Position of interventricular septum during heart cycle in anesthetized dogs

H. Piene and E. S. Myhre Department of Biomedical Engineering, Faculty of Medicine, University Hospital, Trondheim, Norway. The impact of the transseptal pressure gradient (TSP) during the entire heart cycle has not been assessed. This study explores in anesthetized open-chest dogs the interventricular septum's relative position to the anteroposterior transverse diameter (Dap) of the left ventricle (LV). By varying preload and afterload for both LV and right ventricle (RV) and inotropy in LV, a wide range of TSP was generated. By ultrasonomicrometry the distance between the Dap and the midpoint of septum was recorded. 1) Loops of the septal-LV free-wall diameter vs. Dap showed that the LV transverse cross section independent of loading conditions tended to reach a circular form during systole. 2) Paradoxical movement of the intraventricular septum occurred when TSP was less than zero but was not dependent on an increase of TSP. 3) TSP's influence on septum's position was reduced to one-tenth when changing from end diastole to end systole. 4) During systole independent of level of LV performance, LV pressure had approximately 50% less influence on septal position than RV pressure. 5) During depressed LV performance a similar pattern was evident during diastole. 6) The pericardium had no influence on septum's position. This study demonstrates that the position of the intraventricular septum is significantly influenced by the time-varying elastance of LV and the septum itself, and that RV pressure changes have a greater impact on septal position than LV pressure changes.