Evaluation of Diastolic Function by Tissue Doppler, Strain, and Torsion Analysis

Introduction

Early Doppler echocardiographic indices of left ventricular (LV) diastolic function have studied the mechanics of left atrial (LA) and ventricular filling. These filling indices have been proven to be useful in providing prognostic information in heart failure patients. However, filling indices have limited accuracy predicting intrinsic parameters of diastolic function due to the confounding effects of extrinsic loading conditions and intrinsic cardiac performance. Tissue Doppler imaging (TDI) overcame some of the disadvantages of pulse wave Doppler, but it is angle-dependent and still faces several important limitations. Recently, two-dimensional (2-D) and three-dimensional (3-D) speckle tracking has been applied to study LV myocardial mechanics. Compared to Doppler imaging, they provide mechanical analysis of the whole thickness of myocardium seen in one echocardiographic view or the entire myocardium with the introduction of 3-D speckle tracking. Since myocardial mechanical events precede left atrial and ventricular filling, they may be potentially less dependent on extrinsic variables, and therefore more accurate characterizing intrinsic myocardial properties.

In this chapter, we will review the role of 2-D and 3-D speckle tracking echocardiography (STE) and their derivatives for the assessment of LV diastolic function.

Case Study

A 62-year-old obese man with a past medical history significant for hypertension has been complaining of chronic shortness of breath and fatigue mostly with exertion. He went to see his primary care medical doctor and a diagnosis of asthma was made. He was started on bronchodilators, but his symptoms did not improve. Physical examination was notable for obesity. The remainder of his examination was unremarkable. His electrocardiogram (ECG) showed sinus rhythm with normal voltage. The patient underwent a complete 2-D transthoracic echocardiogram, which revealed normal LV ejection fraction (EF). There was grade I diastolic dysfunction (a mitral inflow pattern revealed abnormal relaxation with a normal range of early filling velocity/early filling contraction E/e′). There was no significant valvular disease. 2-D speckle tracking global longitudinal strain was obtained from apical windows. It was noted to be reduced ( Fig. 10.1 ). Based on these findings, the patient was sent for a bicycle exercise stress echocardiogram. At 50 watts, the exercise mitral inflow pattern dramatically changed from normal to restrictive physiology with a significantly elevated E/e′. A diagnosis of diastolic heart failure was made ( Fig. 10.2 ).

Technical Considerations

Measurements of Myocardial Deformation

Myocardial Velocities

Myocardial velocities can be obtained from multiple locations of the myocardium using TDI.

In technical terms, to display the velocities of the myocardium, TDI differs from standard Doppler by eliminating the high pass filter and using low gain amplification. TDI velocities may be displayed in spectral pulsed mode or in color encoded 2-D maps superimposed over structural images.

In a typical spectral display, the myocardial velocity waveform displays a positive wave representing ventricular systole (s′) and two waves corresponding to early filling (e′) and atrial contraction (a′). From the apical acoustic windows, the diastolic myocardial velocities obtained from any of the LV myocardial segments appear as a mirror image of the mitral inflow early (E) and atrial (A) filling velocities. In normal humans, the peak of e′ precedes the peak of LV filling E velocity, suggesting that active relaxation of the myocardium generates negative pressures in the LV cavity that initiate LV filling.

Fig. 10.1, Bull’s eye image of peak systolic global longitudinal strain.

Fig. 10.2, Mitral flow and annular velocity at rest and during supine bicycle exercise.

TDI velocities are influenced not only by regional LV mechanical events but also by global translation and rotation of the heart. Since LV longitudinal velocities are less affected by translational motion, they are usually obtained from the apical acoustic window ( Figs. 10.3 and 10.4 ).

Fig. 10.3, Longitudinal axis tissue Doppler velocities obtained using spectral Doppler from the basal lateral left ventricular segment. a’ , Atrial contraction velocity; e’ , early diastolic velocity; s’ , systolic velocity.

Fig. 10.4, Color-encoded tissue Doppler velocities obtained from the four-chamber view during (A) systole and (B) diastole.

Alternatively, translational motion may be corrected by off-line analysis from color-encoded tissue Doppler echocardiography (TDE) velocities. One method plots the velocity of each adjacent scan line from the distance from epicardium to endocardium. From a parasternal color M mode image, the rates of circumferential fiber shortening and lengthening are proportional to the slope of the velocity/distance regression line. The value of this slope has been referred to as the myocardial velocity gradient (MVG).

Strain

The term deformation refers to the myocardium changing shape and dimensions during the cardiac cycle. Strain is a unitless measurement of deformational change of the myocardium and is often expressed as a percentage. It is well known that the spiral architecture of the myocardial fiber bundles deforms in multiple directions; strain imaging can identify and measure that deformation. Thus changes in LV geometry during LV systole relate primarily to radial (short axis), longitudinal (long axis), circumferential, and torsional strain. Lengthening or thickening of the myocardium is represented by positive strain values, whereas negative values represent shortening or thinning.

