The mechanism of increase in total lung capacity during acute asthma.

In order to investigate the mechanism underlying the increased total lung capacity (TLC) observed during an acute asthmatic attack, we measured respiratory mechanics in a specially trained, exercise-induced, asthmatic. During the acute attack his TLC (determined plethysmographically) increased from 7.8 to 9.2 liters. The static pressure-volume curve of the lung shifted to the left and expiratory compliance increased from 0.24 to 0.55 liter/cm H2O. There was a parallel shift of the static pressure-volume curve of the chest wall resulting in an increase in the outward recoil of this structure. The maximum inspiratory pressure-volume curve of the total respiratory system was shifted so that the inspiratory muscles were able to generate greater pressures at any given lung volume during the attack. The findings indicate that the increase in TLC during acute bronchospasm results from the combination of loss of lung recoil, increased outward recoil of the chest wall and increased strength of contraction of the inspiratory muscles.