RESPIRATORY PHARMACOLOGY - ASTHMA. Primary Exam Teaching - Westmead ED

Transcription

1 RESPIRATORY PHARMACOLOGY - ASTHMA Primary Exam Teaching - Westmead ED

2 Sympathomimetic agents MOA: relax airway smooth muscle and inhibit broncho constricting mediators from mast cells May also inhibit microvascular leakage and increase mucocilliary transport by increasing ciliary activity The beta agonists stimulate andeylyl cyclase and increase formation of intracellular camp Beta 2 effect causes Relaxation of airway smooth muscle Inhibits mediator release Causes tachycardia and skeletal muscle tremor as side effects Best mode of delivery is via inhalation - results in the greatest local effect on airway smooth muscle with the least systemic toxicity Aerosol deposition depends on particle size, pattern of breathing and geometry of airways - deposition can be increased by holding breath during inspiration

3 ADRENALINE Effective, rapidly acting bronchodilator Maximal bronchodilation 15 min after inhalation Stimulates alpha and beta one as well as beta two Causes tachycardia, arrythmias and worsening of angina pectoris These CV effects are of value for treating the acute vasodilation and shock as well as the bronchospasm of anaphylaxis

4 Beta 2 selective agents Salbutamol Use - asthma, hyperkalemia Pharmacokinetics A % act directly on smooth muscle when inhaled, D - minimal entry into systemic circulation when inhaled, otherwise low Vd M - signifiant first pass metabolism E - excreted really Pharmacodynamics - selective beta two receptor agonist and causes bronchodilation Maximal effect within minutes and persists for 3-4 hours Nebulisers are no more effective than MDI - should only be given when patients cannot coordinate their own breathing Longer acting agents such as salmeterol have a 12 hour duration as a result of HIGH LIPID SOLUBILITY - and appear to interact with corticosteroids to improve asthma control Adverse effects: Cardiac arrythmia and hypoxemia - ma increase the perfusion of poorly ventilated lung and thus transiently decrease arterial oxygenation - this is overcome by the administration of supplemental oxygen Fine tremor + palpitations + cramps

5 Methylxanthine drugs Three important agents Theophylline Theobromine Caffeine Importance as a therapeutic agent in asthma has weaned over time Aminophylline is a commonly used theophylline preparation Mechanism of action At high concentrations > inhibits phosphodiesterase enzyme family This results in higher concentrations of intracellular camp Another proposed mechanism in inhibition of cell surface adenosine receptors - which has been shown to provoke contraction of isolated airway smooth muscle and histamine release

6 Methylxanthine drugs Pharmacokinetics : A - 100% bioavailibilty, serum levels in minutes D - low Vd 0.5L/kg, 60% protein bound M - hepatic E % hepatic elimination, remainder renal, half life 4-8 hours Pharmacodynamics CNS effects - low and moderate doses, especially caffeine, can cause mild cortical arousal with increase alertness Larger doses are required for bronchodilation effects and in very high doses can cause convulsions and death CV effects - positive chronotropic and inotropic effects - at low doses these effects are thought to be mediated by inhibition of adenosine receptors in sympathetic nerves. Higher concentrations are associated with inhibition of phosphodiesterase and increased influx of calcium Results in slight tachycardia, increased CO, increased TPR, HTN GI - stimulates secretions

7 Methylxanthines Clinical use Theophylline is the most important bronchodilator Relieves airway obstruction in acute asthma and reduces severity of symptoms in chronic asthma Should only be used when methods to measure blood levels are available because it has a narrow therapeutic window Toxicity includes: Anorexia, Nausea, Vomiting, Higher levels cause seizures and arrhythmia Levels over 20mcg/mL Plasma clearance of theophylline varies wildly It is metabolised in the liver and should be used in caution in patients with liver disease

8 Antimuscarinic agents Mechanism of action - competitively inhibit the effect of acetylcholine at muscarinic receptors In the airways Ach is released from the parasympathetic nerve endings (muscarinic) and block airway smooth muscle relaxation and increase airway secretions Ipratropium Bromide Pharmacokinetics A: oral route - can be given in high doses because of its poor systemic absorption and lack of CNS penetration, onset of action 15 min D: low Vd and low systemic absorption M: hepatically E: Renal (46%) - half life 2 hours

9 Cromolyn and Nedocromil Mechanism of action: Mast cell stabiliser - inhibits release of histamine, leukotrienes and slow reacting substances of anaphylaxis by inhibiting mast cell degranulation Pharmacokinetics: A: PO bioavailibility 0.5-2% D: Peak plasma time 15 min M: E: 98% in feaces unabsorbed, half life 90 min Clinical use: Blocks bronchoconstriction caused by Allergen inhalation Exercise Sulfure dioxide Both agents reduce symptom severity and need for bronchodilator medications

10 Leukotrine pathway inhibitors Leukotirnes results from the activation of 5 - lipoxygenase on arachadonic acid and are synthesised by a variety of inflammatory mediators LTB 4 is a potent chemoattractant LTC4 and LTD4 are responsible for many of the effects associated with asthma Two pharmacological approaches exist to the inhibition of this pathway Inhibition of 5 - Lipoxygenase - preventing leukotrine synthesis Inhibition of binding of LTD4 to receptor of target tissues Have been shown to improve asthma control and reduce the frequency of exacerbations

11 Corticosteroids Presumed to act by broad anti-inflammatory mechanisms Reduce bronchial reactivity and reduce the frequency of asthma exacerbations if taken regularly Also induce contraction of engorged vessels in the bronchial mucosa and potentiate effects of beta receptor agonists Most important mechanism of action is the inhibition of lymphocytic, eosinophilic mucosal inflammation of asthmatic airways

12 Corticosteroids Effective in improving all indicies of asthma control Severity of symptoms Tests of airway calibre Bronchial reactivity Frequency of exacerbation QOL Oral and parenteral treatments reserved for those people who require urgent treatment - aerosolised treatment is the most effective mode of delivery to avoid systemic side effects Budesonide Fluticasone Main side effect it oral candidiasis which can be avoided by mouth gargling post treatment