A flow-volume loop plots airflow against lung volume during a forced breath. A scooped, concave expiratory limb means obstruction (COPD, asthma); a tall, narrow, convex "witch's hat" means restriction. Pick a preset or drag the sliders below and read the shape back in plain language.
Concave, scooped expiratory limb with a low FEV1/FVC ratio — COPD, asthma. The lower the ratio, the deeper the scoop.
Tall, narrow, convex loop (witch's hat) with a preserved ratio — small FVC, confirmed by a reduced TLC.
Flattened plateau on one or both limbs. FEF50/FIF50 >1 intrathoracic, ~1 fixed, <1 extrathoracic.
Small, rounded loop with blunted flows in both directions — think effort or muscle strength before parenchymal disease.
Every value here is a simplified illustration, not a real measurement. Loop shapes are schematic; predicted values use teaching-level linear equations (height, age, sex), not the GLI-2012 splines a real lab uses. Real interpretation follows ATS/ERS standards with lab-specific reference equations and the actual tracing.
A concave, scooped expiratory limb — caving toward the volume axis — means obstruction (COPD or asthma), alongside a reduced FEV1/FVC ratio. Flow drops fast after the peak and then caves inward.
Obstruction is concave/scooped with a low FEV1/FVC ratio. Restriction is tall, narrow, and convex — a 'witch's hat' — with a preserved ratio because FEV1 and FVC shrink together.
It compares mid-expiratory to mid-inspiratory flow to localize an upper-airway lesion: greater than 1 suggests variable intrathoracic obstruction, about 1 suggests fixed obstruction, and less than 1 suggests variable extrathoracic (or neuromuscular) obstruction.
The loop shows the first second best, while the volume-time (spirogram) curve shows the tail of the exhalation and the forced expiratory time (FET) — together they give the fuller picture.