INTRODUCTION
Cardiac resynchronization therapy (CRT) using atriobiventricular pacing has evolved into a useful therapeutic option for patients with heart failure, refractory to optimal medical therapy. Improvements in quality of life, morbidity, mortality, severity of mitral regurgitation (MR) and left ventricular (LV) function have been demonstrated in large randomized clinical trials but the treatment is invasive and expensive. Besides, about one third of patients remain unresponsive in spite of the device implantation using the existing criteria. There is thus a need to carefully identify the patients most likely to benefit by this intervention using indices of mechanical dyssynchrony. Out of the several imaging and electrophysiological modalities studied, echocardiography has emerged as the modality having
the maximum potential to identify the subset of patients most likely to respond. Although the current selection criteria for CRT do not include assessment of mechanical dyssynchrony1, clinicians and researchers recognize the potential role of echocardiography in carefully selecting patients for this form of therapy and noninvasive cardiologists are frequently called upon to perform a “dyssynchrony analysis” towards this aim. Several echocardiography and ancillary technique based criteria
have been proposed but it has been shown that factors like
reproducibility and feasibility interfere with the results and there is considerable variability when comparisons are made in a multicenter setting2. This article aims to present the current state of the art in quantifying mechanical dyssynchrony so as to minimize these effects and assist in selection of suitable candidates for CRT. However, this is not a comprehensive review but a practical guide to performing some of the commonest measurements in routine practice.
ASSESSMENT OF MECHANICAL
DYSSYNCHRONY
The three types of cardiac dyssynchrony that have been described are: atrioventricular (AV), interventricular (VV), and
intraventricular dyssynchrony. Atrioventricular dyssynchrony Atrioventricular dyssynchrony is said to be present when the diastolic filling period (DFP) occupies less than 40% of cardiac cycle (measured by R-R interval).
1. To obtain DFP, obtain the apical four-chamber view with

mitral valve in the centre of the frame.
2. Using pulsed Doppler with the sample volume placed at the tips of mitral leaflets; obtain a spectral display of mitral inflow pattern.
3. Measure the time from the onset to the end of the spectral
display- this represents DFP.
4. Measure the R-R interval using any two consecutive regular beats. Divide DFP with R-R interval and multiply by hundred to obtain a percentage value (Figure 1).

Interventricular dyssynchrony
Interventricular mechanical dyssynchrony is assessed by
measuring the interventricular mechanical delay. A difference of 40 milliseconds (ms) or more in the onset of ejection of right ventricular (RV) and LV is considered significant.

1. To obtain time to onset of LV ejection, obtain the apical five-chamber view and place the pulsed-Doppler sample volume at the left ventricular outflow tract (LVOT).
2. Measure the time from the onset of QRS complex to the
onset of the pulsed-Doppler flow velocity. This is the LV
pre-ejection period (LVPEP). 3. To obtain the time to onset of RV ejection, obtain the left ventricular outflow tract (RVOT) view (para-sternal shortaxis) and place the pulsed-Doppler sample volume at RVOT. Measure the time interval from onset of QRS complex to the onset of pulse-Doppler velocity curve. This is the RV pre ejection period (RVPEP).
4. The difference of LVPEP and RVPEP gives interventricular
mechanical delay (figure 2).

TDI FOR ASSESSMENT OF
INTRAVENTRICULAR DYSSYNCHRONY
A) Measurement of time to peak- systolic longitudinal
velocity
Assessment of longitudinal shortening velocities from the apical
window with TDI has been studied extensively and several
indices proposed5. There are two approaches possible- pulsed-
TDI or color-coded TDI but owing to several limitations with
pulsed-TDI, color TDI is now the preferred method and is
described here.
1. Obtain a noise free ECG trace and good quality 2D image
from the apical window with the LV cavity in the centre of
image sector and the depth adjusted to include the mitral
annulus.
2. Activate color TDI, adjusting the sector width so as to keep
the frame rate > 90 frames/second.
3. Suspend breathing if possible and acquire 3-5 beats (sinus
rhythm) or more (in case of ectopics).
The same sequence is followed to acquire images in the
three apical views- 4-chamber, 2-chamber and long-axis
view.
4. Apical 5-chamber view is used to record the velocity trace
at LVOT using pulsed-Doppler. Time interval from the
onset of LV ejection to the end of LV ejection is measured.
These time interval will be used for identification of systolic
velocities and their differentiation from velocities during
the isovolumetric contraction period and during post-systolic
period in the subsequent analysis.
Measurement of time-to-peak systolic velocity can now be
performed both online and offline, as described below-
5. Select the view (4C, 2C and apical long-axis) and position
the regions of interest (~5-10mm) in each of the basal and
mid segments thus producing four time-velocity curves for
each view.
6. As mentioned, the ejection time is superimposed on the
velocity curves and helps to identify the various components
of the velocity curve-the isovolumetric contraction wave,
the systolic wave (S), the isovolumetric relaxation wave,
early (E) and late (A) diastolic waves (figure 4).
7. The time from onset of QRS to the peak of systolic wave is
obtained for each segment (total 12 segments in 3 views).
Time difference between the opposing segment peak systolic
waves can also be measured directly in each view (total 6
time intervals in 3 views) (figure 5).
An average of at least 3-5 beats should be obtained, excluding
the sequences with atrial or ventricular premature beats to
improve reproducibility.
The data thus obtained can be used to calculate any of the several
proposed dyssynchrony indices, as below 5-

