Postural changes in rib cage and abdominal volume-motion coefficients and their effect on the calibration of a respiratory inductance plethysmograph.

Volume-motion coefficients were determined for the rib cage and abdomen in normal human subjects in upright, supine, and semirecumbent postures by the isovolume calibration technique of Konno and Mead (J Appl Physiol 1967; 22:407-422, using the respiratory inductive plethysmograph (RIP) to measure displacements of rib cage and abdominal walls. Volume motion coefficients changed systematically with posture; those for the rib cage were smallest in the upright posture, and for the abdomen, greatest in the upright posture. These volume motion coefficients were then used to estimate tidal volume during resting breathing in the different postures, and compared with estimates of tidal volume derived from calibration by the change in posture technique reported by Sackner and coworkers (American Review of Respiratory Disease 1980; 122:867-871). Estimates of tidal volume derived from RIP signals using both calibration techniques were compared with independently measured spirometric volume changes. Errors in tidal volume averaged 6% with the isovolume technique and 9 to 23% with the change in posture technique (depending upon whether the calibrating postures were upright, supine or semirecumbent supine). The larger errors with the change in posture calibration method are attributable to both the change in volume motion coefficients with posture and the change in distribution of tidal volume between rib cage and abdomen compartments with change in posture.