Bronchodilators are indicated for individuals that have lower than optimal airflow through the lungs. The mainstay of treatment is beta-2 agonists that target the smooth muscles in the bronchioles of the lung. Various respiratory conditions may require bronchodilators, including asthma and chronic obstructive pulmonary disease. They are used to either reverse the symptoms of asthma or improve lung function in patients with chronic obstructive pulmonary disease. Pulmonary function tests assess lung function. Thus, bronchodilators have an essential role in the diagnosis and treatment of lung conditions based on their effect on pulmonary function tests. The FEV1/FVC ratio compares how much air flows during the first second of exhalation (forced expiratory volume) to the theoretical amount of air someone can push out in a maximum exhalation (forced vital capacity). A typical ratio is 0.7. In reversible increased airway resistance like asthma, pre-bronchodilator pulmonary function tests will typically be lower than 0.7. However, after the administration of a short-acting bronchodilator, the ratio may normalize. In nonreversible conditions like chronic obstructive pulmonary disease, giving a short-acting bronchodilator may not normalize pulmonary function test levels in patients.

Commonly, inhaled corticosteroids are added to beta-2 agonists to reduce inflammation and pro-inflammatory agents that will further constrict airways. Beta-2 agonist class bronchodilators do not affect the underlying pathology of lung disease; they are only symptomatic treatment. Therefore, adding inhaled corticosteroids to the regimen has been the mainstay of mild to moderate reversible lung diseases with or without long-acting beta-2 agonists. Anticholinergics are the final class of medicine considered bronchodilators. This class's mechanism inhibits the effects of the parasympathetic nervous system mediated by the vagus nerve. A hyperactive parasympathetic nervous system causes bronchial secretions and narrowing of the airways. Medicines that inhibit the actions of the parasympathetic nervous system at the level of the airways will then generate a bronchodilatory effect. These medicines include ipratropium bromide, which is a short-acting medicine (4 to 6 hours), and tiotropium bromide, which is longer acting (24 hours). Anticholinergics primarily function in the setting of chronic obstructive pulmonary disease. Patients with asthma can usually control their symptoms with the combination of a beta-2 agonist and corticosteroid.

The step theory in managing reversible lung diseases like asthma incorporates both short- and long-acting bronchodilators. Those with intermittent asthma should receive a short-acting bronchodilator such as albuterol as needed. Adding a low-dose, inhaled corticosteroid is the next step to more symptomatic disease, followed by adding a long-acting bronchodilator with the inhaled steroid. Increasingly aggressive treatment should defer to those who specialize in asthma and allergy treatment. After achieving control, the patient will consult with their doctor to wean them off these medicines to a smaller dose with fewer adverse effects. Failure to control symptoms with short or long-acting bronchodilators and corticosteroids can cause irreversible lung injury. Frequent monitoring by pulmonary function tests and peak airway flow is the mainstay of treatment success.[1][2]

Bronchodilators' mechanism of action includes targeting the beta-2 receptor, which is a G-protein coupled receptor, in the lung airways. When the beta-2 receptor is activated, the smooth muscle of the airway relaxes. Subsequently, the patient experiences better airflow for a period. Consistent use of beta-2 agonists for an extended amount of time reduces their efficacy due to the down-regulation of the beta-2 receptor in the airways. As such, a higher dose of medicine is necessary to achieve the same result. Bronchodilator metabolism occurs in the gastrointestinal tract by cytochrome P-450 enzymes. About 80% to 100% is excreted in the urine, and less than 20% is excreted in feces. Short-acting bronchodilators have a half-life of 3 to 6 hours, while longer-acting bronchodilators have a half-life of 18 to 24 hours.

Anticholinergics target parasympathetic nervous system receptors in the airways and inhibit their function. Since the parasympathetic nervous system is responsible for increased bronchial secretions and constriction, reversing those should provide bronchodilation and fewer secretions.[3][4][5]

The administration of bronchodilators is primarily through inhalation devices to deliver the drug to the lung's bronchioles. Inhalation devices come in all shapes and sizes, but what is critical is maximizing the amount of drug reaching the bronchioles. Even when used with a perfect technique, the bioavailability of this class of medications remains very low. The best way to achieve maximum bioavailability is by fully exhaling, placing the inhaler in the mouth, and taking a full inhalation. After the patient has inhaled completely, it is followed by 10 seconds of no breathing to wait for the medicine to dissipate into the lung space. Then, a slow exhalation back to normal breathing is advised. Failure to follow the correct steps risks not maximizing the full potential of the inhaled medicines. Patients taking short-acting bronchodilators should benefit from the effects of the medication very quickly, within seconds to minutes, and should have the clinical benefit for around 4 hours. These are sometimes called emergency inhalers due to their immediate effect on bronchodilation. Long-acting bronchodilators do not typically work as quickly and are not useful in an emergency setting.[6][7]

The adverse effects of bronchodilators are due to sympathetic system activation. The most frequent and common adverse effects include trembling, nervousness, sudden, noticeable heart palpitations, and muscle cramps. More severe effects include sudden constriction of the bronchial airways, or paradoxical bronchospasm, hypokalemia, and in rare cases, myocardial infarction. A patient should talk to their primary care physician if they have any comorbidities. For anticholinergics, side effects include symptoms caused by a decrease in vagal tone. These can include dry mouth, urinary retention, tachycardia, constipation, and an upset stomach. Caution is always necessary when administering an anticholinergic to elderly patients due to the possibility of acute delirium.[8][4]

If a patient has a known hypersensitivity to the drug, then physicians should not prescribe it. These hypersensitivities include severe allergic reactions that can cause hemodynamic instability or loss of a patent airway. Use caution when treating patients with ischemic heart disease, arrhythmias, or hypokalemia, as bronchodilators have demonstrated worsening of the effects of these conditions. Exercising care with this class of medication is also essential during labor and delivery and when treating elderly patients. In very high doses, caution is also necessary for patients with renal impairment.[1][9]