poor signal-to-noise ratio, limiting their accuracy and
reproducibility. Hence, their routine use for assessment of
dyssynchrony is not recommended. However, if desired, both S and SR can be measured from the same color TDI dataset acquired for measurement of myocardial velocities. Strain imaging application of the analysis software needs to be turned on for obtaining regional S and SR curves and time-delays among various segments can be measured during offline analysis.
ASSESSMENT OF RADIAL DYSSYNCHRONY
USING SPECKLE-TRACKING
Conventionally, myocardial velocities and deformation have
been measured using TDI. However, being Doppler-based, they are limited by angle dependence which remains a major challenge to its clinical utility. This makes measurement of myocardial velocities difficult if ultrasound beam can not be aligned parallel to the myocardial segment due to altered LV shape or in case of apical segments. In addition, circumferential strain can not be
measured at all using TDI whereas measurement of radial strain is restricted only to anterior septum and posterior wall in shortaxis view and even that is fraught with several fallacies. Recently, 2-dimensional speckle-tracking echocardiography (STE) has been developed to overcome the problem of angle dependency. Recent studies on radial strain, measured using STE, have shown it to have additive value in assessment of myocardial dyssynchrony6. To measure radial S with STE-
1. Obtain Gray-scale images using harmonic B-mode imaging in parasternal short-axis view (mid-ventricular level). Keep the frame rate of 50-70 frames/s. Three consecutive cardiac cycles are acquired during breath hold.
2. For analysis, single cardiac cycle is selected and the
endocardial border is traced manually at end-systole. The
region of interest is manually adjusted to include the entire
myocardial thickness - care should be taken to exclude
pericardium from the region of interest.
3. The analysis software then automatically selects stable
speckles within the myocardium and tracks these speckles
frame-by-frame throughout the cardiac cycle. The adequacy
of tracking can be verified manually and the region of
interest readjusted to achieve optimal tracking.
4. The software then automatically divides the entire LV
circumference in up to six conventional segments to generate
traces depicting regional radial Strain for each segment
(figure 6).
5. A difference of > 130 msec in the time-to-peak systolic
radial S of anterior septum and posterior wall is considered
to be predictive of response to CRT6. Value of other timing
intervals is not known at present.
Three-dimensional echocardiography for assessment
of dyssynchrony-

16 standard myocardial segments, as defined by the American Society of Echocardiography. The data can be represented in the form of global and regional volume curves, parametric maps and the calculated dyssynchrony index7 (figure 7). Quantification of mechanical dyssynchrony with RT3DE takes all myocardial segments into account by examining the composite effect of radial, circumferential, and longitudinal contraction and has been found to be reproducible with a variability of <10%8.

3. For TDI analysis, a sweep speed of 50-100 is recommended to improve temporal resolution.
4. Tissue Doppler analysis is highly angle dependent and misaligned images produce velocity curves consisting of not only the longitudinal but also the radial velocities. The sample volume should be as parallel as possible to the incident beam.
5. While analyzing color TDI data, the region-of-interest should be moved within the myocardial segment to obtain the most reproducible velocity curve with minimum artifacts.
6. If more than one systolic peak is obtained in the tissue velocity curve, the most reproducible peak of maximum amplitude should be used for analysis. If two or more peaks have same amplitude, the earlier peak should be taken into consideration.
7. For 3D imaging, optimum gain settings with clear delineation of the endocardial border, especially at the apex should be ensured.
8. A complete echocardiographic study along with quantitation of mitral regurgitation should always be performed in all cases.
9. Right ventricular systolic function should also be assessed routinely during dyssynchrony analysis and the findings should be reported.
10. No single parameter is predictive and there is no standard recommended approach. Hence, all the parameters possible depending on the laboratory and expertise should be evaluated.

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