Failure of body plethysmography in bronchial asthma

To determine whether acute changes in lung volumes in asthma are accurately measured by body plethysmography, we induced acute changes in lung mechanics with acetylcholine in nine asthmatic patients and with salbutamol in six others. Total lung capacity (TLC) was measured with a body plethysmograph and derived from mouth pressure vs. box volume (Vbox) changes (TLCm) or esophageal pressure vs. Vbox changes (TLCes). In six patients (4 after acetylcholine) TLCm was significantly (P less than 0.05) different from prechallenge values (differences ranged from 0.29 to 1.55 liters), but TLCes did not change. In three additional patients both TLCm (mean difference 0.62 liter) and TLCes (mean difference 0.43 liter) changed (P less than 0.05), but no changes occurred in six others. An electrical analog of the lung demonstrates in the presence of intrathoracic airway obstruction overestimation of thoracic gas volume and TLC. This is due to the presence of a compliant extrathoracic airway that acts as a shunt impedance. The magnitude of the overestimation of TGV appears to depend on the distensibility of the extrathoracic airway and the degree of airway obstruction. We conclude that the plethysmographic method may introduce important errors in lung volume measurements in the obstructive syndrome, especially a severe one. Some of previously reported acute increases in TLC in asthmatics may be artifactual.