Figure 25-3.

Spirogram obtained in a normal subject showing maneuvers to determine vital capacity and forced vital capacity.

On the tracing shown on the left, the subject breathes quietly (slow recording speed), then takes a maximal inspiration followed by a maximal expiration without concern for time (vital capacity). On the tracing shown to the right, after a maximal inspiration (not shown), with a rapid recording speed, the subject then exhales completely, forcefully, and as rapidly as possible (forced vital capacity).