Introduction
Normal regional left ventricular (LV) mechanical contraction is uniform and balanced, resulting in efficient ejection. Abnormalities in electrical activation or myocardial diseases may affect the timing of regional contraction, which results in discoordinated or dyssynchronous LV ejection, which is inefficient. Interest in quantifying dyssynchrony has been closely related to the advent of cardiac resynchronization therapy (CRT), also known as biventricular pacing. It was observed in randomized CRT clinical trials that 30–50% of heart failure (HF) patients with reduced ejection fraction (EF) were nonresponders as selected by electrocardiographic (ECG) criteria of QRS widening [1–3]. It has been observed that baseline LV mechanical dyssynchrony measured by various means is associated with favorable response to CRT [4–6]. Patients with QRS widening but with no measurable LV mechanical dyssynchrony did not appear to have as much benefit from CRT. Accordingly, it was initially hoped that measures of mechanical dyssynchrony would play a role in refining patient selection for CRT [7]. PROSPECT was a multicenter observational study that reported variability in echocardiographic dyssynchrony methods [8]. In addition, attempts to use CRT in HF patients with a narrow QRS complex and echocardiographic dyssynchrony have resulted in no clearly defined role for mechanical dyssynchrony in patient selection for CRT [9]. We have subsequently learned that the reasons for dyssynchrony are multifactorial and more complicated than originally thought. For example, LV dyssynchrony may occur from contractile heterogeneity or regional scar which is unresponsive to CRT, and a component of electrical delay is required for CRT response [10]. Imaging of strain patterns may play an important role in identifying the electromechanical substrate responsive to CRT. This newer understanding of dyssynchrony is providing important prognostic information and has the potential to make contributions to the care of HF patients treated with CRT.
Mechanical Discoordination
Several imaging approaches have been applied to assess abnormalities in the timing of regional contraction, usually timing delays from septal to LV free wall or the degree of heterogeneous regional contraction. The terms “dyssynchrony,” “asynchrony,” and “discoordination” have been used interchangeably in the scientific literature. The common feature of these terms is the differences in the timing of regional LV contraction. Frequently used approaches have included tissue Doppler imaging (TDI) measures of longitudinal velocity, in terms of septal to lateral wall delay (Figure 1), and the standard deviation of peak longitudinal velocity in 12 basal and mid segments [11, 12]. Because these velocity methods cannot differentiate active contraction from passive motion that may occur with tethering or scar strain, imaging methods using speckle tracking have gained favor for assessment of mechanical dyssynchrony [13]. The original echocardiographic strain approach was to assess peak-to-peak radial strain from short-axis views. We subsequently observed that strain patterns of electrical delay as usually occurs with left bundle branch block (LBBB) result in a peak-to-peak strain delay (Figure 2) and that strain patterns from LBBB associated with scar (Figure 3) also result in a peak-to-peak strain delay but with a different pattern [10]. Electromechanical delay patterns consist of septal contraction before aortic valve opening, associated with stretch of posterior lateral segments, known as systolic prestretch. LBBB has later contraction of posterior and lateral segments occurring during ejection and typically peaking after aortic valve closure and associated with septal stretch. Table 1 includes a partial list of echocardiographic means to assess mechanical dyssynchrony [10–16]. There have been a multitude of echocardiographic approaches to measure mechanical dyssynchrony. Accordingly, the focus will be on LV dyssynchrony, the measures most commonly used, and those with the most prognostic support in the literature. Routine pulsed Doppler measures of the time difference in right ventricular and LV ejection, known as the interventricular mechanical delay (IVMD), remain of use to predict response to CRT because of their simplicity [14]. IVMD is calculated as the time difference between the onset of the QRS complex to LV ejection and the onset of the QRS complex to right ventricular ejection. TDI measures of opposing septal to lateral wall delay or 12-site standard deviation, known as the Yu index, have been shown to be associated with response to CRT [12]. More recently, TDI signal cross-correlation has been a potential approach to assess LV dyssynchrony [17]. Speckle tracking methods have included peak-to-peak radial strain, described already, and the systolic stretch index (SSI), described in more detail later [10]. Visual methods to describe dyssynchrony have included septal flash and apical rocking, and a combined technique of speckle tracking longitudinal strain using visual assessment of the contraction patterns has been described [16, 18] (Figure 4). Many other echocardiographic approaches to mechanical dyssynchrony have been proposed. Further study and discovery may occur in the future.
