From the Guidelines
Air trapping in COPD primarily causes increased dead space ventilation, not increased shunting. When air becomes trapped in the lungs due to airflow obstruction and loss of elastic recoil, areas of the lung receive ventilation but have reduced perfusion, creating ventilation-perfusion (V/Q) mismatch, as noted in the European Respiratory Society task force report 1. This results in physiologic dead space, where air moves in and out but doesn't participate effectively in gas exchange. The trapped air occupies space that could otherwise be used for effective ventilation, leading to increased work of breathing and decreased exercise tolerance. Unlike shunting (which occurs when blood passes through the lungs without being oxygenated), air trapping represents the opposite problem - ventilated areas with inadequate blood flow. Key points to consider include:
- The increase in functional residual capacity (FRC) due to static and dynamic factors at the end of expiration, leading to dynamic pulmonary hyperinflation 1.
- The presence of high V'/Q' units, which probably represent emphysematous regions with alveolar destruction and loss of pulmonary vasculature, contributing to dead space ventilation 1.
- The absence of significant shunt, suggesting that collateral ventilation and hypoxic pulmonary vasoconstriction are very efficient, or that airway occlusion is not functionally complete 1.
- The correlation between routine lung function tests and respiratory blood gases or patterns of V '/Q' distribution is poor, but significant hypoxaemia or hypercapnia is rare with FEV1 >1.0 L 1. This dead space ventilation contributes to hypercapnia (CO2 retention) in advanced COPD as patients struggle to clear carbon dioxide from poorly perfused lung regions. The mechanism involves both structural changes in small airways and parenchymal destruction, which together create air trapping during expiration.
From the Research
Air Trapping in COPD
- Air trapping in COPD is associated with an increased risk of exacerbation, as shown in a prospective cohort study 2.
- The study found that a 10% increase in the ratio of residual volume to total lung capacity (RV/TLC) increased the risk of moderate to severe exacerbation by 35% and severe exacerbation by 36% 2.
Effects of Air Trapping on Dead Space and Shunting
- There is no direct evidence in the provided studies to suggest that air trapping in COPD causes increased dead space or increased shunting.
- However, air trapping is known to lead to hyperinflation, which can result in increased dead space ventilation 3.
- The use of bronchodilators, such as beta-agonists and anticholinergics, can help improve lung function and reduce hyperinflation, but their effect on dead space and shunting is not clearly established in the provided studies 4, 3, 5.
Treatment of COPD
- The treatment of COPD typically involves the use of bronchodilators, such as beta-agonists and anticholinergics, to improve lung function and reduce symptoms 4, 6, 3, 5.
- The choice of bronchodilator and the addition of other medications, such as inhaled corticosteroids, depend on the severity of symptoms and the frequency of exacerbations 6.