Fig. 16:  Before talking about hyperinflation, it is important to review measurement methods, in this case volume measurements with volume time spirometry.  At present this is usually done with flow-volume spirometry, but volume-time spirometry is actually more useful, as shown in the Figure.  When we breathe in, then breathe out, this is a normal tidal volume.  If a patient takes a full breath in and then breathes all the way out, this is the patient’s vital capacity, and the amount of air the patient cannot exhale is the residual volume.  The volume of air in the lung after a quiet exhalation is the functional residual capacity. 

What is hyperinflation?  When expiratory volume is measured during an exercise test, it is called the end-expiatory lung volume.  The volume from the end of a quiet exhalation to a full inspiration is the inspiratory capacity, and if this volume is diminished then the result is dynamic hyperinflation. 

Another important measurement is the inspiratory reserve volume, abbreviated often as the IRV; the IRV is the quantity measured from the end of inhalation to total lung capacity, and when this volume is <0.5 liter, the patient has intolerable dyspnea, which is related to hyperinflation.