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Isovolume (low-lung volume) flow-pressure relationship in normal subjects, COPD without expiratory flow limitation (NFL) and COPD with expiratory flow limitation (FL). In any case, after P crit, expiratory flow does not increase further on, and its driving pressure becomes P el. In COPD patients with high airflow resistance and very low P el, the P crit occurs early, limiting expiratory flow in the tidal volume range.
