Airway distensibility in normal and asthmatic subjects and partitioning of the Fowler dead space.

Anatomical dead space measured by the Fowler method (VDF) is the sum of 2 serial volume compartments (VDF = VDp1 + VDp2). VDF has been shown to increase linearly with end-inspiratory lung volume (EILV) and the gradient of the relationship (DeltaVDF) has been used as an index of airway distensibility. The aim of this study was to partition VDF into its serial compartments VDp1 and VDp2 to test the hypothesis that, given the greater distensibility of distal airways, VDp2 would demonstrate greater volume dependence than VDp1. The relationships between each measure of VD (VDF, VDp1, and VDp2) and EILV were studied in 16 healthy subjects and 16 mildly asthmatic subjects. Significant (p < 0.05) linear relationships were obtained between each measure of VD and EILV in both subject groups. Changes in VDp1 with EILV (DeltaVDp1) accounted for 78.6% +/- 5.6% (mean +/- SEM) and 72.6% +/- 6.3% of DeltaVDF in the healthy and asthmatic groups, respectively. DeltaVDp1 was greater in the healthy subjects than in asthmatic subjects (18.4 versus 13.1 mL/L, p = 0.005). We conclude that in both asthmatic and healthy subjects, the major component of DeltaVDF was DeltaVDp1 and not DeltaVDp2, as originally hypothesized. We believe our results are reflecting the degree of asynchronous airway emptying.