RIB CAGE AND DIAPHRAGM-ABDOMEN COMPLIANCE IN HUMANS - EFFECTS OF AGE AND POSTURE

The influence of age and posture on compliance of the rib cage (Crc) and diaphragm-abdomen (Cab) compartments of the chest wall was studied in 61 healthy adults(33 men, 28 women) aged 24-75 yr. Chest wall compliance (CW) was measured by the weighted spirometer technique; Crc and Cab were derived from the slope of the relaxation line of the thoracoabdominal system obtained with two pairs of linearized magnetometers. While Cw was being measured, we monitored electrical activity of the abdominal external oblique muscle with a concentric needle electrode and thoracoabdominal configuration. In 52 subjects, the electromyogram did not show any abdominal muscle activity and the end-expiratory level never departed from the relaxed thoracoabdominal configuration, thus suggesting adequate respiratory muscle relaxation. Aging was associated with significant decreases in Crc and Cab. In the upright posture Crc decreased from 0.164 .+-. 0.041 (mean .+-. SD) l/cm H2O in the younger subjects (24-39 yr) to 0.114 .+-. 0.027 l/cm H2O in the older subjects (55-75 yr). Cab concomitantly fell from 0.032 .+-. 0.012 l/cm H2O to 0.020 .+-. 0.007 l/cm H2O. These reductions were statistically significant (P < 0.05-0.01) and were also present in the supine posture. Shifting from the seated to the supine posture did not cause any significant change in Cw but was invariably associated with a decrease in Crc and an increase in Cab. In the 52 subjects Crc averaged 0.141 .+-. 0.043 l/cm H2O in the seated posture and 0.088 .+-. 0.038 l/cm H2O in the supine posture (P < 0.001); corresponding values for Cab were 0.025 .+-. 0.012 and 0.070 .+-. 0.025 l/cm H2O (P < 0.001). We conclude that 1) the decrease in Cw with age involves both the rib cage and diaphragm-abdomen compartments and 2) in normal humans the rib cage becomes stiffer and the diaphragm-abdomen compartment more distensible when changing from the seated to supine posture.