How can hyperinflation due to asthma be differentiated from hyperinflation due to chronic obstructive pulmonary disease (COPD)?

Differentiating Hyperinflation: Asthma vs COPD

The key distinction is that asthma-related hyperinflation is typically reversible and variable, while COPD hyperinflation is persistent and progressive, best differentiated by demonstrating >20% variability in serial peak expiratory flow measurements and substantial bronchodilator reversibility in asthma versus fixed obstruction with little variability in COPD. 1

Clinical History: The Foundation of Differentiation

Age and smoking history are critical discriminators:

• COPD patients typically have significant smoking history (>40 pack-years strongly suggests COPD) and present in middle to late adulthood 2
• Asthma often begins in childhood or has a history of childhood wheeze, atopy, or pertussis 1
• A past history of childhood wheeze, bronchitis, or atopy suggests asthma rather than COPD 1

Symptom patterns differ fundamentally:

• Asthma shows marked day-to-day and diurnal variability in symptoms and wheezing 2
• COPD presents with progressive, persistent breathlessness that develops gradually over years 2
• Morning cough productive of sputum for 3+ months in 2 consecutive years defines chronic bronchitis (COPD feature) 2

Spirometry and Bronchodilator Reversibility: The Definitive Test

Perform formal bronchodilator reversibility testing when patients are clinically stable and free from infection 1:

Asthma characteristics:

• More than 20% variability in absolute measurements of serial peak expiratory flow (PEF) suggests asthma 1
• Substantial increase in FEV1 after bronchodilator administration indicates asthma 1
• The objective is to detect those whose FEV1 increases substantially—these patients are truly asthmatic 1

COPD characteristics:

• An abnormal FEV1 (<80% predicted) with FEV1/VC ratio <70% and little variability in serial PEF strongly suggests COPD 1
• Post-bronchodilator FEV1/FVC ratio <0.70 confirms COPD diagnosis 2
• The post-bronchodilator FEV1 is the best predictor of long-term prognosis in COPD 1

Critical caveat: When PEF is low, spontaneous variability of the measurement may exceed 20%, potentially mimicking asthma 1

Pathophysiological Mechanisms: Understanding the Underlying Difference

COPD hyperinflation has two distinct components:

• Static hyperinflation: Results from destruction of alveolar walls (emphysema), causing loss of lung elastic recoil, decreased carbon monoxide transfer capacity, and increased total lung capacity 3
• Dynamic hyperinflation: Caused by expiratory flow limitation leading to incomplete expiration and intrinsic PEEP (PEEPi) 4, 5

Asthma hyperinflation is primarily dynamic:

• Occurs acutely during bronchoconstriction when expiratory flow limitation develops during tidal breathing 6
• FRC increases to allow breathing at higher flows when flow limitation occurs 6
• The increase in FRC during induced bronchoconstriction in asthma is closely associated with the occurrence of flow limitation 6
• Decreases following bronchodilation as flow limitation disappears 6

Physical Examination Findings

Both conditions may show signs of chronic overinflation 1, 3:

• Loss of cardiac dullness on percussion
• Decreased cricosternal distance
• Increased anterior-posterior diameter of the chest
• Hyperresonance on percussion 2

COPD-specific findings suggesting irreversible disease:

• Weight loss and anorexia are common in advanced COPD 2
• Hoover sign (paradoxical inward movement of lower ribcage during inspiration) indicates severe diaphragmatic flattening and predicts exacerbations 2
• "Blue and bloated" phenotype with hypoxemia, hypercapnia, peripheral edema, and cor pulmonale 1
• Flapping tremor, bounding pulse, drowsiness (signs of hypercapnia) may occur in severe COPD 1

Imaging Considerations

Chest radiography helps identify structural changes:

• Emphysematous bullae on chest x-ray suggest COPD 7
• Flattened diaphragms, increased retrosternal airspace, and hyperlucent lung fields indicate COPD 3
• It is not possible to diagnose mild emphysema radiographically 1

Important note: Dynamic hyperinflation can significantly affect quantitative CT measurements, potentially mimicking disease progression in COPD 8

Practical Algorithm for Differentiation

1. Obtain detailed smoking and atopy history 1, 2
2. Perform spirometry with bronchodilator testing when patient is stable 1
3. Measure serial PEF over 2-4 weeks to assess variability 1
4. If >20% PEF variability OR substantial FEV1 improvement: Diagnose asthma 1
5. If FEV1/FVC <0.70 with minimal variability: Diagnose COPD 1, 2
6. Consider chest radiography to identify emphysematous changes or exclude alternative diagnoses 7

Common Pitfalls to Avoid

• Do not rely on symptoms or physical signs alone—the degree of airways obstruction cannot be predicted from clinical findings 1
• Do not assume all wheezing is asthma; COPD commonly presents with wheezes (rhonchi), especially on forced expiration 1, 3
• Do not perform reversibility testing during acute exacerbations or active infections 1
• Do not forget that some patients may have both conditions (asthma-COPD overlap), requiring careful longitudinal assessment 1