Verapamil

Verapamil In Dogs & Cats: Uses, Dosage and Side Effects

Overview

Verapamil works by blocking the inward movement of calcium ions through slow (L-type) calcium channels in myocardial cells, cardiac conduction tissue, and vascular smooth muscle. This action leads to several effects:

• Reduction in myocardial contractility, resulting in negative inotropy.
• Depression of electrical activity, leading to slowed atrioventricular (AV) conduction.
• Vasodilation of cardiac vessels, peripheral arteries, and arterioles.

Uses of Verapamil

• Verapamil is mainly used for supraventricular tachyarrhythmias like accessory pathway-mediated SVT, atrial tachycardia, and flutter.
• It’s considered a secondary option among calcium-channel blockers, after diltiazem, due to its stronger negative inotropic effect.
• Patients with severe hepatic disease may need a 70% dose reduction due to reduced drug metabolism capacity.

Dose of Verapamil in Dogs and Cats

Dogs:

• 0.5–3 mg/kg p.o. q8h or 0.05 mg/kg slowly i.v. over 5 minutes (with ECG monitoring).
• Up to 4 repeat i.v. administrations at a reduced dose of 0.025 mg/kg q5min if necessary.

Cats:

• 0.5–1 mg/kg p.o. q8h or 0.025 mg/kg slowly i.v. over 5 minutes (with ECG monitoring).
• Up to 3 repeat i.v. administrations q5min if necessary.

Side Effects of Verapamil in Dogs and Cats

• Verapamil in humans can result in hypotension, bradycardia, dizziness, exacerbation or precipitation of congestive heart failure, nausea, constipation, and fatigue.

Contraindications of Verapamil in Dogs and Cats

• Do not use in patients with 2nd or 3rd-degree AV block, hypotension, sick sinus syndrome, left ventricular dysfunction or heart failure.

Some Notes:

• Avoid concurrent use of verapamil with beta-blockers due to their combined negative inotropic and chronotropic effects, which can be significant.
• Co-administration with sodium-channel blockers may lead to cardiovascular depression and hypotension.
• Verapamil activity may be negatively impacted by vitamin D or calcium salts.
• Cimetidine may enhance the effects of verapamil.
• Verapamil may elevate blood levels of digoxin, digitoxin, or theophylline, potentially causing toxicity.
• Calcium-channel blockers may increase intracellular levels of vincristine.
• Verapamil may enhance the neuromuscular blocking effects of non-depolarizing muscle relaxants.