Verapamil
Verapamil is a nondihydropyridine calcium-channel blocker used infrequently in veterinary medicine. Its principal potential application is control of selected supraventricular tachyarrhythmias. Effects on cardiac conduction are accompanied by reduced contractility and vascular relaxation. Veterinary clinical experience is smaller than that for some other drugs used for similar cardiac objectives.
Group
Nondihydropyridine calcium-channel blocker
Mechanism of Action
Verapamil blocks L-type calcium channels in cardiac nodal tissue, myocardium and vascular smooth muscle. Reduced calcium entry slows atrioventricular nodal conduction, increases nodal refractoriness and reduces sinoatrial activity. It also decreases myocardial contractility and relaxes resistance vessels. These effects can suppress selected nodal-dependent tachyarrhythmias or reduce ventricular response to rapid atrial activity.
Dose Rates
Dogs
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Supraventricular arrhythmia | Reference only | IV | 0.05 mg/kg IV every 10-30 minutes up to cumulative 0.15 mg/kg. The cumulative course cap must not be misrepresented as a per-dose cap. | |
| Supraventricular arrhythmia oral alternative | 0.5–2 mg/kg | PO | Every 8 h | Historical reported dose; oral efficacy and myocardial tolerance remain limitations. |
| Hypertension | 1–5 mg/kg | PO | Every 8 h | |
| Supraventricular tachycardia maintenance infusion | 2–10 mcg/kg/min | IV | Continuous | After monitored bolus protocol; marked cardiac-depression risk. |
Hamsters
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Reported cardiac therapy | 0.25–0.5 mg/animal | SC |
Indications
- Specialist treatment of selected supraventricular tachycardias involving atrioventricular nodal conduction.
- Ventricular rate control in selected atrial flutter or fibrillation cases in dogs or cats, with limited veterinary clinical evidence.
Contraindications
- Decompensated congestive heart failure
- Advanced heart block
- Not recommended in cats.
Interactions
- P-glycoprotein and cytochrome P450 interactions can alter exposure.
Adverse effects
- Hypotension, bradycardia, AV block, myocardial depression and possible cardiac arrest with IV use.
Toxicity
Excess causes bradycardia, hypotension, AV block, myocardial depression and possible hyperglycemia. Intensive ECG and perfusion monitoring, targeted cardiovascular support and sometimes pacing are required.
Preparations
| Preparation | Form | Strength | Route |
|---|---|---|---|
| Verapamil injection 2.5 mg/mL | Injection | 2.5 mg / mL | IV |
Monitoring and Administration
Monitor ECG and blood pressure continuously during IV treatment. Oral active-isomer exposure is inadequate for an established dose; diltiazem is generally preferred.
Feline older alternatives 0.025 mg/kg IV repeated to 0.15-0.2 mg/kg total, or 0.5-1 mg/kg orally every 8 hours, conflict with guidance against feline use. Equine 0.025-0.05 mg/kg IV every 30 minutes requires a cumulative amount below 0.2 mg/kg. Rabbit combined therapy 8-16 mg/kg orally plus 0.5-2 mg/kg SC daily is a joint protocol, not either dose alone.