Expiratory flow limitation during tidal breathing.

Dr KOULOURIS and co-workers [1] found that some patients with chronic obstructive pulmonary disease (COPD) and apparent expiratory flow-limitation during tidal breathing had increased expiratory flow during tidal breathing when negative pressure was applied to the airway. They concluded that flow could not have been limited during control tidal expirations. I thought that the shape of the flow-volume curve in such patients during tidal breathing indicates expiratory flow-limitation. That is, the tidal expiratory flow-volume curve is not half of a oval, which is the normal shape; instead, it has a flow peak early in expiration and decreases in parallel to the expiratory flow-volume curve of a forced vital capacity. The authors show a tidal breathing loop like this in their figure 2 [1]. Why should the tidal loop be chopped if flow is not limited? I suggest that the application of negative pressure to the airways moves the choke point back into slightly larger airways, with a resultant increase in flow. The increase in flow with application of negative pressure may not, therefore, prove the absence of flow-limitation. A much less likely explanation for chopped expiratory tidal breathing curves is that expiration is not flow-limited, but that the patients subconsciously brake expiratory flow rates just below actual flow-limitation