Introduction
Bronchodilators are medicines that relax bronchial smooth muscle and improve airflow. They provide quick relief during an asthma attack and are therefore called reliever or rescue drugs. These drugs do not treat the underlying inflammation but help open the airways to reduce wheezing, breathlessness and chest tightness.
Classes of Bronchodilators
The main groups include:
- Selective β2 agonists
- Non-selective sympathomimetics
- Antimuscarinic drugs
- Methylxanthines
1. Selective β2 Agonists
These are the most widely used bronchodilators today. They activate β2-receptors on airway smooth muscle causing relaxation. They also inhibit mediator release from mast cells, reduce microvascular leakage and improve mucociliary clearance.
Short-Acting β2 Agonists (SABA)
Examples: Salbutamol, Terbutaline, Fenoterol, Pirbuterol, Bitolterol
Onset: 5–15 min | Peak: 30–60 min | Duration: 4–6 hrs.
Best used as quick relievers during acute asthma attacks or before exercise.
Long-Acting β2 Agonists (LABA)
Examples: Salmeterol, Formoterol, Indacaterol, Bambuterol, Olodaterol
They provide prolonged bronchodilation and are used for maintenance therapy, usually along with inhaled corticosteroids.
Adverse Effects
- Tremors (stimulation of β2 receptors in skeletal muscle)
- Tachycardia (β2 and high-dose β1 stimulation)
- Restlessness
- Hypokalaemia when combined with steroids or methylxanthines
Desensitization
Continued LABA use may down-regulate β2 receptors. Concomitant use of inhaled corticosteroids helps reduce this effect.
2. Non-Selective Sympathomimetics
Examples: Epinephrine, Ephedrine, Isoproterenol, Orciprenaline (Metaproterenol)
These older drugs act on both α and β receptors. Because they also stimulate the heart, their use has declined.
Epinephrine
Previously used for acute severe asthma due to fast bronchodilation. Stimulates α, β1 and β2 receptors.
Adverse Effects
- Tachycardia
- Hypertension
- Angina
- Arrhythmias
Isoproterenol and Metaproterenol
Preferential β activity but still cause significant cardiac stimulation. Largely replaced by selective β2 agonists.
3. Antimuscarinic (Anticholinergic) Drugs
These drugs block M3 receptors on airway smooth muscle and reduce cholinergic bronchoconstriction. They are useful in asthma and particularly in COPD.
Short-Acting Muscarinic Antagonists (SAMA)
Examples: Ipratropium, Oxitropium
Long-Acting Muscarinic Antagonists (LAMA)
Examples: Tiotropium, Glycopyrronium, Aclidinium, Umeclidinium
Key Points
- Poor absorption from airways → minimal systemic side effects
- Reduce mucus secretion but less risk of mucus plug formation
- Do not cross the blood–brain barrier
- Combination of SABA + Ipratropium is effective in severe asthma
Adverse Effects
- Dry mouth
- Bitter taste
- Urinary retention in elderly (LAMA)
4. Methylxanthines
Examples: Theophylline, Aminophylline, Etophylline, Doxophylline
Mechanism of Action
- Inhibit phosphodiesterase → increased cAMP → bronchodilation
- Block adenosine receptors → prevents bronchoconstriction
Pharmacokinetics
- Well absorbed orally
- Cross placenta and blood–brain barrier
- Metabolised in liver, excreted in urine
Notes on Individual Drugs
- Theophylline: Oral use; narrow safety margin
- Aminophylline: Water-soluble; IV use in emergencies
- Etophylline: Oral/IM/IV; smoother response
- Doxophylline: Once or twice daily; fewer GI and CNS side effects
Adverse Effects
Methylxanthines have a narrow therapeutic window.
- Tachycardia and palpitations
- Hypotension
- Arrhythmias (may cause sudden death)
- Nausea, vomiting, CNS stimulation
Drug Interactions
- Enhanced metabolism (↓ effect): Phenytoin, Rifampicin, Phenobarbitone
- Inhibited metabolism (↑ toxicity): Cimetidine, Ciprofloxacin, Erythromycin
- With sympathomimetics: Increased bronchodilation but ↑ cardiac risk
Uses
- Moderate to severe asthma (as add-on therapy)
- COPD
- Apnoea of prematurity (Aminophylline/Caffeine)
Detailed Notes:
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