Increased Lung Compliance is Associated with Obstructive Lung Disease
The correct answer is A. Increased lung compliance. Obstructive lung disease characterized by decreased FEV1/FVC ratio and decreased vital capacity is associated with increased lung compliance due to loss of elastic recoil in the lungs 1.
Pathophysiology of Obstructive Lung Disease
Obstructive lung disease is characterized by:
- Decreased FEV1/FVC ratio (defining feature)
- Airflow limitation that is not fully reversible
- Abnormal inflammatory response of the lungs to noxious particles or gases
In this 35-year-old patient with decreased vital capacity and decreased FEV1/FVC ratio, the pathophysiological changes include:
Increased lung compliance: The reduced elastic recoil in obstructive lung disease results in increased lung compliance 1. This allows for hyperinflation with increases in residual volume (RV) and total lung capacity (TLC).
Airway obstruction mechanism: The obstructive impairment (reduced FEV1/FVC ratio) is primarily due to loss of elastic recoil from parenchymal disease (emphysema) with dynamic collapse of airways 1.
Flow-volume curve changes: The flow–volume curves typically show a marked decrease in flow with decreasing lung volumes, evidenced by concavity of the expiratory portion of the flow curve 1.
Why Other Options Are Incorrect
B. Decreased lung compliance: This is characteristic of restrictive lung diseases, not obstructive diseases. Restrictive diseases show decreased TLC with preserved or increased FEV1/FVC ratio 1.
C. Weak respiratory muscles: While respiratory muscle weakness can occur in advanced COPD as a secondary effect 1, it is not the primary mechanism causing the decreased FEV1/FVC ratio. Diaphragmatic atrophy may occur in some cases of advanced disease 1, but this is a consequence rather than a cause.
D. Increased TLC: While TLC is often increased in obstructive lung disease due to air trapping 1, this is a consequence of the increased lung compliance, not an independent association. The question asks what is associated with the condition, and increased lung compliance is the primary physiological abnormality.
Clinical Implications
The increased lung compliance in obstructive lung disease leads to several important clinical consequences:
- Hyperinflation with increased residual volume
- Air trapping (static lung volumes measured by body plethysmography are usually greater than those measured by dilution techniques) 1
- Impaired gas exchange with reduction in diffusing capacity
- Widening of the alveolar–arterial gradient for oxygen
Classification of Severity
According to the GOLD classification, obstructive lung disease severity can be categorized based on post-bronchodilator FEV1 percentage predicted 1:
| Severity | Post-bronchodilator FEV1 % predicted | FEV1/FVC |
|---|---|---|
| Mild COPD | ≥80% | ≤0.7 |
| Moderate COPD | 50-80% | ≤0.7 |
| Severe COPD | 30-50% | ≤0.7 |
| Very severe COPD | <30% | ≤0.7 |
For this 35-year-old patient, further evaluation including complete pulmonary function tests with lung volumes and diffusing capacity would help determine the severity and specific phenotype of obstructive lung disease.