The volume-time graph

The beginning of modern pulmonary function testing can be attributed to John Hutchinson in 1846. Hutchinson, was an English physician who turned a common gasometer into a device for measuring exhaled air from human beings – ultimately, inventing the spirometer.

When performing spirometry, volume can be plotted against time to attain a graphical pattern, known as a volume-time trace or Tiffeneau graph. Your spirometer may also allow measurements of the inspired volume of air as well as a graphical display of the flow rates with respect to volume, which is known as a flow-volume loop.

When performing a FVC manoeuvre, following a full inspiration the subject is required to blow as sharply as possible. This produces a curve on the graph. For a normal volume-time curve it has a rapid upslope. It then reaches a point where it plateaus.

Volume-time graph

When the curve plateaus the subject has expired completely and has therefore reached their residual volume (RV). The maximum volume attained, at the point where the expiratory trace flattens, or plateaus, represents the forced vital capacity (FVC).

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