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.

Keywords: Blood filling waves; Blood transport; Cardiac resynchronization therapy; Echocardiography; Intracardiac flow; Intraventricular flow patterns.

Fig. 1

Pulse wave Doppler inflow velocity as a function of time in a patient without CRT (a), and undergoing CRT at AVOPT (b), AVMAX (c), AVMIN (d), and atrial pacing at 100 bpm (e). In all the panels, the inflow velocity envelope is shown in green and the mitral valve closure time is represented by a red-dashed vertical line. Notice the truncation of the A wave for AVMIN, shortening the timing of atrial driven filling, and the fusion of the E and A waves for tachycardia

Fig. 2

Evolution of filling transport regions in the LV of the same patient shown in Figs. 1, 2. Each column represents a different CRT setting: a CRT-OFF, b AVOPT, c AVMAX, d AVMIN and e Tachycardia (100 bpm). Each row represents a different instant during diastole: the 1st, 2nd, 3rd and 4th rows correspond respectively to peak E-wave, A-wave onset, peak A-wave and mitral valve closing (MVC). In each panel, the yellow and red regions track respectively the fluid that enters the LV during the E-wave and the A-wave, whereas the blue background tracks the residual volume of blood that occupies the LV at the onset of diastole. Instantaneous velocity vectors are shown in black. In the Tachycardia setting the E/A-waves fusion is depicted in green. (Color figure online)

Fig. 3

%LV volume occupied by E-wave (a) and A-wave (b) filling transport at mitral valve closing in patients (N = 9) undergoing CRT with different AV delay settings, compared with healthy volunteers (N = 3). The data in each column are plotted as univariate scatter plots and summarized in the form of boxplots. In the patients, red and blue boxplots refer respectively CRTOFF and different AV delay cases. Each symbol type refers to a different patient, and is colored in green (red) if CRT makes the corresponding variable more (less) similar to the healthy subjects. The latter are represented by a green boxplot. (Color figure online)

Fig. 4

End-diastolic distribution of different transport regions in the LV of the same patient shown in Figs. 1, 2, 3, plotted for different CRT settings (CRTOFF, AVOPT, AVMIN, AVMAX and Tachycardia). The different regions represented are direct flow (green), retained inflow (yellow), delayed ejection (blue) and residual flow (red). (Color figure online)

Fig. 5

Statistics of intraventricular blood redirection efficiency at aortic valve opening in patients (N = 9) undergoing CRT with different AV delay settings, compared with healthy volunteers (N = 3). a Fraction of LV volume that undergoes direct flow in the imaged plane, η_DF = S_DF/S_LV . b Fraction of LV occupied by residual volume in the imaged plane, λ_RF = S_RF/S_LV . c Fraction of total kinetic energy in the LV contained in the direct flow region, η_K = K_DF/K_LV . d Fraction of total kinetic energy in the LV contained in the residual volume, λ_K = K_RF/K_LV . e Net acceleration communicated to the direct flow region in the direction of the outflow tract, normalized by the total acceleration magnitude, ${\eta }_{\mathit{M}}=\left({\mathit{M}}_{DF}\cdot {\mathit{e}}_{LVOT}\right)/\left|{\mathit{M}}_{DF}\right|$. The data in each column are plotted as univariate scatter plots and summarized in the form of boxplots. In the patients, red and blue boxplots refer respectively CRTOFF and different AV delay cases. Each symbol type refers to a different patient, and is colored in green (red) if CRT makes the corresponding variable more (less) similar to the healthy subjects. The latter are represented by a green boxplot. (Color figure online)