What is air trapping in the context of respiratory conditions like asthma and chronic obstructive pulmonary disease (COPD)?

Air Trapping in Respiratory Conditions

Air trapping is the abnormal retention of air in the lungs due to airway obstruction, particularly in small airways, resulting in areas of hyperinflation that fail to empty properly during exhalation. 1

Pathophysiology of Air Trapping

Air trapping occurs through several mechanisms:

• Small airway luminal narrowing caused by inflammation and remodeling, leading to increased airway resistance and reduced expiratory flow 1
• Small airway collapse due to loss of supporting alveolar attachments, particularly in COPD 1
• Destruction of peripheral airways and parenchyma, reducing elastic recoil of the lungs during expiration 1
• Expiratory flow limitation with small airway collapse during tidal breathing 1

These pathological changes result in:

• Increased small airway resistance 1
• Reduced expiratory flow 1
• Gas trapping on expiration 1

Clinical Significance

Air trapping has significant clinical implications:

• Associated with lung function impairment in patients with small airways disease, including asthma and COPD 1
• Contributes to dyspnea, especially during exertion, as gas trapping worsens with exercise due to hyperventilation-related reduction in expiratory time 1
• Causes dynamic lung hyperinflation and increased work of breathing 1
• Creates a vicious cycle of activity avoidance, physical deconditioning, and reduced quality of life 2
• May have implications for early development of comorbidities such as cardiovascular disease 2

Detection and Measurement

Air trapping can be detected through various imaging techniques:

• Computed tomography (CT) is the primary method for visualizing air trapping 1

  • Appears as geographic areas of differing attenuation within lung parenchyma 3
  • Inspiratory and full expiratory phase CT studies are required for accurate diagnosis 3
  • Quantitative CT analysis can measure the extent of air trapping 4

• Chest radiography may show:

  • Lung hyperinflation 1
  • Hyperlucent areas in the lungs 1
  • Peripheral trimming of vascular markings 1
  • However, chest radiography is often normal in early disease 1

• Pulmonary function tests:

  • Decreased FEV1/FVC ratio indicating airflow obstruction 1
  • Increased residual volume 5
  • Ratio of forced expiratory vital capacity (FVC) to slow inspiratory vital capacity (iVC) can assess compressive air trapping 5

Air Trapping in Specific Respiratory Conditions

COPD

• Air trapping in COPD results from small airway narrowing, loss of elastic recoil, and premature airway closure 1
• Can lead to both static and dynamic lung hyperinflation 6
• Static hyperinflation occurs due to destruction of pulmonary parenchyma and loss of elastic recoil 6
• Dynamic hyperinflation develops when patients breathe in before achieving full exhalation, becoming more pronounced during exercise and exacerbations 6

Asthma

• Air trapping in asthma is associated with disease severity and instability 5
• Compressive air trapping (airway closure more extensive during forced exhalation than slow exhalation) occurs in most asthma patients over 50 years of age 5
• Affects men more than women and persists after bronchodilator treatment 5
• Associated with a history of asthma-related hospitalizations, ICU visits, and/or mechanical ventilation 4
• Risk factors include longer duration of asthma, history of pneumonia, high levels of airway neutrophils, airflow obstruction, and atopy 4

Other Conditions

• Air trapping can be seen in patients with history of COVID-19 infection 1
• Other small airways disease subtypes may also present with air trapping 1
• Common causes include constrictive bronchiolitis, hypersensitivity pneumonitis, and post-infectious conditions 3

Clinical Management

Several strategies can reduce the impact of air trapping:

• Long-acting bronchodilators reduce hyperinflation by decreasing flow limitation and improving lung emptying 2
• Exercise programs decrease respiratory rate and reduce ventilatory demand 2
• Combination of optimal bronchodilation and exercise can reduce exertional dyspnea and increase exercise capacity 2
• Early intervention with appropriate treatments is recommended, even in mild-to-moderate disease with persistent dyspnea 2

Imaging Considerations

• Inspiratory and expiratory CT chest may have a specific role in evaluating air trapping 1
• Xenon-MRI has shown promise in demonstrating impaired ventilation in patients with COPD, COVID-19 history, and small airways disease 1
• No consensus currently exists on accurate assessment of air trapping severity 3
• The ratio of mean lung density between expiration and inspiration on CT correlates with small airway disease 3