Atenolol

Key Notes

Practical clinical points to support safe and effective use of atenolol in veterinary patients:

• Beta-1 selectivity matters: At recommended doses, atenolol is relatively cardioselective, making it a preferred beta-blocker in patients with concurrent bronchospastic disease.
• Symptom control, not disease modification: Atenolol improves heart rate and arrhythmia control, but evidence for survival benefit in conditions such as HCM or subaortic stenosis is limited.
• Start low, titrate slowly: Initiate therapy at the lowest effective dose and adjust based on heart rate, blood pressure, ECG findings, and clinical response.
• Negative inotrope: Use cautiously in patients with compromised systolic function or borderline heart failure.
• Renal clearance: Dose adjustments are often required in cats with chronic kidney disease due to renal elimination.
• Endocrine masking: Atenolol may mask clinical signs of hypoglycemia or hyperthyroidism without addressing the underlying disease.
• Withdrawal risk: Abrupt discontinuation after chronic therapy may precipitate rebound tachycardia or hypertension; taper gradually whenever possible.
• Monitoring is essential: Regular reassessment of heart rate, blood pressure, and ECG findings is critical to avoid overt beta-blockade.

Indications

Atenolol is used in dogs and cats primarily for cardiovascular conditions where reduction of heart rate, myocardial oxygen demand, and arrhythmia control are desired. Its relative beta-1 selectivity makes it suitable for small animal patients, including those with concurrent respiratory disease.

• Tachyarrhythmias: Management of supraventricular tachyarrhythmias and ventricular premature complexes (VPCs/PVCs) in dogs and cats.
• Hypertrophic cardiomyopathy (cats): Used to reduce heart rate, left ventricular outflow tract (LVOT) obstruction, and myocardial oxygen demand in cats with hypertrophic cardiomyopathy, particularly when tachycardia or dynamic obstruction is present.
• Hyperthyroidism-associated tachycardia (cats): Adjunctive therapy to control excessive heart rate and clinical signs related to thyrotoxicosis while definitive therapy is pursued.
• Obstructive cardiac disease (dogs): Commonly prescribed for subaortic or pulmonic stenosis to reduce myocardial workload and improve diastolic filling, although survival benefit is unproven.
• Arrhythmogenic right ventricular cardiomyopathy (dogs): Used alone or in combination with antiarrhythmics (eg, mexiletine) to control ventricular arrhythmias.
• Systemic hypertension (adjunctive use): May be used as part of a multimodal antihypertensive protocol, particularly when tachycardia is present; rarely effective as monotherapy in cats.

Dosage (Reference)

Dog

In dogs, atenolol is administered orally and dosing is titrated based on heart rate, arrhythmia control, and blood pressure. Therapy is typically started at the low end of the range and adjusted gradually.

Cat

In cats, atenolol dosing is typically based on a fixed dose per cat rather than mg/kg. Careful titration is required, particularly in cats with chronic kidney disease or heart failure.

Warnings & Precautions

Atenolol is a negative inotropic and chronotropic agent. Careful patient selection, dose titration, and monitoring are essential to minimize adverse cardiovascular and metabolic effects in dogs and cats.

• Bradycardia and heart block: Contraindicated in patients with sinus bradycardia, greater than first-degree AV block, or sick sinus syndrome.
• Congestive heart failure: Use cautiously in patients with compensated CHF; avoid use in overt or decompensated heart failure due to negative inotropic effects.
• Hypotension: May cause or worsen hypotension, especially during dose escalation or when combined with other hypotensive agents.
• Renal disease: Atenolol is primarily eliminated by the kidneys; dose reduction is recommended in cats with chronic kidney disease.
• Diabetes mellitus: May mask clinical signs of hypoglycemia and can cause hypo- or hyperglycemia; use cautiously in brittle or insulin-dependent diabetics.
• Hyperthyroidism: Atenolol may mask clinical signs such as tachycardia and hypertension without treating the underlying disease.
• Respiratory disease: Although relatively beta-1 selective, loss of selectivity at higher doses may cause bronchoconstriction; use cautiously in animals with asthma.
• Abrupt discontinuation: Sudden withdrawal after chronic therapy may precipitate rebound tachycardia, hypertension, or arrhythmias; taper gradually when discontinuing.
• Pregnancy and nursing: Crosses the placenta and is excreted in milk; use only when benefits outweigh potential risks to offspring.

Drug Interactions

Clinically significant interactions with atenolol are primarily related to additive cardiovascular depression, altered blood pressure or heart rate control, and interference with glucose regulation. Careful monitoring and dose adjustment are recommended when atenolol is used concurrently with the following agents.

