Beta 2 agonists

Question 1

Example of a beta 2 agonist

Answer 1

Salbutamol

Question 2

How do beta 2 agonists work?

Answer 2

Beta2-receptors are found in smooth muscle of the bronchi, gastrointestinal tract, uterus and blood vessels. Stimulation of this G protein-coupled receptor activates a signalling cascade that leads to smooth muscle relaxation. This improves airflow in constricted airways, reducing the symptoms of breathlessness. Like insulin, β2-agonists also stimulate Na+/K+-ATPase pumps on cell surface membranes, thereby causing a shift of K+ from the extracellular to intracellular compartment. This makes them a useful adjunct in the treatment of hyperkalaemia, particularly when IV access is difficult. However, their effect is less reliable than other therapies, so they should not be used in isolation.

Question 3

Indications for beta 2 agonists

Answer 3

Asthma: short-acting β2-agonists are used to relieve breathlessness. Long-acting β2-agonists are used as ‘step 3’ treatment for chronic asthma, but must always be given in combination with inhaled corticosteroids.

Chronic obstructive pulmonary disease (COPD): short-acting β2-agonists are used to relieve breathlessness. Long-acting β2-agonists are an option for second-line therapy of COPD.

Hyperkalaemia: nebulised salbutamol may be used as an additional treatment (alongside insulin, glucose and calcium gluconate) for the urgent treatment of a high serum potassium concentration.

Question 4

Contraindication of beta 2 agonists

Answer 4

Long-acting β2-agonists should be used in asthma only if an inhaled corticosteroid is also part of therapy. This is because, without a steroid, long-acting β2-agonists are associated with increased asthma deaths.

Care should be taken when prescribing β2-agonists for patients with cardiovascular disease, in whom tachycardia may provoke angina or arrhythmias. This is especially pertinent in the treatment of hyperkalaemia, when high doses may be necessary.

Question 5

Side effects of beta 2 agonists

Answer 5

Activation of β2-receptors in other tissues accounts for the common ‘fight or flight’ adverse effects of tachycardia, palpitations, anxiety and tremor. They also promote glycogenolysis, so may increase the serum glucose concentration. At high doses, serum lactate levels may also rise. Long-acting β2-agonists can cause muscle cramps.

Question 6

Interactions of beta 2 agonists

Answer 6

Beta-blockers may reduce the effectiveness of β2-agonists. Concomitant use of high-dose nebulised β2-agonists with theophylline and corticosteroids can lead to hypokalaemia, so serum potassium concentrations should be monitored.

Question 7

Elimination of beta 2 agonists

Answer 7

Renal

Question 8

Patient information on beta 2 agonists

Answer 8

Inhaled short-acting β2-agonists are prescribed for ‘as required’ administration. A common choice in adults is salbutamol 100–200 micrograms inhaled as required. In asthma and COPD exacerbations requiring hospital treatment, nebulised therapy is more often used (e.g. salbutamol 2.5 mg nebulised 4-hrly (Clinical tip—When prescribing nebuliser therapy, you should always indicate whether the nebuliser should be driven by oxygen or air. In general, oxygen should be used in asthma, whereas medical air should be used in COPD, due to the risk of CO2 retention.)

Explain that the medicine will make their airways relax and therefore improve their breathing. Make sure that they understand that this treats the symptoms, not the disease. Consequently, if they find themselves needing to use the β2-agonists very frequently, then they should seek medical advice, or increase their other treatment (e.g. inhaled corticosteroid) in accordance with a written action plan. Make sure that they are clear on how and when to take the inhaler (e.g. for acute symptoms, pre-emptively before exercise or regularly for long-acting medication).