The ratio that sorts diseases
The single most useful spirometry number is the FEV1/FVC ratio — what fraction of your full breath you can blow out in the first second. In a healthy adult this is roughly 0.7 to 0.8: most of the tank empties fast. A low ratio means the air is coming out too slowly relative to how much there is, which points to an obstructive pattern — narrowed or floppy airways, as in asthma or COPD.
When the ratio is normal or even high but both FEV1 and FVC are reduced together, the airways are fine — the lungs are simply small or stiff. That is a restrictive pattern, seen in scarring diseases and chest-wall problems. The trick to remember: obstruction lowers the ratio; restriction usually preserves it but shrinks the volumes.
Example A — FEV1 1.6 L (55% pred), FVC 3.6 L (95% pred) Ratio = 1.6 / 3.6 = 0.44 -> LOW Pattern: OBSTRUCTIVE (air comes out slowly; tank is normal-sized) Example B — FEV1 1.8 L (60% pred), FVC 2.2 L (58% pred) Ratio = 1.8 / 2.2 = 0.82 -> normal/high Pattern: RESTRICTIVE (both volumes small; emptying speed is fine) Rule of thumb: Low ratio -> obstruction Normal ratio + low FVC -> restriction (confirm with lung volumes)
Seeing the shape: the flow–volume loop
The flow–volume loop plots how fast air moves (flow) against how much has been breathed (volume), for both the breath out and the breath in. A healthy expiratory curve shoots up to a sharp peak — the peak expiratory flow — then descends in a nearly straight line back to zero. The whole loop looks clean and roughly triangular on top.
In obstruction the descending limb sags inward into a scooped or concave shape, because the small airways collapse and flow drops away faster than volume — this is where FEF25-75, the mid-expiratory flow, falls early. In restriction the loop keeps its normal shape but is narrow and tall, like a healthy loop that has been squeezed from the sides. A fixed bump or plateau on the loop can even hint at a blockage in the windpipe itself.