Effects of body position change on thoracoabdominal motion.

With a linearized respiratory magnetometer, measurements of anteroposterior and lateral diameters of both the rib cage and the abdomen were made at functional residual capacity and continuously during tidal breathing. Twenty-five subjects with normal respiratory systems were studied in the sitting, supine, lateral decubitus, and prone body positions. When subjects changed from sitting to supine position anteroposterior diameters of both rib cage and abdomen decreased while their lateral diameters increased. Both anteroposterior and lateral tidal excursions of the rib cage decreased; those of the abdomen increased. When subjects turned from supine to lateral decubitus position both anteroposterior diameters increased and the lateral diameters decreased. This was associated with an increase in both lateral excursions and a decrease in the abdominal anteroposterior excursions. Diameters and tidal excursions in the prone position resembled those in the supine position. Diameter changes could be explained by gravitational effects. Differences in tidal excursions accompanying body position change were probably related to 1) differences in the distribution of respiratory muscle force, 2) differences in the activity or mechanical advantage of various inspiratory muscles, and 3) local compliance changes in parts of the rib cage and abdomen.

Compliances of human rib cage and diaphragm-abdomen pathways in relaxed versus paralyzed states.

The respiratory magnetometer method of Konno and Mead was used to measure separately the rib cage and the diaphragm-abdomen components of the total respiratory system compliance in 11 subjects with normal respiratory systems. Measurements made in the awake, relaxed state by the method of Heaf and Prime were compared with similar measurements made in the anesthetized, paralyzed state by the supersyringe method. The rib cage component was greater in the paralyzed than the relaxed state in 9 of 11 subjects, but the diaphragm-abdomen component was greater in the relaxed than the paralyzed state in 8 of 11 subjects. We believe that these differences can be explained by respiratory muscle activity in the presumed relaxed state. The fraction of the tidal volume attributable to rib cage displacement compared to abdominal displacement was greater during mechanical ventilation in the paralyzed state than during awake, spontaneous breathing. This can be explained by the different distribution of inflating forces produced by diaphragmatic contraction compared to positive airway and alveolar pressure, in particular by the very different patterns of diaphragmatic displacement in the 2 states.