Tidal expiratory flow patterns in airflow obstruction.

Tidal expiratory flow pattern was analysed in 99 subjects with a view to assessing it as a quantitative measurement of airflow obstruction. Fifteen normal volunteers, nine patients with dyspnoea referred for investigation in whom airway resistance was within normal limits, 24 patients with restrictive lung disorders, and 51 patients with airway obstruction were studied. The expiratory flow pattern against time had a quadrilateral configuration in airway obstruction, which differed from the more sinusoidal form that is seen in subjects without airflow obstruction. The rapid rise to tidal peak flow was analysed in two ways, percentage of volume expired at tidal peak flow (delta V/V) and percentage of expiratory time to tidal peak flow (delta t/t). Both these indices correlated significantly with conventional measurements of airway obstruction. The pattern of expiratory flow in airflow obstruction during quiet breathing resembles that of a forced expiratory maneuver at similar lung volumes. In some cases this may be caused by dynamic compression occurring during tidal breathing. In others, the pattern may result from the static recoil of the lung being permitted to drive flow freely in expiration, rather than being braked by postinspiratory contraction of inspiratory musculature.