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By: Dr. Nicholas Dodman

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  • Verapamil is a calcium channel blocker used principally for its effects in cardiac tissue.
  • This class of drugs demonstrates diverse cardiovascular effects and a variety of clinical uses. Verapamil primarily acts to impair the entry of calcium ions into cardiac muscle, specialized cardiac nodal tissues, or vascular smooth muscle.
  • Drug effects include a decreased heart rate, slowing of AV nodal conduction and other less well–defined antiarrhythmic effects.
  • In addition to cardiac effects, verapamil also has vascular effects. These effects are related to impaired calcium entry into vascular smooth muscle with resultant vasodilation.
  • Verapamil can be administered by the intravenous route for rapid control of arrhythmias(e.g. treatment of severe supraventricular tachycardia, peak effect within about 15 minutes), or chronically by the oral route for treatment of hypertension, chronic arrhythmias, or diastolic heart failure.
  • Verapamil is a prescription drug and can only be obtained from a veterinarian or by prescription from a veterinarian.
  • This drug is not approved for use in animals by the Food and Drug Administration but it is prescribed legally by veterinarians as an extra-label drug.

    Brand Names and Other Names

  • This drug is registered for use in humans only.
  • Human formulations: Calan® (Searle), Isoptin® (Knoll), Verelan® (Ledere), Covera-HS® (Searle) and generic preparations
  • Veterinary formulations: None

    Uses of Verapamil

  • Verapamil is primarily used in the treatment of supraventricular tachycardia.
  • Verapamil also slows the ventricular rate response in primary atrial arrhythmias such as ectopic atrial tachycardia, flutter, or fibrillation.
  • In pets with congestive heart failure (CHF), verapamil can be combined with digoxin to obtain better heart rate control.
  • Verapamil can also be used for long-term control of atrial arrhythmias in the dog and cat (e.g. atrial tachycardia or fibrillation) and is often given in combination with digoxin.
  • It has also been used in the control of atrial arrhythmias in hypertrophic cardiomyopathy of cats.

    Precautions and Side Effects

  • While generally safe and effective when prescribed by a veterinarian, verapamil can cause side effects in some animals.
  • Verapamil should not be used in animals with known hypersensitivity or allergy to the drug.
  • Verapamil should not be used in pregnant animals.
  • Verapamil should also be used with caution in animals with hepatic dysfunction as animals with pre-existing hepatic dysfunction may have an increased risk of toxicity due to delayed metabolism of the drug and subsequent accumulation.
  • Various adverse effects have been associated with verapamil administration. The most common side effect is hypotension, which is caused by the calcium blocking effects of verapamil.
  • Other potential adverse effects include bradycardia, peripheral edema, pulmonary edema, AV block and nausea.

    Drug Interactions

    Verapamil may interact with other medications. Consult with your veterinarian to determine if other drugs your pet is receiving could interact with verapamil. Possible interactions may include:

  • Verapamil is a negative cardiac inotrope and chronotrope and should not be used with other negative inotropes, such as beta-adrenergic blockers. When used with these drugs, such as propranolol, the negative inotropic effects are enhanced and cardiac function can be significantly negatively affected.
  • When used concurrently with cimetidine, the effects of verapamil may be enhanced.
  • When used with rifampin, the effects of verapamil may be reduced.
  • Verapamil may increase the blood levels of digoxin and theophylline, increasing the risk of toxicity.

    How Verapamil is Supplied

  • Verapamil is available in sustained release tablets and capsules are available in concentrations of 120 mg, 180 mg, 240 mg and 360 mg. It is also available in 40 mg, 80 mg and 120 mg tablets.
  • Verapamil is available as an injection in a concentration of 5 mg/2ml.
  • Verapamil tablets and capsules should be stored at room temperature. The injectable formulation should also be stored at room temperature and protected from light.

    Dosing Information

  • Medication should never be administered without first consulting your veterinarian.
  • The dosage prescribed may vary depending on the reason for prescribing.
  • The duration of administration depends on the condition being treated, response to the medication and the development of any adverse effects. Be certain to complete the prescription unless specifically directed by your veterinarian. Even if your pet feels better, the entire treatment plan should be completed to prevent relapse.
  • In dogs, verapamil is dosed at 0.05 mg/kg intravenously every 10 to 30 minutes to a maximum cumulative dose of 0.15 mg/kg. The oral dose is 0.05 to 0.1 mg per pound (0.1 to 0.2 mg/kg) orally ever 8 hours, titrated to effect.
  • In cats, verapamil is dosed at 0.025 mg/kg intravenously (IV) every 5 minutes up to a total dose of 0.15 to 0.2 mg/kg IV. The oral feline dose is 0.25 to 0.5 mg per pound (0.5 to 1 mg/kg) orally every 8 hours.

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