PRESSURE-VOLUME ANALYSIS OF THE LUNG WITH AN EXPONENTIAL AND LINEAR-EXPONENTIAL MODEL IN ASTHMA AND COPD

The prevalence of abnormalities in lung elasticity in patients with as thma or chronic obstructive pulmonary disease (COPD) is still unclear, This might be due to uncertainties concerning the method of analysis of quasistatic deflation long pressure-volume curves. Pressure-volume curves were obtained in 99 patients with moderately severe asthma or C OPD, These patients were a subgroup from a Dutch multicentre trial; th e entire group was selected on the basis of a moderately lowered % pre dicted forced expiratory volume in one second (FEV1), and a provocativ e concentration of histamine producing a 20% decrease in FEV1 (PC20) < 8 mg . mL(-1) obtained with the 2 min tidal breathing technique. The c urves were fitted with an exponential (E) model and an exponential mod el which took the Linear appearance in the mid vital capacity range in to account (linear-exponential (LE)). The linear-exponential model sho wed a markedly better fit ability, yielding additional parameters, suc h as the compliance at functional residual capacity (FRC) level as slo pe of the linear part (b), and the volume at which the linear part cha nged into the exponential part of the curve (transition volume (Vtr)), Vtr (mean value Vtr/total lung capacity (TLC) = 0.79 (so 0.07)) showe d a close positive linear correlation with obstruction and hyperinflat ion variables, which might be due to airway closure, already starting at elevated lung volumes. The exponential shape factor K was closely c orrelated with b and mean values (K=1.32 (so 0.05) kPa(-1); b=2.96 (so 1.16) L . kPa(-1)) and the relationship with age was comparable with data reported in healthy individuals, The shape factor of the linear-e xponential fit showed no correlation with any elasticity related varia ble. Neither the elastic recoil at 90% TLC, as obtained from the linea r-exponential fit, nor its relationship with age were significantly di fferent from healthy individuals. We conclude that, for a more accurat e description of the lung pressure-volume curve, a linear-exponential fit is preferable to an exponential model However, the physiological r elevance of the shape parameter (LLE) is still unclear, These results indicate that patients with moderately severe asthma or COPD had, on a verage, no appreciable loss of elastic lung recoil as compared with he althy individuals.