Assessment of hemodynamic load components affecting optimization of cardiac resynchronization therapy by lumped parameter mode

Timing of biventricular pacing devices employed in cardiac resynchronization therapy (CRT) is a critical determinant of efficacy of the procedure. Optimization is done by maximizing function in terms of arterial pressure (BP) or cardiac output (CO). However, BP and CO are also determined by the hemodynamic load of the pulmonary and systemic vasculature. This study aims to use a lumped parameter circulatory model to assess the influence of the arterial load on the atrio-ventricular (AV) and inter-ventricular (VV) delay for optimal CRT performance. The model consists of variable elastance components to simulate both left and right ventricles as well as the interventricular septum. The pulmonary and systemic circulations are modeled by lumped parameter Windkessel elements using resistors, inductors and capacitors to represent vascular resistance, blood inertia and arterial and venous compliance, including the coronary circulation. Optimal CRT performance was determined by varying AV and VV delay and the critical delay was obtained for the maximum value of CO. The maximal (optimal) central systolic blood pressure (SBP) was also used to assess the potential use of non-invasive continuous pressure for CRT optimization Model calculations were made for maximal (optimal) CO and SBP with changes in systemic compliance (Cas) and peripheral resistance (Ras). Simulations with the circulatory model indicate that arterial loading parameters have an intrinsic effect on the timing for optimal CRT performance, with a greater relative impact on VV compared to that on AV delay. Load parameter changes for SBP give similar results to using CO as an optimizing parameter, although differences occur with changes in Ras.