This article appears in the following Europace issue: Spotlight Issue: Cardiac Imaging in EP and CRT [View the issue table of contents]
IMAGING IN CRT
Predicting response to CRT. The value of two- and three-dimensional echocardiography
1 Department of Cardiology, Leiden University Medical Center, Albinusdreef 2, 2333 ZA Leiden, The Netherlands; 2 Department of Cardiology, Klinikum Coburg gGmbH, Academic Teaching Hospital of the University of Wuerzburg, Coburg, Germany
* Corresponding author. Tel: +31 71 5262020; fax: +31 71 5266809. E-mail address: n.ajmone{at}lumc.nl
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Abstract |
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 Top Abstract IntroductionTwo-dimensional echocardiographyThree-dimensional...Clinical implications and future...FundingReferences |
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Recently, it has been suggested that a direct assessment of left ventricular (LV) mechanical dyssynchrony may improve the selection of candidates to cardiac resynchronization therapy (CRT). In fact, when the established clinical and electrocardiographic selection criteria are applied, response to CRT may vary widely and up to one-third of the patients fail to benefit from CRT. Echocardiography has been extensively applied to assess LV dyssynchrony and to predict favourable response to CRT, using different two- and three-dimensional modalities. In this review, the value of these echocardiographic modalities will be discussed, highlighting the advantages and drawbacks of each technique and evaluating the clinical implications and future perspectives of LV dyssynchrony assessment.
Key Words: Cardiac resynchronization therapy, Left ventricular dyssynchrony, Three-dimensional echocardiography
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Introduction |
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TopAbstractIntroductionTwo-dimensional echocardiographyThree-dimensional...Clinical implications and future...FundingReferences |
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In patients with severe, symptomatic drug-refractory heart failure and wide QRS complex, cardiac resynchronization therapy (CRT) has been demonstrated to have significant favourable effects on left ventricular (LV) remodelling and function and on clinical outcomes.1
However, a consistent percentage of patients fail to benefit from CRT when the established clinical and electrocardiographic selection criteria are applied.2 In particular, 20–30% of the patients do not experience any clinical improvement after CRT and 40–50% of the patients do not show any significant LV reverse remodelling or improvement in LV function. Therefore, at present, attention has shifted towards a more accurate selection of CRT candidates, beyond the standard use of NYHA functional class, LV ejection fraction, and QRS complex. Several contributory factors to CRT non-response have been reported, such as inappropriate LV lead positioning and, in patients with ischaemic cardiomyopathy, the extent and location of scar tissue and viable myocardium.3,4 Furthermore, the presence of significant baseline LV mechanical dyssynchrony has been demonstrated to be highly predictive for a positive response to CRT.5 Echocardiography has been extensively used for a direct assessment of LV dyssynchrony with different approaches. In this review, the value of two-dimensional (2D) and three-dimensional (3D) echocardiographic modalities to predict response to CRT will be discussed, highlighting the advantages and drawbacks of each modality. |
Two-dimensional echocardiography |
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TopAbstractIntroductionTwo-dimensional echocardiographyThree-dimensional...Clinical implications and future...FundingReferences |
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Two-dimensional echocardiography is widely applied in the selection and management of CRT patients, due to the low cost, broad availability, non-invasive approach, and the extensive information that this technique can provide: (i) 2D echocardiography confirms the presence of an impaired LV systolic function at baseline and can evaluate CRT success at follow-up in terms of LV reverse remodelling and/or improvement of LV function; (ii) 2D echocardiography evaluates the presence of other significant cardiac structural abnormalities that might affect therapy success, such as valvular pathologies and the extent of akinetic/thin (probably scar) myocardium; (iii) 2D echocardiography can accurately assess the presence of LV dyssynchrony with adequate temporal resolution.
