Giovanni Tonti

Characterization and Quantification of Vortex Flow in the Human Left Ventricle by Contrast Echocardiography Using Vector Particle Image Velocimetry

OBJECTIVES The aims of this study were to: 1) assess the feasibility of left ventricular (LV) vortex flow analysis using contrast echocardiography (CE); and 2) characterize and quantify LV vortex flow in normal subjects and patients with LV systolic dysfunction. BACKGROUND Vortices that form during LV filling have specific geometry and anatomical locations that are critical determinants of directed blood flow during ejection. Therefore, it is clinically relevant to assess the vortex flow patterns to better understand the LV function. METHODS Twenty-five patients (10 normal and 15 patients with abnormal LV systolic function) underwent CE with intravenous contrast agent, Definity (Bristol-Myers Squibb Medical Imaging, Inc., North Billerica, Massachusetts). The velocity vector and vorticity were estimated by particle image velocimetry. Average vortex parameters including vortex depth, transverse position, length, width, and sphericity index were measured. Vortex pulsatility parameters including relative strength, vortex relative strength, and vortex pulsation correlation were also estimated. RESULTS Vortex depth and vortex length were significantly lower in the abnormal LV function group (0.443 +/- 0.04 vs. 0.482 +/- 0.06, p < 0.05; 0.366 +/- 0.06 vs. 0.467 +/- 0.05, p < 0.01, respectively). Vortex width was greater (0.209 +/- 0.05 vs. 0.128 +/- 0.06, p < 0.01) and sphericity index was lower (1.86 +/- 0.5 vs. 3.66 +/- 0.6, p < 0.001) in the abnormal LV function group. Relative strength (1.13 +/- 0.4 vs. 2.10 +/- 0.8, p < 0.001), vortex relative strength (0.57 +/- 0.2 vs. 1.19 +/- 0.5, p < 0.001), and vortex pulsation correlation (0.63 +/- 0.2 vs. 1.31 +/- 0.5, p < 0.001) were significantly lower in the abnormal LV function group. CONCLUSIONS It was feasible to quantify LV vorticity arrangement by CE using particle image velocimetry in normal subjects and those with LV systolic dysfunction, and the vorticity imaging by CE may serve as a novel approach to depict vortex, the principal quantity to assess the flow structure.

Emerging trends in CV flow visualization.

Blood flow patterns are closely linked to the morphology and function of the cardiovascular system. These patterns reflect the exceptional adaptability of the cardiovascular system to maintain normal blood circulation under a wide range of workloads. Accurate retrieval and display of flow-related information remains a challenge because of the processes involved in mapping the flow velocity fields within specific chambers of the heart. We review the potentials and pitfalls of current approaches for blood flow visualization, with an emphasis on acquisition, display, and analysis of multidirectional flow. This document is divided into 3 sections. First, we provide a descriptive outline of the relevant concepts in cardiac fluid mechanics, including the emergence of rotation in flow and the variables that delineate vortical structures. Second, we elaborate on the main methods developed to image and visualize multidirectional cardiovascular flow, which are mainly based on cardiac magnetic resonance, ultrasound Doppler, and contrast particle imaging velocimetry, with recommendations for developing dedicated imaging protocols. Finally, we discuss the potential clinical applications and technical challenges with suggestions for further investigations.

Echocardiographic particle image velocimetry: a novel technique for quantification of left ventricular blood vorticity pattern.

BACKGROUND In this study, the functionality of echocardiographic particle imaging velocimetry (E-PIV) was compared with that of digital particle imaging velocimetry (D-PIV) in an in vitro model. In addition, its capability was assessed in the clinical in vivo setting to obtain the ventricular flow pattern in normal subjects, in patients with dilated cardiomyopathy, and in patients with mechanical and bioprosthetic mitral valves. METHODS A silicon sac simulating the human left ventricle in combination with prosthetic heart valves, controlled by a pulsed-flow duplicator, was used as the in vitro model. Particle-seeded flow images were acquired (1) using a high-speed camera from the mid plane of the sac, illuminated by a laser sheet for D-PIV, and (2) using a Siemens Sequoia system at a frame rate of 60 Hz for E-PIV. Data analysis was performed with PIVview software for D-PIV and Omega Flow software for E-PIV. E-PIV processing was then applied to contrast echocardiographic image sets obtained during left ventricular cavity opacification with a lipid-shelled microbubble agent to assess spatial patterns of intracavitary flow in the clinical setting. RESULTS The velocity vectors obtained using both the E-PIV and the D-PIV methods compared well for the direction of flow. The streamlines were also found to be similar in the data obtained using both methods. However, because of the superior spatial resolution of D-PIV, some smaller scale details were not revealed by E-PIV. The application of E-PIV to the human heart resulted in reproducible flow patterns in echocardiographic images taken within different time frames or by independent examiners. CONCLUSIONS The E-PIV technique appears to be capable of evaluating the major flow features in the ventricles. However, the bounded spatial resolution of ultrasound imaging limits the small-scale features of ventricular flow to be revealed.

Fluid dynamics of the left ventricular filling in dilated cardiomyopathy

Modifications in diastolic function occur in a broad range of cardiovascular diseases and there is an increasing evidence that abnormalities in left ventricular function may contribute significantly to the symptomatology. The flow inside the left ventricle during the diastole is here investigated by numerical solution of the Navier-Stokes equations under the axisymmetric assumption. The equation are written in a body-fitted, moving prolate spheroid, system of coordinates and solved using a fractional step method. The system is forced by a given volume time-law derived from clinical data, and varying the two-degrees-of-freedom ventricle geometry on the basis of a simple model. The solution under healthy conditions is analysed in terms of vorticity dynamics, showing that the flow field is characterised by the presence of a vortex wake; it is attached to the mitral valve during the accelerating phase of the E-wave, and it detaches and translate towards the ventricle apex afterwards. The flow evolution is discussed, results are also reported as an M-mode representation of colour-coded Doppler velocity maps. In the presence of ventricle dilatation the mitral jet extends farther inside the ventricle, propagation velocity decreases, and the fluid stagnates longer at the apex.

