Pharm 101: Salbutamol
Class
Bronchodilator
Pharmacodynamics
- Selective beta-2 adrenergic receptor agonist (abundant on airway smooth muscle cells)
- Binding increases formation of intracellular cAMP via stimulation of adenylyl cyclase, which:
- Relaxes airway smooth muscle (bronchodilation)
- Inhibits release of bronchoconstricting mediators from mast cells
- Inhibits microvascular leakage
- Increases mucociliary transport
Pharmacokinetics
- Principle routes of administration are either inhaled or IV. Less commonly can be given PO, SC or IM.
- Rapid absorption in both GIT and lungs
- Via inhaled route, effects are maximal at 15-30 minutes post administration and persist for 3-4 hours
- Metabolism:
- No lung metabolism
- 50% first pass metabolism
- Sulphated in the liver to inactive metabolites
- Urinary elimination of active (30%) drug and inactive metabolites
- Half life is 3-6 hours
Clinical uses
- Acute exacerbation of asthma or COPD
- Hyperkalaemia
- Previously used as a tocolytic in obstetric medicine to relax uterine smooth muscle to delay premature labour
Formulations
- Nebulised salbutamol
- Advantages: no patient education required, no first pass metabolism
- Disadvantages: larger particles and hence dose required, higher incidence of systemic side effects
- Spacer/inhaler
- Advantages: as effective as nebulised salbutamol when used properly, can be used at lower dose, fewer side effects, no first pass metabolism
- Disadvantages: patient education required
- IV salbutamol:
- Advantages: no first pass metabolism, potentially efficacious in severe asthma
- Disadvantages: requires IV access (can be problematic in young children), more systemic side effects, expensive
- Oral salbutamol:
- Advantages: longer half-life, easier administration in disabled/very young patients
- Disadvantages: larger dose required, 50% first pass metabolism, higher side effect profile, no advantages over inhaled route
Adverse effects
- Adverse effects relate to stimulation of beta-2 receptors, which are located in skeletal vascular and bronchial smooth muscle, the liver, and on cell membranes
- Musculoskeletal:
- Tremor due to stimulation of beta-2 receptors in skeletal muscle
- Cardiovascular:
- Tachycardia/palpitations/arrhythmias
- Direct stimulating action of atrial beta-2 receptors
- Peripheral vasodilation causes reflex cardiac stimulation
- Respiratory:
- V/Q mismatch and transient decrease in PaO2
- Vasodilatory action of beta-2 agonist causes transient decrease in PaO2 due to increased perfusion of poorly ventilated lung units
- Metabolic:
- Hypokalaemia due to potassium entry into skeletal muscle
- Other systemic metabolic effects, such as increase in FFA, insulin, glucose, pyruvate and lactate are generally only seen after large systemic doses
- Lactic acidosis is rare
- Gastrointestinal:
- Vomiting
Precautions/contraindications
- Arrhythmias
- Hypokalaemia
Further Reading
- Nickson C. Hyperkalaemia Management
- Nickson C. Acute Severe Asthma
Pharmacology 101
Top 200 drugs
Adult/Paediatric Emergency Medicine Advanced Trainee in Melbourne, Australia. Special interests in diagnostic and procedural ultrasound, medical education, and ECG interpretation. Co-creator of the LITFL ECG Library. Twitter: @rob_buttner