What is the most commonly prescribed beta agonist?
Salbutamol
What is the overall effect of salbutamol?
it is a short-acting beta-2-agonist
it works by causing relaxation of bronchial smooth muscle (bronchodilation)
How does salbutamol work?
- it binds to B2 adrenergic receptors on the bronchial smooth muscle
- activation of B2 receptors causes adenylyl cyclase to convert ATP to cAMP
- this stimulates an intracellular signalling cascade that ends with the inhibition of myosin phosphorylation, which lowers intracellular concentration of calcium ions
myosin phosphorylation and calcium ions are necessary for muscle contraction
What is another effect of increasing cAMP as a result of salbutamol administration?
increase in cAMP inhibits inflammatory cells in the airway, particularly mast cells, from releasing inflammatory mediators
this assists in relaxation of bronchial smooth muscle in the airways
What are the indications for use for salbutamol?
it is typically used to treat bronchospasm due to any cause
this may be induced by acute asthma, exercise or allergies
it is commonly used to treat acute asthma attacks and for symptomatic relief in COPD
When may IV salbutamol be used?
to relax the uterine smooth muscle to delay premature labour
Which patients should extra care be paid attention to when giving salbutamol and why?
high doses or prolonged use can lead to hypokalaemia
this is particularly concerning in patients with kidney failure or those on certain diuretics and xanthine derivatives (theophylline)
In which conditions should salbutamol only be used with caution and if strictly indicated?
- serious cardiac disorders, particularly a recent MI
- coronary heart disease / tachyarrhythmia (due to positive ionotropic effect of B2 agonists)
- severe / untreated hypertension
- poorly controlled diabetes
- pheochromocytoma
- anuerysm
- hyperthyroidism
- glaucoma
What are the common side effects associated with salbutamol?
- tremor / anxiety
- headache
- muscle cramps
- dry mouth
- palpitations
What are less common side effects of salbutamol?
- tachycardia / arrhythmias
- flushing of the skin
- myocardial ischaemia (v rare)
- disturbances of sleep and behaviour
What are allergy related side effects of salbutamol?
- urticaria (hives) / itchy skin rash
- angioedema
- hypotension
- collapse
What drug must salbutamol not be used with?
B-agonists should not be used alongside Beta-blockers or any other B-receptor blocking drug
these cause bronchoconstriction, so will reduce the effectiveness of the salbutamol
What drugs, when used alongside salbutamol, can potentiate hypokalaemia?
- xanthine derivatives (theophylline)
- glucocorticoids
- diuretics
- cardiac glycosides (digoxin)
What other medications should caution be taken when being given with salbutamol?
- tricyclic antidepressants - can increase the risk of cardiovascular side effects
- corticosteroids (e.g. prednisolone) - can increase the risk of hyperglycaemia
- ipratropium bromide - used to treat asthma
How is salbutamol eliminated?
it is either filtered out directly by the kidneys
or it is metabolised into 4’-O’-sulphate, which is then excreted in the urine