Selected Echocardiographic Markers of Mechanical Dyssynchrony.
| Doppler methods | Approach and cutoffs |
|---|---|
| Interventricular mechanical delay | Pulsed Doppler time difference between RV preejection and LV preejection ≥40 ms |
| LV outflow tract and RV outflow tract | |
| Tissue Doppler longitudinal velocity from apical four-chamber view (two sites) | Time from peak septal to peak lateral wall velocity ≥65 ms or ≥80 ms |
| Yu index tissue Doppler longitudinal velocity from apical four-, two-, and three-chamber views (12 sites) | Standard deviation of 12-site peak velocity measures of 33 ms or greater |
| Tissue Doppler cross-correlation of myocardial acceleration from apical four-chamber view | Maximum activation delay from opposing septal and lateral walls greater than 35 ms |
| Speckle tracking methods | Approach and cutoffs |
| Speckle tracking radial strain | Time difference in peak septal to peak posterior wall strain ≥130 ms |
| Midventricular short-axis view | |
| Visual assessment of longitudinal strain pattern of typical left bundle branch from the apical four-chamber view | Early septal peak shortening; early stretching in lateral wall; lateral wall peak shortening after aortic valve closure |
| Systolic stretch index | Posterolateral prestretch (before aortic valve opening) and septal systolic stretch (to aortic valve closure) ≥9.7 % |
| Radial strain | |
| Midventricular short-axis view | |
| Visual echocardiographic methods | Approach |
| Septal flash from parasternal or apical views: M-mode, color tissue Doppler M-mode, or two-dimensional imaging | Presence or absence of brief inward and outward motion of the septum with respect to the LV during preejection |
| Apical rocking from the apical four-chamber view | Visual movement of apex toward septum early during preejection, followed by lateral motion of apex during ejection |
LV, left ventricular; RV right ventricular.
An Example of Color-coded Tissue Doppler Longitudinal Velocity from a Four-Chamber View in a Patient with Left Bundle Branch Block Demonstrating Early Septal Peak Velocity (Yellow) and Later Peak Lateral Wall Velocity (Turquoise).
AVC, aortic valve closure; AVO, aortic valve opening.
An Example of Radial Strain from Speckle Tracking Imaging of the Midventricular Short View in a Patient with Left Bundle Branch Block.
Six segments are color coded as labeled, demonstrating peak-to-peak dyssynchrony. Early septal thickening before aortic valve opening (AVO) is associated with posterolateral stretch, and delayed posterolateral thickening is associated with systolic septal stretch up to aortic valve closure (AVC). This pattern has been associated with favorable response to cardiac resynchronization therapy.
An Example of Radial Strain from Speckle Tracking Imaging of the Midventricular Short View in a Patient with QRS Widening and Anteroseptal Myocardial Infarction.
Six segments are color coded as labeled. There is peak-to-peak dyssynchrony. However, the timing and pattern of peak-to-peak dyssynchrony differ from those of typical left bundle branch block, and this pattern has not been associated with response to cardiac resynchronization therapy.
The Top Panels Show a Computer Simulation of Electrical Activation Delay as Occurs with a Left Bundle Branch Block with Early Septal Shortening Before Aortic Valve Opening (AVO) Associated with Posterolateral Stretch.
This is followed by posterolateral shortening associated with septal stretch, resulting in apical rocking as seen in the drawing of the echocardiographic four-chamber view. Modified from Gorcsan et al. [18].
