Configuration of the chest wall during increased gravitational stress in erect humans

Abstract

Using magnetometers we measured changes in the dimensions of the rib cage and abdomen in three seated subjects at + 1 G Z, + 2 G Z and two subjects at + 3 G Z. Measurements were made during normal tidal breathing and at a variety of lung volumes during relaxation. We found that changes in thoraco-abdominal shape could be summarized by measuring antero-posterior and lateral diameters at the angle of Louis, the xyphisternal junction and a level 1 cm cephalad to the umbilicus. During increased acceleration, at all lung volumes, antero-posterior diameters decreased and lateral diameters increased at both the angle of Louis and the xyphisternum, the greater change occuring at the lower level. At the umbilicus, a-p diameter decreased while lateral diameter changed little. Changes were small, always being less than 10% of the dimensions measured at + 1 G Z. Cross-sectional area of the rib cage increased slightly. Changes in diameter and cross-section of the rib cage were similar t those observed during a Muller maneuver, which decreased gravitational differences in regional lung volume, while increased acceleration is known to increase these regional volume differences. We conclude that in erect humans the vertical distribution of regional volumes, and by implication regional pleural surface pressure, is independent of chest wall shape.