Anti – arrhythmics

Electrophysiology of cardiac muscle: the pathophysiological mechanisms responsible for the
genesis of cardiac arrhythmias are not clearly understood. However, it is generally accepted
that cardiac arrhythmias arise as the result of either of
a) Disorders of impulse formation and/ or
b) Disorders of impulse conduction.
Pharmacotherapy of cardiac arrhythmias
Antiarrhythmic drugs are used to prevent or correct cardiac arrhythmias (tachyarrhythmias).
Drugs used in the treatment of cardiac arrhythmias are traditionally classified into:
Class (I): Sodium channel blockers which include quinidine, lidocaine, phenytion,
flecainide, etc.
Class (II): Beta adrenergic blockers which include propranolol, atenolol, etc.
Class (III): Potassium channel blockers e.g. amiodarone, bretylium.
Class (IV): Calcium channel blockers e.g. verapamil, etc.
Class (V): Digitalis e.g.digoxin.

Class – I drugs
Quinidine: It blocks sodium channel so that there is an increase in threshold for excitability. It is
well absorbed orally
Adverse effects: It has low therapeutic ratio. Main adverse effects are SA block, cinchonism,
severe headache, diplopia and photophobia.
Lidocaine, which is used commonly as a local anaesthetic blocks both open and inactivated
sodium channel and decreases automaticity. It is given parenterally.
Adverse effects: excessive dose cause massive cardiac arrest, dizziness, drowsiness,
seizures, etc.
Flecainide: It is a procainamide analogue and well absorbed orally. It is used in ventricular
ectopic beats in patients with normal left ventricular function.
Class –II drugs: Beta-adrenergic receptor blockers
Propranolol: Myocardiac sympathetic beta receptor stimulation increases automaticity,
enhances A.V. conduction velocity and shortens the refractory period. Propranolol can reverse
these effects. Beta blockers may potentiate the negative inotropic action of other
antiarrhythmics.
Therapeutic uses: This is useful in tachyarrhythmias, in pheochromocytoma and in
thyrotoxicosis crisis. It is also useful in patients with atrial fibrillation and flutter refractory to
digitalis.
Class – III: Potassium channel blockers
AMIODARONE: This drug is used in the treatment of refractory supraventriculat
tachyarrhythmias and ventricular tachyarrhythmias. It depresses sinus, atrial and A.V nodal
function.
The main adverse effects of this drug are anorexia, nausea, abdominal pain, tremor,
hallucinations, peripheral neuropathy, A.V. block
Class IV drugs: Calcium channel blockers
Verapamil: this drug acts by blocking the movement of calcium ions through the channels. It is
absolutely contraindicated in patients on beta blockers, quinidine or disopyramide.

It is the drug of choice in case of paroxysmal supraventricular tachycardia for rapid conversion
to sinus rhythm.
Class – V drugs:
Digoxin causes shortening of the atrial refractory period with small doses (vagal action) and a
prolongation with the larger doses (direct action). It prolongs the effective refractory period of
A.V node directly and through the vagus. This action is of major importance in slowing the rapid
ventricular rate in patients with atrial fibrillation