Bronchodilators are a group of medications that help breathing by keeping the airways dilated. That being said, they are typically used in obstructive lung diseases, like asthma and chronic obstructive pulmonary disease, or COPD for short, where clients suffer from narrowing and obstruction of the airways.
Asthma is characterized by chronic inflammation in the lungs, as well as asthma exacerbations or attacks, where certain triggers, such as viruses, allergens, stress, aspirin or other NSAIDs and exercise, lead to reversible bronchial smooth muscle spasms and mucus production, both of which make it hard to breathe. As a result, clients experience symptoms like dyspnea, wheezing, chest tightness, and coughing.
On the other hand, in COPD, there’s chronic inflammation and fibrosis in the lungs, most commonly due to smoking. As a result, the airways become irreversibly obstructed and the lungs are not able to empty properly, which leaves air trapped inside the lungs. As a result, clients experience symptoms like dyspnea and a productive cough.
Now, COPD generally refers to a group of progressive lung diseases that includes chronic bronchitis and emphysema. These two differ in that chronic bronchitis is defined by long-term inflammation of the bronchial tubes, whereas emphysema is defined by destruction and enlargement of the alveoli.
Although the airway obstruction in COPD is irreversible, bronchodilators can often help prevent the complete closure of the airway during expiration, which provides mild symptomatic relief.
Now, based on their mechanism of action, bronchodilators can be broadly divided into three main groups; β2-agonists; anticholinergics; and methylxanthines.
The effect of all these medications is bronchial smooth muscle relaxation, which in turn results in dilation of the narrowed airways and improved air flow.
In particular, β2-agonists, like albuterol and salmeterol, come in an aerosolized form, and can be taken via metered dose inhalers or MDIs, or nebulizers.
Once in the lungs, they bind to and activate the β2 adrenergic receptors on bronchial smooth muscle cells, ultimately promoting relaxation of the smooth muscle. β-2 agonists can be classified, based on the duration of action, into short acting β-2 agonists, or SABAs, such as albuterol, and long acting β-2 agonists, or LABAs, like salmeterol.
SABAs are typically the treatment of choice for quick symptom relief in acute asthmatic attacks, whereas LABAs are often used in combination with an inhaled corticosteroid like budesonide as prophylactic or maintenance treatment for asthma and COPD.
On the other hand, commonly used anticholinergics include ipratropium and tiotropium, and can also be given via inhalers or nebulizers. Once in the airways, they bind to M3 muscarinic receptors on the tracheal and bronchial smooth muscles.
This blocks acetylcholine from binding to the receptors, decreasing smooth muscle constriction. In comparison to β2-agonists, anticholinergics are less effective for asthma, but more effective for COPD, where they are the bronchodilators of choice.
However, for severe cases of asthma or COPD, anticholinergics are often given in combination with LABAs for an additive effect, leading to stronger and longer lasting bronchodilation.
Finally, methylxanthines, such as theophylline, are usually taken orally, but can also be administered intravenously. Once methylxanthines reach the airways, they inhibit the enzyme phosphodiesterase, or PDE, and ultimately lead to smooth muscle relaxation. These medications can be used in asthma and COPD.
Okay, now each group of bronchodilators has its own set of side effects. With β-2 agonists, the most common ones are muscle tremors, restlessness, and insomnia, as well as tachycardia, and palpitations.
Some clients may even develop arrhythmias, especially with LABAs, which could result in heart failure or even death. As a result, β-2 agonists should be used with caution in clients with concurrent heart or renal disease, hyperthyroidism, diabetes mellitus, and pregnancy.
Moving on to anticholinergics, common side effects include pupil dilation, dry mouth, tachycardia, and restlessness. For that reason, anticholinergics should be used with caution in clients with narrow angle glaucoma, heart disease, and hyperthyroidism, and are contraindicated in clients with a previous hypersensitivity or allergic reaction.