Inhaled Asthma Medication

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Chapter: Essential pharmacology : Drugs for Cough and Bronchial Asthma

Four classes of antiasthma drugs, viz. β2 agonists, anticholinergics, cromoglycate and glucocorticoids are available for inhalational use. They are aimed at delivering the drug to the site of action so that lower dose is needed and systemic side effects are minimized. Most asthma patients are now maintained on inhaled medication.


INHALED ASTHMA MEDICATION

Four classes of antiasthma drugs, viz. β2 agonists, anticholinergics, cromoglycate and glucocorticoids are available for inhalational use. They are aimed at delivering the drug to the site of action so that lower dose is needed and systemic side effects are minimized. Most asthma patients are now maintained on inhaled medication only. 

Aerosols are of two types:
i. use drug in solution: metered dose inhaler, nebulizer.
ii. use drug as dry powder: spinhaler, rotahaler

Metered dose inhalers use chlorofluorocarbon (are being banned now for their effect on ozone layer) or hydro-fluroalkane (HFA) propellants and deliver a specified dose of the drug in spray form per actuation. Device actuation has to be properly coordinated with deep inspiration, which many patients are unable to learn. A ‘spacer’ (chamber interposed between the inhaler and the patient’s mouth) can be used to improve drug delivery. Nebulizers produce a mist of the drug solution generated by pressurized air or oxygen which can be inhaled through a mouth piece, face mask or in a tent. Metered dose inhalers are convenient handheld devices which can be carried along, while nebulizers are used at patient’s bed side. Nebulizers are preferred for severe episodes of asthma as well as for children and elderly. More than one drug can be nebulized simultaneously.

Dry powder inhalers are also portable devices in which the capsule (rotacap) containing the drug is punctured/ cut across and the powder is aerosolized by the inspiratory air flow of the patient. It requires high velocity inspiration which children, elderly and the very sick may not be capable of. The dry powder is also more likely to irritate the air passage—producing cough and broncho constriction.

Efficacy of aerosolized drug depends on the particle size: 1–5 μm diameter particles deposit on the bronchioles and effectively deliver the drug. Larger particles settle on the oropharynx, while very fine particle do not settle anywhere and are exhaled out. On an average only 4–10% of the inhaled drug reaches the site of action. A considerable fraction is swallowed. Therefore, to minimize systemic action, the drug should have low oral bioavailability. Spacer devices improve inhaled to swallowed drug ratio. Slow and deep inbreathing after device actuation and holding the breath after inhalation also enhances efficacy of the inhaler.

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