 Spirometry

Spirometry measures the volume of air moved into or out of the respiratory system as a function of time. A spirometer records displaced gas directly or derives volume by integrating volumetric flow rate. The resulting trace describes gas movement at the mouth; it does not independently determine alveolar pressure, pleural pressure or lung compliance.^[ref]

Two important outputs of a forced expiration following maximal inspiration are forced vital capacity, the total volume expelled in that maneuver, and forced expiratory volume in one second, the volume expelled during its first second. Their ratio compares early emptying with the eventual expelled volume.^[ref] Equal final volumes can accompany substantially different emptying rates.

An original mathematical example sets expelled volumewhere the numerical time $t$ and time constant $\tau$ are in seconds. Both $\tau =1$ and $\tau =2$ give an asymptotic expelled volume of 4 L. Their first-second volumes are instead $4(1-e^{-1})\approx 2.53$ L and $4(1-e^{-1/2})\approx 1.57$ L. The corresponding ratios to the limiting volume are approximately 0.632 and 0.393. These smooth toy curves explain the measurements; they are not measured traces or diagnostic reference limits.

Spirometry measures volume differences. The gas left after maximal expiration is residual lung volume, which has not crossed the instrument. Consequently a volume trace alone does not determine total lung capacity: different absolute starting volumes can produce the same expelled-volume trace. This is the absolute-volume ambiguity in spirometry.

Timing, effort, leaks and early termination can alter what a trace represents. Distinguish an instrumentally repeatable signal from a maneuver that answers the intended physiological question. The primary technical statement treats equipment quality assurance and maneuver acceptability separately.^[ref]