Atenolol

Beta-adrenergic receptor antagonist (beta-blocker) Cardiovascular agent, antiarrhythmic (Class II)

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog PO 0.25-1 mg/kg (up to 2 mg/kg for refractory cases) q12h (every 12 hours) Duration: Long-term, as needed
Notes: Start at low end and titrate based on heart rate and clinical response. Monitor heart rate and blood pressure.
Cat PO 0.25-1 mg/kg (up to 2 mg/kg for refractory cases) q12h (every 12 hours) Duration: Long-term, as needed
Notes: Use cautiously in cats with asthma or bronchospastic disease. Titrate slowly.
Horse PO 0.5-1 mg/kg q12h (every 12 hours) Duration: As needed for rate control
Notes: Not commonly used; monitor heart rate and ECG. May cause hypotension.
Cattle PO 0.5-1 mg/kg (extrapolated) q12h Duration: As needed
Notes: Extra-label use; not FDA-approved for food animals. Observe withdrawal times.
Small Ruminants (sheep, goats) PO 0.5-1 mg/kg (extrapolated) q12h Duration: As needed
Notes: Extra-label use; not FDA-approved for food animals. Observe withdrawal times.
Rabbit PO 0.5-1 mg/kg q12h Duration: Long-term
Notes: Use with caution; monitor for bradycardia and hypotension.
Bird (pet birds) PO 0.5-2 mg/kg (empirical) q12h Duration: As needed
Notes: Limited data; use with caution and monitor heart rate.

Clinical Indications & Species Uses

General Indications
  • Management of tachyarrhythmias (e.g., atrial fibrillation, supraventricular tachycardia)
  • Hypertrophic cardiomyopathy (HCM) in dogs and cats
  • Systemic hypertension (adjunctive therapy)
Dog (Canine)
  • Hypertrophic cardiomyopathy (HCM) - to reduce heart rate and outflow obstruction
  • Chronic atrial fibrillation - for rate control
  • Supraventricular tachycardia (SVT)
  • Systemic hypertension (adjunctive therapy)
  • Dynamic left ventricular outflow tract obstruction (DLVOTO)
  • Prevention of exercise-induced arrhythmias
Cat (Feline)
  • Hypertrophic cardiomyopathy (HCM) - to reduce heart rate and outflow obstruction
  • Hypertrophic obstructive cardiomyopathy (HOCM)
  • Supraventricular tachycardia
  • Systemic hypertension (adjunctive therapy)
  • Hyperthyroidism-related tachycardia (adjunctive)
Horse (Equine)
  • Atrial fibrillation (rate control, though not first-line)
  • Supraventricular tachycardia
  • Exercise-induced pulmonary hemorrhage (EIPH) - controversial, not standard
  • Hypertension (rare)
Rabbit & Small Mammals
  • Hypertrophic cardiomyopathy (HCM) - used off-label
  • Tachyarrhythmias
Exotic & Other Species
  • Small mammals (e.g., ferrets, guinea pigs) - off-label for cardiac disease
  • Reptiles - limited data, not standard

Pharmacology & Mechanism of Action

Drug Class: Beta-adrenergic receptor antagonist (beta-blocker) | Pharmacological Group: Cardiovascular agent, antiarrhythmic (Class II)

Mechanism of Action: Atenolol is a selective beta-1 adrenergic receptor antagonist that competitively blocks the effects of catecholamines (epinephrine and norepinephrine) at beta-1 receptors, which are predominantly located in cardiac tissue. By inhibiting beta-1 receptors, atenolol reduces heart rate, myocardial contractility, and cardiac output, thereby decreasing myocardial oxygen demand. It also suppresses renin release from the juxtaglomerular cells, leading to reduced angiotensin II and aldosterone levels, which contributes to its antihypertensive effects. At therapeutic doses, atenolol has minimal effect on beta-2 receptors (bronchial and vascular smooth muscle), making it relatively cardioselective.

Pharmacodynamics: Atenolol produces a dose-dependent reduction in heart rate and contractility, leading to decreased cardiac output and blood pressure. It prolongs sinoatrial node recovery time, slows atrioventricular conduction, and increases the refractory period of the AV node. These effects are beneficial in managing tachyarrhythmias and hypertrophic cardiomyopathy. Atenolol also reduces myocardial oxygen demand, which is beneficial in ischemic heart disease. Its antihypertensive effect is mediated through multiple mechanisms, including reduced cardiac output, inhibition of renin release, and central nervous system effects. The onset of action after oral administration is within 1-2 hours, with peak effects at 2-4 hours. The duration of action is approximately 12-24 hours, allowing once-daily dosing in most species.

