Amlodipine
Amlodipine is a **dihydropyridine calcium channel blocker (CCB)** primarily utilized in veterinary medicine as a potent **peripheral arterial vasodilator**. - **Feline Medicine**: It is considered the **gold standard** and first-line therapy for managing **feline systemic hypertension**, which is often secondary to chronic kidney disease (CKD), hyperthyroidism, or hypertrophic cardiomyopathy. It is highly effective as a single agent in cats. - **Canine Medicine**: In dogs, it is typically used as an adjunctive afterload reducer in refractory congestive heart failure (CHF) or as a second-line antihypertensive agent. When used alone in dogs at higher doses, it can activate the renin-angiotensin-aldosterone system (RAAS), so it is frequently combined with an ACE inhibitor (e.g., enalapril) to mitigate this effect. - **Clinical Pearl**: It has a relatively slow onset of action, which helps prevent acute hypotensive crises, but missing doses can lead to rapid rebound hypertension.
Mechanism: Amlodipine selectively inhibits the transmembrane influx of calcium ions into vascular smooth muscle and cardiac muscle by blocking **L-type voltage-gated calcium channels**. - **Vascular Smooth Muscle** โ Relaxation and vasodilation โ Significant reduction in **peripheral vascular resistance (afterload)** โ Decreased systemic blood pressure. - It has a much higher affinity for vascular smooth muscle than cardiac muscle, meaning it produces profound vasodilation with minimal negative inotropic (contractility) or chronotropic (heart rate) effects at therapeutic doses. - **Cardiac Muscle** โ Mildly depresses impulse formation (automaticity) and conduction velocity.
Dosing by species
- Adjunctive therapy for refractory heart failure (advanced mitral valve degeneration as an afterload reducer) ยท 0.2-0.4 mg/kg PO twice daily. Initiate therapy at 0.1 mg/kg PO twice daily and up-titrate weekly while monitoring blood pressure. ยท PO ยท q12h ยท After ACE inhibitor maintenance therapy has been established.
- Arterial vasodilator for refractory CHF secondary to mitral regurgitation ยท 0.1 mg/kg q12-24h initially; titrate up as needed to 0.25 mg/kg PO q12-24h ยท PO ยท q12-24h ยท Monitor blood pressure.
- Systemic hypertension in dogs with chronic renal disease ยท 0.1-0.5 mg/kg PO once daily (q24h) ยท PO ยท q24h ยท Most often combined with an ACE inhibitor. May require weeks to months to achieve satisfactory control.
- Systemic hypertension (2nd step drug) ยท 0.1 mg/kg q24h; step up to 0.2 mg/kg q24h if needed ยท PO ยท q24h ยท Used after enalapril/benazepril. Each step added if after 1-2 weeks systolic BP > 160 mmHg.
- Systemic hypertension ยท 0.05-0.1 mg/kg (titrate upwards weekly as required, up to 0.5 mg/kg) ยท PO ยท q12-24h ยท Long-term ยท Monitor blood pressure regularly. Consider concurrent ACE inhibitor.
- Systemic hypertension (1st step drug) ยท 0.625 mg per cat q24h, if cat greater then 6 kg, 1.25 mg/cat q24h ยท PO ยท q24h ยท Add ACE inhibitor if proteinuric.
- Systemic hypertension ยท 0.625-1.25 mg (total dose) PO once daily ยท PO ยท q24h ยท Drug of choice; often successful as a single agent. Maximum effect seen within 7 days.
- Systemic hypertension ยท 0.625-1.25 mg/cat ยท PO ยท q24h (may increase slowly or increase frequency to q12h if necessary) ยท Long-term ยท Blood pressure monitoring is essential. Appears safe even with concurrent renal failure.
Routes of administration
Contraindications
- Patients with cardiogenic shock
- Advanced aortic stenosis (relative contraindication)
- Use with caution in patients with heart failure (due to slight negative inotropic effects)
- Use with caution in patients with hepatic dysfunction
Adverse effects
- Cats: Infrequent azotemia, lethargy, hypokalemia, reflex tachycardia, weight loss
- Dogs: Gingival hyperplasia (with long-term use)
- Anorexia and hypotension (early in therapy)
Drug interactions
- Diuretics ยท Concomitant use may cause additive hypotension. ยท moderate
- Beta-blockers ยท Concomitant use may cause additive hypotension. ยท moderate
- Vasodilators ยท Concomitant use may cause additive hypotension.
- Fentanyl ยท May cause hypotension if used concurrently.
- Grapefruit juice/powder ยท May alter bioavailability of amlodipine.
- Cimetidine ยท May impair hepatic metabolism of amlodipine, increasing circulating levels ยท moderate
- Cyclosporine ยท May impair hepatic metabolism of amlodipine, increasing circulating levels ยท moderate
- Ketoconazole ยท May impair hepatic metabolism of amlodipine, increasing circulating levels ยท moderate
- Itraconazole ยท May impair hepatic metabolism of amlodipine, increasing circulating levels ยท moderate
- Rifampin ยท CYP3A4 inducer; may reduce circulating amlodipine levels ยท moderate
- ACE inhibitors ยท Increased risk of hypotension when combined ยท moderate
Monitoring
- Systemic blood pressure (frequent monitoring during dose titration)
- Ophthalmic exam (to assess for hypertensive retinopathy/retinal detachment)
- Renal values and electrolytes (especially if combined with ACE inhibitors)
- Clinical signs of hypotension (lethargy, weakness)
Overdose
Overdoses pose a significant risk for **profound hypotension** and **reflex tachycardia** (though bradycardia is also possible). Common clinical signs in dogs and cats include lethargy and vomiting. **Treatment**: - When possible, massive overdoses should be managed with gut emptying (emesis/lavage) and activated charcoal. - Provide aggressive supportive treatment (IV fluids). - **Specific therapies** may include beta-agonists, intravenous lipid emulsion (ILE) therapy, and intravenous calcium to counteract the calcium channel blockade.
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.