High tidal end expiratory flow – an index of dynamic hyperinflation?

Dynamic hyperinflation is considered an important mechanism behind shortness of breath and reduced exercise capacity in chronic obstructive pulmonary disease (COPD) patients. Prevailing methods to assess dynamic hyperinflation are crude because of the large normal variation of both functional residual capacity and inspiratory capacity (IC). In the present study, we hypothesized that expiratory flow on a relatively high level near the end of tidal expiration is an indication of dynamic hyperinflation. A method to measure tidal end expiratory flow (TEEF), i.e. the flow between 0·6 s and 0·04 s before start of inspiration is presented and evaluated in 15 healthy subjects and 16 COPD patients. The COPD patients had more than twice as high TEEF values compared with the healthy subjects (45·4 ± 23·8 and 20·4 ± 7·3 ml s⁻¹ respectively; mean ± SD; P = 0·0002, for TEEF at 0·4 s before start of inspiration). TEEF values correlated to IC, e.g. TEEF at 0·4 s before start of inspiration expressed as a fraction of mean expiratory flow, correlated to IC (% pred.) (r = 0·74; P<0·0001). These results justifies further testing of the relationship between TEEF and dynamic hyperinflation.