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Figure 3 Top panel: Flow (top trace), volume (middle trace) and mouth pressure (Pm, bottom trace) vs. time. Negative expiratory pressure (NEP) is applied during the last expiration shown (Pm trace). The ensuing expiratory flow is not different from that observed during the preceding control expiration, defining flow limitation (FL). Bottom panel: Examples of flow-volume curves in response to NEP (arrows) with corresponding curves of preceding control breaths. In the patient without FL (nFL, left), the expiratory flow-volume curve with NEP is above that of the control breathing cycle throughout expiration. In the patient with partial FL (pFL, middle), the expiratory flow-volume curve with NEP is superimposed on the latter part of the corresponding control expiratory flow-volume curve. In the patient with complete FL (cFL, right), the expiratory flow-volume curve with NEP is superimposed on the whole control curve, except for an initial transient increase in flow. See text for further details. cFL = completely flow limited; FRC = functional residual capacity; NEP = negative expiratory pressure; nFL = not flow limited; pFL = partially flow limited; Pm = mouth pressure.
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