Effect of posture on lung and regional chest wall mechanics.

BACKGROUND: Little is known about the extent to which changes in postures in clinical situations affect respiratory mechanics, even in humans with healthy respiratory systems. This study tested the hypothesis that posture has only small effects on overall respiratory system mechanics in healthy subjects, despite changes in parts of the respiratory system in some postures.

METHODS: Measurements were made of airway flow, airway and esophageal pressures, and rib cage and abdominal volume displacements (with inductance plethysmography) of awake, healthy subjects, relaxed at functional residual capacity, during external forcing at 0.2 Hz with a tidal volume of 8-10 ml/kg. From these measurements, discrete Fourier transform was used to calculate elastances (E) and resistances (R) of the total respiratory system, lungs, total chest wall, and compartments of the chest wall (rib cage, diaphragm-abdomen, and belly wall). Measurements were made while the subjects were in nine different postures: in six of these, the torso was straight; in three, the torso was bent or twisted.

RESULTS: Although changes in mechanics of parts of the respiratory system were evident in certain postures, overall respiratory mechanics were not greatly affected by posture. Changing from sitting to supine decreased E and R of the diaphragm-abdomen about 50% (P < .05), but total chest wall E and R changed only slightly. Lung E increased 24% (P < .05), but total respiratory E did not change (P < .05). Lung and total respiratory R increased 40-50% (P < .05) with this same change in posture. As long as the torso was straight, however, changes in orientation of 30 degrees from the horizontal or a shift to lateral posture resulted in only minor changes in the variables measured. Postures in which the torso was twisted or bent increased E of the total chest wall 20-30% compared to supine (P < .05), due to increases in E of one or more compartments. Respiratory system E also increased, at most 14%. Although lung R decreased 30-45% (P < .05) in these postures compared to supine with a straight torso, chest wall and total respiratory R generally were unchanged.

CONCLUSIONS: Changes in respiratory system mechanics over a wide range of postures that may be encountered clinically are relatively small in healthy awake subjects due to adaptability of total chest wall mechanical behavior.