Beta 2 agonists

Question 1

What is the most commonly prescribed beta agonist?

Answer 1

Salbutamol

Question 2

What is the overall effect of salbutamol?

Answer 2

it is a short-acting beta-2-agonist

it works by causing relaxation of bronchial smooth muscle (bronchodilation)

Question 3

How does salbutamol work?

Answer 3

• 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

Question 4

What is another effect of increasing cAMP as a result of salbutamol administration?

Answer 4

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

Question 5

What are the indications for use for salbutamol?

Answer 5

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

Question 6

When may IV salbutamol be used?

Answer 6

to relax the uterine smooth muscle to delay premature labour

Question 7

Which patients should extra care be paid attention to when giving salbutamol and why?

Answer 7

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)

Question 8

In which conditions should salbutamol only be used with caution and if strictly indicated?

Answer 8

• 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

Question 9

What are the common side effects associated with salbutamol?

Answer 9

• tremor / anxiety
• headache
• muscle cramps
• dry mouth
• palpitations

Question 10

What are less common side effects of salbutamol?

Answer 10

• tachycardia / arrhythmias
• flushing of the skin
• myocardial ischaemia (v rare)
• disturbances of sleep and behaviour

Question 11

What are allergy related side effects of salbutamol?

Answer 11

• urticaria (hives) / itchy skin rash
• angioedema
• hypotension
• collapse

Question 12

What drug must salbutamol not be used with?

Answer 12

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

Question 13

What drugs, when used alongside salbutamol, can potentiate hypokalaemia?

Answer 13

• xanthine derivatives (theophylline)
• glucocorticoids
• diuretics
• cardiac glycosides (digoxin)

Question 14

What other medications should caution be taken when being given with salbutamol?

Answer 14

• 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

Question 15

How is salbutamol eliminated?

Answer 15

it is either filtered out directly by the kidneys

or it is metabolised into 4’-O’-sulphate, which is then excreted in the urine