Understanding Airway Pharmacology

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There are three main phases of drug action that can be applied to the use of” bronchoactive” inhaled aerosols. The drug administration phase looks at the method of how the drug is given. Aerosol medications are directly inhaled into the respiratory tract through the form of dry powder inhalers (DPI) and aerosol liquid solutions delivered via devices such as small volume nebulizers (SVN) and metered dose inhalers (MDI). This efficient method of drug administration avoids long onset time of drug action with smaller dosing due to the localized effect, while reducing the systemic side effects1, 2.

Next is the pharmacokinetic phase, which looks at the duration of time it takes for a medication to be fully absorbed, distributed, metabolized, and eliminated in the body. Aerosol medications that are fully ionized, such as ipratropium, cause little to no systemic effects. On the other hand,      non-ionized aerosol medications, such as atropine, can diffuse into the bloodstream causing a variety of systemic side effects. Ideally we would like the aerosol to be completely absorbed into the respiratory tract, but some of the drug may be absorbed within the stomach.  The lung availability-to-total systemic availability ratio or “L/T ratio” is a quantifiable method to measure the efficiency of the inhaled aerosol by determining how much of the medication is being absorbed into the   lungs2.

Finally, the pharmacodynamic phase looks at how the molecules within the aerosol medication bind and respond to the targeted receptors in the body. Short-acting and long-acting adrenergic bronchodilators are beta-2 agonists that cause bronchodilation in the lungs through the mediation of G protein- linked receptors. Adrenergic bronchodilators are typically used to improve air flow in patients suffering from asthma, emphysema, cystic fibrosis, acute and chronic bronchitis. Short-acting adrenergic bronchodilators are used as “rescue inhalers” and are the first line of therapy for patients suffering from acute asthma2. Studies have shown that using a MDI with a spacer or valved holding chamber is just as effective as a SVN treatment. The MDI is more cost effective and can provide faster sequential dosing in comparison to the SVN. Intravenous administration of beta-2 adrenergic bronchodilators may be used for patients in emergency situations, “who are unable to use or are unresponsive to the inhaled route”3. Long-acting adrenergic bronchodilators are used on a daily basis to provide and maintain bronchodilation for patients with asthma or other obstructive diseases. These medications have a slower peak onset, which means that more time is required before the effects of the treatment can be felt.  Thus, long-acting adrenergic bronchodilators should not be used in emergency situations. Some asthmatic patients may benefit from the combination of a long-acting adrenergic bronchodilator with anti-inflammatory medication to reduce bronchospasm and airway inflammation2.

 

 

References:

  1. Cairo JM. Chapter Humidity and Aerosol Therapy. In: Mosby’s Respiratory Care Equipment, 9th edition Elsevier; 2014:183-191.
  2. Kacmarek R, Stoller J, Heur A, Egan DF. Principles of Pharmacology. In: Egans’ Fundamentals of Respiratory Care, 10th edition Elsevier; 2013:708-711.
  3. Hodder R, Lougheed MD, Rowe BH, FitzGerald JM, Kaplan AG, McIvor RA. Management of acute asthma in adults in the emergency department: nonventilatory management. CMAJ 2010; 182(2):E55-67.

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