The visual assessment of wall motion synchrony by 2D echocardiography is probably the first and most straightforward step to approach a possible CRT candidate. However, the human eye with its limited temporal resolution is not able to precisely quantify the extent and location of myocardial dyssynchrony.6 Nevertheless, it is possible to identify typical phenomena that are associated with myocardial dyssynchrony. In the presence of delayed electrical conduction, the interventricular septum typically shows a characteristic multiphasic motion pattern (abnormal relaxation pattern) and the LV often demonstrates a rotating motion (typically oriented counter-clockwise in late systole, often also referred to as ‘rocking motion’ or ‘apical shuffle’), as seen in the apical four-chamber view. In a small study on 53 patients, Jansen et al.7 were able to demonstrate that the identification of these two phenomena predicted the presence of LV dyssynchrony (defined by tissue Doppler criteria) and LV reverse remodelling after CRT with a sensitivity of more than 90% and a positive predictive value above 85%. The septal multiphasic motion pattern can be also detected and quantified from the M-mode short-axis view at the level of the papillary muscles. The ‘septal-to-posterior wall motion delay’ is calculated as the shortest interval between the maximal posterior displacement of the septum and the maximal inward displacement of the posterior wall.8 However, this parameter demonstrated a limited predictive value and, more importantly, a poor feasibility especially in ischaemic patients.9
Two-dimensional echocardiography also permits the quantification of myocardial dyssynchrony based on the analysis of endocardial wall motion. Breithardt et al.10 evaluated a semi-automated contour detection algorithm, which had been originally developed for stress-echocadiography, in a small cohort of CRT patients. Semi-automatically contoured septal and lateral wall motion curves were constructed from digitized video recordings of the apical four-chamber view and averaged over several cardiac cycles to calculate the phase angle difference between the opposing walls. Patients with significant dyssynchrony, defined by a phase angle difference below –25° or above +25°, were better haemodynamic CRT responders, defined by a >10% increase in LV peak positive dP/dt and/or pulse pressure. This quantitative approach is obviously limited by the restriction to two opposing walls, but similar algorithms have later been transferred to 3D technology (see below).
Kawaguchi et al.11 quantified LV dyssynchrony before and during CRT with contrast enhanced 2D echocardiography. Temporal and spatial pixel intensity changes were analysed in up to 50 sequential beats (contrast variability imaging) and demonstrated improved septal inward motion and reduced LV septal-lateral dyssynchrony by CRT. This novel approach is clearly limited by its time-consuming and costly image acquisition, which necessitates the intravenous administration of contrast agents and the lack of a widely commercially available software tool.
Modern quantitative analyses, such as tissue Doppler imaging (TDI) and speckle tracking analysis, are also based on 2D echocardiography and are discussed in more detail elsewhere in this supplement.
Briefly, TDI is at present the most frequently used technique for LV dyssynchrony assessment.12 In particular, this technique allows the measurement of myocardial regional velocities with a very high temporal resolution and, in relation to the electrical activity (QRS complex), enables the assessment of the electro-mechanical delay of two or more LV segments. Both two- and multiple-segmental approaches demonstrated their value for the prediction of clinical and echocardiographic response to CRT with high sensitivity and specificity.13,14 Recently, tissue synchronization imaging (TSI) has become available to simplify this approach, providing an intuitive colour-coded image of the myocardium according to the automatically detected time-to-peak systolic velocity (Ts) of each LV segment, together with the possibility of an automated quantitative assessment.15
Furthermore, when myocardial velocities are known, several other parameters can be derived off-line. Integrating velocities over time results in myocardial displacement, i.e. the total amount of tissue movement during systole. Alternatively, the spatial derivative of velocity and displacement leads to strain rate and strain calculation, respectively, as myocardial deformation measurements16 (Figure 1). However, these parameters have proved not to be as robust as myocardial velocity parameters for predicting response to CRT,14,17 probably due to the relatively poor reproducibility, independent of the operator experience. Furthermore, for all the myocardial motion and deformation TDI measurements, the angle-dependency of the analysis has to be considered as the major drawback that limits the application of this technique to few LV segments and to one component of contraction (longitudinal, radial, and circumferential) at a time.