⚑ Pharmacokinetics Summary

Absorption: Atenolol is rapidly and completely absorbed from the gastrointestinal tract after oral administration, with an oral bioavailability of approximately 50-60% in dogs and cats due to first-pass metabolism. Absorption is not significantly affected by food, but may be slightly reduced. Peak plasma concentrations are achieved within 2-4 hours in dogs and cats, and within 2-3 hours in horses.
Distribution: Atenolol is hydrophilic and has a low volume of distribution (approximately 0.7-1.2 L/kg in dogs). It distributes into extracellular fluid and penetrates poorly into the central nervous system due to its low lipid solubility. It crosses the placenta and is excreted in milk. Protein binding is low, approximately 5-15% in most species.
Metabolism: Atenolol undergoes minimal hepatic metabolism; it is primarily excreted unchanged by the kidneys. A small fraction is metabolized to inactive metabolites. In animals with hepatic impairment, metabolism is not significantly altered, but renal impairment can lead to accumulation.
Excretion: Atenolol is primarily eliminated by renal excretion via glomerular filtration and active tubular secretion. Approximately 85-90% of an oral dose is excreted unchanged in the urine. Biliary excretion is minimal. The elimination half-life is approximately 3-4 hours in dogs, 3-5 hours in cats, and 6-8 hours in horses. In animals with renal insufficiency, the half-life is prolonged, requiring dose adjustment.
Half-Life: Dog: 3-4 hours; Cat: 3-5 hours; Horse: 6-8 hours; Cattle: 4-6 hours (estimated)
Bioavailability: Oral: 50-60% in dogs and cats; approximately 50% in horses
Protein Binding: 5-15%

Available Formulations & Strengths

Oral Tablet 25 mg, 50 mg, 100 mg (PO)
Oral Tablet (scored) 25 mg, 50 mg, 100 mg (PO)

Contraindications & Clinical Warnings

Contraindications:
  • Hypersensitivity to atenolol or other beta-blockers
  • Sinus bradycardia (heart rate < 60 bpm in dogs/cats)
  • Second- or third-degree atrioventricular block
  • Cardiogenic shock
  • Uncompensated congestive heart failure
  • Severe hypotension
  • Concurrent use with MAO inhibitors (except MAO-B inhibitors)
  • Bronchospastic disease (e.g., asthma, chronic obstructive pulmonary disease) - use with extreme caution due to beta-2 blockade at higher doses
Warnings & Clinical Precautions:
  • Use with caution in animals with renal impairment; dose adjustment may be necessary.
  • Use with caution in animals with diabetes mellitus; beta-blockers may mask signs of hypoglycemia.
  • Use with caution in animals with hyperthyroidism; abrupt withdrawal may exacerbate thyroid storm.
  • Do not discontinue abruptly; taper dose gradually to avoid rebound tachycardia or hypertension.
  • Use with caution in animals with peripheral vascular disease; may exacerbate symptoms.
  • In food animals, observe withdrawal times; extra-label use requires veterinary oversight.
  • Monitor heart rate, blood pressure, and ECG during therapy.
  • In cats, use with caution in those with asthma or other respiratory diseases.
  • May cause fatigue, lethargy, or depression in some animals.

Adverse Effects & Reactions

Common:

  • Bradycardia
  • Hypotension
  • Lethargy or depression
  • Gastrointestinal upset (vomiting, diarrhea)
  • Decreased exercise tolerance

Serious / Severe:

  • Severe bradycardia or heart block
  • Congestive heart failure (precipitated or worsened)
  • Bronchospasm (especially in animals with respiratory disease)
  • Hypoglycemia (especially in diabetic animals)
  • Syncope or collapse

Rare:

  • Allergic reactions (rash, urticaria)
  • Paresthesia
  • Mental depression
  • Hematologic disorders (thrombocytopenia, agranulocytosis)
  • Hepatic enzyme elevations

