Routine Lung Function Tests

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17 Table 3. Lung Function Indices Capable of Differentiating Extrathoracic from Intrathoracic Obstruction in Adults Extrathoracic Obstruction Intrathoracic Obstruction Fixed Variable PEF Decreased Normal or decreased Decreased Forced inspiratory flow (FIF) 50 Decreased Decreased Normal or decreased FIF 50 /Forced expiratory flow (FEF) 50 ~1 < 1 > 1 Table 2. Summary of Types of Spirometrically Defined and Lung Volume Defined Ventilatory Impairments Ventilatory Impairments Patterns Restriction • FVC < 5th percentile. • Reduced FVC does not prove restrictive impairment but may be suggestive of restriction when FEV 1 /FVC is normal or increased. • Low TLC from single breath test not reliable, especially with low FEV 1 /FVC. • A normal FVC usually excludes restriction. Obstruction • FEV 1 /FVC < 5th percentile. • Decrease in flow at low lung volume may reflect small airway disease in individuals. • Concomitant decrease in FEV 1 and FVC most commonly due to poor effort but may reflect airflow obstruction. Recommend lung volumes. • Measurement of absolute lung volumes may assist in diagnosis and assessment of hyperinflation. • Measurement of airflow resistance may assist in diagnosis. Mixed FEV 1 /FVC and TLC both < 5th percentile.

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