Strain Patterns and Response to Cardiac Resynchronization Therapy
From work with computer modeling, strain patterns of myocardial substrates responsive to CRT were observed. Differences in regional timing may occur from three principal reasons: (1) electrical delay, (2) regional contractile heterogeneity, or (3) regional scar. A component of electrical delay is required to be associated with CRT response. Regional contractile heterogeneity from myocardial disease or regional scar without electrical delay may result in dyssynchrony that is not responsive to CRT, and that may potentially be harmful, as was observed in the EchoCRT study [9]. The characteristic strain pattern that represents electromechanical substrate of CRT response can be quantified by the systolic prestretch of the posterolateral wall (before aortic valve opening) from unopposed septal contraction and systolic septal stretch associated with delayed posterolateral contraction (Figure 5). The SSI combines the degree of systolic prestretch (expressed as a percentage) with the degree of systolic septal stretch (expressed as a percentage). For radial strain, an SSI cutoff of 9.7% was associated with favorable response to CRT [10]. The exact pathophysiological reason for SSI to be associated with response to CRT is not clear. However, nonelectrical myocardial substrates of peak-to-peak dyssynchrony, such as contractile heterogeneity, which may occur in nonischemic disease and scar occurring in ischemic cardiomyopathy, have low SSI values and are nonresponsive to CRT. Similar patterns of SSI with systolic prestretch and systolic septal stretch may be observed in longitudinal strain (Figure 6). Determining SSI from longitudinal strain has promise for future studies, because longitudinal strain measures have been considered stabler and less variable than radial strain measures.
An Example of Speckle Tracking Longitudinal Strain from the Four-chamber View in a Patient with Left Bundle Branch Block.
Six segments are color coded as labeled. Early septal shortening before aortic valve opening (AVO) is associated with posterolateral stretch (red arrow), and delayed posterolateral shortening is associated with systolic septal stretch (turquoise arrow) up to aortic valve closure (AVC). This pattern has been associated with favorable response to cardiac resynchronization therapy.
Examples of Different Dyssynchrony Patterns Associated with Different Myocardial Substrates.
The electromechanical substrate responsive to cardiac resynchronization therapy (CRT) (left) is characterized by early septal contraction and posterolateral stretch before aortic valve opening (AVO) and systolic septal stretch to aortic valve closure (AVC). Nonelectrical dyssynchrony substrates not associated with response to CRT include contraction heterogeneity from worse posterior contraction (middle) and posterior scar (right). All three examples are from patients with modest QRS widening (130–135 ms). Modified from Lumens et al. [10].
Dyssynchrony as a Marker of Prognosis with a Narrow QRS Complex
Mechanical dyssynchrony, with the broad definition as different timing of regional cardiac contraction, plays a different role in the prognosis of HF patient subsets with a narrow QRS complex versus those with a widened QRS complex. In a series of 201 patients with recent-onset nonischemic cardiomyopathy with an EF of (23±8)% and a QRS width of 98±21 ms, 108 (54%) had significant LV dyssynchrony at presentation [19]. When a speckle tracking method known as velocity vector imaging was used, LV opposing wall delay in the four-chamber view was increased to 89±51 ms compared with 35±11 ms seen in a healthy control group (P<0.001). At the 6-month follow-up in the cardiomyopathy group, the mean EF increased to (40±12)% and dyssynchrony reduced to 52±35 ms in maximum opposing wall delay, with the prevalence of dyssynchrony decreasing to 12% (all P<0.001) [19] (Figure 7). These observations show an association of dyssynchrony with the severity of LV dysfunction that can lessen with subsequent LV recovery in acute nonischemic cardiomyopathy. The EchoCRT randomized trial enrolled patients with a QRS width less than 130 ms, an EF of 35% or less, and echocardiographic dyssynchrony to CRT off versus CRT on [9]. Baseline dyssynchrony required for study inclusion was either TDI peak-to-peak longitudinal velocity delay of 80 ms or greater or speckle tracking radial strain peak-to-peak delay of 130 ms or greater. The trial was stopped early because patients in the CRT-on treatment arm had no benefit compared with controls with respect to the primary end point of HF hospitalization or death, and also had increased mortality as a secondary end point. In a subgroup analysis of 614 patients who had 6-month follow-up echocardiography for dyssynchrony analysis. Persistent dyssynchrony at 6 months was observed similarly in both groups (77% in the CRT-on group; 76% in the CRT-off group) [20]. Persistent dyssynchrony was associated with a higher incidence of death or HF hospitalization as a combined end point (hazard ratio [HR] 1.54, 95% confidence interval [CI] 1.03–2.30, P=0.03), and in particular the end point of HF hospitalization (HR 1.66, 95% CI 1.07–2.57, P=0.02) (Figure 8). HF hospitalization was also associated with both increasing TDI longitudinal dyssynchrony (HR 1.45, 95% CI 1.02–2.05, P=0.037) and increasing speckle tracking radial dyssynchrony (HR 1.81, 95% CI 1.16–2.81, P=0.008). The associations of persistent or increasing dyssynchrony with outcomes were similar in the CRT-off and CRT-on groups. These data suggest that heterogeneity in regional contraction in HF patients with reduced EF without significant QRS widening results in mechanically inefficient contraction that is associated with an increase in HF hospitalizations compared with those who had reduction in dyssynchrony over time.
