Effect of the cheeks and the compliance of alveolar gas on the measurement of respiratory variables.

The distensibility of the upper airways was found to range in 8 subjects between 0.01 and 0.001 L/cm H2O. This distensibility may cause a number of errors in standard respiratory measurements in normal subjects and in patients with airway obstruction due to delays in pressure and flow equilibration between alveoli and mouth. The error include overestimation of thoracic gas volume with the body plethysmograph; of airway inertance and of dynamic lung compliance; underestimation of respiratory resistance with the forced oscillation technique and with interruption pressure; underestimation of total pressure exerted by respiratory muscles with the P o.1 technique during occlusions of the airway opening. The errors are often quite appreciable and may exceed 100%. Ways to reduce them or to compensate for them are discussed.