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Speckle tracking analysis is a novel echocardiographic technique that allows myocardial deformation measurements (strain and strain rate) from standard 2D grey scale images, tracking frame-to-frame the movement of natural myocardial acoustic markers (‘speckles’). The advantage of this technique, compared with TDI, is the angle-independency, allowing for the assessment of the three components of myocardial contraction (longitudinal, radial, and circumferential direction) in all LV segments. Initial studies have demonstrated the value of radial dyssynchrony, assessed with this technique, to predict response to CRT.18
,19 Furthermore, Gorcsan et al.20 suggested that combining radial dyssynchrony, measured by speckle tracking, with longitudinal dyssynchrony, measured by TDI, predicted echocardiographic response to CRT (88% sensitivity and 80% specificity) significantly better than either technique alone. |
Three-dimensional echocardiography |
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TopAbstractIntroductionTwo-dimensional echocardiographyThree-dimensional...Clinical implications and future...FundingReferences |
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Early approaches to 3D echocardiography were based on the principle that a 3D data set is reconstructed from a series of 2D images obtained using either freehand scanning or a mechanically driven rotating transducer that sequentially recorded images at predefined intervals. This approach was limited by several technical aspects during image acquisition and required significant time-consuming off-line data processing.21
Nowadays, 3D echocardiography is a real-time technique that permits a rapid (5–7 min) post-processing of the 3D data set off-line or on the scanner itself. In addition, the post-processing of the 3D data set provides, in one single analysis, highly accurate quantification of LV volumes and function as well as the assessment of LV dyssynchrony with different modalities. In particular, 3D echocardiography, applying a semi-automated contour-tracing algorithm in multiple planes, has been validated against magnetic resonance imaging and found to be more accurate and reproducible than 2D echocardiography for the assessment of LV volumes and function.22,23 These are crucial measurements in the evaluation of CRT success and carry out important prognostic implications.24 Furthermore, a 3D assessment of LV dyssynchrony has the advantage of a simultaneous acquisition of all LV segments, allowing for more extensive intersegment comparison, when compared with other 2D techniques, and avoiding the problem of heart rate variability during image acquisition.
Tri-plane tissue Doppler imaging
Recently, colour-coded TDI has been used in combination with tri-plane imaging, which allows for analysis of the 3D data set along three major planes with a simultaneous visualization of the apical four-, two-, and three-chamber views (Figure 2). Sample volumes can be placed simultaneously in 12 LV segments, and Ts of any LV segment can be compared during the same heartbeat. Van de Veire et al.25 applied this technique in 60 patients undergoing CRT and demonstrated that the standard deviation of Ts of 12 (six basal and six mid segments) LV segments (Ts-SD-12), with a cut-off value of 
33 ms, was able to predict echocardiographic response with a sensitivity of 90% and a specificity of 83%. These findings further support the previous results of Yu et al.14 using single-plane TDI.
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Similarly, it has become possible to apply TSI to the tri-plane view and this approach was found useful to identify the area of latest activation within the LV (Figure 3). Further studies are needed to confirm the feasibility of this technique in the clinical practice and to explore the value to predict response to CRT.25
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Analysis of volumetric changes by real-time three-dimensional echocardiography
Real-time three-dimensional echocardiography (RT3DE) has emerged as a novel echocardiographic technique for the assessment of LV dyssynchrony based on the analysis of regional volumetric changes. Briefly, the LV 3D model is subdivided by the software in 17 wedge-shaped (apart from the apex) subvolumes according to the standard segmentation. For the whole LV and for each volumetric segment, it is possible to derive time-volume data for the entire cardiac cycle and to assess the time taken to reach the minimum systolic volume (Tmsv) (Figure 4). Segments should achieve the minimum volume at the same point in the cardiac cycle. However, when significant LV dyssynchrony is present, a wide dispersion of the Tmsv of the various segments occurs. The standard deviation of the Tmsv for 16 segments has been proposed as a measure of LV dyssynchrony and expressed as a percentage of the cardiac cycle (the systolic dyssynchrony index, SDI).
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Furthermore, parametric images, derived from over 800 virtual waveforms, are also provided by the software with a visual summary of LV regional contraction timings as a polar plot. The global Tmsv is used as timing reference and segments with a Tmsv that is approximately the same as global Tmsv are coded in green. Early segments are colour-coded in blue, whereas late segments are coded in red/yellow, providing a rapid and intuitive assessment of the area of latest LV activation (Figure 5). This may be of particularly importance to guide an optimal LV lead placement.
