Diagrammatic analysis of the respiratory action of the diaphragm.

During isolated phrenic nerve stimulation, the muscles of the diaphragm shorten by 40-50% of their optimal length, and the force in the muscle and transdiaphragmatic pressure (Pdi) depend on the final muscle length. The muscle shortening depends on the load imposed on the diaphragm by pleural and abdominal pressures during a particular maneuver. The mechanics of the interaction between the diaphragm and the load is well understood, but the force-length properties of the diaphragm are nonlinear, and an algebraic analysis of the interaction is clumsy. Here we describe a graphical analysis of the interaction. The variable muscle length is transformed into an equivalent variable, i.e., volume displaced by the diaphragm (Vdi), to obtain the characteristic line for the diaphragm, a graph of Pdi vs. Vdi for a given level of activation. The load is described by the same variables. Therefore, load lines can be drawn on the same graph, and the equilibrium point for the diaphragm is given by the intersection of the load line with the characteristic line of the diaphragm. Graphical analyses of the volume dependence of the respiratory effects of diaphragm and intercostal muscle activation and for the interaction between them are shown.