Class II Antiarrhythmic Drugs

Class II drugs are β-adrenergic receptor antagonists.

Action:
These agents have the ability of decreasing Phase 4 depolarization thus suppresses the automaticity, causing a prolonged AV conduction and decreased cardiac rate and contractility.

Therapeutic uses:
Class II drugs are used in the treatment of tachycardia due to promoted activity of sympathetic system.

They are also useful in the treatment of atrial flutter and fibrillation.

They are used in AV nodal re-entrant tachycardia.

Propafenone

Same as that of flecainide. It has the ability of allowing conduction in all the tissues of heart.

Flecainide

It binds to sodium channels and slowly detaches from sodium channels.

Mechanism of action:
It is a sodium channel blocker. It suppresses phase 0 of action potential in purkinje fibers and fibers of the muscles of the heart.

Actions:
It does not affect greatly the action potential.

1. In this case, automaticity of the ectopic pacemaker is decreased by the promoted threshold potential (not by a reduction in the slope of phase 4 depolarization).
2. It causes a decreased conduction and excitability and promoted refractory period (pronounced in the depolarized tissue).

Pharmacokinetics:
It is absorbed orally and it has a half life of about 16-20 hours.

Therapeutic uses:
1. It is used in premature ventricular contractions, refractory ventricular tachycardia and supraventricular tachycardia.
2. It is used for the inhibition of sudden outburst of atrial fibrillation or flutter.

Adverse effects:
It may cause dizziness, blurred vision, anorexia, nausea, vomiting, impotence and in some cases may worsen preexisting arrhythmias.

Mexiletine and tocainide

These are almost similar to lidocaine. But they can be administered orally.
Therapeutic uses:
They are used for ventricular arrhythmias.

Adverse effects:
Tocainide may cause pulmonary toxicity which may cause pulmonary fibrosis.