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
Selecting CRT candidates: the value of intracardiac mapping
Department of Cardiology, Institute for Clinical and Experimental Medicine, Víde
ská 1958/9, 140 21 Prague 4, Czech Republic
* Corresponding author. Tel: +4202 4172 2011; fax: +4202 6136 2985. E-mail address: joka{at}medicon.cz
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Abstract |
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Cardiac resynchronization therapy (CRT) has become standard therapy for selected patients with congestive heart failure and dyssynchrony of cardiac contraction that is a consequence of electrical dyssynchrony. Intracardiac mapping enables detailed analysis of electrical activation sequences far beyond the standard ECG and allows description of specific activation associated with the benefit of CRT. Intracardiac mapping can also localize the region of the latest ventricular activation and areas of slow conduction, and thus potentially assist in selection of optimal pacing site for CRT. Precise description of electrical activation sequences, and correlation with parameters of mechanical dyssynchrony, may contribute to understanding the principles and mechanisms underlying the effect of CRT.
Key Words: Cardiac mapping, Resynchronization therapy, Conduction disturbances
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Introduction |
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Over the last decade, cardiac resynchronization therapy (CRT) has become standard therapy for selected patients with congestive heart failure and dyssynchrony of cardiac contraction. In this population, CRT improves heart failure symptoms and quality of life, leads to reverse remodelling, and reduces the risk of death.1
Despite these undisputable achievements, clinical benefit varies individually and optimal criteria for patient selection are still sought. Currently, different techniques that should reflect mechanical dyssynchrony are being evaluated with a goal to further improve identification of eligible candidates for CRT. However, primary pathophysiological moment in these patients appears to be electrical dyssynchrony, and CRT alters primarily the pattern of electrical activation of both ventricles. This article reviews the potential role of intra-cardiac mapping with respect to CRT.
Mapping of conduction abnormalities
Pioneering works describing activation patterns in left bundle branch block (LBBB) were performed back in the 1980s by Vassallo et al.2,3 In a typical LBBB, ventricular activation originates from the distal branching of the right bundle, and the activation of the left endocardium starts with a significant delay (more than 60 ms) due to the slow conduction throughout the inter-ventricular septum. Within the left ventricle, the impulse propagates variably depending on the aetiology of underlying heart disease. However, the late activated region is located in the vast majority of cases laterally or postero-laterally (Figure 1A). The development of CRT has revived the interest in ventricular activation sequences in relation to underlying conduction abnormalities.4–8 Both electroanatomical4,5 and non-contact6–8 mapping systems were employed to describe ventricular activation in a CRT population in detail. Despite some differences among the studies, they all showed that patients eligible for CRT represent a broad spectrum of underlying conduction disturbances with a high variability of activation patterns, especially in patients with ischaemic heart disease. Therefore, it is clear that the electrocardiogram (ECG) pattern of LBBB may result from various combinations of conduction disturbances within the ventricular myocardium and, thus, the standard ECG is of limited value for a more detailed description.
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The above observed variability can be explained by the heterogeneity of areas of fibrosis and/or dense myocardial scars that generally lead to anisotropy and slow conduction. In addition, compared with the right bundle, the left bundle is a complex fan-like structure that covers large areas of the septum and transits into the Purkinje fibres. Therefore, a conduction defect at different levels of this His-Purkinje network can generate a wide spectrum of activation patterns. In this respect, a large proportion of patients may show early activation of the left ventricular endocardium, which indicates preserved conduction through the left bundle despite the pattern of complete LBBB on 12-lead ECG. The major conduction abnormality in these patients is within the left ventricle. On the other side of the spectrum may be a true complete proximal LBBB (that is often used in animal models by catheter ablation) with slow trans-septal conduction and significant inter-ventricular electrical delay. In addition, Auricchio et al.6
described variable lines of block within the left ventricle, some of them purely on a functional basis as they shifted during ventricular pacing at different sites and pacing rates.
Prediction of response to cardiac resynchronization therapy
Practical implications of these different conduction abnormalities may be less apparent due to the complex electromechanical coupling in heart failure and due to the fact that electrical synchrony may not be required for the improvement of ventricular systolic function in CRT.9 In an attempt to achieve practical use of the mapping data, Tse et al.10 used the so-called electromechanical mapping (NOGA, Biosense Webster, Diamond Bar, CA, USA) in CRT candidates. In this case, the electromechanical mapping system is used for three-dimensional mapping of mechanical contraction in relation to electrical activation. They found that the improvement in contractility during left ventricular pacing was not influenced by the left ventricular ejection fraction or by dP/dt at baseline. Similarly, total left ventricular activation time, baseline QRS duration or QRS shortening during CRT did not predict the haemodynamic effect of CRT. The only parameter that correlated with haemodynamic improvement was the amount of late activated regions (areas with activation time >2/3 of the total left ventricular endocardial time) and the extent of preserved (contracting) myocardium.