Left Panels: Example of a Patient with Narrow QRS Complex and Acute Nonischemic Cardiomyopathy Demonstrating Dyssynchrony (top) by Speckle Tracking Velocity Vector Imaging at the Baseline Presentation and Resolution of Dyssynchrony (bottom) 6 months Later when the Ejection Fraction Increased.
Right panels: Group data from 201 patients with acute-onset nonischemic cardiomyopathy showing increases in ejection fraction and decreases in dyssynchrony assessed as opposing wall delay. Modified from Tanaka et al. [19].
Kaplan-Meier Plots of 536 Patients with a Narrow QRS Complex from the EchoCRT Trial Who Had Follow-up Echocardiography at 6 months.
Patients were grouped by the presence or absence of significant peak-to-peak radial dyssynchrony at 6 months. Persistent radial dyssynchrony was associated with a higher rate of heart failure hospitalizations. CI, confidence interval; HR, hazard ratio. Modified from Gorcsan et al. [20].
Dyssynchrony as a Marker of Prognosis with Prolonged QRS Duration
Although measures of echocardiographic dyssynchrony have not become a part of mainstream clinical practice to select patients for CRT, their role as markers of prognosis after CRT have been clearly documented. The vast majority of data have demonstrated that dyssynchrony at the baseline before CRT is associated with subsequent favorable clinical outcomes. Measures of the timing of mechanical dyssynchrony are a continuous variable, but several cutoffs have been tested to consistently show that patients with electrical delay and lesser degrees or no apparent dyssynchrony before CRT have a lower response rate, and less favorable prognosis following CRT. We studied 229 consecutive New York Hear Association class III–IV HF patients with EF of 35% or less, and a QRS duration of 120 ms or more referred for CRT with baseline echocardiography [21]. Dyssynchrony before CRT was defined as follows: TDI velocity opposing wall delay of 65 ms or greater and 80 ms or greater; 12-site standard deviation (Yu index) of 32 ms or greater; speckle tracking radial strain anteroseptal to posterior wall delay of 130 ms or greater; or pulsed Doppler IVMD of 40 ms or greater. Outcome was defined as freedom from death, heart transplant, or LV assist device (LVAD) implantation. Of 210 patients with dyssynchrony data available, there were 62 events: 47 deaths, 9 transplants, and 6 LVAD implantations in 4 years, demonstrating the high-risk nature of this patient population. Event-free survival was associated with the Yu index (P=0.003), speckle tracking radial strain (P=0.003) (Figure 9), and IVMD (P=0.019). When adjusted for confounding baseline variables of ischemic cause and QRS duration, the Yu index and radial strain dyssynchrony remained independently associated with outcome (P<0.05). Lack of radial dyssynchrony was particularly associated with unfavorable outcome in those with a QRS duration of 120–150 ms (P=0.002). The precise reason for having no significant measureable dyssynchrony in patients with a widened QRS complex remains unknown. One would anticipate mechanical dyssynchrony to be uniformly observed with QRS duration prolongation. Potential explanations include that particular patterns of disease or fibrosis of the conduction system may result in QRS widening without regional mechanical dyssynchrony, and that our echocardiographic methods are insensitive to subtler forms of mechanical dyssynchrony. Despite this gap in knowledge, numerous studies overall have consistently shown that patients who have significant echocardiographic dyssynchrony determined before CRT have better outcomes following CRT than those who do not.
Kaplan-Meier Plots of 197 Patients with Routine Cardiac Resynchronization Therapy (CRT) Indications (QRS Width 120 ms or greater).
