bronchodilation

Bronchodilation is the widening of the bronchi and bronchioles, produced by relaxation of the smooth muscle in the airway walls. This expansion lowers airway resistance, allows greater airflow to the lungs, and can improve ventilation and gas exchange. Bronchodilation can occur naturally in response to certain autonomic signals and can be induced pharmacologically by various medications.

Physiology and mechanisms: Airway tone is regulated by the autonomic nervous system. Stimulation of beta-2 adrenergic receptors on airway smooth muscle activates cyclic adenosine monophosphate (cAMP) signaling, leading to muscle relaxation and bronchodilation. Conversely, acetylcholine acting on M3 receptors promotes constriction. Inflammation and constrictive mediators in conditions such as asthma and chronic obstructive pulmonary disease (COPD) contribute to airway hyperresponsiveness and episodic bronchoconstriction; bronchodilators alleviate these episodes and improve airflow.

Bronchodilator medications: Short-acting beta-2 agonists (SABAs), such as albuterol, provide rapid relief of acute symptoms by quickly relaxing airway smooth muscle. Long-acting beta-2 agonists (LABAs), such as salmeterol and formoterol, offer extended bronchodilation and are used for maintenance therapy, usually in combination with inhaled corticosteroids in asthma and/or COPD. Anticholinergic agents (muscarinic antagonists) like ipratropium (short-acting) and tiotropium (long-acting) reduce acetylcholine-driven constriction and are commonly used in COPD, with adjunctive use in asthma. Theophylline, a phosphodiesterase inhibitor, has bronchodilatory effects but is limited by systemic side effects. Inhaled corticosteroids reduce airway inflammation but are not bronchodilators themselves.

Clinical use and considerations: Bronchodilators are central to symptom relief in asthma and COPD and are evaluated for efficacy by improvements in lung function, such as an increase in FEV1 after inhalation. Side effects vary by agent, with tachycardia and tremor common with beta-agonists and dry mouth with anticholinergics. Proper device technique and adherence to therapy are important for effectiveness.