In normal situations there is a phenomenon called ventricular coupling or interventricular dependence wherein the function of one ventricle is altered by changes in the filling of the other. As a result, the septum always maintains a right-ward bowing.
In constrictive pericarditis, this coupling is lost. As a result, the septum shows dyskinetic and often paradoxical movement. Specifically in inspiration, when there is a reduction in intra-thoracic pressure with an increase in systemic venous return and a reduction in pulmonary venous return, the dilated RV results in left-ward bowing of the septum.
Real-time MRI can assess this well. With this technique, described by Francone M et al in the March issue of the Journal of Magnetic Resonance Imaging, during the onset of inspiration, the real-time study can easily show the abrupt left-ward shift or flattening of the septum. This reduces in expiration and with successive heart-beats.
Flattening is also seen with pulmonary hypertension, but this is fixed and does not change with the phase of respiration.