Short Acting Beta 2 Agonists

Short acting beta 2 agonists (salbutamol and terbutaline) are bronchodilators with onset within minutes and duration of 4 to 6 hours. They are the reliever for acute bronchospasm in asthma and COPD, and they treat symptoms only.

Mechanism. Selective beta 2 receptor agonism raises cyclic AMP in airway smooth muscle and causes relaxation within minutes.

Pharmacokinetics. Inhaled onset is 3 to 5 minutes, with peak effect at 15 to 60 minutes. Only 10 to 20 percent of a pressurised inhaler dose reaches the lung, and a spacer improves this. Oral salbutamol bioavailability is about 50 percent, with an inactive sulphate conjugate excreted in urine. Salbutamol elimination half life is 4 to 6 hours.

Bronchoprotection. Protection against exercise or allergen challenge is shorter than bronchodilation and falls with regular use.

Salbutamol. Pressurised inhaler 100 mcg each actuation, nebuliser solution 2.5 mg and 5 mg, tablets 2 mg and 4 mg, syrup 2 mg/5 mL, and injection.

Terbutaline. Dry powder 500 mcg each inhalation, nebuliser solution 5 mg/2 mL, tablets 2.5 mg, and injection for subcutaneous or intravenous use.

Differences. Efficacy of salbutamol and terbutaline is similar, so the choice follows availability and device. Levosalbutamol has no proven advantage. Fenoterol is not preferred because of greater cardiac effect.

Representative regimens.

• Reliever for adults: salbutamol 100 to 200 mcg (1 to 2 puffs) as needed, with a routine maximum of 8 puffs (800 mcg) in 24 hours. Needing more signals loss of control.
• Exercise-induced bronchoconstriction: salbutamol 200 mcg 10 to 15 minutes before exercise.
• Acute severe asthma in adults: 4 to 10 puffs of 100 mcg through a spacer, each puff inhaled separately, or 5 mg nebulised, repeated every 15 to 20 minutes. Use 5 to 10 mg/h by continuous nebulisation in life-threatening disease.
• Acute severe asthma in children: 2 to 10 puffs of 100 mcg through a spacer, repeated every 10 to 20 minutes, or nebulised 2.5 mg under 5 years and 5 mg from 5 years.
• COPD exacerbation: 2.5 to 5 mg nebulised, or 4 to 6 puffs through a spacer, every 4 to 6 hours and more often at first.
• Intravenous salbutamol is specialist use when inhalation fails: 250 mcg slowly over 10 minutes, then an infusion of 5 mcg/min adjusted within 3 to 20 mcg/min to heart rate and response.
• Hyperkalaemia: salbutamol 10 to 20 mg nebulised over 10 minutes with insulin and glucose.

Not indicated. Infant bronchiolitis shows no routine benefit.

Reliever overuse. Use of three or more reliever canisters in a year marks higher exacerbation risk. Rising need calls for review of controller treatment, not more reliever.

Adverse effects. Tremor, tachycardia, palpitations, headache and cramps. High dose causes hypokalaemia, hyperglycaemia, lactic acidosis and arrhythmia.

Contraindications and cautions. Hypersensitivity. Caution in ischaemic heart disease, tachyarrhythmia, thyrotoxicosis, uncontrolled hypertension and diabetes.

Interactions. Non-selective beta blockers are contraindicated in asthma. Diuretics, corticosteroids and theophylline add to hypokalaemia.

Monitoring. Canisters used each year, night symptoms, technique with spacer, heart rate and potassium after repeated high doses. Check lactate if breathlessness persists despite improving air entry.

Special groups. Pregnancy and breastfeeding: salbutamol may be used. Children: spacer with face mask under 4 years, and improvised bottle spacer where none is available. Older adults: tremor and arrhythmia more likely. Renal impairment: accumulation is modest, but watch potassium.

Referral and escalation. No improvement after 3 doses within the first hour, peak flow under 50 percent of best or predicted, or oxygen saturation under 92 percent needs admission. Silent chest, cyanosis, exhaustion or peak flow under 33 percent is life threatening.