8.1 Obstructive Airway Diseases: Asthma & COPD
Key Takeaways
- Spirometry differentiates asthma (reversible airflow obstruction with ≥12% and ≥200 mL increase in FEV1 post-bronchodilator) from COPD (irreversible post-bronchodilator FEV1/FVC < 0.70).
- A normal or elevated PaCO2 in an acute asthma exacerbation indicates respiratory muscle fatigue and impending respiratory failure requiring immediate ICU evaluation and intubation prep.
- Smoking cessation and long-term home oxygen therapy (resting PaO2 ≤ 55 mmHg or SaO2 ≤ 88%) are the only non-surgical interventions proven to reduce mortality in COPD.
- Indications for antibiotics in acute COPD exacerbations require at least 2 cardinal symptoms (one being increased sputum purulence) or mechanical ventilation.
- Noninvasive positive pressure ventilation (BiPAP) is first-line for acute hypercapnic respiratory acidosis (pH < 7.35, PaCO2 > 45 mmHg) in COPD exacerbations.
8.1 Obstructive Airway Diseases: Asthma & COPD
Obstructive airway diseases are characterized by limitation of expiratory airflow due to partial or complete obstruction at any level of the bronchial tree. On the USMLE Step 2 CK, differentiating asthma from chronic obstructive pulmonary disease (COPD), applying current Global Initiative for Asthma (GINA) and GOLD treatment algorithms, and managing acute life-threatening exacerbations are high-yield testable topics.
Differential Diagnosis: Asthma vs COPD
The fundamental physiological distinction between asthma and COPD lies in the reversibility of airflow limitation and the underlying inflammatory process. Spirometry remains the gold standard diagnostic test for both conditions.
| Feature | Asthma | COPD (Chronic Bronchitis / Emphysema) |
|---|---|---|
| Primary Age of Onset | Typically childhood or young adulthood | Typically >40 years old (history of smoking) |
| Airflow Obstruction | Reversible (spontaneous or post-bronchodilator) | Irreversible or partially reversible |
| Post-Bronchodilator Spirometry | FEV1 increases by ≥12% AND ≥200 mL | Post-bronchodilator FEV1/FVC < 0.70 |
| Predominant Inflammation | Eosinophils, CD4+ T-cells, IgE, Mast cells | Neutrophils, CD8+ T-cells, Macrophages |
| DLCO (Diffusing Capacity) | Normal or Elevated (increased pulmonary blood flow) | Decreased in Emphysema; normal in Chronic Bronchitis |
| Primary Risk Factor | Atopy, family history, environmental allergens | Tobacco smoking, Alpha-1 antitrypsin deficiency |
| Diurnal Variation | Worsening at night or early morning | Persistent symptoms with gradual progression |
Diagnostic Evaluation & Spirometry
Diagnostic Tests
- Spirometry: Demonstrates an obstructive defect (FEV1/FVC < 0.70). Reversibility is confirmed if post-bronchodilator testing shows an increase in FEV1 by ≥12% AND ≥200 mL.
- Bronchoprovocation (Methacholine Challenge): Utilized when baseline spirometry is normal but asthma is clinically suspected. A positive challenge is defined by a ≥20% decrease in FEV1 at low methacholine concentrations (high sensitivity, effectively rules out asthma if negative).
- Diffusing Capacity for Carbon Monoxide (DLCO):
- Emphysema: Decreased DLCO due to destruction of alveolar-capillary membrane interalveolar septa.
- Chronic Bronchitis: Normal DLCO (airway disease without alveolar destruction).
- Asthma: Normal or elevated DLCO (increased pulmonary capillary blood volume driven by negative intrathoracic pressure swings).
Asthma Management Guidelines (GINA / NAEPP)
Modern GINA guidelines emphasize avoiding SABA monotherapy due to increased risk of severe exacerbations and asthma-related death. Inhaled corticosteroid (ICS)-containing therapy is indicated for all adults and adolescents with asthma.
Stepwise Pharmacotherapy Algorithm
- Step 1 (Mild Intermittent): As-needed low-dose ICS-formoterol (SMART strategy: Single Maintenance and Reliever Therapy) OR low-dose ICS taken whenever SABA (albuterol) is administered.
- Step 2 (Mild Persistent): Daily low-dose ICS + as-needed SABA OR as-needed low-dose ICS-formoterol.
- Step 3 (Moderate Persistent): Daily low-dose ICS-LABA (e.g., budesonide-formoterol or fluticasone-salmeterol) + as-needed SABA/SMART.
- Step 4 (Severe Persistent): Daily medium-dose ICS-LABA + as-needed controller/reliever.
- Step 5 (Severe Refractory): Daily high-dose ICS-LABA + add-on therapy (LAMA e.g., tiotropium; biologic agents such as mepolizumab/benralizumab for anti-IL-5 eosinophilic asthma, omalizumab for anti-IgE, or dupilumab for anti-IL-4Rα).
Acute Asthma Exacerbation Triage & Red Flags
Clinical Presentation & Alarm Features
- Symptoms: Severe dyspnea, tachypnea (RR >30/min), accessory muscle use, diaphoresis, inability to speak in full sentences, and pulsus paradoxus (>10 mmHg decrease in systolic blood pressure during inspiration).
- CRITICAL USMLE ALARM SIGN: A normal or elevated PaCO2 (e.g., PaCO2 ≥ 40-45 mmHg) on arterial blood gas in a patient with severe acute asthma indicates impending respiratory muscle fatigue and respiratory arrest! Hyperventilation during an acute asthma attack normally causes a low PaCO2 (respiratory alkalosis). A "normal" PaCO2 reflects fatigue and requires immediate intensive care unit (ICU) admission and preparation for rapid sequence intubation.
