The influence of reflection artifacts on apparent phase velocity measurements in the trachea

Models of the pulmonary airway network were analyzed to determine the effect of reflections on apparent phase velocity measurements in the trachea of dogs. A stiffly constrained elastic tube filled with a viscous fluid was used to model each airway segment, and a series resistance-capacitance combination served as a terminal element for each pathway. Reflection, transmission and attenuation coefficients at each branching site and reflection coefficients at each termination were computed. Composite sinusoidal pressure waves at two sites in the trachea where computed for frequencies from 5 to 150 Hz, and crosscorrelograms were used to determine apparent phase velocities. In an asymmetrical model, the randomness of the airway diameters caused phase shifts in the reflection coefficients to vary between approximately 0° and 180° resulting in significant destructive interference between the reflected waves. Calculated apparent phase velocities agreed with the assumed values within 40% at frequencies of 70 Hz and below and within 20% at higher frequencies in this model.