Determinants of midwall circumferential segmental length of the canine ventricular septum at end diastole

The ventricular septum attaches to the insertion points of the ventricular free walls, separates, and is shared by both the left (LV) and right (RV) ventricles. Changes in the transseptal pressure gradient (P(TS), the difference between LV and RV pressures) will change the stress in the septum and, therefore, the length of the septal segment (L(s)). However, since most of the septal myocardial fibers are continuous with those of LV free wall and the septum appears to be an integral part of LV, one might also expect that if the LV transmural pressure (P(lvtm); the difference between LV and pericardial pressure) is raised, L(s) would increase even in the absence of any change in P(TS). Therefore, we hypothesized that at end diastole, L(s) depends on both P(lvtm) and P(TS). To test this hypothesis, we measured L(s) (sonomicrometry), LV and RV pressures (micromanometers), and pericardial pressure (flat liquid-containing balloon) in seven anesthetized open-chest dogs. P(lvtm) was increased through volume loading, whereas P(TS) was maintained constant at 10, 5, 0, -5, -10, and -15 mmHg by adjusting the degree of constriction of the pulmonary artery or aorta. These procedures were performed first with pericardium reapproximated and then after the pericardium had been opened widely. At each controlled P(TS) level, L(s) increased linearly with the increase in P(lvtm). Both the slope and the intercept of this L(s)-P(lvtm) relation were affected by P(TS) in a nonlinear fashion. With use of a model in which L(s) is a linear function of P(lvtm) and a quadratic function of P(TS), there was an excellent correlation between predicted and observed L(s) (R² = 0.97 ± 0.02, mean ± SD), providing a clearer picture of the dependence of L(s) on P(lvtm) and P(TS). We conclude that L(s) depends both on P(lvtm), as indicated by the linear relation between L(s) and P(lvtm), and on P(TS), as indicated by the curvilinear relation between L(s) and P(TS).