• Calcium channel blockers (e.g., diltiazem, verapamil, amlodipine):
  Increased risk of bradycardia, AV block, and hypotension due to additive negative chronotropic effects.
• Other beta-blockers:
  Concurrent use may result in excessive beta-adrenergic blockade and severe bradycardia or hypotension.
• Antiarrhythmic agents (e.g., amiodarone, disopyramide):
  Increased risk of conduction disturbances, profound bradycardia, and myocardial depression.
• Digoxin:
  Additive effects on AV node conduction may increase the risk of bradycardia and heart block; close monitoring is required.
• Alpha-1 blockers (e.g., prazosin, tamsulosin):
  Increased risk of hypotension and syncope, especially at initiation of therapy.
• Anesthetic agents:
  Myocardial depressant anesthetics may enhance hypotension and bradycardia; dose reduction or withholding atenolol on the day of anesthesia may be considered.
• Antidiabetic agents (e.g., insulin, sulfonylureas):
  Atenolol may mask clinical signs of hypoglycemia and alter glucose control; blood glucose monitoring is recommended.
• NSAIDs:
  May reduce the antihypertensive effect of atenolol by inhibiting prostaglandin-mediated vasodilation.
• Sympathomimetic agents (e.g., albuterol, terbutaline):
  Beta-agonist effects may be antagonized, potentially reducing bronchodilation or cardiovascular support.
• Phenothiazines (e.g., acepromazine):
  Additive hypotensive effects may occur.
• Clonidine:
  Abrupt withdrawal of clonidine in patients receiving atenolol may precipitate rebound hypertension.

Side Effects & Overdose

Side Effects

Atenolol is generally well tolerated in dogs and cats when used at appropriate doses. Adverse effects are usually related to its beta-adrenergic blocking activity and are more likely in geriatric patients, animals with underlying cardiac disease, or those receiving higher doses.

• Bradycardia:
  Dose-dependent reduction in heart rate; may be clinically significant in patients with conduction disease.
• Hypotension:
  May occur secondary to reduced cardiac output, particularly in dehydrated or compromised patients.
• Lethargy and weakness:
  Commonly reported, especially during initiation of therapy or dose escalation.
• Worsening heart failure:
  Negative inotropic effects may exacerbate CHF, particularly in cats with hypertrophic cardiomyopathy and left-sided failure.
• Gastrointestinal effects:
  Inappetence, vomiting, or diarrhea have been reported, usually mild and self-limiting.
• Hypoglycemia or hyperglycemia:
  May occur due to altered glucose regulation; atenolol can also mask clinical signs of hypoglycemia.
• Bronchoconstriction:
  Uncommon at recommended doses due to beta-1 selectivity, but possible at higher doses.
• Syncope:
  May occur secondary to excessive bradycardia or hypotension.
• Ocular effects:
  Rare reports of exacerbation of keratoconjunctivitis sicca in dogs.

Overdose

Atenolol overdose results in exaggerated pharmacologic effects related to excessive beta-blockade. Clinical severity depends on the dose ingested, concurrent medications, and underlying disease.

• Cardiovascular effects:
  Severe bradycardia, hypotension, heart block, reduced cardiac output, and possible cardiogenic shock.
• Neurologic signs:
  Profound lethargy, weakness, depression, collapse; seizures are uncommon but possible.
• Respiratory effects:
  Bronchospasm, particularly at high doses or in susceptible patients.
• Metabolic disturbances:
  Hypoglycemia, hyperglycemia, and hyperkalemia may occur.
• Gastrointestinal signs:
  Vomiting may be seen following oral overdose.
• Management:
  Treatment is supportive and based on clinical signs. Recent oral ingestion may warrant GI decontamination. Monitor ECG, blood pressure, glucose, and electrolytes.
• Therapeutic interventions:
  IV fluids and vasopressors (e.g., dopamine, norepinephrine) for hypotension; atropine or isoproterenol for bradycardia; glucagon may improve heart rate and blood pressure.
• Severe cases:
  Temporary cardiac pacing and intensive care monitoring may be required.

Key Notes

Practical clinical pearls for the safe and effective use of atenolol in veterinary patients:

• Beta-1 selectivity:
  At standard doses, atenolol primarily affects cardiac beta-1 receptors, making it preferable to nonselective beta-blockers in patients with concurrent airway disease.
• Heart rate–driven therapy:
  Clinical response should be guided by heart rate control and rhythm stability rather than dose alone.
• Limited CNS penetration:
  Low lipid solubility results in minimal central nervous system effects compared with more lipophilic beta-blockers.
• Renal elimination:
  Because a significant portion is excreted unchanged in urine, dose adjustments may be required in patients with reduced renal function.
• Adjunctive role in hypertension:
  Atenolol is rarely effective as sole therapy for systemic hypertension and is commonly combined with other antihypertensive agents.
• Arrhythmia-specific utility:
  Particularly useful for rate control in tachyarrhythmias and conditions associated with dynamic outflow tract obstruction.
• Delayed maximal effect:
  Full therapeutic response may not be immediate; reassessment after several dosing intervals is recommended before dose escalation.
• Chronic therapy consideration:
  Long-term administration requires periodic reassessment of cardiovascular status to ensure continued benefit.