Which antiarrhythmic class prolongs the cardiac action potential by blocking potassium channels?

• A. Sodium Channel Blockers
• B. Potassium Channel Blockers
• C. Calcium Channel Blockers
• D. Beta Blockers

Answer: (B)

Blocking potassium channels slows repolarization, which lengthens the duration of the cardiac action potential and the QT interval. This prolonged repolarization increases the refractory period, helping to prevent re-entrant arrhythmias. That’s the hallmark of class III antiarrhythmics, the potassium channel blockers. Clinically, you’d see a longer QT on the ECG, and there’s a risk of torsades de pointes if the QT prolongation is excessive. Other antiarrhythmic classes affect different parts of the action potential: sodium channel blockers alter depolarization, calcium channel blockers influence the plateau phase, and beta blockers reduce sympathetic drive and AV conduction.