Beta Agonists ๐
๐จ ฮฒโ-Agonists (Bronchodilators)
โ ๏ธ In asthma, LABAs must never be used without inhaled corticosteroids (ICS).
Over-reliance on SABAs is a recognised marker of poor control and increased mortality risk (BTS/SIGN).
โ๏ธ Mode of Action
- ๐ Stimulate ฮฒโ-adrenoceptors on airway smooth muscle.
- ๐ Activate adenylate cyclase โ โ cAMP.
- ๐งช cAMP activates protein kinase A โ โ intracellular Caยฒโบ.
- ๐จ Result: bronchodilation and improved airflow.
- โก Stimulate Naโบ/Kโบ-ATPase โ shift Kโบ intracellularly (temporary โ serum potassium).
๐ง Clinical insight: The potassium shift explains both the usefulness in hyperkalaemia and the risk of hypokalaemia during high-dose nebulisation.
๐ฉบ Indications
- ๐ฌ๏ธ Asthma (BTS/SIGN):
- SABA = first-line reliever.
- LABA only in combination with ICS.
- ๐ฌ COPD (NICE):
- Symptom control and airflow improvement.
- Often combined with LAMA or ICS.
- ๐งช Hyperkalaemia (Emergency):
- High-dose nebulised salbutamol (temporary effect).
- ๐คฐ Tocolysis:
- Terbutaline, ritodrine (rare in UK now; nifedipine preferred).
๐ Drug Classification
- ๐ต Short-Acting ฮฒโ-Agonists (SABA)
- Salbutamol, Terbutaline.
- Onset 1โ2 minutes.
- Duration 4โ6 hours.
- ๐ฃ Long-Acting ฮฒโ-Agonists (LABA)
- Salmeterol (slower onset).
- Formoterol (rapid onset).
- Duration ~12 hours.
- Must be combined with ICS in asthma.
- ๐ข Ultra-LABA (COPD)
- Indacaterol, Vilanterol, Olodaterol.
- Once-daily maintenance therapy.
๐จ Delivery Systems
- ๐ Metered Dose Inhaler (MDI) โ common reliever device.
- ๐ฆ Spacer โ improves lung deposition, reduces side effects.
- ๐ฌ๏ธ Dry Powder Inhaler (DPI) โ breath-actuated.
- ๐ซ Nebuliser โ high doses in severe attacks.
- ๐ IV Salbutamol โ refractory severe asthma.
โ ๏ธ Side Effects
- ๐ซ Tachycardia & Palpitations
- Due to ฮฒโ spillover.
- Higher risk in cardiac disease.
- ๐คฒ Tremor
- ๐งช Hypokalaemia
- More likely with high-dose nebulisation.
- Enhanced with diuretics or steroids.
- ๐งฌ Lactic Acidosis
- Seen in prolonged high-dose therapy in severe asthma.
- May worsen apparent breathlessness.
- โ ๏ธ Asthma Safety
- Frequent SABA use โ โ mortality risk.
- ๐คฐ Tocolysis Risks
- Pulmonary oedema (rare in UK use).
๐ Drug Interactions
- ๐ซ Non-selective ฮฒ-blockers (e.g. propranolol) blunt effect.
- โก Theophylline โ โ arrhythmia risk.
- ๐ง Loop/thiazide diuretics โ additive hypokalaemia.
- ๐ง MAOIs / TCAs โ โ cardiovascular effects.
๐งช Monitoring & Precautions
- ๐งช Monitor serum potassium with high-dose therapy.
- ๐ซ Monitor HR and rhythm in cardiac patients.
- โ ๏ธ Use caution in thyrotoxicosis.
- ๐ง Can worsen glycaemic control in diabetes.
- ๐ฆ >3 SABA inhalers/year = poor control marker.
๐งโโ๏ธ Patient Education
- ๐ซ Demonstrate and check inhaler technique regularly.
- ๐ฆ Encourage spacer use with MDI.
- ๐ต Explain โrelieverโ vs โpreventerโ inhalers clearly.
- โ ๏ธ Report worsening palpitations or tremor.
- ๐จ Never use LABA alone in asthma.
