Procainamide
Procainamide is a **Class 1A antiarrhythmic** agent structurally related to the local anesthetic procaine. It is primarily used in veterinary medicine to manage **ventricular tachyarrhythmias** (such as ventricular premature complexes [VPCs] and ventricular tachycardia) and certain **supraventricular tachycardias** (SVTs). Because of its broad spectrum of activity for both atrial and ventricular arrhythmias, it is often considered an ideal first-line IV therapeutic agent when a wide QRS complex tachycardia cannot be definitively identified as supraventricular or ventricular in origin. > **Clinical Pearl:** Unlike humans, dogs are poor acetylators and do not form appreciable amounts of the active metabolite N-acetyl-procainamide (NAPA). Therefore, therapeutic monitoring in dogs should focus solely on procainamide levels.
Mechanism: Procainamide acts primarily as a **fast sodium channel blocker** (Class 1A), similar to quinidine. * **Phase 0 Blockade:** Slows the influx of sodium during Phase 0 depolarization of the cardiac action potential. * **โ** Prolongs the refractory period in both the atria and ventricles. * **โ** Decreases myocardial excitability and depresses automaticity and conduction velocity. * **Anticholinergic Effects:** Exhibits mild vagolytic properties which may cause slight increases in heart rate or unpredictable rate changes. * **ECG Effects:** Commonly causes widening of the QRS complex and prolongation of the PR and QT intervals.
Dosing by species
- Chronic management of SVTs ยท 3-8 mg/kg PO q6-8h ยท PO ยท q6-8h
- Chronic management of SVTs ยท 1-2 mg/kg slowly IV; 10-20 micrograms/kg/minute constant rate IV infusion ยท IV ยท Bolus then CRI
- Chronic management of SVTs ยท 7.5-20 mg/kg q 6-8h ยท PO ยท q6-8h
- Atrial fibrillation / Ventricular tachycardia ยท 1 mg/kg/min IV, not too exceed 20 mg/kg (20 minutes) total dose ยท IV ยท Once ยท 20 minutes max ยท Not as effective as quinidine for atrial fibrillation
- Atrial fibrillation / Ventricular tachycardia ยท 25-35 mg/kg PO q8h ยท PO ยท q8h
- V-Tach ยท 1 mg/kg/minute IV up to a total dose of 20 mg/kg; or 25-35 mg/kg PO q8h ยท IV/PO ยท Once or q8h
- Ventricular tachyarrhythmias ยท 2-4 mg/kg IV slowly (over two minutes) up to a total dose of 12-20 mg/kg until arrhythmia controlled and then a CRI may be started at 10-40 micrograms/kg/minute ยท IV ยท Intermittent boluses then CRI
- Ventricular tachyarrhythmias ยท 20-30 mg/kg PO q6-8h ยท PO ยท q6-8h ยท Previous recommendations of 8-20 mg/kg PO q6-8h are almost certainly too low
- Acute management of SVTs ยท 6-8 mg/kg IV over 3 minutes or 6-20 mg/kg IM ยท IV/IM ยท Once ยท After drugs have been used to slow AV nodal conduction (i.e., diltiazem)
Routes of administration
Contraindications
- Myasthenia gravis
- Hypersensitivity to procainamide, procaine, or chemically related drugs
- Systemic lupus erythematosus (SLE) in humans (unknown in dogs)
- Torsade de pointes
- 2nd or 3rd degree heart block (unless artificially paced)
- Doberman pinschers and boxers with dilated cardiomyopathy (relative - proarrhythmic risk)
- Dogs with subaortic stenosis (relative - proarrhythmic risk)
Adverse effects
- Anorexia
- Vomiting
- Diarrhea
- Weakness
- Hypotension (especially with rapid IV injection)
- Negative inotropism
- Widened QRS complex
- Prolonged QT interval
- AV block
- Multiform ventricular tachycardias
- Fevers
- Leukopenias
Drug interactions
- Amiodarone ยท May increase procainamide levels; procainamide dose may need to be reduced
- Anticholinesterase agents (e.g., pyridostigmine, neostigmine) ยท Procainamide may antagonize effects in patients with myasthenia gravis
- Cimetidine ยท May increase procainamide levels
- Hypotensive drugs ยท Procainamide may enhance hypotensive effects
- Lidocaine ยท Toxic effects may be additive, and cardiac effects unpredictable
- Neuromuscular blocking agents ยท Procainamide may potentiate or prolong the neuromuscular blocking activity
- Quinidine ยท Toxic effects may be additive, and cardiac effects unpredictable
- Phenytoin ยท Toxic effects may be additive, and cardiac effects unpredictable
- Propranolol ยท Toxic effects may be additive, and cardiac effects unpredictable
- Ranitidine ยท May increase procainamide levels
- Trimethoprim ยท May increase procainamide levels
Monitoring
- ECG (continuously with IV dosing)
- Blood pressure (during IV administration)
- Clinical signs of toxicity (GI signs, weakness)
- Serum drug levels (Trough levels for oral therapy. Note: Request lab to not run NAPA for dogs. Target range: 3-8 to 8-20 mcg/mL in dogs; 4-10 mcg/mL in horses)
Overdose
Clinical signs of overdosage can include **hypotension, lethargy, confusion, nausea, vomiting, and oliguria**. Cardiac signs may include widening of the QRS complex, junctional tachycardia, ventricular fibrillation, or intraventricular conduction delays. * **Oral Ingestion:** Emptying of the gut and charcoal administration may be beneficial to remove unabsorbed drug. * **Hypotension:** IV fluids, plus dopamine, phenylephrine, or norepinephrine could be considered. * **Cardiotoxicity:** A 1/6 molar intravenous infusion of sodium lactate may be used in an attempt to reduce cardiotoxic effects. * **Elimination:** Forced diuresis using fluids and diuretics along with reduction of urinary pH can enhance renal excretion. * **AV Block:** Temporary cardiac pacing may be necessary should severe AV block occur.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.