COPR Advanced Care Paramedic (ACP) Practice Exam

Which option correctly describes Vaughan Williams Class 3 effects on the cardiac action potential?

Shortens action potential duration Increases depolarization threshold Slows repolarization; prolongs the action potential and ERP Lengthens phase 0

Correct Answer:

Slows repolarization; prolongs the action potential and ERP

Explanation:

Class III antiarrhythmic effects come from blocking potassium channels responsible for the heart’s repolarization. By inhibiting this outward K+ current during phase 3, repolarization is slowed, so the action potential lasts longer. That lengthened action potential duration also prolongs the effective refractory period, making the tissue less capable of re-exciting and helping prevent re-entrant arrhythmias. These drugs do not lengthen the upstroke phase 0 (driven by sodium channels) or raise the depolarization threshold, which would be more characteristic of other drug effects. Keep in mind the trade-off: QT prolongation, with a risk of torsades de pointes in some settings.

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