In another study, Fung et al.8 correlated different types of activation patterns obtained by a non-contact mapping system with tissue Doppler imaging and outcome during CRT. A significantly higher degree of mechanical dyssynchrony was found only in patients with an activation pattern containing a line of conduction block within the left ventricle; also, more responders to CRT during follow-up were noted in this subgroup. This study confirmed that the benefit of CRT is more dependent on specific left ventricular activation patterns rather than on total left ventricular activation time. Therefore, this finding underlines the limited role of the QRS duration (as a correlate of left ventricular activation time) in the prediction of CRT response.
Selection of optimal pacing site
Besides identification of eligible CRT candidates, intra-cardiac mapping can be used for localization of an optimal pacing site. In this respect, the available data seem to be contradictory. In an animal model, the highest haemodynamic improvement could be observed when pacing in a relatively large region of the left ventricular lateral wall, irrespective of underlying conduction abnormality.11 However, the situation in heart failure patients appears to be different from the experimental model of heart failure created by rapid pacing and ablation of the left bundle. Some clinical studies showed that the location of the left ventricular lead is the primary determinant of haemodynamic benefit in patients with heart failure and should be individualized.12
In theory, the latest activated regions possess the greatest dyssynchrony and should be the target site for left ventricular lead placement. Although such regions are located most commonly on the lateral/postero-lateral wall, they can be shifted variably in individual patients. Using non-contact mapping, Lambiase et al.13 showed that pacing within regions of slow conduction may result in the slow propagation of the activation wavefront and is associated with decreased cardiac output. Thus, the common wisdom about the optimal pacing site being in the mid-lateral wall may not reflect the sites with the most haemodynamic benefit. Although this can be partially corrected by optimization of the inter-ventricular delay, intra-cardiac mapping can be used in studies aimed at identification of optimum pacing sites by non-invasive methods. An example could be the use of tissue synchronization imaging.14 Whether such selective positioning of the left ventricular lead can improve the clinical benefit of CRT in a typical population of candidates identified by a QRS width 
120 ms remains to be proved.
Besides the site selection for the left ventricular lead, Kiuchi et al.15 deployed electroanatomical mapping in an attempt to optimize the right ventricular pacing site. They found that the latest activated region within the right ventricle is highly dependent on the position of the left ventricular lead. However, the highest haemodynamic response was obtained most commonly during pacing in the right ventricular mid-septum, irrespective of the most delayed region. Such data indicate that the mid-septal region may be recommended as the optimal pacing site for all patients undergoing CRT. This reflects our clinical observation about the preference of the mid-septal position of the right ventricular lead over right ventricular apical pacing in a population of patients undergoing CRT.16
Another possible utility of the mapping system in CRT candidates is to describe activation patterns during different pacing modes used for cardiac resynchronization. In our study,17 we compared the activation patterns during single-site left ventricular pacing, biventricular pacing, and right ventricular bifocal pacing. Only biventricular pacing and single-site left ventricular pacing together with fusion with spontaneous conduction resulted in fundamental changes of activation patterns and in the shortening of left ventricular activation (Figure 1B). In contrast, right ventricular bifocal pacing resulted in an activation pattern similar to that observed during LBBB with a right-to-left direction of the left ventricular activation. From this point of view, this pacing modality cannot be considered as an alternative to biventricular pacing.
Correlation of electrical patterns and mechanical dyssynchrony parameters
Finally, intra-cardiac mapping can be used as a tool for a better understanding of the relationship between electrical and mechanical dyssynchrony in less defined populations for CRT. An example could be patients with a relatively narrow QRS complex and echocardiographically documented mechanical dyssynchrony. Until now, selection of such CRT candidates relied solely on echocardiographic parameters. However, recent studies have failed to demonstrate any relevant benefit of CRT in this patient population.18 This may indicate either the limited role of several tissue Doppler techniques for the detection of true mechanical dyssynchrony or the fact that not all detectable mechanical dyssynchrony can be corrected by CRT. Therefore, it seems logical that identification of the ‘right’ dyssynchrony parameter can be made only by better understanding the relationship between electrical activation and mechanical contraction and, thus, by a direct comparison of electrical activation patterns with parameters of mechanical dyssynchrony.
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The clinical value of intra-cardiac mapping for the selection of CRT candidates is limited by the invasive nature of these techniques and their complexity. However, intra-cardiac mapping enables a detailed analysis of electrical activation sequences far beyond that by the standard ECG. In this respect, some specific types of activation patterns have been associated with a superior benefit with CRT. In addition, intra-cardiac mapping can allocate the latest activated region and areas of slow conduction and, thus, potentially assist in selection of optimal pacing site within the left ventricle. Finally, precise characterization of electrical activation sequences and its correlation with the parameters of mechanical dyssynchrony form a solid background for understanding the principles and mechanisms underlying the effect of CRT. Using such comprehensive studies, the ‘true’ dyssynchrony is likely to be identified in the near future, which can be subsequently corrected by CRT.
Conflict of interest: J.K. is a member of the advisory board for Biosense-Webster.
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This study was supported in part by the research grant NR8541-3/2005 of the Internal Grant Agency of the Ministry of Health of the Czech Republic.
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