Pitfalls in spirometry: Clinical relevance

Figure 1 Relationship between maximum flow, volume and pleural pressure. Left panel: Flow-volume curves during inspiratory and expiratory manoeuvres. The arrows indicate the volume at 75%, 50%, and 25% vital capacity; Right panel: flow and pleural pressure (Ppl) relationships during expiratory manoeuvres at different lung volumes. At 25% and 50% vital capacity (VC) flow plateaus at different pressures (oblique arrows). In contrast, at 75% VC flow keeps increasing with the increase of pressure. The lack of increased expiratory flow at 25% and 50% VC despite the increase in Ppl supports the concept of expiratory flow limitation.