Measurement of acoustic input impedance of an excised canine pulmonary system by transient excitation

Abstract

The acoustic input impedance of a freshly excised canine lung was measured in the frequency range of 80 to 10 000 Hz. The pulmonary system was coupled at the trachea to a long acoustic transmission line which was excited at the distal end by an electrical spark discharge. The waveforms incident upon and reflected from the pulmonary system were digitized and used to determine the complex reflection coefficient and input impedance of the termination. At 300 Hz, a zero of the reactance and peak in the resistance occurs. Other system resonances are observed below 1000 Hz. Above 1000 Hz. the input impedance is primarily resistive. Decrease in lung volume systematically reduces the peaks in the resistance and reactance but alters the resonant frequencies only slightly. To aid in interpreting this data the airways of the lung were modelled as a nonuniform transmission line with reactive walls. This model includes the effects of rapid flare of airway cross sectional area, mechanical properties of the wall (mass, ...