Which statement describes Vaughan Williams Class 3 effects on the cardiac action potential?

Which statement describes Vaughan Williams Class 3 effects on the cardiac action potential?

• A. Increases depolarization threshold
• B. Slows repolarization; Prolongs action potential and ERP
• C. Shortens ERP
• D. Inhibits sodium channels

Answer: (B)

Class III antiarrhythmics work by blocking potassium channels that carry the repolarizing current. This reduces the outward K+ flow during the final phase of the action potential, so repolarization slows. As a result, the overall action potential duration lengthens and the effective refractory period increases. This makes it harder for re-entrant circuits to sustain tachyarrhythmias. On the ECG, this shows up as a prolonged QT interval. These drugs don’t primarily raise the depolarization threshold or inhibit sodium channels, which is why prolonging repolarization and ERP is the distinguishing effect. Some Class III agents can have other actions, but the hallmark is slowed repolarization with longer APD and ERP, matching the statement described.

Class III antiarrhythmics work by blocking potassium channels that carry the repolarizing current. This reduces the outward K+ flow during the final phase of the action potential, so repolarization slows. As a result, the overall action potential duration lengthens and the effective refractory period increases. This makes it harder for re-entrant circuits to sustain tachyarrhythmias. On the ECG, this shows up as a prolonged QT interval. These drugs don’t primarily raise the depolarization threshold or inhibit sodium channels, which is why prolonging repolarization and ERP is the distinguishing effect. Some Class III agents can have other actions, but the hallmark is slowed repolarization with longer APD and ERP, matching the statement described.