Cardiac resynchronization therapy (CRT) is recommended for patients with chronic heart failure who remain symptomatic (NYHA class II-IV) despite optimal medical treatment, with severe systolic dysfunction (left ventricular ejection fraction (LVEF) ≤35%) and widened QRS complex.
Echocardiographic assessment of CRT candidates comprises:
- evaluation of left ventricular global function (ejection fraction and volumes)
- evaluation of left ventricular regional function (extent and localization of infarct scar) and
- evaluation of cardiac dyssynchrony.
The measurement of LVEF determines the candidacy for CRT, whereas the end-systolic volume reduction of ≥ 10-15% after 3-6 months of device implantation usually defines echocardiographic response to CRT. Since patients with high scar burden (especially those with posterolateral scar) are less likely to respond to CRT, regional function assessment should also be included in the echocardiographic report.
The definition and evaluation of cardiac dyssynchrony are still subject to debate. An electrical criterion, QRS duration >120 ms on surface electrocardiogram (ECG), is currently the only guideline-recommended dyssynchrony parameter for patient selection for cardiac resynchronization therapy (CRT), but allows identification of only 60-70% of responders. In order to improve CRT success rate, a plethora echocardiographic parameters to identify true mechanical dyssynchrony, i.e. uncoordinated contraction in different myocardial regions, has been proposed, with no consensus on the incremental value of any of these parameters.
Various echocardiographic modalities have been used for dyssynchrony assessment - from conventional M-mode and pulsed-wave Doppler techniques to myocardial deformation imaging and three-dimensional echocardiography.
Echocardiographic evaluation of dyssynchrony involves assessment of atrioventricular, inter- and intraventricular dyssynchrony.
Atrioventricular dyssynchrony can be assessed by using pulsed-wave Doppler recording of transmitral flow. Diastolic filling time (LVFT), defined as the sum of E-wave and A-wave duration, is divided by the RR interval duration (Figure 1) to obtain a diastolic filling ratio (LVFT/RR).
Significant atrioventricular dyssynchrony is assumed if LVFT/RR is <40%. Short- and long-AV delays can also be visually identified from the transmitral flow patterns (Figure 2), which is frequently used strategy for atrioventricular delay optimization after CRT pacemaker implantation.
Atrioventricular dyssynchrony has limited predictive value for CRT response.
Interventricular dyssynchrony refers to dyssynchrony between the left and the right ventricle and can be measured using conventional pulsed-wave Doppler or Tissue Doppler imaging (Figures 3 and 4, respectively).
The presence of inter-ventricular dyssynchrony is indicated by the difference of >40 ms between left ventricular and right ventricular pre-ejection time (measured by pulsed-wave Doppler) or by a delay of >56 ms between the onset of systolic motion in the basal right ventricular free wall versus the most delayed basal LV segment (measured by tissue Doppler).
Recent studies have reported a limited value of inter-ventricular dyssynchrony for predicting CRT response.
Intraventricular dyssynchrony can be evaluated by conventional echocardiography, tissue velocity measurements and deformation imaging (Table 1).
|Septal to posterior wall motion delay||M-mode||≥ 130 ms|
|Septal to lateral Ts delay||Tissue velocity imaging||≥ 60 ms|
|Max delay in Ts in 4 basal LV segments||Tissue velocity imaging||> 65 ms|
|SD of Ts of 6 basal LV segments||Tissue velocity imaging||≥ 36.5 ms|
|Max delay in Ts in 12 basal and mid LV segments||Tissue velocity imaging||≥ 100 ms|
|SD of Ts in 12 basal and mid LV segments (Dyssynchrony Index)||Tissue velocity imaging||≥ 32.6 ms|
|Anteroseptal to posterior time to peak strain difference (radial strain)||2D speckle tracking||≥ 130 ms|
|SD of time-to peak longitudinal strain in 12 basal and mid LV segments||Colour –Tissue Doppler imaging||> 60 ms|
|SD of time to minimum systolic volume of 16 LV segments (systolic dyssynchrony index)||3D echocardiography||> 5.6 %|
Conventional echocardiographic markers of dyssynchrony comprise:
- septal to posterior wall motion delay (cut-off >130 ms) and
- left ventricular electromechanical delay (cut-off >140 ms)
Septal to posterior wall motion delay is assessed by M-mode echocardiography from parasternal short-axis view at the papillary muscle level. It is calculated as the interval between the maximal posterior displacement of the septum and the maximal displacement of the left posterior wall (Figure 5). This method is not applicable in patients with previous septal or posterior wall myocardial infarction.
