In asthma positive phase III slopes can result from structural heterogeneity of the bronchial tree.

We have previously identified bronchial generations 5-7 as the locus of maximum contribution to the convective portion of the phase III slope in computed tomography (CT)-based lung models of patients with asthma. In the present study, we examined how phase III slope is generated locally, by specifically interrogating at individual branch points, the necessary condition for a phase III slope to occur: some degree of convective flow sequencing between any two daughter branches that have a heterogeneity in gas washout concentration between them. Flow sequencing at individual branch points showed a wide range of values, including branch points where flow sequencing was such that phase III slopes were negative locally. Yet, the net effect in the 24 bronchial trees that we studied was that flow sequencing between pairs of less and better ventilated units most frequently drove positive phase III slopes in generations 5-7. By investigating the link of local flow sequencing between any two daughter branches to the corresponding heterogeneity of mechanical lung properties, heterogeneity of compliance was seen to be a major determinant of flow sequencing. In these bronchial structures, compliance heterogeneity was essentially brought about by volume asymmetry resulting from terminating pathways within the three-dimensional (3-D) confines of the lung contours. We conclude that the serial and parallel combination of lung mechanical properties at individual branch points in an asymmetrical branching network generates flow sequencing in mid-range conductive airways, leading to a positive at-mouth phase III slope. NEW & NOTEWORTHY Conceptually, the simplest way to obtain a sloping phase III during a washout exhalation is when there is convective flow sequencing between two lung units, such that the better ventilated unit contributes relatively more to exhaled flow at the beginning of phase III in the exhalation. Here, we show how compliance heterogeneity across the serial and parallel arrangement of branch points in bronchial trees of patients with asthma leads to flow sequencing, and thus phase III slopes of positive sign at the patient's mouth.