Procainamide Hydrochloride

Antiarrhythmic Agent (Class IA) Sodium Channel Blocker

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog IV 6-15 mg/kg (slow IV bolus over 5 minutes) Loading dose, then CRI at 25-50 mcg/kg/min Duration: As needed for arrhythmia control
Notes: Monitor ECG and blood pressure; discontinue if QRS widens >50% or hypotension occurs.
Dog PO 10-30 mg/kg q6-8h (every 6-8 hours) Duration: For chronic management
Notes: Sustained-release formulations may be given q8-12h.
Cat IV 2-5 mg/kg (slow IV) Loading dose, then CRI at 25-50 mcg/kg/min Duration: As needed
Notes: Use with caution in cats due to narrow therapeutic index.
Cat PO 5-10 mg/kg q6-8h Duration: For chronic management
Notes: Monitor for gastrointestinal upset.
Horse IV 0.5-1 mg/kg (slow IV) Loading dose, then CRI at 1-2 mg/kg/hr Duration: As needed
Notes: Monitor ECG and blood pressure; may cause hypotension.
Horse PO 20-40 mg/kg q6h Duration: For atrial fibrillation conversion
Notes: Oral absorption is variable; monitor plasma levels if possible.

Clinical Indications & Species Uses

General Indications
  • Treatment of ventricular and supraventricular arrhythmias in small animals and horses
Dog (Canine)
  • Ventricular arrhythmias (ventricular tachycardia, premature ventricular contractions)
  • Supraventricular arrhythmias (atrial fibrillation, atrial flutter) when other agents fail
  • Emergency treatment of life-threatening ventricular arrhythmias
Cat (Feline)
  • Ventricular arrhythmias
  • Supraventricular arrhythmias (less common)
Horse (Equine)
  • Ventricular arrhythmias
  • Atrial fibrillation (especially in performance horses)
Cattle (Bovine)
  • Ventricular arrhythmias (rarely used)

Pharmacology & Mechanism of Action

Drug Class: Antiarrhythmic Agent (Class IA) | Pharmacological Group: Sodium Channel Blocker

Mechanism of Action: Procainamide is a Class IA antiarrhythmic agent that blocks cardiac sodium channels, slowing the rate of rise of the action potential (phase 0) and prolonging the effective refractory period. It also has mild potassium channel blocking effects, prolonging repolarization (phase 3) and increasing the action potential duration. These effects stabilize the myocardial membrane, suppress abnormal automaticity, and terminate reentrant arrhythmias. Procainamide also has vagolytic (anticholinergic) properties and can cause peripheral vasodilation via ganglionic blockade.

Pharmacodynamics: Procainamide depresses myocardial excitability, slows conduction velocity, and prolongs the refractory period in atrial, ventricular, and His-Purkinje tissues. It suppresses automaticity in ectopic pacemakers and raises the threshold for ventricular fibrillation. In therapeutic doses, it may cause mild negative inotropic effects and hypotension, especially after rapid IV administration. Its metabolite, N-acetylprocainamide (NAPA), has Class III antiarrhythmic activity, contributing to the overall antiarrhythmic profile.

⚡ Pharmacokinetics Summary

Absorption: Procainamide is well absorbed after oral administration, with bioavailability of 75-95% in most species. Food may delay absorption but not the extent. In horses, oral absorption is variable and may be less reliable.
Distribution: Procainamide is widely distributed throughout the body, with a volume of distribution of approximately 2 L/kg in dogs and cats. It crosses the placenta and is distributed into milk. Protein binding is low (15-20%) in most species.
Metabolism: Procainamide is metabolized in the liver primarily by N-acetylation to N-acetylprocainamide (NAPA), which is pharmacologically active. The rate of acetylation is genetically determined and varies among species and individuals. In dogs, acetylation is slow, while in cats it is intermediate.
Excretion: Both procainamide and NAPA are excreted primarily by the kidneys via glomerular filtration and tubular secretion. Renal impairment can lead to accumulation and toxicity. In dogs, the elimination half-life is approximately 2.5-3.5 hours; in cats, it is about 2-3 hours; in horses, it is around 3-4 hours.
Half-Life: Dogs: 2.5-3.5 hours; Cats: 2-3 hours; Horses: 3-4 hours; Cattle: 2-3 hours (estimated)
Bioavailability: Oral: 75-95% in dogs and cats; variable in horses
Protein Binding: 15-20% in most species

Available Formulations & Strengths

Oral Tablet 250 mg, 375 mg, 500 mg (PO)
Oral Capsule (Sustained-Release) 250 mg, 500 mg, 750 mg (PO)
Injectable Solution 100 mg/mL, 500 mg/mL (IV/IM)

