repolarization, extends the duration of the action potential, and increases the refractory period (which is why the QT interval often lengthens). The initial depolarization, phase 0, is driven mainly by sodium entry, not potassium efflux, so blocking potassium channels does not alter phase 0. That’s why the best description is prolonging repolarization without changing phase 0. Shortening repolarization would shorten the QT interval, blocking sodium channels would slow phase 0 and reduce conduction, and increasing phase 0 depolarization isn’t how these blockers work.