Rib cage vs. abdominal displacement in dogs during forced oscillation to 32 Hz.

Allen et al. (J. Clin. Invest. 76: 620-629, 1985) reported that during oscillatory forcing the base of isolated canine lungs distends preferentially relative to the apex as frequency and tidal volume increase. The tendency toward such nonuniform phasic lung distension might influence phasic displacement of the rib cage (RC) relative to the abdomen (ABD). To test this hypothesis we measured RC and ABD displacement in four anesthetized dogs during forced oscillation. Sinusoidal volume changes were delivered through a tracheostomy at 1-32 Hz and measured by body plethysmography. RC and ABD displacements were measured by inductive plethysmography. During oscillation with air at fixed tidal volumes (10-80 ml) RC, normalized to unity at 1 Hz, increased to 2.06-2.22 at 8 Hz (P less than 0.001) and then decreased to 1.06-1.35 (P less than 0.0025) at 32 Hz. ABD, normalized to unity at 1 Hz, was 1.12-1.16 at 4 Hz (P less than 0.001) and decreased to 0.12-0.14 at 32 Hz (P less than 0.001). Displacement of ABD relative to RC did not increase systematically with increasing tidal volume during sinusoidal forcing at any frequency. Thus we found no discernible influence of nonuniform phasic lung distension on chest wall behavior. We infer that in the dog the nonuniform mechanical behavior of the chest wall dominates the nonuniform (but opposing) mechanical tendency of the lung.