Contraindications & Clinical Warnings

Contraindications:
  • Known hypersensitivity to procainamide or other amide local anesthetics
  • Complete heart block or second-degree AV block (unless pacemaker in place)
  • Severe myasthenia gravis (may exacerbate weakness)
  • Lupus erythematosus (may exacerbate symptoms)
  • Severe renal impairment (requires dose adjustment)
  • Hypotension or cardiogenic shock
Warnings & Clinical Precautions:
  • Use with caution in animals with pre-existing heart failure, renal or hepatic disease, or electrolyte imbalances (hypokalemia, hypomagnesemia).
  • Monitor ECG continuously during IV administration; discontinue if QRS duration widens by >50% or if arrhythmia worsens.
  • Monitor blood pressure during IV administration; may cause hypotension.
  • In horses, oral absorption is erratic; consider IV therapy for acute management.
  • In food animals, observe withdrawal times; not approved for use in food animals in many countries.
  • May cause lupus-like syndrome in dogs with chronic use; discontinue if signs develop.
  • Use with caution in animals with myasthenia gravis; may worsen muscle weakness.

Adverse Effects & Reactions

Common:

  • Gastrointestinal upset (nausea, vomiting, diarrhea)
  • Hypotension (especially with rapid IV administration)
  • Cardiac conduction disturbances (QRS widening, AV block)
  • Anorexia

Serious / Severe:

  • Ventricular arrhythmias (torsades de pointes)
  • Heart block
  • Cardiac arrest
  • Lupus-like syndrome (with chronic therapy)
  • Blood dyscrasias (agranulocytosis, thrombocytopenia)

Rare:

  • Hypersensitivity reactions (rash, fever)
  • Hepatotoxicity
  • Neurotoxicity (tremors, seizures) at high doses

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
Other antiarrhythmics (e.g., lidocaine, quinidine, amiodarone) Additive cardiac depression and proarrhythmic effects; use with caution. High
Beta-blockers (e.g., propranolol) Additive negative inotropic and chronotropic effects; may cause bradycardia or heart block. Moderate
Calcium channel blockers (e.g., diltiazem) Additive hypotension and AV block. Moderate
Neuromuscular blocking agents Procainamide may potentiate neuromuscular blockade. Moderate
Anticholinergic drugs (e.g., atropine) Additive vagolytic effects. Mild
Cimetidine May reduce renal clearance of procainamide, increasing plasma levels. Moderate
Trimethoprim-sulfamethoxazole May decrease renal excretion of procainamide and NAPA. Moderate

Overdose & Toxicity Management

Signs of Toxicity:

  • Severe hypotension
  • Widening of QRS complex
  • Ventricular arrhythmias (including torsades de pointes)
  • AV block
  • Asystole
  • CNS depression, confusion, seizures
  • Respiratory depression

Emergency Treatment Protocol: Discontinue drug immediately. Provide supportive care: IV fluids for hypotension, atropine for bradycardia, sodium bicarbonate for QRS widening (alkalinization may reverse sodium channel blockade), lidocaine or magnesium sulfate for ventricular arrhythmias (if not due to procainamide toxicity), and cardiac pacing if necessary. In severe cases, consider hemodialysis or hemoperfusion to enhance elimination.

Food Animal Withdrawal Times

Procainamide is not approved for use in food animals in the US and many other countries. If used off-label in food animals, a prolonged withdrawal period (e.g., 30 days for meat and 7 days for milk) is recommended, but due to lack of data, it is safer to avoid use in food animals.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature 20-25°C (68-77°F), excursions permitted between 15-30°C (59-86°F).

Handling & Special Conditions: Protect from moisture. Keep container tightly closed. Injectable solution should be stored at room temperature and protected from freezing.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Not FDA-approved for veterinary use; approved for human use.

Extra-Label (Off-Label) Use: In the US, procainamide is not FDA-approved for veterinary use; extra-label use is permitted under AMDUCA for non-food animals. For food animals, extra-label use is prohibited if an approved animal drug exists that is effective and available, and if the drug is not approved for food animals, it cannot be used in an extra-label manner in feed or water. Use in food animals is generally discouraged.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Procainamide is a valuable antiarrhythmic for managing ventricular arrhythmias in dogs and horses, and occasionally cats. It is particularly useful for emergency control of life-threatening ventricular tachycardia. However, its use requires careful monitoring due to a narrow therapeutic index and potential proarrhythmic effects. In dogs, chronic therapy may be associated with a lupus-like syndrome; therefore, it is often reserved for cases where other antiarrhythmics (e.g., sotalol, mexiletine) are ineffective or contraindicated. In horses, procainamide is used for atrial fibrillation, but oral absorption is erratic, and IV administration is preferred for acute conversion. Always monitor ECG and blood pressure during IV administration. Adjust doses in patients with renal or hepatic impairment. For food animals, avoid use due to lack of withdrawal data and regulatory restrictions.

References & Literature

  • 📚 Plumb's Veterinary Drug Handbook
  • 📚 Papich Veterinary Pharmacology and Therapeutics
  • 📚 Compendium of Veterinary Products (CVP)