Characteristics of a simple measure of respiratory impedance

The ratio of mouth pressure developed 0.1 sec after occlusion at end-expiration (P0.1) to average inspiratory flow rate (VT/TI) has been proposed as 'effective inspiratory impedance', Imeff. We have studied a simple mathematical model of lung mechanics, consisting of an effective resistance, an effective compliance and a single pressure generator, to learn how Imeff is altered by changes in resistance (R), compliance (C), inspiratory duration (TI), and the degree of curvature of the inspiratory pressure wave form. The degree of curvature was varied between concave with respect to the time axis to convex and included a linear inspiratory muscle pressure function. Assuming a linear pressure function, we obtained an explicit equation for Imeff as a function of R, C, and TI. Using the same model we also studied the classical impedance as a function of R, C and frequency (of a sinusoidal excitation pressure). We found that Imeff was increased by increases in R, increasing degrees of concavity, decreases in C, and by decreases in TI. For this model the classical impedance was about 5 times larger than Imeff. Classical impedance was increased by increases in R, decreases in C, and decreases in excitation frequency. In conclusion, measurements of effective inspiratory impedance need to be interpreted in terms of R, C, TI, and the shape of the inspiratory muscle pressure function.