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

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

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

Answer: (C)

Calcium channel blockers fall into Class 4 and mainly affect tissue where the action potential upstroke is calcium-driven—the SA and AV nodes. Blocking L-type calcium channels reduces calcium influx, which slows the phase 0 upstroke in these nodal cells and slows conduction through the AV node. Because less inward current is available to reach threshold, the depolarization threshold effectively increases. So, you have a slower nodal upstroke (lengthened phase 0) and a higher threshold to trigger depolarization. In ventricular tissue, where phase 0 is driven by sodium, these drugs have less direct effect, which is why the dominant impact is on nodal conduction. This combination is why describing both an increased depolarization threshold and a lengthened phase 0 fits best.

Calcium channel blockers fall into Class 4 and mainly affect tissue where the action potential upstroke is calcium-driven—the SA and AV nodes. Blocking L-type calcium channels reduces calcium influx, which slows the phase 0 upstroke in these nodal cells and slows conduction through the AV node. Because less inward current is available to reach threshold, the depolarization threshold effectively increases. So, you have a slower nodal upstroke (lengthened phase 0) and a higher threshold to trigger depolarization. In ventricular tissue, where phase 0 is driven by sodium, these drugs have less direct effect, which is why the dominant impact is on nodal conduction. This combination is why describing both an increased depolarization threshold and a lengthened phase 0 fits best.