Respiratory

Flow-volume loops

Obstruction, restriction and a lesion in the large airway produce three unmistakable shapes, and the exam asks you to tell them apart from the picture alone. Every trace here is generated from a description of the lung, not traced from a printout.

The trace

Obstructive

Normal shown dashed

FVC

4.0 L

vital capacity

FEV1

1.8 L

first second

FEV1 / FVC

0.44

below 0.70

These three are integrated off the curve above rather than written in by hand, so if the shape changes the numbers change with it.

What you are looking at

The shape

Peak flow is blunted and the expiratory limb is scooped out, sagging well below the straight line. The whole loop sits shifted toward higher lung volumes.

Why it looks like that

The mechanism

Loss of elastic recoil and inflammation leave small airways without the radial traction that holds them open, so they narrow as the lung empties and flow falls away faster than volume. Air trapped behind them raises residual volume, which is the leftward shift.

Asthma, chronic obstructive pulmonary disease. Ratio falls well below 0.7.

All six

The comparison is the point

Same scale

Drawn on one shared axis so the sizes are honest. The restrictive loop really is that much smaller, and the obstructive loop really does sit further to the left, hanging on to air it cannot push out.

The two questions

How to read one in ten seconds

Is a limb flat? If either limb is flattened into a plateau, stop thinking about asthma and fibrosis: this is a large airway. Inspiration flat means the lesion is above the sternal notch. Expiration flat means it is inside the chest. Both flat means it is rigid, and the loop cannot tell you where it sits.
Otherwise, is the expiratory limb scooped or just small? Scooped and shifted toward bigger volumes is obstruction. The same shape in miniature, shifted toward smaller volumes, is restriction. The ratio settles it: obstruction drops it below 0.70, restriction leaves it alone because both numbers shrink together.

These curves are generated from a small model of the lung that we wrote, and the derived numbers are integrated from the curves themselves. They are teaching illustrations, not measurements from a real patient, and nothing here is medical advice. How this content is written.