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Definitions

During pulmonary function testing, patients blow through tubing connected to a flowmeter.  This produces a graph of exhaled volume against time.  The graph is often presented as an idealized spirogram, showing tidal breathing followed by a forced vital maneuver ( Figure 1).  From the spirogram, 4 lung volumes and 4 capacities can be defined.

Lung volumes

Lung volumes are fundamental units (ie, they cannot be expressed in terms of each other):

  • Tidal volume (TV):  the volume of a normal, quiet breath.
  • Inspiratory reserve volume (IRV):  the volume that can be further inspired after normal tidal inhalation.
  • Expiratory reserve volume (ERV):  the volume that can be further expired after normal tidal exhalation.
  • Residual volume (RV):  the volume remaining in the lungs after maximal expiration. 
Lung capacities

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