Volume dependence of high-frequency respiratory mechanics in healthy adults.

Respiratory system input impedance (Zrs) at low to medium frequencies below 100 Hz, and study of its volume dependence, have been used extensively to quantify airway and tissue mechanics. Zrs at high oscillation frequencies including the first antiresonant frequency (far,1) may contain important information about airway mechanics. Changes in high-frequency Zrs with lung volume have not been studied. The volume-dependent behavior of high-frequency Zrs, specifically far,1 and respiratory system resistance at first antiresonance (Rrs(far,1)), was characterized in 16 healthy adults. Zrs was measured with a forced oscillation signal (5-302.5 Hz) through a wavetube setup. To track Zrs, subjects performed slow deep inspiratory and expiratory maneuvers over 30-s measurements, during which average impedance was calculated over 0.4-s intervals, with successive overlapping estimates every 0.156 s. Flow was measured using a pneumotachometer and integrated to obtain volume. Transpulmonary pressure dependence (Ptp) of Zrs was separately determined in five subjects. Both far,1 and Rrs(far,1) decreased with increasing lung volume and Ptp, consistent with an increase in airway caliber and decreased airway wall compliance as volume increased. These characterizations provide insight into airway mechanics, and are furthermore a necessary first step toward determining whether volume dependence of the first antiresonance is altered in disease.