Propafenone

Propafenone

Propafenone (Rythmol) exhibits predominantly class IC properties with conduction slowing due to sodium channel blockade. Additionally, propafenone is a weak β-receptor and L-type calcium channel blocker.

Electrophysiological Actions

As with all members of its class, propafenone has its ma-jor effect on the fast inward sodium current. The IC agents depress V_max over a wide range of heart rates and shift the resting membrane potential in the direction of hyperpolarization. The IC agents bind slowly to the sodium channel and dissociate slowly. Therefore, they exhibit rate-dependent block. Inhibition of the sodium channel throughout the cardiac cycle will result in a de-crease in the rate of ectopy and trigger ventricular tachycardia.

Sinoatrial Node

Propafenone causes sinus node slowing that could lead to sinoatrial block. It may lengthen the sinus node recovery time with minimal effects on sinus cycle length.

Atrium

The action potential duration and ERP of atrial mus-cle are both prolonged by propafenone. The electrophys-iological effects persist beyond removal of the drug from the tissue. In patients with atrial flutter, fibrillation, or tachycardia, propafenone can slow the atrial rate, result-ing in a change from 2:1 or 4:1 A-V block to 1:1 A-V con-duction with a subsequent increase in the ventricular rate.

A-V Node

The IV administration of propafenone slows con-duction through the A-V node.

His-Purkinje System and Ventricular Muscle

Propafenone slows conduction and inhibits auto-matic foci.

Electrocardiographic Changes

Propafenone causes dose-dependent increases in the PR and QRS intervals.

Hemodynamic Effects

The IV administration of propafenone is accompanied by an increase in right atrial, pulmonary arterial, and pulmonary artery wedge pressures in addition to an in-crease in vascular resistance and a decrease in the car-diac index. A significant decrease in ejection fraction may be observed in patients with preexisting left ven-tricular dysfunction. In the absence of cardiac abnor-malities, propafenone has no significant effects on car-diac function.

Pharmacokinetics

The pharmacokinetic characteristics of propafenone:

Oral bioavailability : Nearly complete

Onset of action : 1 hour

Peak response : 2–3 hours

Duration of action : 8–12 hours

Plasma half-life : 2–10 hours

Primary route of metabolism : Hepatic

Primary route of excretion : 18.5–38% renal (unchanged)

Therapeutic serum concentration : <1/mL

Clinical Uses

Approved indications for propafenone include treatment of supraventricular arrhythmias and life-threatening ventricular arrhythmias in the absence of structural heart disease. Propafenone has been shown to increase mor-tality in patients with structural heart disease, and so ex-treme caution must be used in this subset of patients. As with flecainide, the patient should be hospitalized for initiation of therapy.

Adverse Effects and Drug Interactions

Concurrent administration of propafenone with digoxin, warfarin, propranolol, or metoprolol increases the serum concentrations of the latter four drugs. Cimetidine slightly increases the propafenone serum concentrations. Additive pharmacological effects can occur when lidocaine, procainamide, and quinidine are combined with propafenone.

As with other members of class IC, propafenone may interact in an unfavorable way with other agents that depress A-V nodal function, intraventricular con-duction, or myocardial contractility.

Overall, 21 to 32% of patients have adverse effects. The most common are dizziness or light-headedness, metallic taste, nausea, and vomiting; the most serious are proarrhythmic events.

Contraindications

Propafenone is contraindicated in the presence of se-vere or uncontrolled congestive heart failure; cardio-genic shock; sinoatrial, A-V, and intraventricular disor-ders of conduction; and sinus node dysfunction, such as sick sinus syndrome. Other contraindications include severe bradycardia, hypotension, obstructive pulmo-nary disease, and hepatic and renal failure. Because of its weak β-blocking action, propafenone may cause pos-sible dose-related bronchospasm. This problem is great-est in patients who are slow metabolizers.