Abdominothoracic configuration and maximal static pressures in children

The relationship of abdominothoracic configuration (AT config) to maximal static inspiratory and expiratory mouth pressure (Pimax, Pemax) was studied in 9 children. Pmax manoeuvres were performed from the relaxed end-expiratory position (FRC relax) and after contracting the abdomen (Abd) and expanding the rib cage (RC), (FRC isovol). Anteroposterior diameters (AP) of RC and Abd during Pmax were measured with magnetometers and compared to AT config at FRC relax. During both Pimax and Pemax from FRC relax the RC AP increased and Abd AP decreased. The pressures generated did not correlate with the change in either diameter. Moving to FRC isovol prior to the onset of pressure generation had no effect on AT config obtained for Pemax, but for Pimax resulted in further increase in RC AP and decrease in Abd AP as compared to FRC relax. Despite these differences in AT config, Pimax was not significantly affected. This suggests that changes in AT config expected to place the diaphragm at a mechanical advantage were countered by an opposite effect on the other inspiratory muscles. Because the AT config for Pemax was fixed, there appears to be a unique balance of forces between the expiratory muscles acting on the rib cage and the abdomen during a maximal static expiratory effort.