Clinical assessment of intraventricular blood transport in patients undergoing cardiac resynchronization therapy

Abstract

In the healthy heart, left ventricular (LV) filling generates different flow patterns which have been proposed to optimize blood transport by coupling diastole and systole. This work presents a novel image-based method to assess how different flow patterns influence LV blood transport in patients undergoing cardiac resynchronization therapy (CRT). Our approach is based on solving the advection equation for a passive scalar field from time-resolved blood velocity fields. Imposing time-varying inflow boundary conditions for the scalar field provides a straightforward method to distinctly track the transport of blood entering the LV in the different filling waves of a given cardiac cycle, as well as the transport barriers which couple filling and ejection. We applied this method to analyze flow transport in a group of patients with implanted CRT devices and a group of healthy volunteers. Velocity fields were obtained using echocardiographic color Doppler velocimetry, which provides two-dimensional time-resolved flow maps in the apical long axis three-chamber view of the LV. In the patients under CRT, the device programming was varied to analyze flow transport under different values of the atrioventricular conduction delay, and to model tachycardia (100 bpm). Using this method, we show how CRT influences the transit of blood inside the left ventricle, contributes to conserving kinetic energy, and favors the generation of hemodynamic forces that accelerate blood in the direction of the LV outflow tract. These novel aspects of ventricular function are clinically accessible by quantitative analysis of color-Doppler echocardiograms.

Age-Dependence of Flow Homeostasis in the Left Ventricle

“…In non-ischemic dilated cardiomyopathy intraventricular stasis is assumed to be the main risk factor related to mural thrombosis (Bakalli et al, 2013). Assistance devices and prosthetic valves always disrupt intraventricular flow to some degree, which may occasionally lead to blood stagnation and/or to a remarkable increase in blood shear stresses (May-Newman et al, 2013; Rossini et al, 2017). The lack of suitable quantitative methods has heavily limited addressing the alterations in flow homeostasis in these conditions.…”

“…Therefore, one could hypothesize that increasing heart-rate in adults would be an effective way to facilitate blood clearing. However, tachycardia increases the velocity of the A-wave at the expense of fusing it with the E-wave (Oniki et al, 1992; Santhanakrishnan et al, 2016; Rossini et al, 2017). Thus, tachycardia and age-related changes in heart-rate induce different effects on blood mixing.…”

“…Thus, tachycardia and age-related changes in heart-rate induce different effects on blood mixing. In fact, shortening the AV delay does not generally improve intraventricular blood transport in patients undergoing cardiac resynchronization therapy (Rossini et al, 2017). Further investigations are needed, ideally combining experiments of pacing and exercise, to uncouple the effects on blood transport of heart rate, atrioventricular delay, inotropism, and lusotropism.…”

“…Regions smaller than 5% of LV end-diastolic volume (area in 2D) were dismissed. Based on forwarded temporal events, we thresholded RT maps during the diastole of the first beat to isolate the blood fractions carried by E- and A- waves (Rossini et al, 2017) (Figure 1C). The front location of these volumes relative to the normalized long-axis of the LV was measured to determine the degree of penetration in the LV of E and A-waves.…”

“…However, blood does not transit the LV following a first-in-first-out rule; even in normal hearts, a significant fraction of the blood entering the LV is not ejected during the ensuing systole (Bolger et al, 2007; Eriksson et al, 2011). This fraction increases in diseased hearts, potentially resulting in blood stagnation and eventually thrombosis inside the chamber (Eriksson et al, 2011; Hendabadi et al, 2013; Rossini et al, 2017). Thus, understanding the physiological foundations of intraventricular blood transit is of major clinical relevance.…”