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

Anatomic dead space (VD) is known to increase with end-inspiratory lung volume (EILV), and the gradient of the relationship has been proposed as an index of airway distensibility (Delta VD). The aims of this study were to apply a rapid method for measuring Delta VD and to determine whether it was affected by lung volume history. VD of 16 healthy and 16 mildly asthmatic subjects was measured at a number of known EILVs by using a tidal breathing, CO2-washout method. The effect of lung volume history was assessed by using three tidal breathing regimens: 1) three discrete EILVs (low/medium/high; LMH); 2) progressively decreasing EILVs from total lung capacity (TLC; TLC-RV); 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 obviously affected by lung volume history.