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Chapter: Modern Medical Toxicology: Cardiovascular Poisons: Diurets, Antihypertensives and Antiarrhythmics

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Propafenone (Fenopraine) - Antiarrhythmic Cardiovascular Poison

Propafenone is a class IC antiarrhythmic drug which blocks the fast sodium channel of the myocardial cell.

Propafenone (Fenopraine)

·              Propafenone is a class IC antiarrhythmic drug which blocks the fast sodium channel of the myocardial cell. It has some negative inotropic and beta-adrenoceptor blocking activity. Propafenone is structurally related to propranolol and is administered orally for the treatment of life-threatening ventricular arrhythmias. It is indicated in patients without structural heart disease to treat paroxysmal atrial fibrillation/flutter (PAF) or paroxysmal supraventricular tachycardia (PSVT) associated with disa-bling or life-threatening symptoms. It can have proarrhythmic effects and its use is not recommended in lesser ventricular arrhythmias.

·              It is 95% absorbed, peaks in 2 to 3 hours, has a plasma half-life varying from 2 to 32 hours, and less than 1% is excreted unchanged in the urine. Apparent volume of distribution is 1.1 to 3.6 L/kg and protein binding is to the extent of 97%. Therapeutic plasma levels should preferably not exceed 2 mcg/ml. Propafenone is metabolised by the cytochrome P-450 pathway which produces the major metabolites 5-hydroxy propafenone and N-desalkyl propafenone.

Adverse Effects

·              Bradycardia, cardiac conduction anomalies, hypoten- sion, proarrhythmias, worsening of heart failure, vertigo, headache, GI distress, alteration of taste (metallic or bitter taste), visual blurring. Constipation and nausea have been commonly reported side effects with propafenone therapy. Liver damage has been reported rarely with therapeutic use.

Drug Interactions

·              Potentiation of effects with local anaesthetics, beta blockers.

·              Increases digoxin plasma levels.

·              Poentiates anticoagulant effects of warfarin.

Clinical (Toxic) Features

·              The adult therapeutic dose range is 600 to 900 mg/day. Therapeutic levels range from 90 to 3,000 ng/ml.

·      Toxicity is usually most severe within 3 hours of inges-tion. Effects of overdose can include gastrointestinal upset, blurred vision, hypotension, drowsiness, prolonga-tion of the QRS interval, atrioventricular blocks, brady-cardia, A-V dissociation and conduction disturbances, ventricular arrhythmias, asystole, convulsions, acidosis and coma.

·      Neurologic disturbances are relatively common with propafenone overdose, and several types of convul-sions have been described including minor motor and tonic-clonic seizures. Dizziness, amnesia, disorientation, neuropathy, paraesthesias, coma and mania have also been reported.

Treatment

·      Admit to intensive care and monitor cardiac function and tidal volume. Obtain an ECG, institute continuous cardiac monitoring and administer oxygen. Evaluate for hypoxia, acidosis, and electrolyte disorders (particularly hypoka-laemia, hypocalcaemia, and hypomagnesaemia).

·      Lignocaine and amiodarone are generally first line agents for stable monomorphic ventricular tachycardia, particu-larly in patients with underlying impaired cardiac function. Sotalol is an alternative for stable monomorphic ventricular tachycardia. Amiodarone and sotalol should be used with caution. Unstable rhythms require cardioversion. Atropine may be used when severe bradycardia is present and PVCs are thought to represent an escape complex.

·      The class IC agents have been shown to be moderately refractory to conventional antiarrhythmic drug therapy, cardioversion, and ventricular pacing. Because of cross-over in electrophysiologic properties, class IA agents (quinidine, procainamide, disopyramide) are relatively contraindicated. Class IB agents (lignocaine, phenytoin, mexiletine, tocainide) may be the best alternative based on electrophysiologic properties, but could also exacerbate toxicity.

·      Stomach wash and/or activated charcoal, upto 4 to 6 hours post-ingestion.

·      Sodium loading : molar sodium lactate, sodium bicarbonate, or hypertonic saline may be administered. Serum alkalini-sation with sodium bicarbonate may be useful in treating arrhythmias. A reasonable starting dose is 1 to 2 mEq/kg as an intravenous bolus, repeated as needed to maintain arterial pH 7.45 to 7.55. Monitor ECG, arterial blood gases and electrolytes.

·      Diazepam for convulsions. If seizures persist or recur administer phenobarbitone.

·      For hypotension: Infuse 10 to 20 ml/kg of isotonic fluid and place in Trendelenburg position. If hypotension persists, administer dopamine or noradrenaline. Consider central venous pressure monitoring to guide further fluid therapy. Atropine and pressor amines (dopamine, dobutamine) may be used as necessary.

·              There are a few cases reported of beneficial effects following the use of infused adrenaline and temporary internal pacemaker.

 

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