Asthma & Chronic Obstructive Pulmonary Disease (COPD)

Both asthma and COPD present with an obstructive spirometric pattern defined by a post-bronchodilator FEV1/FVC ratio <0.70 (with FEV1 reduced disproportionately more than FVC, accompanied by elevated Residual Volume [RV] and Total Lung Capacity [TLC] from air trapping). However, asthma is characterized by reversible bronchial smooth muscle hyperresponsiveness—demonstrated by an increase in FEV1 of >=12% and >=200 mL following inhaled albuterol (or a >=20% drop in FEV1 during a methacholine bronchial provocation test when baseline spirometry is normal between attacks)—and a normal or elevated DLCO. Conversely, COPD exhibits largely irreversible airflow limitation, and emphysematous alveolar septal destruction obliterates the pulmonary capillary bed, producing a markedly decreased DLCO.

Allergic (atopic) asthma is driven by a CD4+ Th2 (and ILC2) immune cascade: IL-4 induces B-cell class switching to IgE, IL-5 recruits and activates eosinophils (whose major basic protein damages respiratory epithelium and forms Charcot-Leyden crystals alongside Curschmann spirals of shed mucus plugs), and IL-13 stimulates goblet cell metaplasia and airway hyperresponsiveness. Consequently, inhaled corticosteroids (ICS) are the mandatory controller backbone of asthma therapy (LABA monotherapy in asthma is black-box contraindicated due to asthma-related death). In contrast, COPD is driven by cigarette smoke or alpha-1 antitrypsin deficiency activating alveolar macrophages and CD8+ T cells to recruit neutrophils, which release neutrophil elastase and matrix metalloproteinases that destroy elastin fibers (centriacinar emphysema in upper lobes from smoking vs. panacinar emphysema in lower lobes with PAS-positive hepatic globules in SERPINA1 PiZZ alpha-1 antitrypsin deficiency).

A classic board physiology question asks why administering 100% FiO2 to a chronic CO2-retaining COPD patient precipitates acute hypercapnic narcosis (rising PaCO2 and somnolence). While historically attributed solely to 'blunting the hypoxic respiratory drive,' two physiological mechanisms account for over 80% of oxygen-induced hypercapnia: (1) Reversal of Hypoxic Pulmonary Vasoconstriction (HPV), where high alveolar O2 dilates arterioles supplying poorly ventilated emphysematous bullae, shunting blood away from well-ventilated alveoli and dramatically increasing physiological dead space (Vd/Vt); and (2) The Haldane Effect, whereby deoxygenated hemoglobin binds CO2 (as carbaminohemoglobin and H+ buffer) far more avidly than oxygenated hemoglobin, so saturating hemoglobin to 100% unloads bound CO2 directly into the plasma.

In acute COPD exacerbations complicated by acute respiratory acidosis (pH <=7.35 and PaCO2 >45 mmHg), immediate initiation of Non-Invasive Positive Pressure Ventilation (BiPAP) provides inspiratory pressure support (IPAP) to unload fatigued respiratory muscles and expiratory positive airway pressure (EPAP) to stent open collapsing small airways, reducing endotracheal intubation and mortality by more than 50%. However, BiPAP is contraindicated in patients with respiratory arrest, severe encephalopathy/coma (unable to protect airway), copious purulent secretions, facial trauma, or hemodynamic instability—who require immediate endotracheal intubation. On mechanical ventilation, obstructive patients develop dynamic hyperinflation ('breath stacking' or Auto-PEEP), which is corrected by decreasing the respiratory rate, decreasing tidal volume (permissive hypercapnia), and increasing inspiratory flow rate to maximize expiratory time (prolonged I:E ratio of 1:3 to 1:4).

