TOTAL AND LOCAL IMPEDANCES OF THE CHEST-WALL UP TO 10 HZ

To understand how local mechanical chest wall (CW) properties are related to those of the CW as a whole, we measured eosophageal and gastric pressures, CW volume changes (measured with a head-out body plethysmograph), and anteroposterior and transverse CW diameter changes (measured with magnetometers attached to the surface) during sinusoidal forcing at the mouth (2.5% vital capacity, 0.5-10 Hz) in four healthy subjects. Total CW resistance decreased sharply as frequency rose to 3-4 Hz and remained relatively constant at higher frequencies. Total CW reactance became less negative with increasing frequency but showed no tendency to change sign. Above 2 Hz, diameters measured at different locations changed asynchronously between and within the rib cage and abdomen. 'Local pathway impedances' (ratios of esophageal or gastric pressure to a rate of diameter change) showed frequency dependence similar to that of the total CW < 3 Hz. Local pathway impedances increased during contraction of respiratory muscles acting on the pathway. We conclude that 1) total CW behavior is mainly a reflection of its individual local properties at ≤ 3 Hz, 2) local impedances within the rib cage or within the abdomen can change independently in some situations, and 3) asynchronies that develop within the CW during forcing > 3 Hz suggest that two compartments may be insufficient to describe CW properties from impedance measurements.