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Kapetanakis et al. demonstrated the feasibility of LV dyssynchrony assessment with RT3DE in a large group of patients and normal subjects using SDI.25
,26 In 26 patients referred for CRT implantation, the authors observed that clinical responders (reduction in NYHA functional class) had significantly higher SDI at baseline when compared with non-responders (16.6 ± 1.1% vs. 7.1 ± 2.0%, P < 0.001). In addition, after a mean follow-up of 10 months, a reduction in SDI was noted in responders, whereas an increase in SDI was observed in non-responders. In a more recent study, Marsan et al.27 addressed the predictive value of this parameter and found that an SDI 5.6% had a sensitivity of 88% and a specificity of 86% to predict acute volumetric response to CRT.
Currently, no data are available on the predictive value of long-term response to CRT and the technique needs further validation for this application. Furthermore, RT3DE has still some limitations. First, the temporal resolution is suboptimal, 
30–35 ms. Nonetheless, the feasibility of LV dyssynchrony assessment has been demonstrated in a large group of normal individuals and unselected patients with and without QRS prolongation.26 Furthermore, the image quality is somewhat lower than 2D echocardiography and the incomplete visualization of large ventricles may affect the quantitative analysis. However, several studies have reported a good feasibility in heart failure patients.27
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Clinical implications and future directions |
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TopAbstractIntroductionTwo-dimensional echocardiographyThree-dimensional...Clinical implications and future...FundingReferences |
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Limitations of current left ventricular dyssynchrony evaluation
Different echocardiographic approaches, with relative strengths and limitations, are available for the assessment of LV dyssynchrony and prediction of response to CRT. However, LV dyssynchrony assessment is a challenging goal, since it cannot be considered an all-or-none phenomenon but a dynamic condition that may vary widely and can be dependent on loading conditions.28
Consequently, it is still not clear which parameters, among those of myocardial motion, myocardial deformation or volume changes, would best reflect the complex pathophysiologic substratum of LV dyssynchrony in heart failure and whether the two-segmental model should be preferred over the multiple-segmental model. Therefore, a consensus on the definition of LV dyssynchrony is still lacking.
In addition, the predictive value of these echocardiographic indices have been generated from small, single-centre, non-randomized studies that used non-uniform definitions of response to CRT and evaluated a relatively short-term follow-up.5 Consequently, the results need to be confirmed in larger prospective multi-centre studies. Ideally, the optimal predictor of a favourable response to CRT must have high sensitivity and specificity. In addition, it should also be simple to perform, widely available, easily reproducible, and obtainable before and after CRT implantation.
Future directions
The PROSPECT trial is currently evaluating in 426 patients whether specific echocardiographic measurements of dyssynchrony, including M-mode, Doppler, and TDI, could be used to better predict a favourable response to CRT.29 The first, preliminary results revealed that none of the echocardiographic parameters was associated with a relevant additional response rate.30 However, these findings may be explained by the fact that a marked inter-core lab variability was found for these parameters, highlighting the challenges that the interpretation of these exams offer even to experienced personnel.31 Importantly, it should be noted that the most recent, 3D, and automated imaging techniques were not included in this trial. Furthermore, a combined approach using different and complementary parameters of LV dyssynchrony, rather than using single parameters, may provide better results.20 More large prospective studies are needed to fully appreciate the role of LV dyssynchrony assessed with echocardiography in the prediction of response to CRT. Consequently, at present, LV dyssynchrony assessment is not yet recommended for patient selection for CRT.12 Furthermore, it should be kept in mind that other pathophysiological issues may also be important to consider before CRT, including extent and location of scar tissue, optimal LV lead position, and availability of coronary veins.
Conflict of interest: none declared.
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Funding |
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TopAbstractIntroductionTwo-dimensional echocardiographyThree-dimensional...Clinical implications and future...FundingReferences |
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N.A.M. is financially supported by the Research Fellowship of the European Society of Cardiology. O.A.B. has received speaker's honoraria of GE Vingmed, Medtronic, and Philips.
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[1] Freemantle N, Tharmanathan P, Calvert MJ, Abraham WT, Ghosh J, Cleland JG. Cardiac resynchronisation for patients with heart failure due to left ventricular systolic dysfunction—a systematic review and meta-analysis. Eur J Heart Fail (2006) 8:433–40.
[2] Mehra MR, Greenberg BH. Cardiac resynchronization therapy: caveat medicus! J Am Coll Cardiol (2004) 43:1145–8.
[3] Becker M, Franke A, Breithardt OA, Ocklenburg C, Kaminski T, Kramann R, et al. Impact of left ventricular lead position on the efficacy of cardiac resynchronisation therapy: a two-dimensional strain echocardiography study. Heart (2007) 93:1197–203.
