Problems in the Plethysmographic Assessment of Changes in Total Lung Capacity in Asthma1–3

We studied the effect of abdominal gas compression on plethysmographically determined total lung capacity (TLC) in asthmatic patients before, during, and after treatment of induced bronchospasm. TLC was derived from panting maneuvers near residual volume, at functional residual capacity, and near TLC. Significant differences among these “derived TLC” values increased significantly during bronchospasm. Whether or not TLC appeared to increase, and by how much, depended on the level of the vital capacity from which it was derived. Individual increases in TLC during bronchospasm could not be explained by increases in abdominal gas volume or in the extent to which it was compressed and decompressed during panting. We postulate that during the Boyle's Law panting maneuver, pleural, and therefore alveolar, pressure swings may be nonhomogeneous and greater over lung regions subtended by closed airways than over regions in communication with the mouth. This would result in an underestimation of alveolar pressure s...