Immediately after the peak expiratory flow, the flow drops to low values until the end of the forced expiration. This pattern is compatible with severe airflow limitation. Expiratory flow during normal tidal breathing is at the same level as during a forced expiration; this signifies the presence of a flow limiting segment during normal respiration, so that each normal expiration is in fact a forced expiration at the expense of considerable work of breathing. The ventilatory reserves in this patient are therefore minimal. During physical exercise inspiratory flow can be relatively easily increased, unlike expiratory flow; therefore end-expiratory volume will increase sharply, exacerbating the hyperinflation in this patient.
This pattern is encountered in the case of loss of elastic lung recoil associated with dynamic compression of intrathoracic airways, such as occurs in pulmonary emphysema.
See also: Added value of tidal flow-volume curve After bronchodilatation