Emphysema is defined anatomically as permanent enlargement and destruction of airspaces distal to the terminal bronchiole without fibrosis. Centriacinar (centrilobular) emphysema is caused by cigarette smoking (90% of cases): inhaled oxidants recruit neutrophils and alveolar macrophages that release elastase and metalloproteinases while inactivating endogenous antiproteases, selectively destroying the respiratory bronchioles in the upper lobes first. Conversely, Panacinar (panlobular) emphysema destroys the entire acinus uniformly (respiratory bronchioles, alveolar ducts, and alveoli) predominantly in the lower lobes and is the hallmark of Alpha-1 Antitrypsin (AAT) Deficiency. Inherited as an codominant mutation in the SERPINA1 gene (PiZZ homozygote carries the severest phenotype), misfolded AAT polymerizes within hepatocyte endoplasmic reticulum—appearing as PAS-positive, diastase-resistant magenta globules on liver biopsy—causing neonatal hepatitis, adult micronodular cirrhosis, hepatocellular carcinoma, and early-onset (< 45 years) lower-lobe panacinar emphysema. Treat with IV pooled human AAT augmentation and hepatitis vaccination.

During forced expiration in healthy lungs, positive pleural pressure compresses both the alveoli and the intrathoracic airways, while alveolar elastic recoil maintains a driving lumen pressure that places the Equal Pressure Point (EPP)—where intraluminal airway pressure equals surrounding pleural pressure—in cartilaginous bronchi that resist collapse. In emphysema, destruction of elastin fibers drastically reduces alveolar elastic recoil pressure (increasing lung compliance). Consequently, during expiration, intraluminal pressure drops rapidly, shifting the Equal Pressure Point distally toward the fragile, non-cartilaginous terminal bronchioles, causing premature dynamic airway collapse and air trapping. Patients instinctively adopt pursed-lip breathing, which generates resistance at the mouth, creating positive back-pressure (extrinsic PEEP) throughout the bronchial tree that shifts the EPP proximally back into cartilaginous airways and prevents premature alveolar collapse.

Patients with classic episodic asthma frequently present to the outpatient clinic when asymptomatic, exhibiting completely normal baseline spirometry (FEV₁/FVC > 0.75). To confirm the diagnosis of asthma when baseline spirometry is normal, perform a bronchoprovocation challenge using inhaled Methacholine (a direct muscarinic M₃ receptor agonist). A ≥ 20% decrease in FEV₁ at a low provocative concentration (PC₂₀ ≤ 8 mg/mL) confirms bronchial hyperresponsiveness; a negative methacholine challenge has a > 95% negative predictive value to rule out asthma. Additionally, Fractional Exhaled Nitric Oxide (FeNO > 40–50 ppb) and peripheral blood eosinophilia (≥ 300 cells/μL) serve as non-invasive biomarkers of Type 2 (T_H2: IL-4, IL-5, IL-13) eosinophilic airway inflammation and predict an excellent clinical response to inhaled corticosteroids.

Asthma symptom control is assessed at every visit using the 'Rule of 2s' over the past 4 weeks: daytime symptoms > 2 times/week, nighttime awakenings due to asthma (≥ 2 times/month), rescue reliever needed > 2 times/week, or any activity limitation. Under modern GINA guidelines (Track 1), a single ICS-Formoterol inhaler (MART: Maintenance and Reliever Therapy) serves as both daily controller and PRN rescue: Step 1–2 (mild intermittent/persistent): As-needed low-dose ICS-formoterol (or low-dose daily ICS + PRN SABA); Step 3 (moderate persistent): Low-dose maintenance ICS-formoterol daily + PRN; Step 4 (severe persistent): Medium-dose maintenance ICS-formoterol; and Step 5: High-dose ICS-LABA + add-on LAMA (tiotropium) + phenotypic assessment for targeted biologics (anti-IgE, anti-IL-5, anti-IL-4Rα). Before stepping up therapy, always verify inhaler technique, medication adherence, trigger avoidance, and gastroesophageal reflux (GERD).

