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
Pharm 101 700

Pharmacology 101

Top 200 drugs

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