FVC
FVC is the grand total of air you can force out after filling your lungs as full as they will go — keep blowing until nothing more will come, and the cumulative volume is your forced vital capacity. It is the size of the breath you can move, from the very top to the very bottom of a maximal exhalation.
FVC shrinks whenever the lungs cannot expand fully — stiff, scarred lungs (fibrosis), a chest wall that will not move, or weak breathing muscles all cut it down. That is the fingerprint of restriction: a reduced FVC with a normal or even high FEV1/FVC ratio.
FVC can also read low in severe obstruction, not because the lungs are small but because air gets trapped and the patient gives up blowing before the lungs truly empty. To complete the blow, technicians often ask for at least six seconds of continuous exhalation, and a slow-vital-capacity manoeuvre may capture more in obstructed patients.
A patient with pulmonary fibrosis has an FVC of 2.1 L (about 60 percent of predicted) but an FEV1/FVC ratio of 0.85, the classic restrictive picture.
A reduced FVC with a preserved ratio suggests restriction.
A low FVC on spirometry only suggests restriction — it must be confirmed by directly measuring a reduced total lung capacity, because trapped air can mimic restriction in obstructive disease.