Terbutaline
Terbutaline is a synthetic, selective **beta-2 adrenergic agonist** primarily used in veterinary medicine as a bronchodilator. It is frequently prescribed for the adjunctive management of cardiopulmonary diseases such as **feline asthma, canine chronic bronchitis, collapsing trachea, and pulmonary edema**. Beyond its respiratory applications, terbutaline is occasionally utilized off-label as a **tocolytic agent** to inhibit premature labor by relaxing uterine smooth muscle, and as a chronotropic agent to manage certain **bradyarrhythmias**. In equine medicine, it has diagnostic utility for quantifying anhidrosis via intradermal injection. > **Clinical Pearl:** While highly selective for beta-2 receptors at standard doses, higher doses can cause beta-1 spillover, leading to direct cardiac stimulation. Reflex tachycardia secondary to peripheral vasodilation is also a common clinical observation.
Mechanism: Terbutaline selectively binds to and stimulates **beta-2 adrenergic receptors** located predominantly in bronchial, vascular, and uterine smooth muscle. **Mechanism Pathway:** Beta-2 receptor activation โ Stimulation of **adenylyl cyclase** โ Increased intracellular **cyclic AMP (cAMP)** โ Activation of **protein kinase A (PKA)** โ Decreased intracellular calcium concentrations โ **Smooth muscle relaxation**. This results in significant bronchodilation and reduced airway resistance. At therapeutic doses, it has minimal affinity for **beta-1 receptors** (cardiac muscle) and virtually no **alpha-adrenergic** activity.
Dosing by species
- For acute exacerbations of feline asthma treated at home ยท 0.01 mg/kg SC or IM ยท SC/IM ยท PRN ยท Beneficial response occurs in 15-30 minutes. A heart rate approaching 240 BPM indicates absorption.
- For feline asthma ยท 0.312-0.625 mg (total dose) per cat PO two to three times daily ยท PO ยท q8-12h ยท May adjust dose up to 1.25 mg in larger cats if needed
- For bradyarrhythmias ยท 0.625 mg PO q8-12h ยท PO ยท q8-12h ยท Improvement usually partial and often temporary
- For acute bronchoconstriction (initial crisis) ยท 0.01 mg/kg IV, SC, IM ยท IV/SC/IM ยท Single dose
- For use as a quantitative intradermal terbutaline sweat test (QITST) to identify anhidrosis ยท Eight 0.1 mL intradermal injections of serial 10-fold dilutions of terbutaline sulfate in 0.9% saline (0.001, 0.01, 0.1, 1, 10, 100 and 1000 mg/L) ยท Intradermal ยท Single dose ยท ARCI UCGFS Class 3 Drug
- General bronchodilation ยท 0.0033 mg/kg IV ยท IV ยท Not specified ยท ARCI UCGFS Class 3 Drug
- For a trial to treat intrathoracic tracheal collapse, expiratory cough or dyspnea and marked exercise intolerance ยท 1.25-5 mg (total dose) PO two to three times daily ยท PO ยท q8-12h
- As a bronchodilator in chronic bronchitis ยท Small dogs: 0.625-1.25 mg (total dose) PO q12h; medium-sized dogs: 1.25-2.5 mg (total dose) PO q12h; large dogs: 2.5-5 mg PO q12h ยท PO ยท q12h
Routes of administration
Contraindications
- Known hypersensitivity to terbutaline
- Heart disease (especially congestive heart failure or cardiomyopathy)
- Known hypersensitivity to terbutaline or sympathomimetic amines
- Severe cardiac arrhythmias
- Hypertrophic cardiomyopathy (relative contraindication)
Adverse effects
- Increased heart rate (tachycardia)
- Tremors
- CNS excitement (nervousness)
- Dizziness
- Sweating (reported in horses after parenteral injection)
- Transient hypokalemia
- Tachycardia
- Muscle tremors
- Restlessness or excitement
- Hypokalemia (with high doses or prolonged use)
- Hypertension or hypotension
Drug interactions
- Inhalation Anesthetics (e.g., halothane, isoflurane, methoxyflurane) ยท May predispose the patient to ventricular arrhythmias, particularly with preexisting cardiac disease; use cautiously.
- Beta-Adrenergic Blocking Agents (e.g., propranolol) ยท May antagonize the bronchodilating and cardiovascular actions of terbutaline.
- Digoxin ยท Concurrent use with digitalis glycosides may increase the risk of cardiac arrhythmias. ยท moderate
- Monoamine Oxidase Inhibitors (MAOIs) ยท May potentiate the vascular effects of terbutaline.
- Other Sympathomimetics ยท May increase the risk of developing adverse cardiovascular effects.
- Tricyclic Antidepressants ยท May potentiate the vascular effects of terbutaline.
- Beta-blockers (e.g., propranolol) ยท Antagonize the bronchodilating effects of terbutaline ยท major
- Sympathomimetics ยท Additive cardiovascular toxicity and CNS stimulation ยท moderate
- Inhalant anesthetics (e.g., halothane) ยท Sensitize the myocardium, increasing the risk of severe arrhythmias ยท major
- Tricyclic antidepressants (TCAs) / MAOIs ยท May potentiate vascular effects ยท moderate
Monitoring
- Clinical symptom improvement (respiratory rate/effort)
- Auscultation of lung sounds
- Cardiac rate and rhythm (especially if indicated for arrhythmias or if tachycardia develops)
- Serum potassium (early in therapy if the animal is susceptible to hypokalemia)
- Heart rate and rhythm
- Respiratory rate and effort
- Serum potassium levels (with prolonged or high-dose therapy)
Overdose
Clinical signs of significant systemic overdose may include **arrhythmias** (bradycardia, tachycardia, heart block, extrasystoles), **hypertension, fever, vomiting, mydriasis, and CNS stimulation**. **Treatment:** - For recent oral ingestion without significant cardiac/CNS effects: Empty gut, administer activated charcoal, and a cathartic. - For severe cardiac arrhythmias: A beta-blocking agent (e.g., propranolol) can be used, but **caution is advised** as it may precipitate severe bronchoconstriction.
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.