Clinical significance of abnormal rib cage-abdominal motion.

A major conceptual breakthrough occurred in 1967 when KoNNo and MEAD proposed that, under the condition of a fixed spinal position, the chest wall can be considered as a system with two degrees of freedom [1]. They stated that any change in lung volume is accommodated by the sum of the changes in dimensions of the rib cage and of the anterior abdominal wall. When the airways are closed, the chest wall becomes a system with a single degree of freedom, so that changes in dimensions of the rib cage and abdominal wall are equal and opposite in direction. Rib cage and abdominal motion can be measured by two methods. As originally described, magnetometry can be used to measure the anteroposterior diameter of the rib cage and abdomen. Respiratory inductance plethysmography is now more widely used to measure the changes in cross-sectional area of the rib cage and abdomen [2]. The signals of either method are usually displayed on an X-Y plot, with the rib cage dimension on the ordinate and the abdominal dimension on the abscissa, as in the Konno-Mcad diagram. In reality, it has been shown that the chest wall, in particular the rib cage, possesses a greater degree of freedom [3, 4]. Nevertheless, the model of two degrees of freedom has allowed deeper insight into chest wall mechanics and is a practical tool for clinical interpretation of respiratory movements.