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Difference between functional residual capacity and elastic equilibrium volume in patients with chronic obstructive pulmonary disease.

BACKGROUND: A study was performed to determine the elastic equilibrium volume (Vr) of the respiratory system in patients with chronic obstructive pulmonary disease (COPD). METHODS: Voluntary relaxed expiration from total lung capacity (TLC) was studied in three groups of subjects: seven patients with severe chronic airways obstruction (COPD), 10 normal subjects, and 15 subjects with restrictive disease. RESULTS: In the normal subjects and the patients with restrictive disease voluntary relaxed expiration from TLC stopped close to end tidal volume (FRC) and the volume expired in this manoeuvre was less than that expired in a slow vital capacity manoeuvre (SVC). In the patients with COPD the voluntary relaxed expiration continued beyond the end tidal volume (FRC) and the volume expired was not different from the SVC. Oesophageal (pleural) pressures and surface diaphragmatic EMG recordings in the patients with COPD supported the premise that relaxation was achieved. CONCLUSIONS: In patients with COPD, end tidal volume (FRC) is higher than the elastic equilibrium volume, Vr, of the respiratory system. This is in contrast to patients with restrictive disease and normal subjects in whom end tidal volume (FRC) is close to Vr. This study shows that, in patients with severe chronic obstructive pulmonary disease, Vr is at least as small as residual volume (RV).