Procainamide
Procainamide
is classified as a Vaughn Williams’ Class IA antiarrhythmic agent. In contrast,
its active metabolite, N-acetylprocainamide (NAPA; acecainide) has been
described as a Class III antiarrhythmic drug. Procainamide is an antiarrhythmic
agent with electrophysiologic properties similar to that of quinidine. Its
primary effects on the heart are to decrease electrical impulse conduction
velocity through atrial and ventricular tissue manifested by a widened QRS and
PR interval in the ECG, and to prolong the effective refractory period. While
N-acetylprocainamide retains some of the same clinical effects as procainamide,
it has a slightly different electrophysiologic profile. It increases the
effective refractory period with a selective lengthening of the action
potential by prolonging repolarisation. There is no effect on depolarisation,
which is thought to result from its inability to block fast sodium channels and
depress phase 4 depolarisation. Thus, the drug has been described as a Class
III antiarrhythmic.
·
To suppress ventricular ectopy.
·
To suppress atrial and ventricular
tachyarrhythmias.
Procainamide
can be given orally but absorption is often delayed in overdose situations. The
apparent volume of distri-bution is 1.7 to 2.22 L/kg. Peak plasma level: 1 to 2
hours; protein binding: 15%. It is metabolised by the liver and under-goes
biotransformation by acetylation to N-acetylprocainamide (NAPA) which is
pharmacologically active. Excretion occurs in the urine (approximately 40 to
60% procainamide and 84% NAPA are excreted unchanged in the urine). Elimination
half-life is approximately 8 hours.
The
usual oral adult dose of procainamide is 1 gram initially, followed by 50
mg/kg/day in divided doses every 3 hours. Therapeutic range for procainamide
serum levels varies from 6 to 14 mcg/ml (25.5 to 59.5 mmol/L).
·
Nausea, vomiting, diarrhoea,
anorexia.
·
Drowsiness, blurred vision,
paraesthesias, hallucina-tions. Toxic psychosis has been reported.
·
Proarrhythmic
events : torsade de pointes,
ventriculartachycardia, and fibrillation.
·
Skin rash.
·
Thrombocytopenia.
·
Drug-induced systemic lupus erythematosus (SLE),
characterised by fever, arthralgias, myalgias, pleural effusion and pain, and
serositis.
·
Elevated liver enzyme and serum bilirubin levels have been
reported.
Overdose
results in arrhythmias (ventricular tachycardia, junctional tachycardia),
conduction abnormalities (QRS and QTc prolongation), torsade de pointes,
hypotension, mental status depression, seizures,
anticholinergic effects, respiratory depression, nausea, vomiting and
diarrhoea.
· Monitoring drug levels of
procainamide and its active metabolite, N-acetyl procainamide, may be helpful
in diagnosis of procainamide toxicity.
· Decontamination : stomach wash and activated
charcoal.
· Elimination enhancement: haemoperfusion,
continuousarteriovenous haemofiltration, or continuous arteriovenous
haemodiafiltration. Resin haemoperfusion or haemodialysis are the methods of
choice for removal of procainamide and N-acetylprocainamide.
· Continuous ECG monitoring, airway
and circulatory support, and IV access.
· Sodium bicarbonate or sodium lactate
may help in reversing the sodium blockade. Cardiac toxicity often responds to
intravenous sodium bicarbonate. A reasonable starting dose is 1 to 2 mEq/kg as
an intravenous bolus repeated as needed to reverse QRS widening and arrhythmias
and maintain an arterial pH of 7.45 to 7.55. Monitor serial ECGs, arterial
blood gases and serum potassium.
· Lignocaine: 1 to 1.5 mg/kg IV push.
For refractory VT/VF an additional bolus of 0.5 to 0.75 mg/kg can be given over
3 to 5 minutes. Total dose should not exceed 3 mg/kg or more than 200 to 300 mg
during a one hour period. Only bolus therapy is recommended during cardiac
arrest. Once circulation has been restored begin maintenance infusion of 1 to 4
mg/min. If arrhythmias recur during infusion, repeat 0.5 mg/kg bolus, and
increase the infusion rate incremen-tally (maximal infusion rate is 4 mg/min).
For a child: 1 mg/kg initial bolus intravenously; followed by a continuous
infusion of 20 to 50 mcg/kg/min.
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