Emergency Medical Management
- Oxygen Therapy: Supplemental O2 targeting SpO2 93-95% (92-95% in pregnancy).
- Inhaled Short-Acting Beta-2 Agonist (SABA): Albuterol nebs or MDI every 20 minutes for the first hour.
- Inhaled Anticholinergic: Ipratropium bromide added to albuterol for moderate-to-severe exacerbations.
- Systemic Corticosteroids: Oral prednisone (40-50 mg) or IV methylprednisolone administered early to reduce hospital admission and relapse rates.
- Intravenous Magnesium Sulfate: Single IV dose (2 g over 20 min) for severe exacerbations unresponsive to initial inhaled bronchodilator therapy after 1 hour.
- Intubation & Mechanical Ventilation: Indicated for altered mental status, silent chest (complete absence of wheezing due to severe airflow limitation), worsening hypercapnia/acidosis, or cardiac arrest.
Chronic Obstructive Pulmonary Disease (COPD)
Etiology & Pathophysiology
COPD encompasses chronic bronchitis (airway inflammation, hypersecretion of mucus, cough for ≥3 consecutive months in 2 consecutive years) and emphysema (permanent enlargement of airspaces distal to terminal bronchioles with wall destruction).
- Alpha-1 Antitrypsin (AAT) Deficiency: Suspect in non-smokers <45 years old or patients presenting with panacinar emphysema predominant in the lower lobes, often accompanied by liver disease (cirrhosis/hepatocellular carcinoma).
GOLD Classification & Management
- Diagnostic Requirement: Post-bronchodilator FEV1/FVC < 0.70 is mandatory to establish non-reversible airflow limitation.
- GOLD Staging (based on FEV1 % predicted):
- GOLD 1 (Mild): FEV1 ≥ 80% predicted
- GOLD 2 (Moderate): 50% ≤ FEV1 < 80%
- GOLD 3 (Severe): 30% ≤ FEV1 < 50%
- GOLD 4 (Very Severe): FEV1 < 30%
- GOLD ABE Groups:
- Group A (Low symptoms, 0-1 exacerbation/yr without hospitalization): Single bronchodilator (SABA or LABA/LAMA).
- Group B (High symptoms, 0-1 exacerbation/yr without hospitalization): Dual LABA + LAMA combination.
- Group E (High risk, ≥2 exacerbations/yr OR ≥1 hospitalization): Dual LABA + LAMA combination (add ICS if blood eosinophils ≥300 cells/μL).
Interventions Proven to Reduce COPD Mortality
- Smoking Cessation: The single most effective intervention to slow the rate of FEV1 decline.
- Long-Term Oxygen Therapy (LTOT):
- Indications:
- Resting PaO2 ≤ 55 mmHg OR SaO2 ≤ 88%.
- Resting PaO2 56-59 mmHg OR SaO2 89% IF there is evidence of cor pulmonale, right heart failure, or secondary erythrocytosis (hematocrit > 55%).
- Goal: Maintain SaO2 ≥ 90% or PaO2 ≥ 60 mmHg during rest, sleep, and exercise for at least 15 hours/day.
- Indications:
- Lung Volume Reduction Surgery (LVRS): Selected patients with predominant upper-lobe emphysema and low baseline exercise capacity.
Acute COPD Exacerbation Management
- Triggers: Tracheobronchial viral or bacterial infections (H. influenzae, S. pneumoniae, M. catarrhalis, Pseudomonas in severe cases).
- Cardinal Symptoms: Increased dyspnea, increased sputum volume, increased sputum purulence.
- Indications for Antibiotics (e.g., Azithromycin, Amoxicillin-clavulanate, Doxycycline):
- Presence of all 3 cardinal symptoms.
- Presence of 2 cardinal symptoms IF one symptom is increased sputum purulence.
- Any patient requiring mechanical ventilation (invasive or non-invasive).
- Indications for Noninvasive Positive Pressure Ventilation (NIV / BiPAP):
- Acute hypercapnic respiratory acidosis (pH < 7.35 and PaCO2 > 45 mmHg) despite maximal medical therapy.
- Reduces intubation rate, length of stay, and mortality (first-line intervention before endotracheal intubation).
- Supplemental Oxygen Target: Target SpO2 88-92% (PaO2 ~60 mmHg) to avoid worsening hypercapnia caused by loss of hypoxic pulmonary vasoconstriction (V/Q mismatching) and the Haldane effect.
A 24-year-old woman presents to the emergency department with acute shortness of breath and wheezing. She has a history of asthma managed with low-dose inhaled budesonide. On examination, she is in moderate respiratory distress, using intercostal accessory muscles. Respiratory rate is 32/min, blood pressure is 120/78 mmHg, and pulse is 118/min. Auscultation reveals diffuse bilateral expiratory wheezes. Arterial blood gas on room air shows: pH 7.42, PaCO2 40 mmHg, PaO2 72 mmHg, and HCO3- 25 mEq/L. Inhaled albuterol and ipratropium have been administered. Which of the following is the most appropriate next step in management?
A 66-year-old man with a 45-pack-year history of cigarette smoking presents for a routine follow-up. He reports progressive dyspnea on exertion over the past 3 years and a daily productive cough. Post-bronchodilator spirometry demonstrates an FEV1/FVC ratio of 0.58 and an FEV1 of 48% of predicted. Pulse oximetry on room air at rest reveals an SpO2 of 87% (PaO2 54 mmHg). Physical examination demonstrates bilateral decreased breath sounds, barrel chest, and trace ankle edema. Which of the following interventions is most likely to prolong this patient's survival?