Patients were grouped by the presence or absence of significant peak-to-peak radial dyssynchrony at the baseline. Significant peak-to-peak radial dyssynchrony at the baseline was associated with a more favorable outcome after CRT, specifically fewer deaths, transplants, or left ventricular assist device implantations. Modified from Gorcsan et al. [21].
As discussed so far in detail, mechanical discoordination or dyssynchrony may be observed in HF patients regardless of QRS duration [22]. Our computer modeling coupled with observations in humans led to the basis that nonelectrical dyssynchrony may occur from regional differences in contractility or regional scar, not related to electrical delay. This is the most plausible explanation for mechanical dyssynchrony in patients with a narrow QRS complex by the surface electrocardiogram. Since electrical delay appears to be a minimum component for CRT response, it is important to appreciate how can we determine what the minimum amount of QRS widening is to afford benefit from CRT. We observed that the SSI is a potential means to determine CRT response in patients with more modest QRS widening (QRS duration 120–149 ms) or non-LBBB morphology. This group of patients currently has the most clinical uncertainty about CRT response. In a study of 191 HF patients (QRS duration 120 ms or greater; LV EF 35% or less), SSI was quantified before CRT and patients were followed up for 2 years. Overall, patients with baseline SSI of 9.7% or greater had significantly fewer HF hospitalizations or deaths in the 2 years after CRT (HR 0.32, 95% CI 0.19–0.53, P<0.001), and fewer deaths, transplants, or LVAD implantations (HR 0.28, 95% CI 0.15–0.55, P<0.001). Furthermore, in a subgroup of 113 patients with intermediate ECG criteria (QRS duration 120–149 ms or non-LBBB), SSI of 9.7% or greater was independently associated with significantly fewer HF hospitalizations or deaths (HR 0.41, 95% CI 0.23–0.79, P=0.004), and fewer deaths, transplants, or LVAD implantations (HR 0.27, 95% CI 0.12–0.60, P=0.001) [22] (Figure 10). These data suggest that strain imaging may be additive to the ECG criteria in predicting CRT response in patients with a QRS duration of 120–149 ms or non-LBBB. It is worthwhile continuing to study this topic for future clinical applications.
Kaplan-Meier Plots of 113 Patients who Underwent Cardiac Resynchronization Therapy (CRT) with Intermediate Electrocardiographic Criteria: QRS Width 120–149 ms or Non-left Bundle Branch Block).
Patients were grouped by a higher or lower systolic stretch index (SSI) at the baseline. Higher SSI at the baseline was associated with a more favorable outcome after CRT, specifically fewer deaths, transplants, or left ventricular assist device implantations. Modified from Lumens et al. [10].
Conclusions
CRT is an important therapy where the mechanisms of action remain incompletely understood. Use of the electrocardiogram alone to select patients for CRT results in a high number of nonresponders. A variety of echocardiographic methods have been proposed to measure differences in timing of regional contraction, or dyssynchrony, to improve patient selection for CRT. However, no method has been shown to be convincing to date to influence patient selection. We have learned that echocardiographic dyssynchrony is more complicated than originally thought, and that nonelectrical causes of discoordination from contractile heterogeneity or scar are nonresponsive to CRT. There is a large body of data suggesting that mechanical dyssynchrony has prognostic implications in patients with either a narrow QRS complex or a widened QRS complex (Figure 11). Emerging echocardiographic methods such as strain patterns of systolic stretch are promising to identify the electromechanical substrate responsive to CRT, and further study is warranted.
Summary of Prognosis Associated with Dyssynchrony in Patients with Reduced Ejection Fraction Grouped by QRS Width.
Patients with a narrow QRS complex and no dyssynchrony have a more favorable prognosis than those with nonelectrical dyssynchrony, who in addition do not benefit from cardiac resynchronization therapy (CRT). Patients with a wide QRS complex who have no apparent dyssynchrony or nonelectrical dyssynchrony (from contractile heterogeneity or scar) at the baseline have a worse prognosis. Patients with a wide QRS complex and dyssynchrony from an electromechanical substrate at the baseline have the best prognosis with CRT.