What is Paradoxical Motion of Interventricular Septum?

Interventricular septum is a common wall shared by the left ventricle and right ventricle. Normally it contracts with the left ventricle, that is why when it does not contract in synchrony with the left ventricle it is known as paradoxical septal motion. The paradoxical motion is due to change in the shape of the left ventricle. Normally the left ventricle is circular in cross section in both systole and diastole. In right ventricular volume overload, septum becomes flat or even concave towards the right ventricle in diastole. This curvature reverts in systole, producing the paradoxical motion. The authors of the cited study concluded that the diastolic change in the shape of left ventricle due to right ventricular volume overload is the reason for paradoxical septal motion (Volume overload is also called diastolic overload).