Frequency dependence of plethysmographic measurement of thoracic gas volume

With airways obstruction, panting frequency affects plethysmographically determined thoracic gas volume (Vtg) because the extrathoracic airway acts as a shunt capacitor. Stanescu et al. (19) suggested that in the calculation of Vtg, use of esophageal (delta Pes) rather than mouth pressure (delta Pm) swings might eliminate the problem. We measured total lung capacity (TLC) plethysmographically in 10 subjects with chronic airways obstruction (CAO) and in four normal subjects. TLC (using delta Pm) was derived from Vtg obtained from slow-(approximately 1 Hz) and fast- (approximately 4 Hz) panting frequencies. In the normal subjects and four subjects with CAO, TLC was also obtained using delta Pes. In these subjects abdominal gas compression and decompression did not contribute significantly to the frequency dependence of TLC. In CAO, TLC was frequency dependent in direct proportion to the severity of obstruction. Although the frequency dependence was greater using delta Pm to calculate Vtg, it also occurred using delta Pes. Thus it could not be explained entirely by the shunt capacitor effect of the extrathoracic airways. The residual and significant overestimations of TLC (reflected by frequency dependency of TLC derived from Vtg calculated from delta Pes) may be explained by interregional nonhomogeneities during the panting maneuver.