Airway distensibility in healthy and asthmatic subjects: effect of lung volume history

Anatomic dead space (VD) is known to increase with end-inspiratory lung vol ume (EILV), and the gradient of the relationship has been proposed as an in dex of airway distensibility (Delta VD). The aims of this study were to app ly a rapid method for measuring Delta VD and to determine whether it was af fected by lung volume history. VD of 16 healthy and 16 mildly asthmatic sub jects was measured at a number of known EILVs by using a tidal breathing, C O2-washout method. The effect of lung volume history was assessed by using three tidal breathing regimens: 1) three discrete EILVs (low/medium/high; L MH); 2) progressively decreasing EILVs from total lung capacity (TLC; TLC-R V); and 3) progressively increasing EILVs from residual volume (RV; RV-TLC) . Delta VD was lower in the asthmatic group for the LMH (25.3 +/- 2.24 vs. 21.2 +/- 1.66 ml/l, means +/- SE) and TLC-RV (24.3 +/- 1.69 vs. 18.7 +/- 1. 16 ml/l) regimens. There was a trend for a lower Delta VD in the asthmatic group for the RV-TLC regimen (23.3 +/- 2.19 vs. 18.8 +/- 1.68 ml/l). There was no difference in Delta VD between groups. In conclusion, mild asthmatic subjects have stiffer airways than normal subjects, and this is not obviou sly affected by lung volume history.