The unreliability of indirect lung compliance in healthy subjects and patients with chronic lung disorders

The clinical usefulness of lung compliance calculated indirectly was reassessed in a large number of healthy subjects and patients with chronic airflow obstruction or restrictive lung disorders. Indirect compliance was calculated from maximal expiratory flow-volume (MEFV) curve and airway resistance measured plethysmographically according to two approaches. In the first (approach A) all calculations were done at the functional residual capacity, whereas in the second (approach B) they were obtained over the 50-75% volume range of the forced vital capacity; values were compared to those of direct compliance measured concurrently. For the group as a whole, the correlations between indirect and direct values were poor regardless of the approach. Examined separately, the best correlations were found for the healthy group using approach A (r = 0.501) and for the obstructive group using approach B (r = 0.312). Failure to derive a valuable indirect compliance is due to the fact that there is a very poor correlation between upstream resistance and airway resistance measured by body plethysmography.U