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Answer

How would you distinguish obstructive lung disease from restrictive lung disease by spirometry?

Answer:

Table 1.1 summarizes the distinctions between the two types of lung diseases. In restrictive lung disease (e.g., pulmonary fibrosis and ankylosing spondylitis), the FVC is low because of limited expansion of the lungs or chest wall, although the FEV1 is often not reduced proportionately, because airway resistance is normal. Therefore, the FEV1/FVC percentage is normal or high.

In obstructive lung disease, the FEV1/FVC is grossly reduced because the airway resistance is high. Normally, FEV1 is more than 80% of FVC, and VC should be more than 80% of predicted value. The predicted values depend on body size, age, and sex. The FEV1/FVC is less than 0.70 in COPD. The severity of airflow limitation in COPD is determined by the FEV1. Patients with an FEV1 less than predicted but greater than or equal to 80% of predicted are considered to have mild COPD (Global Initiative for Chronic Obstructive Lung Disease [GOLD] 1). Those with FEV1 greater than or equal to 50% but less than 80% of predicted are classified as having moderate COPD (GOLD 2). Severe cases of COPD have an FEV1 that is greater than or equal to 30% but less than 50% of predicted (GOLD 3). Finally, patient with an FEV1 that is less than 30% of predicted have very severe COPD (GOLD 4).

The TLC is increased in obstructive lung disease and decreased in restrictive lung disease. However, TLC cannot be obtained by routine screening spirometry.


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