The concept of strain is complex, but linear strain can be defined by the Lagrangian formula:

ɛ = L - L_{0} / L_{0} = Δ L / L_{0},

$ɛ = L - L_{0} / L_{0} = Δ L / L_{0},$

where ɛ is strain, L ₀ is baseline length, and L is the instantaneous length at the time of measurement ( Figs. 10.5 and 10.6 ).

Fig. 10.6, The three main directions of the left ventricle: longitudinal (L), transverse/transmural or radial (T), and circumferential (C).

Multiple noninvasive techniques can quantify myocardial strain, including using magnetic resonance imaging (MRI), TDI, or high frame rate 2-D echocardiography with speckle tracking.

The first ultrasound-based strain measurements were derived from TDI but were limited to the interrogation of a few segments aligned with the Doppler beam. With the introduction of speckle tracking imaging, interrogation from all strain components became feasible from most myocardial segments, becoming as of today the preferred method. Global longitudinal strain can be calculated from apical views, while radial and circumferential strain are calculated from short axis views ( Figs. 10.7 and 10.8 ). With the introduction of 3-D speckle tracking imaging, regional and global mechanics may be obtained in the full thickness of myocardium. 3-D provides assessment of area strain that represents a combination of longitudinal and circumferential strain. However, one of the major limitations in 3-D strain is lower temporal and spatial resolution, thus in the clinical setting strain is obtained from multiple 2-D views.

Fig. 10.7, (A) Global longitudinal strain, which is calculated from apical views from most commercial available systems. (B) Deformation curves and curved anatomic M mode mapping were also provided by the software. (C) Bull’s eye of peak systolic strain.

Fig. 10.8, Longitudinal, radial, circumferential strains curves with torsion.

Strain Rate

The strain rate (SR) is the instantaneous strain (or change in strain) per time unit.

ɛ = Δ ɛ / Δ t

$ɛ = Δ ɛ / Δ t$

The unit of strain rate is cm/s, or s ⁻¹ . The strain rate has the same direction as strain (i.e., negative strain rate during shortening, positive strain during elongation). SR corresponds to the velocity profile of a myocardial segment through the cardiac cycle. SR reflects the shortening/lengthening rate of the studied myocardial segment relatively independently of possible tethering or whole-body effects, an advantage over simple velocity measurements ( Fig. 10.9 ).

Fig. 10.9, Strain and strain rate from normal (blue) and ischemic myocardium (red) . There is an early peak positive strain (PPS) in ischemic myocardium, and peak systolic strain (PSS) is typically lower than end-systolic strain (ESS) . A postsystolic shortening is often seen, and peak strain (PS) therefore occurs after end systole. The thicker and thinner tracings are obtained from ischemic and normal myocardium, respectively.

Twist and Torsion

In addition to radial and longitudinal deformation, there is twist and torsional deformation of the LV, which play an important role in LV filling. There is a systolic twist and an early diastolic untwist of the LV of oppositely directed apical and basal rotations, which is expressed in degrees. The term torsion is the twist normalized value to the distance between the LV apex and base and refers to the base-to-apex gradient in rotation angle, expressed in degrees per centimeter (°/cm).

During initial isovolumic contraction, the apex and the base both rotate in a counterclockwise direction when viewed from apex to base. Subsequently, during systole, the base changes direction and starts to rotate in a clockwise direction, while the apex continues to rotate in a counterclockwise direction.

LV torsion is followed by rapid untwisting, which contributes to ventricular filling. Because LV torsion is directly related to fiber orientation, it might reflect subclinical abnormalities in heart function, especially diastolic dysfunction. Initially, LV twist and torsion were measured invasively using implanted sonomicrometer units. Later, for LV deformation and rotation, tissue tagging cardiac magnetic resonance was regarded as the noninvasive reference standard. Recently, speckle tracking echocardiography was introduced for quantification of LV twist and torsion, providing higher spatial and temporal resolution compared to MRI. STE measurements of LV twist and torsional deformation have been tested and validated with sonomicrometry in animal models and with MRI tagging in humans ( Fig. 10.10 ). However, 2-D echocardiography does not provide an accurate determination of the distance between the two image planes required for quantification of torsional deformation. Because of this 2-D limitation, recent investigational efforts have shifted to speckle tracking analysis using real-time 3-D echocardiographic data sets. Validation of 3-D STE has been performed in animal studies using sonomicrometry as the reference standard and in simulated models and in vitro as well.

Fig. 10.10, Diagram indicating the vectors of apical counterclockwise and basal clockwise twist. Torsion is the sum of both components.

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