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
Other beta-blockers (e.g., propranolol) Additive beta-blockade, increased risk of bradycardia and hypotension. High
Calcium channel blockers (e.g., diltiazem, verapamil) Additive negative inotropic and chronotropic effects, risk of severe bradycardia, AV block, and heart failure. High
Digoxin Additive bradycardia and AV conduction slowing; increased risk of digoxin toxicity due to reduced renal clearance. Moderate
Antiarrhythmic agents (e.g., amiodarone, sotalol) Additive effects on cardiac conduction and contractility; increased risk of proarrhythmia. High
Sympathomimetics (e.g., epinephrine, dobutamine) Antagonism of beta-adrenergic effects; may result in unopposed alpha-adrenergic vasoconstriction leading to hypertension. Moderate
NSAIDs (e.g., indomethacin) Reduced antihypertensive effect due to inhibition of renal prostaglandins. Mild
Phenothiazines (e.g., chlorpromazine) Additive hypotensive effects. Moderate
Insulin or oral hypoglycemics May potentiate hypoglycemic effect and mask signs of hypoglycemia (tachycardia). Moderate

Overdose & Toxicity Management

Signs of Toxicity:

  • Severe bradycardia
  • Hypotension
  • AV block
  • Heart failure
  • Bronchospasm
  • Hypoglycemia
  • Seizures (in severe cases)
  • Coma

Emergency Treatment Protocol: Treatment is symptomatic and supportive. Induce emesis if recent ingestion and animal is conscious. Administer activated charcoal to reduce absorption. For bradycardia and hypotension, administer atropine (0.02-0.04 mg/kg IV) and fluids. If unresponsive, consider glucagon (50-150 mcg/kg IV bolus followed by continuous rate infusion) as it bypasses beta-receptor blockade. In severe cases, use positive inotropes (e.g., dobutamine) and temporary cardiac pacing. Monitor ECG, blood pressure, and blood glucose. Provide respiratory support if needed.

Food Animal Withdrawal Times

πŸ₯© Meat: 7 daysπŸ₯› Milk: 3 days

Atenolol is not approved for food animals. Withdrawal times are extrapolated from human data and general beta-blocker guidelines. Consult a veterinary pharmacologist for specific recommendations. For cattle and small ruminants, a meat withdrawal of at least 7 days and milk withdrawal of at least 3 days is suggested, but longer may be prudent. For poultry, not recommended; if used, a withdrawal of 14 days is suggested.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature (20-25Β°C or 68-77Β°F).

Handling & Special Conditions: Protect from moisture and keep in a tightly closed container. Keep out of reach of children and animals.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Not approved for veterinary use in the US; approved for human use only.

Extra-Label (Off-Label) Use: In the US, atenolol is not FDA-approved for veterinary use; it is used extra-label under the Animal Medicinal Drug Use Clarification Act (AMDUCA). Extra-label use requires a valid veterinarian-client-patient relationship and must be in accordance with regulations. For food animals, a withdrawal time must be established and observed.

Clinical Pearls & Practice Notes

πŸ’‘ Clinical Insights: Atenolol is a cardioselective beta-blocker commonly used in small animal practice for the management of hypertrophic cardiomyopathy, particularly in cats, and for rate control in atrial fibrillation and other supraventricular tachyarrhythmias. It is preferred over non-selective beta-blockers like propranolol due to its lower risk of bronchospasm and fewer central nervous system effects. Dosing should be individualized based on heart rate and clinical response; the goal is to reduce resting heart rate to 120-160 bpm in dogs and 140-180 bpm in cats, depending on the condition. Atenolol should be used with caution in animals with renal impairment, as it is renally excreted. In food animals, extra-label use is possible but requires careful consideration of withdrawal times. Monitoring includes heart rate, blood pressure, and ECG. Abrupt discontinuation should be avoided to prevent rebound tachycardia or hypertension. Atenolol is generally well-tolerated, but adverse effects such as bradycardia, hypotension, and lethargy can occur. It is contraindicated in animals with significant bradyarrhythmias or uncompensated heart failure. Overall, atenolol is a valuable therapeutic option for managing certain cardiac conditions in veterinary patients.

References & Literature

  • πŸ“š Plumb's Veterinary Drug Handbook
  • πŸ“š Papich Veterinary Pharmacology and Therapeutics
  • πŸ“š Compendium of Veterinary Products (CVP)