Asthma Pathophysiology and Pharmacological Interventions

Assessing Severity and Function

To manage asthma effectively, clinicians must assess both the current impairment and the future risk of exacerbations.

Pulmonary Function Tests (PFTs)

Objective measurement of lung function is performed through various tests:

• FEV1 (Forced Expiratory Volume): The amount of air forcibly exhaled in one second. A normal value is 80% or greater.
• FVC (Forced Vital Capacity): The total amount of air that can be forcibly exhaled after a deep breath.
• FEV1/FVC Ratio: A ratio used to assess airway obstruction; normal ranges are typically between 0.75 and 0.85.
• PEF (Peak Expiratory Flow): The maximum flow rate during forced exhalation.

Asthma Classification

Asthma is classified based on the frequency and intensity of presenting symptoms into four categories:

1. Intermittent
2. Mild persistent
3. Moderate persistent
4. Severe persistent

Asthma Therapy Goals and Stepwise Treatment

The primary goals of asthma therapy are to achieve minimal or no symptoms, maintain a normal PEF rate (with variations $< 20\%$), prevent emergency visits, and minimize the need for "as needed" rescue medications. Crucially, therapy aims to decrease long-term airway remodeling and reduce morbidity and mortality.

Stepwise Management

Treatment follows a stepwise approach, where therapy is escalated based on severity and "stepped down" if the patient remains well-controlled for at least three months:

Clinical Pearl: If a patient requires the use of a Short-Acting Beta Agonist (SABA) more than twice a week for symptom relief, it generally indicates inadequate control and the need to step up treatment.

Pharmacological Interventions

Bronchodilators (Sympathomimetics)

Beta-2 ($\beta_2$) agonists work by stimulating $\beta_2$ adrenergic receptors in the smooth muscle, increasing levels of cyclic adenosine monophosphate (cAMP), which results in bronchodilation.

• SABA (Short-Acting Beta Agonists): Known as "rescuers," these provide immediate relief by opening airways and clearing mucus. Examples include Albuterol (Ventolin) and Levalbuterol (Xopenex). Common side effects include tremors, tachycardia, and nervousness.
• LABA (Long-Acting Beta Agonists): Used for maintenance and prevention, these must always be used in conjunction with an inhaled corticosteroid (ICS) to avoid increasing the risk of asthma-related death. Examples include Salmeterol and Formoterol.

Anti-inflammatory Agents

• Inhaled Corticosteroids (ICS): The treatment of choice for long-term management of persistent asthma. They reduce airway inflammation by decreasing white blood cell activity and epithelial cell inflammatory activity. Common side effects include oropharyngeal candidiasis (thrush) and dysphonia; patients should rinse their mouths after use.
• Leukotriene Modifiers: These include receptor antagonists (e.g., Montelukast) and synthesis inhibitors (e.g., Zileuton). They decrease both inflammation and bronchoconstriction but have a slow onset and are not used for acute attacks.
• Mast Cell Stabilizers: Drugs like Cromolyn prevent the release of histamine and interleukins.

Other Classes

• Methylxanthines (Theophylline): Inhibit phosphodiesterase to prevent the degradation of cAMP. They have a narrow therapeutic index (goal 10-15 mcg/mL) and require careful monitoring.
• Anticholinergics (Muscarinic Antagonists): Block acetylcholine to increase cAMP and induce bronchodilation. Examples include Ipratropium and Tiotropium.
• Immunomodulators: For severe asthmatics with allergies, anti-IgE agents (e.g., Omalizumab) can inhibit IgE binding to mast cells.

Exam Tip: When reviewing drug combinations, remember that $\beta_2$-agonists and corticosteroids have synergistic effects. Using a $\beta_2$-agonist 5 minutes before inhaling a corticosteroid can enhance the steroid's delivery to the lungs.