The vortex—an early predictor of cardiovascular outcome?

Blood motion in the heart features vortices that accompany the redirection of jet flows towards the outlet tracks. Vortices have a crucial role in fluid dynamics. The stability of cardiac vorticity is vital to the dynamic balance between rotating blood and myocardial tissue and to the development of cardiac dysfunction. Moreover, vortex dynamics immediately reflect physiological changes to the surrounding system, and can provide early indications of long-term outcome. However, the pathophysiological relevance of cardiac fluid dynamics is still unknown. We postulate that maladaptive intracardiac vortex dynamics might modulate the progressive remodelling of the left ventricle towards heart failure. The evaluation of blood flow presents a new paradigm in cardiac function analysis, with the potential for sensitive risk identification of cardiac abnormalities. Description of cardiac flow patterns after surgery or device therapy provides an intrinsic qualitative evaluation of therapeutic procedures, and could enable early risk stratification of patients vulnerable to adverse cardiac remodelling.

On the Left Ventricular Vortex Reversal after Mitral Valve Replacement

The blood flow in the human left ventricle is known to develop a vortical motion that should facilitate the ejection of blood into the primary circulation. This study shows that such a rotary motion can be totally reversed after the implant of a prosthetic valve. This phenomenon, in agreement with clinical observation, appears mostly imputable to the symmetry of the implant. The reversed rotation increases energy dissipation and modifies the pressure distribution with the potential development of new pathologies. The results provide preliminary, physically based, elements for the improvement of surgical procedures or prosthesis.

Imaging Cardiac Resynchronization Therapy

Although a prognostic benefit has been shown from cardiac resynchronization therapy, questions are often directed toward the prediction of symptomatic or functional benefit. Recent multicenter trials have shown the pitfalls of current mechanical markers of left ventricular synchrony, but these negative trial results have not marked the conclusion of efforts to predict outcome. Potential new contributors to the assessment of mechanical synchrony include echocardiographic and magnetic resonance techniques for the assessment of myocardial deformation. Nonsynchrony markers that seem promising include assessment of the location and extent of myocardial scar and imaging of the coronary venous and phrenic nerve anatomy.

Cardiac fluid dynamics anticipates heart adaptation

Hemodynamic forces represent an epigenetic factor during heart development and are supposed to influence the pathology of the grown heart. Cardiac blood motion is characterized by a vortical dynamics, and it is common belief that the cardiac vortex has a role in disease progressions or regression. Here we provide a preliminary demonstration about the relevance of maladaptive intra-cardiac vortex dynamics in the geometrical adaptation of the dysfunctional heart. We employed an in vivo model of patients who present a stable normal heart function in virtue of the cardiac resynchronization therapy (CRT, bi-ventricular pace-maker) and who are expected to develop left ventricle remodeling if pace-maker was switched off. Intra-ventricular fluid dynamics is analyzed by echocardiography (Echo-PIV). Under normal conditions, the flow presents a longitudinal alignment of the intraventricular hemodynamic forces. When pacing is temporarily switched off, flow forces develop a misalignment hammering onto lateral walls, despite no other electro-mechanical change is noticed. Hemodynamic forces result to be the first event that evokes a physiological activity anticipating cardiac changes and could help in the prediction of longer term heart adaptations.

Space and time dependency of inertial and convective contribution to the transmitral pressure drop during ventricular filling

We have read with much interest the recent article by Firstenberg et al. [(1)][1], which provides new insights into the comprehension of noninvasive assessment of transmitral pressure drop. In their Figure 2, the investigators display a graph demonstrating how the inclusion of inertial term in the

Changes in electrical activation modify the orientation of left ventricular flow momentum: novel observations using echocardiographic particle image velocimetry

AIMS Changes in electrical activation sequence are known to affect the timing of cardiac mechanical events. We aim to demonstrate that these also modify global properties of the intraventricular blood flow pattern. We also explore whether such global changes present a relationship with clinical outcome. METHODS AND RESULTS We investigated 30 heart failure patients followed up after cardiac resynchronization therapy (CRT). All subjects underwent echocardiography before implant and at follow-up after 6+ months. Left ventricular mechanics was investigated at follow-up during active CRT and was repeated after a temporary interruption <5 min later. Strain analysis, performed by speckle tracking, was used to assess the entity of contraction (global longitudinal strain) and its synchronicity (standard deviation of time to peak of radial strain). Intraventricular fluid dynamics, by echographic particle image velocimetry, was used to evaluate the directional distribution of global momentum associated with blood motion. The discontinuation of CRT pacing reflects into a reduction of deformation synchrony and into the deviation of blood flow momentum from the base-apex orientation with the development of transversal flow-mediated haemodynamic forces. The deviation of flow momentum presents a significant correlation with the degree of volumetric reduction after CRT. CONCLUSION Changes in electrical activation alter the orientation of blood flow momentum. The long-term CRT outcome correlates with the degree of re-alignment of haemodynamic forces. These preliminary results suggest that flow orientation could be used for optimizing the biventricular pacing setting. However, larger prospective studies are needed to confirm this hypothesis.