Aspirin-Exacerbated Respiratory Disease (AERD), classically known as Samter triad, consists of: (1) Asthma, (2) Chronic rhinosinusitis with recurrent bilateral nasal polyps, and (3) Acute bronchospasm, rhinorrhea, and facial flushing within 30–180 minutes of ingesting Aspirin or any COX-1 inhibiting NSAID. It is not an IgE-mediated allergy, so skin testing is negative. Instead, COX-1 inhibition blocks prostaglandin E₂ (PGE₂, which normally inhibits 5-lipoxygenase and stabilizes mast cells) and shunts arachidonic acid metabolism exclusively down the 5-lipoxygenase pathway, causing a massive surge in bronchoconstrictor cysteinyl leukotrienes (LTC₄, LTD₄, LTE₄). Treatment requires strict avoidance of all COX-1 NSAIDs (acetaminophen at low doses ≤ 650 mg or selective COX-2 inhibitor celecoxib is usually tolerated), intranasal and inhaled corticosteroids, leukotriene receptor antagonists (montelukast), or aspirin desensitization (if daily aspirin is required for CAD).

Two board-tested syndromes must be suspected in patients with refractory asthma and prominent peripheral blood eosinophilia: (1) Allergic Bronchopulmonary Aspergillosis (ABPA) is a mixed Type I and Type III hypersensitivity reaction to noninvasive Aspergillus fumigatus colonizing the airways of patients with asthma or cystic fibrosis. It presents with recurrent exacerbations, expectoration of brownish mucus plugs, fleeting pulmonary infiltrates, central (proximal) bronchiectasis on chest CT, positive Aspergillus skin test/specific IgE/IgG precipitins, and Total Serum IgE > 1,000 IU/mL. Treat with systemic oral corticosteroids + Itraconazole. (2) Eosinophilic Granulomatosis with Polyangiitis (EGPA / Churg-Strauss) presents with adult-onset asthma, chronic sinusitis/nasal polyps, marked eosinophilia (> 10–15%), necrotizing small-vessel vasculitis, mononeuritis multiplex (foot drop/wrist drop), palpable purpura, and p-ANCA (anti-MPO) positivity; treat with systemic glucocorticoids plus mepolizumab or cyclophosphamide.

High-yield board questions repeatedly ask which intervention prolongs survival in a patient with COPD. Pharmacologic inhalers (albuterol, ipratropium, tiotropium, salmeterol, theophylline) improve FEV₁, exercise tolerance, and exacerbation frequency, but only Smoking Cessation and Long-Term Oxygen Therapy (LTOT ≥ 15 hours/day) are universally recognized to decrease all-cause mortality (alongside lung volume reduction surgery in upper-lobe emphysema with low exercise capacity). Memorize the exact arterial blood gas thresholds on room air for home oxygen qualification: (1) Resting PaO₂ ≤ 55 mmHg OR SaO₂ ≤ 88% in any stable COPD patient; OR (2) Resting PaO₂ 56–59 mmHg OR SaO₂ ≤ 89% when accompanied by evidence of chronic hypoxia-induced end-organ strain: cor pulmonale (pulmonary hypertension, P pulmonale on ECG), right-sided heart failure (pedal edema), or secondary erythrocytosis (hematocrit > 55%).

A fundamental distinction between asthma and COPD pharmacotherapy lies in the role of Inhaled Corticosteroids (ICS). In asthma (eosinophilic/T_H2 inflammation), ICS is the mandatory first-line controller at every step, and LABA monotherapy is lethal. In contrast, COPD inflammation is driven predominantly by neutrophils, CD8⁺ cytotoxic T lymphocytes, and alveolar macrophages, which are largely resistant to corticosteroids. Furthermore, chronic ICS use in COPD patients significantly increases the risk of severe bacterial pneumonia, oral candidiasis, and osteoporotic fractures. Therefore, ICS monotherapy is never used in COPD, and triple therapy (LABA + LAMA + ICS) is reserved strictly for GOLD Group E patients with frequent/severe exacerbations AND peripheral blood eosinophils ≥ 300 cells/μL (or ≥ 100 cells/μL if exacerbations persist despite LABA+LAMA) or concomitant asthma.

