FEV1/FVC ratio

The FEV1/FVC ratio is a key spirometric parameter in pulmonary function testing. FEV1 measures the volume of air exhaled in the first second of a forced expiration, while FVC is the total volume exhaled forcefully from total lung capacity. In obstructive diseases like COPD and asthma, air trapping occurs due to loss of elastic recoil or airway collapse, causing FEV1 to disproportionately decrease relative to FVC, resulting in a ratio <0.70 (70%). In restrictive diseases (pulmonary fibrosis, chest wall disorders), both FEV1 and FVC decrease proportionally, maintaining a normal or elevated ratio. This distinction is crucial for differential diagnosis and guides treatment: obstructive patterns warrant bronchodilators and corticosteroids, while restrictive patterns require investigation of the underlying cause (interstitial lung disease, neuromuscular weakness). The FEV1 alone also predicts mortality in COPD—an FEV1 <25% predicted indicates severe GOLD Stage IV disease requiring aggressive management.