Which effect is associated with Vaughan Williams Class 1a on the cardiac action potential?

Which effect is associated with Vaughan Williams Class 1a on the cardiac action potential?

• A. Shortens phase 0
• B. Lengthens phase 0 and effective refractory period
• C. Blocks calcium channels
• D. Speeds up conduction through the AV node

Answer: (B)

Class 1A antiarrhythmics work by blocking fast sodium channels, which slows the rapid depolarization that defines phase 0. They also partially inhibit potassium channels, delaying repolarization (phase 3). The combination slows the upstroke (lengthening the time of phase 0) and prolongs repolarization, leading to a longer overall action potential duration and a longer effective refractory period. This is why this option best reflects their dual effect: slower phase 0 depolarization and an extended ERP. Calcium channel blockade or speeding AV conduction aren’t features of this class, so those statements aren’t consistent with their action.

Class 1A antiarrhythmics work by blocking fast sodium channels, which slows the rapid depolarization that defines phase 0. They also partially inhibit potassium channels, delaying repolarization (phase 3). The combination slows the upstroke (lengthening the time of phase 0) and prolongs repolarization, leading to a longer overall action potential duration and a longer effective refractory period. This is why this option best reflects their dual effect: slower phase 0 depolarization and an extended ERP. Calcium channel blockade or speeding AV conduction aren’t features of this class, so those statements aren’t consistent with their action.