[4] Bleeker GB, Kaandorp TA, Lamb HJ, Boersma E, Steendijk P, de Roos A, et al. Effect of posterolateral scar tissue on clinical and echocardiographic improvement after cardiac resynchronization therapy. Circulation (2006) 113:969–76.
[5] Bax JJ, Abraham T, Barold SS, Breithardt OA, Fung JW, Garrigue S, et al. Cardiac resynchronization therapy: Part 1—issues before device implantation. J Am Coll Cardiol (2005) 46:2153–67.
[6] Kvitting JP, Wigstrom L, Strotmann JM, Sutherland GR. How accurate is visual assessment of synchronicity in myocardial motion? An in vitro study with computer-simulated regional delay in myocardial motion: clinical implications for rest and stress echocardiography studies. J Am Soc Echocardiogr (1999) 12:698–705.[CrossRef][Web of Science][Medline]
[7] Jansen AH, van DJ, Bracke F, Meijer A, Peels KH, van den Brink RB, et al. Qualitative observation of left ventricular multiphasic septal motion and septal-to-lateral apical shuffle predicts left ventricular reverse remodeling after cardiac resynchronization therapy. Am J Cardiol (2007) 99:966–9.[CrossRef][Web of Science][Medline]
[8] Pitzalis MV, Iacoviello M, Romito R, Massari F, Rizzon B, Luzzi G, et al. Cardiac resynchronization therapy tailored by echocardiographic evaluation of ventricular asynchrony. J Am Coll Cardiol (2002) 40:1615–22.
[9] Marcus GM, Rose E, Viloria EM, Schafer J, De MT, Saxon LA, et al. Septal to posterior wall motion delay fails to predict reverse remodeling or clinical improvement in patients undergoing cardiac resynchronization therapy. J Am Coll Cardiol (2005) 46:2208–14.
[10] Breithardt OA, Stellbrink C, Herbots L, Claus P, Sinha AM, Bijnens B, et al. Cardiac resynchronization therapy can reverse abnormal myocardial strain distribution in patients with heart failure and left bundle branch block. J Am Coll Cardiol (2003) 42:486–94.
[11] Kawaguchi M, Murabayashi T, Fetics BJ, Nelson GS, Samejima H, Nevo E, et al. Quantitation of basal dyssynchrony and acute resynchronization from left or biventricular pacing by novel echo-contrast variability imaging. J Am Coll Cardiol (2002) 39:2052–8.
[12] Gorcsan J III, Abraham T, Agler DA, Bax JJ, Derumeaux G, Grimm RA, et al. Echocardiography for cardiac resynchronization therapy: recommendations for performance and reporting—a report from the American Society of Echocardiography Dyssynchrony Writing Group endorsed by the Heart Rhythm Society. J Am Soc Echocardiogr (2008) 21:191–213.[CrossRef][Web of Science][Medline]
[13] Bax JJ, Bleeker GB, Marwick TH, Molhoek SG, Boersma E, Steendijk P, et al. Left ventricular dyssynchrony predicts response and prognosis after cardiac resynchronization therapy. J Am Coll Cardiol (2004) 44:1834–40.
[14] Yu CM, Gorcsan J III, Bleeker GB, Zhang Q, Schalij MJ, Suffoletto MS, et al. Usefulness of tissue Doppler velocity and strain dyssynchrony for predicting left ventricular reverse remodeling response after cardiac resynchronization therapy. Am J Cardiol (2007) 100:1263–70.[CrossRef][Web of Science][Medline]
[15] Van de Veire N, Bleeker GB, De Sutter J, Ypenburg C, Holman ER, van der Wal EE, et al. Tissue synchronisation imaging accurately measures left ventricular dyssynchrony and predicts response to cardiac resynchronisation therapy. Heart (2007) 93:1034–9.
[16] Sun JP, Popovic ZB, Greenberg NL, Xu XF, Asher CR, Stewart WJ, et al. Noninvasive quantification of regional myocardial function using Doppler-derived velocity, displacement, strain rate, and strain in healthy volunteers: effects of aging. J Am Soc Echocardiogr (2004) 17:132–8.[CrossRef][Web of Science][Medline]
[17] Yu CM, Fung JW, Zhang Q, Chan CK, Chan YS, Lin H, et al. Tissue Doppler imaging is superior to strain rate imaging and postsystolic shortening on the prediction of reverse remodeling in both ischemic and nonischemic heart failure after cardiac resynchronization therapy. Circulation (2004) 110:66–73.
