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

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

• A. Increases depolarization threshold
• B. Slows nodal action potential
• C. Prolongs action potential and ERP
• D. Lengthens phase 0

Answer: (D)

Class 1c antiarrhythmics are potent fast sodium channel blockers, so they mainly slow the rapid upstroke of the action potential in atrial and ventricular myocardium. By blocking these channels, the slope of phase 0 is reduced, meaning the depolarization takes longer to reach its peak. That translates to slower conduction through the heart tissue and, on the ECG, a widened QRS. This effect centers on the beginning of the action potential (phase 0) rather than prolonging repolarization or ERP, and nodal tissue is less affected because nodal cells depend more on calcium currents for depolarization. The situation described—slowing the phase 0 upstroke and thus lengthening phase 0—is the best fit here.

Class 1c antiarrhythmics are potent fast sodium channel blockers, so they mainly slow the rapid upstroke of the action potential in atrial and ventricular myocardium. By blocking these channels, the slope of phase 0 is reduced, meaning the depolarization takes longer to reach its peak. That translates to slower conduction through the heart tissue and, on the ECG, a widened QRS. This effect centers on the beginning of the action potential (phase 0) rather than prolonging repolarization or ERP, and nodal tissue is less affected because nodal cells depend more on calcium currents for depolarization. The situation described—slowing the phase 0 upstroke and thus lengthening phase 0—is the best fit here.