Final answer:
The medication that can increase the QT interval by 50% or more is Procainamide (D). Procainamide is a potassium channel blocker that impedes the movement of K+ through voltage-gated K+ channels, affecting the repolarization phase of the cardiac action potential and thus prolonging the QT interval on an ECG.
Step-by-step explanation:
The question asks which of the following drugs may increase the QT interval by 50% or more: A. Verapamil, B. Metoprolol, C. Lidocaine, or D. Procainamide. The correct answer is D. Procainamide.
Potassium channel blockers like procainamide are used to treat cardiac dysrhythmia. These medications impede the movement of K+ through voltage-gated K+ channels. This inhibition affects the repolarization phase of the action potential in the heart. Since potassium channels are responsible for facilitating repolarization, blocking these channels prolongs the repolarization phase, which results in an increased QT interval on an ECG.
It's important to note that drugs like verapamil, which are calcium channel blockers, have a different mechanism of action. They are first and foremost used to decrease heart rate and do not typically cause a significant increase in the QT interval.