B2 receptor Agonists Flashcards
Give Examples of Beta2 Receptor Agonist ?
- Short Acting B2 receptor agonists (SABA)
- Long Acting B2 receptor agonists (LABA)
What the the short acting B2 receptor agonists (SABA)?
- Salbutamol
- Terbutaline
What are the Long Acting B2 receptor agonists (LABA) ?
- Salmeterol
- Formoterol
- Vilanterol
What are the indications of SABA ?
- Relief of symptoms in acute and chronic asthma, used on an ‘as required’ basis
- Prophylaxis (treatment given or action taken to prevent disease.) of allergen or exercise induced bronchospasm (narrowing of the bronchi.)
- Relief of symptoms in COPD (only in patients who demonstrate reversibility, note the reversibly in this disease is often small and therefore β2 agonists have a limited role).
- A tocolytic agent (also called anti-contraction medications or labor suppressants), used to halt premature labour. Specialist use only.
- Rarely, treatment of hyperkalemia (abnormal high blood potassium) (nebulised).(unlicensed) (In medicine, a nebulizer or nebuliser is a drug delivery device used to administer medication in the form of a mist inhaled into the lungs)
What are the indications of LABA ?
- Treatment of chronic bronchospasm. Long acting β2 agonists are used as an adjuvant therapy in asthmatic patients who are inadequately controlled with inhaled steroids alone. They are given as regular therapy.
- Relief of symptoms in COPD (only in patients who demonstrate reversibility, note the reversibly in this disease is often small and therefore β2 agonists have a limited role).
What the contraindications of Beta 2 receptor agonists ?
Caution
- In cardiovascular disease high doses of β2 receptor agonists may cause tachy-arrhythmias ( abnormal high heart rate)
- Avoid in Pregnancy, because of the tocolytic actions (unless under specialist care).
********** In many situations, it will be difficult to avoid using β2 agonists as there are not alternatives.*****
How SABA works (Machanism of SABA) ?
β agonists act by stimulating the sympathetic nervous system. One of the actions of the sympathetic system is bronchodilation.
When pre-ganglionic sympathetic nerves are stimulated they release acetylcholine which acts on nicotinic receptors to release norepinephrine from the post ganglionic sympathetic nerve. Norepinephrine then acts on α and β adrenoceptor sub types (α 1, 2 and β 1, 2, 3) to produce sympathetic effects on various tissues around the body (see figure 1). β2 agonists work by mimicking the effect of norepinephrine on β2 receptors. This produces sympathetic effects on tissues containing β2 receptors.
See Autonomic and Neuromuscular physiology 1 - 2 Dr Davies and Autonomic and Neuromuscular pharmacology Dr Rod Scott, science for medicine.
β2 adrenoceptors are also found on skeletal muscle, nerve terminals, mast cells and other smooth muscle including (figure 2):
- Blood vessels
- GI tract
- Uterus
- Bladder detrusor (muscle)
- Seminal tract
- Cillary muscle
Administration Beta 2 receptor agonist ?
Inhalation
- Aerosol: Taken as required, via an MDI (metered dose inhaler) or a dry powder device +/- a spacer device. Note: there are a number of dry powder formulations available.
- Nebulised solutions: Used in acute severe asthma, repeated as required to control symptoms. Review the treatment and clinical response regularly.
- Nebulised solutions are also used in chronic COPD unresponsive to inhaled therapy.
Intravenous infusion
- Specialist use only for the treatment of acute asthma and premature labour.
What the adverse reactions of Beta 2 receptor agonists ?
- The dose delivered by inhalation rarely causes adverse effects.
- In high doses β2 agonists stimulate skeletal muscle adrenoceptor causing a fine tremor.
- Hypokalaemia. β2 agonists decrease the plasma potassium concentration by causing a shift of ions into the cell. This may be important when high doses are used.
- Rarely, paradoxical bronchospasm can occur.
Interactions of Beta 2 receptor agonists?
It is strongly advised to avoid prescribing beta-blockers for patients who require beta-2 agonists as these completely contradict one another and can precipitate an asthma attack.
Common
- Theophyllines, diuretics and corticosteroids increase the risk of developing hypokalaemia.
Rare
- Other sympathomimetic drugs may lead to cardiovascular side effects, some are available over the counter (ephedrine), or may be a component of herbal medications.
- Atomoxetine (used for ADHD); if given with parenteral salbutamol, increases the risk of cardiovascular side effects.
Info on Beta 2 receptor agonists !
- Most asthmatic patents receive dual therapy with corticosteroids and β2 agonists. Ensure the patient understands that short acting β2 agonists only relieve the symptoms of asthma, patients often referred to this as a ‘reliever’ (or the ‘BLUE’ one). In comparison corticosteroids are referred to as a ‘preventer’ (or the BROWN one). To achieve maximum efficacy, it is important to stress the different roles these drugs play in the treatment of asthma.
- Ensure that the patent knows how to use an inhaler device (+/- spacer) correctly. Ask them to demonstrate how to use it. It is generally now recommended that most patients with asthma would benefit from use of a spacer as this increases distribution of the drug into the correct location rather than the back of the throat.
- Warn patients to seek medical help if they require increasing doses of inhaled short acting β2 agonists to relieve symptoms.
- All patents with asthma require an annual asthma review, with either a doctor or asthma nurse specialist.
What the Pharmacokinetics of Beta 2 receptor agonists ?
Metabolism
- Salbutamol has significant first pass metabolism in the liver. This is not important in with normal inhaled therapy.
- There is no dosage adjustment required in hepatic or renal insufficiency.
