Lung distensibility. The static pressure-volume curve of the lungs and its use in clinical assessment.

The main points of interest in the static expiratory PV curve are the changes in TLC (and to a lesser extent in RV), Pst(L) at standard volumes and particularly at TLC,and compliance (delta V/delta P) particularly close to FRC. More subtle changes in curvature may be present but have not as yet achieved any clinical or diagnostic significance. Although any presentation short of the whole PV curve inevitably conceals information a useful summary of the major changes can be obtained by considering only three points--the changes in TLC, Pst(L)max and compliance close to FRC (Fig. 22). In conditions associated with an increased TLC, four distinct patterns of change in the PV curve have been recognized resulting in different combinations of changes in Pst(L)max and compliance at FRC (Fig. 22, a, b, c, d). There are two main patterns of PV curve in restrictive lung disorders--one due to stiffening of the lung (Fig. 22, g, i, j) and the second due to extrapulmonary factors which prevent a normal distending pressure being applied to the pleural surface of the lung (Fig. 22, h). In practice it appears that lack of distending pressure leads to a secondary reduction in lung compliance. Nevertheless differences in Pst(L)max remain. The general patterns of abnormality may be summarized as follows: 1. Increases in TLC are almost always associated with a normal or increased compliance but Pst(L)max may be increased, normal or decreased. 2. Decreases in TLC are almost always associated with a decreased compliance but again Pst(L)max may be increased, normal or decreased. 3. When TLC is normal, it is theoretically possible that the whole PV curve may be displaced on the pressure axis and compliance may be altered (as in ageing) but such changes have not been identified in clinical practice.