Metoprolol
**Metoprolol** is a cardioselective (**beta1-specific**) adrenergic receptor blocker widely used in veterinary medicine for the management of various cardiovascular conditions. Key clinical applications include: * Management of **supraventricular tachyarrhythmias** and premature ventricular contractions (PVCs/VPCs). * Treatment of **systemic hypertension**. * Management of **hypertrophic cardiomyopathy (HCM)** in cats, where it helps reduce dynamic left ventricular outflow tract obstruction and improves diastolic filling time. Because of its relative beta1-selectivity, metoprolol is generally considered safer than non-selective beta-blockers (like propranolol) in patients with concurrent **bronchoconstrictive disease** (e.g., feline asthma), though caution is still warranted at higher doses where selectivity may be lost.
Mechanism: Metoprolol competitively blocks **beta1-adrenergic receptors** located primarily in the myocardium. **Mechanism Pathway:** Blockade of beta1-receptors → decreased activation of adenylyl cyclase → reduced intracellular cAMP → decreased intracellular calcium influx. This results in: * **Negative chronotropy**: Decreased sinus heart rate. * **Negative dromotropy**: Slowed atrioventricular (AV) conduction. * **Negative inotropy**: Decreased myocardial contractility and cardiac output. * Decreased myocardial oxygen demand and reduced blood pressure. *Clinical Pearl*: Metoprolol lacks intrinsic sympathomimetic activity (ISA) and membrane-stabilizing activity. At higher dosages, its cardioselectivity diminishes, leading to **beta2-receptor blockade** in bronchial smooth muscle → potential bronchoconstriction.
Dosing by species
- As an oral beta blocker · 2-15 mg (total dose) PO q8h · PO · q8h
- For rate control in chronic atrial fibrillation · 0.25-1 mg/kg PO q1224h. · PO · q1224h · Note: 'q1224h' is transcribed exactly as it appears in the source text, likely a typo for q12-24h.
- For CHF (early/mild or well-controlled) · 0.2 mg/kg PO twice daily, with slow titration upwards every 2-3 weeks up to 0.4-6.6 mg/kg PO three times a day. · PO · BID to TID · Many dogs will not tolerate this upward titration.
- To decrease the incidence of atrial fibrillation and flutter in dogs undergoing valve surgery · 0.4-1 mg/kg PO q24h · PO · q24h · Using sustained release metoprolol (ToprolXR) administered before and as soon as feasible after surgery.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Routes of administration
Contraindications
- Overt or unstable heart failure
- Hypersensitivity to beta-blockers
- Greater than first-degree heart block
- Sinus bradycardia
Adverse effects
- Bradycardia
- Lethargy and depression
- Impaired AV conduction
- Congestive heart failure (CHF) or worsening of heart failure
- Hypotension
- Hypoglycemia
- Bronchoconstriction (at high doses)
- Syncope
- Diarrhea
Drug interactions
- General Anesthetics · Increased risk for heart failure and hypotension due to additive myocardial depressant effects.
- Calcium-Channel Blockers (e.g., diltiazem, verapamil, amlodipine) · Concurrent use should be done with caution; additive negative inotropic and chronotropic effects, particularly in patients with preexisting cardiomyopathy or CHF.
- Digoxin · May increase negative effects on SA or AV node conduction.
- Diuretics (thiazides, furosemide) · May increase the hypotensive effect of metoprolol.
- Hydralazine · May increase the risks for pulmonary hypertension in uremic patients.
- Quinidine · May increase metoprolol plasma concentrations.
- Reserpine · Potential for additive effects including hypotension and bradycardia.
- SSRI Antidepressants (e.g., fluoxetine, sertraline, paroxetine) · May increase metoprolol plasma concentrations.
- Sympathomimetics (e.g., metaproterenol, terbutaline, epinephrine, phenylpropanolamine) · May have their actions blocked by metoprolol, and they may in turn reduce the efficacy of the beta-blocker.
Monitoring
- Cardiac function
- Pulse rate
- ECG (if necessary)
- Blood pressure (if indicated)
- Signs of toxicity (bradycardia, hypotension, lethargy)
Overdose
Overdosage primarily results in extensions of the drug's pharmacologic effects: **hypotension, bradycardia, bronchospasm, cardiac failure, and potentially hypoglycemia**. **Treatment:** * **Decontamination**: If recent oral ingestion, consider emptying the gut and administering activated charcoal. *Caution*: Inducing emesis can be risky as coma and seizures may develop rapidly. * **Monitoring**: Continuous ECG, blood pressure, blood glucose, and potassium. * **Cardiovascular Support**: Treat symptomatically. Use IV fluids and pressor agents for hypotension. * **Bradycardia**: Treat with atropine. If atropine fails, isoproterenol may be given cautiously. A transvenous pacemaker may be necessary. * **Cardiac Failure**: May be treated with digitalis glycosides, diuretics, and oxygen. * **Antidote**: Glucagon (5-10 mg IV - human dose) may increase heart rate and blood pressure and reduce the cardiodepressant effects of metoprolol.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.