Pre-CRT echocardiographic testing used to determine dyssynchrony is fraught with limitations, mostly inter-observer variability. Unusual anatomy of CHD patients further imposes technical limitations on the utility of standard echocardiographic methods for objective evaluation. ECGI provides an objective rather than a subjective method in the assessment of dyssynchrony. The use of ECGI in selecting patients likely to benefit from CRT presupposes that mechanical dyssynchrony is preceded by ED, and identification of patients with substantial ED inherently selects patients with mechanical dyssynchrony. For patients with heart failure in absence of appreciable systemic ventricle dyssynchrony (ED in the normal range), CRT may not be the appropriate therapy and may account for part of the high non-responder rate (9
). ED index is expected to be higher for cases with isolated late areas of ventricular activation (the dysynchronous pattern thought to be commonly associated with heart failure) rather than generalized slow conduction. For example, in patient #2, a wide QRS (125 ms) reflected generalized slow conduction associated with pacing but ED index was still low (20 ms). In spite of a long QRS duration, a very small area of the ventricle activated later than 80th
percentile of QRS duration (deep blue, Patient #2, ), preserving the general synchrony of ventricular activation as reflected by a lower ED index. In comparison, patient # 4 had substantially large isolated areas of late activation (deep blue, Patient #4, ) and had an elevated ED index.
It has been reported before that RV pacing has deleterious effects on LV function in adults and one of the causes may be the pacing induced dyssynchrony arising from the the ‘left bundle branch block’ pattern of activation associated with RV pacing. There has been a lack of similar studies in the congenital heart disease population. In fact, the present study is one of the first to show that single site pacing in congenital heart block patients (# 5,6,7) produces dysynchronous intraventricular activation and may be one of the causes of subsequent heart failure observed in these patients.
Resynchronization of ventricular myocardium, the goal of CRT, has been difficult to reliably achieve in the pediatric population with non-responder rates of 12-13% (1
). Currently there are no objective criteria for placement of the resynchronization lead with the CRT resynchronization leads usually placed 180 degrees apart on the epicardial surface (1
). Placement of the resynchronization lead at a site of late electrical activation is more likely to achieve resynchronization, as demonstrated in patient #5 and 6. No objective registration was performed between the epicardial area of latest activation indicated on the ECGI-CT images and the location of the lead placed by the surgeons. ECGI images showed the surgeons the general anatomic area for placement of the lead. For patient #5, the empirical area for the resynchronization lead would be left basal anterior which is 180 degrees apart from the previously implanted right-sided posterior lead. But ECGI maps showed a more inferior location as the area of the latest activation in the baseline rhythm; hence the resynchronization lead was placed inferiorly rather than anteriorly. Though this area is still close to the empirical area that would be chosen, ECGI provides an objective basis for the selection.
Previous ECGI studies (5
) have demonstrated that defining the electrophysiologic substrate may be important for patients before CRT implant. One of the vital considerations pre-implant should be electrical viability of the cardiac muscle. If the underlying myocardial substrate has altered electrical characteristics (i.e., regions of slow conduction, inexcitable tissue, or lines of conduction block) which inhibit the pacing wavefront from activating a substantial target area of the myocardium as in Patient #8, CRT will not yield the desired response. This is demonstrated by the identical activation patterns and similar ED indices during the different conditions of pacing in Patient #8. For this patient, the resynchronization lead was placed adjacent to an area of slow conduction. Pacing from this area resulted in a functional line of block, leaving the half of the ventricular mass devoid of CRT. As such, only half the ventricular mass was activated early resulting in persistent dyssynchrony and insufficient resynchronization. Ideally, CRT leads should be placed at sites within the EP substrate conducive to rapid activation of a substantial target area of the myocardium. Performing ECGI in patients with complex congenial lesions prior to CRT implant may identify regions of slow conduction and block and help circumvent these areas while selecting a site for lead placement. ECGI may be used to noninvasively assess the electrical viability of the ventricular muscle pre-implant and thus avoid inappropriate placement of CRT devices.
ECGI images obtained in a CRT clinical responder with single ventricle physiology (Patient #7, ) imply that electrical synchrony may correlate with clinical response. While this has been demonstrated in adult patients (5
), this is the first case of a clinical responder and improvement in ED in a pediatric patient with complex congenital heart disease.
QRS duration on the body-surface ECG is often used as a surrogate marker for electrical dyssynchrony with the assumption that longer QRS durations correlate with ED. We had the ability to correlate QRS duration with ED index in 21 conditions and demonstrated lack of strong positive correlation (r=0.58) between them. Data from clinical CRT responders in the study (Patients #5, 6, and 7) suggest that improvement in clinical response is accompanied by more synchronized ventricular activation by CRT (lower ED).
ECGI was not used in this study to optimize CRT devices or study the relation between mechanical and electrical synchrony. The present study showed that this novel imaging modality can be applied to the congenital heart disease (CHD) patients to study the ventricular activation patterns and underlying EP substrate. It may be used to decide whether a prospective CHD patient may benefit from CRT based on the electrical intraventricular dyssynchrony and substrate in the baseline rhythm. It may be also used to follow-up CRT patients to evaluate their electrical dyssynchrony post-implant. An important aspect of CRT is optimization of the device including A-V delay, V-V delay, etc which may lead to better response in suitable patients. ECGI may be used in conjunction with the existing echo techniques for device optimization in prospective patients in future. This preliminary study shows that ECGI may be an important tool to add to the clinical armamentarium available for diagnosis and treatment of CHD patients, tailored to each patient.
The normal range for ED was calculated from patients with 2 ventricles and a systemic LV. This normal range may not apply to patients with systemic right ventricles and single ventricles as discussed in this manuscript. This study was performed at a single center in a unique patient population. The relatively small number of patients (n=8) precludes statistical conclusions to be drawn from the study. Unlike routine surface 12-lead ECG or echocardiography, ECGI is not yet an established routine clinical procedure, and requires multi-disciplinary expertise and time for careful analysis. With ECGI evolving as a clinical tool, large-scale multi-center studies will become possible, permitting statistical analysis of CRT response in large groups of CHD patients.