Class IV Antiarrhythmic Drugs

These are calcium entry blockers. Its major effect is seen in vascular smooth muscle and the heart.


Action:
It reduces the influx of calcium leading to reduced rate of Phase 4 (spontaneous depolarization).

It also causes a reduction of conduction in those tissues which are dependent on calcium flow i.e. AV node.

Dofetilide

Pharmacokinetics:
Its half life is about 10 hours. It is excreted in urine.

Therapeutic uses:
1. First line agent in sustained atrial fibrillation. Alongwith, amiodarone and β-blocking agents it is mostly used for atrial fibrillation.
2. In coronary disease of artery when left ventricular function is disturbed.

Sotalol

It also has a potent non-selective β-blocking agent.

As indicative of β-blocking agents, it also has the ability of decreasing mortality from acute myocardial infarction.

Actions:
It causes the inhibition of potassium outflow (also referred to as delayed rectifier). This inhibition increases both
1. Repolarization and
2. Action potential duration

And have the ability of prolonging the effective refractory period.

It has the ability of suppressing the ectopic beats and decreases myocardial oxygen demand. In the case of myocardial ischemia, it has prominent anti-fibrillatory affect.

Therapeutic uses:
1. Myocardial Ischemia
2. Sustained ventricular tachycardia

Adverse effects:
Prolonging the QT interval leading to torsade de pointes syndrome.

Class III Antiarrhythmic Drugs

These are potassium channel blockers.


Actions:
They reduce the outward flow of potassium during repolarization phase of cardiac cells.

They have the ability of increasing the duration of action potential without changing Phase 0 of depolarization or the resting membrane potential.

They also have the ability of prolonging the effective refractory period.

Class III antiarrhythmic agents have the potential of induction of arrhythmias.