Objective To reveal the exact mechanical principle of the respiration-driven variation of cardiac function by observing the effect of respiratory intrathoracic pressure change (RIPC) on the mechanical models using echocardiography.Methods Model 1 was designed to observe the influence of RIPC on systemic and pulmonary venous return systems (SVR and PVR) respectively.Model 2,as an equivalent mechanical model of septal swing,was used to study the influence of RIPC on the motion of the interventricular septum (IVS).Results Model 1 demonstrated that the simulated RIPC had totally different influences on the simulated SVR and PVR.It increased the volume of the simulated right ventricle when the internal pressure was kept constant (8.16 cm H2O),while it had the opposite effect on PVR.Model 2demonstrated that there was a corresponding relationship between RIPC and the position of the simulated IVS which might be called pressure-position relationship.Conclusions The different anatomical arrangement of the two venous return systems leads to a different effect of RIPC on right and left ventricles,and thus generates a pressure gradient across IVS that tends to shift IVS left-and right-wards with respiration.The swing of the IVS changes the short axis diameters of the ventricles,thus their fillings and then their functions reciprocally.