Atenolol
Atenolol is a cardioselective beta-blocker used primarily for the management of systemic hypertension and tachyarrhythmias in small animals. It is frequently utilized in cats with hypertrophic cardiomyopathy (HCM) without accompanying pulmonary edema to reduce dynamic outflow tract obstruction and improve diastolic filling. **Key Highlights:** * **Cardioselective:** Has minimal beta-2 activity at usual doses, making it comparatively safer to use in asthmatic or bronchospastic patients than non-selective beta-blockers like propranolol. * **Negative Inotrope:** Must be used with extreme caution in patients with congestive heart failure (CHF), renal failure, or sinus node dysfunction. * **Metabolic Effects:** Higher dosages may mask clinical signs of hyperthyroidism or hypoglycemia. It can also cause hyper- or hypoglycemia, requiring caution in brittle diabetics. **Clinical Pearl:** Unlike propranolol, atenolol is highly hydrophilic. It has low lipid solubility, meaning it minimally crosses the blood-brain barrier, resulting in significantly fewer central nervous system (CNS) side effects (like lethargy or depression). Because it is primarily excreted unchanged by the kidneys, dosage adjustments may be necessary in patients with significant renal impairment.
Mechanism: Atenolol acts as a competitive, relatively specific antagonist at **β1-adrenergic receptors** located primarily in the myocardium. * **Pathway:** Blockade of β1-receptors → decreased intracellular cAMP → reduced intracellular calcium influx. * **Cardiovascular Effects:** This leads to negative chronotropic (decreased sinus heart rate) and negative inotropic (decreased contractility) effects. It slows AV conduction, diminishes cardiac output at rest and during exercise, and significantly decreases myocardial oxygen demand. * **Blood Pressure:** Reduces systemic blood pressure through decreased cardiac output and potentially decreased renin release from the kidneys. * **Selectivity Loss:** At higher dosages, β1 specificity may be lost, leading to **β2-receptor** blockade (which can cause bronchoconstriction and peripheral vasoconstriction). * Atenolol lacks intrinsic sympathomimetic activity (ISA) and membrane-stabilizing activity.
Dosing by species
- Treatment of hypertension or cardiac conditions (e.g., hypertrophic cardiomyopathy) · 3 mg/kg PO q12h (or 6.25 -12.5 mg total dose) PO q12h · PO · q12h
- Treatment of hypertension or cardiac conditions · 6.25-12.5 mg (total dose per cat) q12h · PO · q12h
- Treatment of choice for hyperthyroid, hypertensive cats · 6.25-12.5 mg (total dose) PO q12-24h · PO · q12-24h · Beta-blockers are rarely sufficient alone to treat hypertension due to other causes.
- Hypertrophic cardiomyopathy · 6.25 mg (total dose) PO once daily · PO · q24h
- Hypertrophic cardiomyopathy · 3.13-6.25 mg (total dose) PO once daily · PO · q24h
- Cardiac arrhythmias, obstructive heart disease, hypertension, myocardial infarction, etc. · 0.3-0.6 mg/kg PO q12h · PO · q12h · For refractory VTach combine with mexiletine (5-8 mg/kg PO q8h)
- Moderate to severe sub-valvular aortic stenosis (SAS) · 0.5-1 mg/kg PO twice a day · PO · q12h
- To attempt to decrease syncopal episodes associated with pulmonic stenosis · 0.25-1 mg/kg PO twice a day · PO · q12h
- Hypertension · 0.25-1 mg/kg PO q12h · PO · q12h
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Routes of administration
Contraindications
- Overt heart failure
- Hypersensitivity to beta-blockers
- Greater than first-degree heart block
- Sinus bradycardia
- Cats with hypertrophic cardiomyopathy with accompanying pulmonary edema
Adverse effects
- Lethargy
- Hypotension
- Diarrhea
- Bradycardia
- Inappetence
- Depression
- Impaired AV conduction
- Worsening of heart failure
- Hypoglycemia
- Syncope
- Bronchoconstriction (rare at normal doses)
Drug interactions
- Anesthetics (myocardial depressants) · Additive myocardial depression may occur with concurrent use.
- Calcium-Channel Blockers (e.g., diltiazem, verapamil, amlodipine) · Concurrent use should be done with caution due to additive negative inotropic effects, particularly in patients with preexisting cardiomyopathy or CHF.
- Clonidine · Atenolol may exacerbate rebound hypertension after stopping clonidine therapy.
- Furosemide, Hydralazine, or other hypotensive drugs · May increase the hypotensive effects of atenolol.
- Phenothiazines · Concurrent use may exhibit enhanced hypotensive effects.
- Reserpine · Potential for additive effects including hypotension and bradycardia.
- Sympathomimetics (e.g., metaproterenol, terbutaline, epinephrine, phenylpropanolamine) · May have their actions blocked by atenolol, and they may in turn reduce the efficacy of atenolol.
Monitoring
- Cardiac function
- Pulse rate
- ECG (if necessary)
- Blood pressure (if indicated)
- Signs of toxicity (lethargy, hypotension, bradycardia)
Overdose
Common clinical signs of overdose include lethargy and vomiting. Severe overdose represents extensions of the drug's pharmacologic effects: **hypotension, bradycardia, bronchospasm, cardiac failure, hypoglycemia, and hyperkalemia**. **Treatment:** * If recent oral ingestion: consider emptying the gut and administering activated charcoal. * **Monitor:** ECG, blood glucose, potassium, and blood pressure. * **Cardiovascular support:** Symptomatic treatment. Use fluids and pressor agents (dopamine or norepinephrine) for hypotension. * **Bradycardia:** Treat with atropine. If atropine fails, isoproterenol given cautiously is recommended. * **Metabolic:** Insulin and dextrose may be needed for hyperkalemia and hypoglycemia. * **Advanced support:** A transvenous pacemaker may be necessary. Cardiac failure can be treated with a digitalis glycoside, diuretics, and oxygen. * **Antidote:** Glucagon (5-10 mg IV; human dose) may increase heart rate and blood pressure and reduce the cardiodepressant effects of atenolol.
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.