[18] Delgado V, Ypenburg C, van Bommel RJ, Tops LF, Mollema SA, Ajmone Marsan N, et al. Assessment of left ventricular dyssynchrony by speckle tracking strain imaging comparison between longitudinal, circumferential, and radial strain in cardiac resynchronization therapy. J Am Coll Cardiol (2008) 51:1944–52.
[19] Suffoletto MS, Dohi K, Cannesson M, Saba S, Gorcsan J III. Novel speckle-tracking radial strain from routine black-and-white echocardiographic images to quantify dyssynchrony and predict response to cardiac resynchronization therapy. Circulation (2006) 113:960–8.
[20] Gorcsan J III, Tanabe M, Bleeker GB, Suffoletto MS, Thomas NC, Saba S, et al. Combined longitudinal and radial dyssynchrony predicts ventricular response after resynchronization therapy. J Am Coll Cardiol (2007) 50:1476–83.
[21] Hung J, Lang R, Flachskampf F, Shernan SK, McCulloch ML, Adams DB, et al. 3D echocardiography: a review of the current status and future directions. J Am Soc Echocardiogr (2007) 20:213–33.[CrossRef][Web of Science][Medline]
[22] Kuhl HP, Schreckenberg M, Rulands D, Katoh M, Schafer W, Schummers G, et al. High-resolution transthoracic real-time three-dimensional echocardiography: quantitation of cardiac volumes and function using semi-automatic border detection and comparison with cardiac magnetic resonance imaging. J Am Coll Cardiol (2004) 43:2083–90.
[23] Corsi C, Lang RM, Veronesi F, Weinert L, Caiani EG, MacEneaney P, et al. Volumetric quantification of global and regional left ventricular function from real-time three-dimensional echocardiographic images. Circulation (2005) 112:1161–70.
[24] Yu CM, Bleeker GB, Fung JW, Schalij MJ, Zhang Q, van der Wall EE, et al. Left ventricular reverse remodeling but not clinical improvement predicts long-term survival after cardiac resynchronization therapy. Circulation (2005) 112:1580–6.
[25] Van de Veire N, Yu CM, Ajmone-Marsan N, Bleeker GB, Ypenburg C, De Sutter J, et al. Triplane tissue Doppler imaging: a novel three-dimensional imaging modality that predicts reverse left ventricular remodelling after cardiac resynchronisation therapy. Heart (2008) 94:e9.
[26] Kapetanakis S, Kearney MT, Siva A, Gall N, Cooklin M, Monaghan MJ. Real-time three-dimensional echocardiography: a novel technique to quantify global left ventricular mechanical dyssynchrony. Circulation (2005) 112:992–1000.
[27] Marsan NA, Bleeker GB, Ypenburg C, Ghio S, Van de Veire N, Holman ER, et al. Real-time three-dimensional echocardiography permits quantification of left ventricular mechanical dyssynchrony and predicts acute response to cardiac resynchronization therapy. J Cardiovasc Electrophysiol (2008) 19:392–9.[CrossRef][Web of Science][Medline]
[28] Kass DA. An epidemic of dyssynchrony: but what does it mean? J Am Coll Cardiol (2008) 51:12–7.
[29] Yu CM, Abraham WT, Bax J, Chung E, Fedewa M, Ghio S, et al. Predictors of response to cardiac resynchronization therapy (PROSPECT)—study design. Am Heart J (2005) 149:600–5.[CrossRef][Web of Science][Medline]
[30] Cleland JG, Abdellah AT, Khaleva O, Coletta AP, Clark AL. Clinical trials update from the European Society of Cardiology Congress 2007: 3CPO, ALOFT, PROSPECT and statins for heart failure. Eur J Heart Fail (2007) 9:1070–3.[CrossRef][Medline]
[31] Nagueh SF. Mechanical dyssynchrony in congestive heart failure: diagnostic and therapeutic implications. J Am Coll Cardiol (2008) 51:18–22.
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