Assessing Intracardiac Vortices with High Frame-Rate Echocardiography-Derived Blood Speckle Imaging in Newborns

Summary

The present protocol uses echocardiography-derived blood speckle imaging technology to visualize intracardiac hemodynamics in newborns. The clinical utility of this technology is explored, the rotational body of fluid within the left ventricle (known as a vortex) is accessed, and its significance in understanding diastology is determined.

Abstract

The left ventricle (LV) has a unique pattern of hemodynamic filling. During diastole, a rotational body or ring of fluid known as a vortex is formed due to the chiral geometry of the heart. A vortex is reported to have a role in conserving the kinetic energy of blood flow entering into the LV. Recent studies have shown that LV vortices may have prognostic value in describing diastolic function at rest in neonatal, pediatric, and adult populations, and may help with earlier subclinical intervention. However, the visualization and characterization of the vortex remain minimally explored. A number of imaging modalities have been utilized for visualizing and describing intracardiac blood flow patterns and vortex rings. In this article, a technique known as blood speckle imaging (BSI) is of particular interest. BSI is derived from high-frame rate color Doppler echocardiography and provides several advantages over other modalities. Namely, BSI is an inexpensive and noninvasive bedside tool that does not rely on contrast agents or extensive mathematical assumptions. This work presents a detailed step-by-step application of the BSI methodology used in our laboratory. The clinical utility of BSI is still in its early stages, but has shown promise within the pediatric and neonatal populations for describing diastolic function in volume-overloaded hearts. A secondary aim of this study is thus to discuss recent and future clinical work with this imaging technology.

Introduction

Intracardiac blood flow patterns play a key role in cardiac development, starting in fetal morphogenesis and continuing throughout the lifespan¹. Hemodynamic shear stress plays a pivotal role in the stimulation of cardiac chamber growth and architecture via the activation of specific genes^2,3. This occurs at both the intrauterine stage and in the early stages of life, thus highlighting the importance of hemodynamic influence on early cardiac development and the carry-over into adulthood³.

The laws of fluid dynamics state that blood passing along a vessel wall move slower when closest to the wall and faster when in the center of a vessel, where resistance is lower. This phenomenon can be demonstrated in any large vessel with pulse wave Doppler as the typical Doppler velocity time integral envelope⁴. When blood enters a larger cavity such as the heart, the blood farthest from the endocardial surface continues to increase its velocity relative to the blood closest to that surface and create a rotational body of fluid, known as a vortex. Once created, vortices are self-propelling flow structures that typically draw in surrounding fluid via negative pressure gradients. Thus, a vortex can move a greater volume of blood than an equivalent straight jet of fluid, promoting greater cardiac efficiency^4,5.

The literature suggests that the evolutionary purpose of vortices is to conserve kinetic energy, minimize shear stress, and maximize flow efficiency^4,5,6. Specifically for the heart, this includes storing hemodynamic energy in a rotary motion, facilitating valve closure, and the propagation of blood flow toward the outflow tract, as seen in Figure 1. Altered intracardiac blood flow patterns are expected in pathological situations such as volume-overloaded states and in cases with artificial valves^7,8. Thus, herein lies the true diagnostic potential of vortices as early predictors of cardiovascular outcomes in adults.

Intracardiac hemodynamics have gained increasing interest in the literature in both adult and pediatric populations. Several modalities are available for the qualitative and quantitative assessment of intracardiac hemodynamics and were comprehensively summarised in a recent review, with a specific emphasis on the intracardiac vortex⁹. One modality with great promise is echocardiography-derived blood speckle imaging (BSI), which offers the ability to noninvasively measure a number of qualitative and quantitative vortex characteristics, described below, at a relatively low cost and with excellent reproducibility¹⁰. BSI is currently commercially available using a high-end cardiac ultrasound system with an S12 or S6 MHz probe. The speckle-tracking features are analogous to those used in tissue speckle tracking to study myocardial deformation^11,12,13. Since red blood cells tend to move faster and with a higher Doppler frequency than the surrounding tissue, the two signals can be separated by applying a temporal filter. BSI uses a best-match algorithm to quantify the movement of blood speckles directly without using contrast agents. The blood velocity measurements can be visualized as arrows, streamlines, or path lines with or without underlying color Doppler images, and can highlight areas of complex flow¹⁰.

BSI has been shown to have good feasibility and accuracy for quantifying intracardiac blood flow patterns, with excellent validity compared to a reference phantom instrument and pulsed-Doppler^7,10,11. Whilst still very novel, BSI is a promising clinical tool for the early diagnosis of various cardiac pathophysiologies. The clinical application of vortex imaging has shown promise in newborn infants. Specifically, the behavior of a vortex in the left ventricle (LV) may have long-term implications on cardiac remodeling and predisposition toward heart failure.

