Final answer:
Potassium channel blockers such as amiodarone and procainamide target the ventricular repolarization phase of the cardiac action potential, which can lead to proarrhythmic effects, where an arrhythmic drug inadvertently causes a new arrhythmia.
Step-by-step explanation:
When considering the effects of potassium channel blockers on the heart, particularly drugs like amiodarone and procainamide, it's important to understand their influence on the action potential of cardiac contractile cells. Potassium channel blockers impede the movement of K+ through voltage-gated K+ channels, which play a critical role in cardiac electrophysiology. Specifically, these drugs affect the phase of the action potential associated with ventricular repolarization.
In a normal cardiac cycle, the action potential initiates a sequence of events that allow the heart to beat in a coordinated manner. Potassium channels contribute to the final phase of the action potential, where they facilitate the exit of K+ from the cells, leading to repolarization and the restoration of the resting membrane potential. When potassium channel blockers are administered, the extended plateau phase due to calcium influx is affected, as the natural efflux of potassium is impeded, prolonging repolarization and potentially preventing the heart from properly relaxing between beats.
This prolonged repolarization phase can lead to an increased risk of developing new proarrhythmic effects, a situation where an antiarrhythmic drug paradoxically induces an arrhythmia instead of suppressing it. The correct answer to the student's question is (C) Proarrhythmic effect.