Clinical meaning
Pulmonary function tests (PFTs) measure lung volumes, airflow rates, and gas exchange capacity to characterize respiratory pathology. Spirometry, the most fundamental PFT, measures FVC (forced vital capacity — total air exhaled forcefully after full inspiration), FEV1 (volume exhaled in the first second), and the FEV1/FVC ratio. The FEV1/FVC ratio differentiates obstructive from restrictive patterns: a ratio < 0.70 (or below lower limit of normal) indicates obstruction (airflow limitation during exhalation — COPD, asthma), while a normal or elevated ratio with reduced FVC suggests restriction (reduced lung volume — pulmonary fibrosis, chest wall disease, neuromuscular weakness). Diffusing capacity (DLCO) measures the lung's ability to transfer gas across the alveolar-capillary membrane, reduced in emphysema (loss of alveolar surface area) and interstitial lung disease (thickened membrane) but preserved or increased in asthma.
Diagnosis & workup
Diagnostics & workup: - Spirometry: FVC, FEV1, FEV1/FVC ratio — the foundation of PFT interpretation - Bronchodilator response: repeat spirometry after albuterol — ≥12% AND ≥200 mL FEV1 improvement = reversible obstruction (asthma) - Lung volumes: TLC (total lung capacity) — elevated in hyperinflation (COPD), reduced in restriction - DLCO: reduced in emphysema and ILD; normal or elevated in asthma - Flow-volume loops: characteristic patterns for upper airway obstruction (variable vs fixed) - Maximal inspiratory/expiratory pressures (MIP/MEP) for neuromuscular respiratory weakness