Left ventricular electromechanical delay (pre-ejection time) is determined as the time from QRS onset to aortic flow onset (Figure 3B). Predictive value of both methods is disputed.
Tissue velocity imaging
With tissue velocity imaging, longitudinal velocities of basal (or basal and mid) myocardial segments are measured from standard apical views.
Tissue velocity- derived dyssynchrony parameters can be broadly divided into:
time delays between opposing walls and
standard deviations of time-to-peak systolic velocities.
Methods involving measurements of longitudinal velocities from 2, 4, 6 and 12 myocardial segments have been described (Table 1). Regional myocardial velocity curves can be created online using spectral pulsed tissue Doppler imaging or offline from digitally stored color tissue Doppler cine-loops. The latter method reduces the time for both online image acquisition and offline image analysis. Since systolic velocity usually refers to the ejection phase, the first step in dyssynchrony measurements is to define the ejection period by indicating the aortic valve opening and closure in the pulsed-wave Doppler recording in the left ventricular outflow tract. Second step is to identify the highest systolic peak between aortic valve opening and closure, which is not necessarily the first peak. If there are several peaks in the ejection phase with the same amplitude, the earliest peak should be selected. Although tissue velocity imaging-based parameters appeared promising in several single-center studies, recent multicenter trial raised doubts regarding the reproducibility and predictive value of these parameters.
In contrast to the timing of myocardial velocity peaks, myocardial deformation parameters (strain, strain rate) have the potential of distinguishing active contraction from passive motion caused by tethering of adjacent myocardial regions. Deformation data can be obtained from color Tissue Doppler or two-dimensional speckle tracking images.
Standard deviation of time-to-peak longitudinal strain is a Doppler-derived parameter which can be calculated after the extraction of regional longitudinal strain curves from 12 basal and mid segments in standard apical views. The standard deviation of time from ECG onset to peak negative strain > 60 ms has been proposed as a strain dyssynchrony index. The data on superiority of this parameter over longitudinal velocity-based indices are contradictory.
At present, the difference ≥130 ms in peak radial strain between the basal anteroseptal and basal posterior wall segments is one of most commonly used speckle tracking-based dyssynchrony parameter (Figure 6).
Apical rocking and septal flash
The direct mechanical consequences of dyssynchronous contraction induced by left bundle branch block can be described by apical rocking and septal flash. A short initial septal contraction within the isovolumic contraction period results of a short inward motion of the septum (septal flash) and causes the apex to move septally. The delayed activation of the lateral wall pulls then the apex laterally during the ejection time while stretching the septum. This typical motion pattern of the apex is described “apical rocking”. The presence of septal flash can be visualized in parasternal (short- and long-axis) and apical (4-chamber) views. It can also be identified on gray scale images or colour M-mode, and quantified by the amplitude of early radial septal velocities (Figure 7).
Apical rocking motion can be easily visualized in apical 4-chamber view and quantified by measuring apical transverse motion (Video 1).
Video 1: Typical apical rocking in a patient with left bundle branch block
Both apical rocking and septal flash have been shown to have predictive value for a CRT response which is superior to velocity-based dyssynchrony parameters.
Although the feasibility of three-dimensional echocardiography to determine the timing of regional volumetric changes has been demonstrated, at present, there is no sufficient evidence to recommend three-dimensional echocardiography for dyssynchrony assessment.