For GOLD Group E patients who continue to suffer exacerbations despite maximal inhaled therapy, two targeted oral anti-inflammatory agents reduce flare rates: (1) Roflumilast, a selective phosphodiesterase-4 (PDE-4) inhibitor that increases intracellular cAMP in neutrophils and macrophages to suppress airway inflammation, is indicated specifically in patients with severe COPD (FEV₁ < 50% predicted) and a chronic bronchitis phenotype (productive cough); and (2) Chronic low-dose Azithromycin (250 mg daily or 500 mg 3 times/week) in former smokers, which exerts immunomodulatory effects (monitor for QTc prolongation and sensorineural hearing loss). All COPD patients require annual inactivated influenza vaccine, PCV20 (or PCV15 followed by PPSV23) pneumococcal vaccination, COVID-19, RSV (≥ 60 yr), and Tdap vaccines.

In an Acute Exacerbation of COPD (AECOPD) complicated by acute hypercapnic respiratory acidosis (arterial pH < 7.35 and PaCO₂ > 45 mmHg), immediate initiation of Bilevel Positive Airway Pressure (BiPAP) is the gold-standard intervention, proven to reduce endotracheal intubation rates by > 60% and cut inpatient mortality in half. BiPAP delivers two distinct pressures: IPAP (Inspiratory Positive Airway Pressure), which unloads the fatigued diaphragm and increases tidal volume to blow off CO₂, and EPAP (Expiratory Positive Airway Pressure / PEEP), which offsets intrinsic auto-PEEP and splints open small airways. Note the absolute contraindications to BiPAP (which mandate immediate endotracheal intubation instead): respiratory or cardiac arrest, severe encephalopathy/obtundation (inability to protect airway), copious secretions or active vomiting (aspiration risk), facial trauma/burns, or hemodynamic shock.

During an acute asthma exacerbation, bronchospasm and hypoxemia stimulate peripheral and central chemoreceptors, driving tachypnea and hyperventilation. Consequently, an uncomplicated mild-to-moderate asthma attack MUST demonstrate acute respiratory alkalosis (e.g., pH 7.48, PaCO₂ 28 mmHg) on arterial blood gas. If a repeat ABG reveals a 'normal' PaCO₂ (38–42 mmHg) and normal pH (7.38–7.40) accompanied by lethargy, diaphoresis, pulsus paradoxus (> 12 mmHg drop in SBP on inspiration), or a 'silent chest' (airflow too minimal to generate audible wheezing), the patient has developed critical respiratory muscle fatigue and requires immediate endotracheal intubation. For severe refractory attacks prior to intubation, administer IV Magnesium Sulfate (2 g) (blocks smooth muscle Ca²⁺ channels) and consider Heliox (helium-oxygen mixture with lower gas density that converts turbulent flow into laminar flow, reducing Reynolds number). Perform Rapid Sequence Intubation using IV Ketamine (dissociative anesthetic that stimulates endogenous catecholamines and directly relaxes bronchial smooth muscle).

When an asthma or COPD patient is intubated and mechanically ventilated, severe expiratory airflow obstruction prevents the lungs from completely emptying before the next machine breath is delivered. Progressive gas trapping with each breath (breath stacking / dynamic hyperinflation) builds dangerously high positive end-expiratory alveolar pressure (intrinsic PEEP or auto-PEEP), visible on the ventilator flow-time waveform when expiratory flow fails to return to zero baseline prior to the next inspiration. If unrecognized, auto-PEEP causes two lethal complications: tension pneumothorax (barotrauma) and acute hypotension / PEA arrest (high intrathoracic pressure crushes the inferior vena cava and blocks venous return to the right heart). If an intubated asthma/COPD patient becomes acutely hypotensive, immediately disconnect the endotracheal tube from the ventilator circuit for 30–60 seconds and gently compress the chest wall to allow trapped air to exhale, then resume ventilation with a low respiratory rate (8–12 breaths/min), high inspiratory flow rate (80–100 L/min), and prolonged expiratory time (I:E ratio 1:4 or 1:5).