Which statement best describes Vaughan Williams Class 4 effects on the cardiac action potential?

Which statement best describes Vaughan Williams Class 4 effects on the cardiac action potential?

• A. Blocks calcium channels
• B. Increases depolarization threshold
• C. Slows nodal action potential
• D. Prolongs repolarization

Answer: (A)

Class IV antiarrhythmics are calcium channel blockers. Their defining action is to block L‑type calcium channels, which reduces calcium entry during the action potential. In the heart, the upstroke of the nodal action potential (SA and AV nodes) depends on calcium influx, so blocking these channels slows nodal depolarization and AV nodal conduction (evident as a longer PR interval). This is the primary way Class IV drugs alter the cardiac action potential. They also dampen the plateau in ventricular cells and decrease contractility, but they do not primarily prolong repolarization like Class III agents. Therefore, blocking calcium channels best describes Vaughan Williams Class 4 effects on the cardiac action potential.

Class IV antiarrhythmics are calcium channel blockers. Their defining action is to block L‑type calcium channels, which reduces calcium entry during the action potential. In the heart, the upstroke of the nodal action potential (SA and AV nodes) depends on calcium influx, so blocking these channels slows nodal depolarization and AV nodal conduction (evident as a longer PR interval). This is the primary way Class IV drugs alter the cardiac action potential. They also dampen the plateau in ventricular cells and decrease contractility, but they do not primarily prolong repolarization like Class III agents. Therefore, blocking calcium channels best describes Vaughan Williams Class 4 effects on the cardiac action potential.