The mechanism linking vortices to left ventricular remodeling is still relatively unexplored, but has been recently investigated in our laboratory and is the subject of ongoing work¹¹. This methodology article aims to describe the use of BSI in exploring intracardiac vortices and discuss the practical and clinical uses of vortices in assessing diastolic function in various populations. A secondary aim is to discuss the clinical relevance of BSI and present some of the work previously performed in neonates.

Protocol

All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards. Informed consent was obtained from all individual participants' families included in the study. All images and video clips were de-identified following the acquisition.

1. Patient preparation

1. Set up the ultrasound machine adjacent to the patient's cot and connect a three-lead electrocardiogram (see Table of Materials).
2. Input the patient code and relevant details, such as body length and weight, and perform the echocardiogram according to the previously described standards¹².

2. Image acquisition

1. Specifically for BSI, obtain a shallow view of the LV in the apical four-chamber view with a narrow sector width, allowing an acquisition frame rate between 400-600 Hz.
2. Open a color box over the left ventricular cavity, narrow maximally to include only the region from the mitral valve to the endocardial apex, and from the septal endocardial border to the lateral wall endocardial border.
3. Increase the color gain to the point of speckling and reduce slightly. Set the color Doppler velocity scale limit to the appropriate diastolic velocity (20-30 cm/s in preterm infants) to maximally fill the color box with the slower-moving diastolic inflow.
4. On the touchscreen control panel of the equipment (see Table of Materials), tap BSI mode to reveal the intracardiac flow directions and vortices in RAW color format. Adjust the BSI box position and size to include the flow region of interest and record at least two cardiac cycles.
5. Repeat the procedure in the apical LV long-axis view or other views where intracardiac hemodynamic assessment is required (Figure 2 and Figure 3).

3. Image analyses

NOTE: The image analysis techniques for the LV vortex have been briefly described in previous work from our laboratory¹¹. The protocol used for assessing intracardiac vortices is as follows (Figure 3 and Figure 4).

1. Save two cardiac cycles from each respective patient to external media in their RAW DICOM format and transfer to a laboratory station with an image processing software (see Table of Materials) installed for detailed offline analyses.
2. Once offline, identify the most prominent or main vortex.
   NOTE: The main vortex is visualized as an elongated, oval-shaped, anti-clockwise rotating structure located in the upper left quadrant of the left ventricle near the septum, with the maximum vortex area found in late diastole (during the transmitral A-wave) in preterm infants (Video 1). The main vortex is usually found during the transmitral E-wave for older infants and children.
3. Record the number of independent, complete oval-shaped vortices forming throughout the cardiac cycle for each clip.
4. Measure the position of the main vortex relative to known landmarks within the LV. To determine the Vortex depth, using the "distance measurement" tool on the analysis software, measure the vertical distance from the vortex eye to the middle of the mitral valve annulus. For Vortex transverse position, measure the horizontal distance from the vortex eye to the endocardial border of the interventricular septum.
5. Measure the vertical and horizontal edge-to-edge distances of the main vortex relative to the LV length and width to obtain the vortex shape.
   NOTE: This also enables estimation of the vortex sphericity index as length divided by width.
6. Using the "tracing measurement" tool on the analysis software, click on and trace the outermost vortex ring at the point where the main vortex is most prominent to determine the main vortex area.
7. To assess Peak Vortex Formation Time (PVFT), record the cardiac frame when the vortex first appears (circular rings delineated) in the cardiac frame where the main vortex is most prominent and calculate the number of frames relative to the total number of frames in one cardiac cycle for the patient.
8. To assess vortex duration, measure the frames from which the vortex first appears when the vortex loses its circular ring formation. Vortex duration is then calculated as the number of frames relative to that patient's total number of frames in one cardiac cycle (Figure 5).

Results

The acquisition of vortex clips is comparable to the standard methodology universally employed in obtaining color Doppler clips. Pioneering studies in adults have described vortices using the apical two-, three-, and four-chamber views¹⁴. The LV vortex is a ring-like structure that moves from base to apex. BSI visualizes the internal diameter of the ring (Figure 2). A vortex ring is usually not symmetrical in shape, hence alternative imaging planes can show variable v...

Discussion

The importance of visualizing and understanding the intracardiac vortex
There are many possible clinical applications of high-frame rate echocardiography-derived vortex imaging. Their ability to provide valuable insight into intracardiac flow dynamics has been the interest of recent studies¹⁶. Moreover, vortex imaging may allow the detection of pre-symptomatic changes in LV architecture and function in neonates, which may have a bearing on long-term cardiac remodeling into a...

Materials

Name	Company	Catalog Number	Comments
Tomtec Imaging Systems GmbH	Phillips	GmbH Corporation	Offline ultrasound image processing tool, used for calculating all vortex measurements
Vivid E95	General Electrics	NA	Cardiac Ultrasound device used to capture Echocardiography-derived Blood Speckle Imaging