Ventricular Synchrony in Pediatric Patients

There is a growing body of evidence that interventricular dyssynchrony results in abnormal
ventricular mechanics that may be detrimental to myocardial performance.(1; 2) Adult
patients with LBBB, dilated cardiomyopathy and CHF are increasingly being treated with a
novel pacing technique in order to restore electrical and mechanical synchrony.(3-5) This
technique, biventricular or resynchronization pacing, involves the insertion of a
specialized cardiac pacing system that not only maintains atrioventricular synchrony, but
also restores interventricular synchrony by simultaneously activating the right and left
ventricles. There have been a number of clinical trials (MIRACLE, MUSTT, INSYNC) that have
revealed marked clinical improvement in adult patients with moderate-to-severe CHF.(6-8)
There have been a few small reports of the acute effects of biventricular pacing in
pediatric patients with acquired interventricular dysynchrony either from repair or
palliation of congenital heart defects, or from chronic standard pacing from one ventricular
chamber (single-site ventricular pacing).(9-11) One of the main hurdles encountered when
evaluating these patients is the lack of reliable, reproducible non-invasive imaging
modalities. Recent reports have shown promising results with a relatively new technique
known as tissue Doppler imaging (TDI).(12-14) TDI echocardiography has recently been
employed as a non-geometric measure of myocardial systolic and diastolic performance for
both the left and the right ventricles. Utilizing low velocity signals from discrete points
on the myocardial walls, motion can be assessed under a variety of conditions
non-invasively. Additionally when two points along a ventricular wall are simultaneously
assayed, the relative motion between the two points can estimate myocardial systolic strain
in the direction of the Doppler sound beam. This technique eliminates any variation due to
the translational motion of the heart.

Tissue synchronization can be assessed by the timing of peak systolic motion of different
points along the myocardium relative either to the QRS of the electrocardiogram or to the
mechanical indicators of systole such as aortic valve opening. Recent advances in software
design have now created technology that simultaneously depicts the peak velocities of the
entire myocardium viewed in a 2-dimensional echo image. This promotes easy identification of
any dysynchronous segments. By subsequently applying discrete cursors to these areas,
graphic patterns of both velocity and direction of myocardial motion can be drawn for
quantitative analysis. The application of these new sophisticated tools will allow better
analysis of electromechanical dysynchrony and may enable definition of criteria, which can
be used to identify those patients who will or will not benefit from cardiac
resynchronization therapy.

We have identified 4 groups of patients that will allow us to prospectively and
systematically evaluate TSI in this young population. The first group is those patients
with supraventricular tachycardia and the Wolff-Parkinson-White (WPW) syndrome. These
patients have an accessory AV connection (AC) that allows electrical activity to bypass the
normal AV node and excite the ventricles prematurely. In doing so, the result is a loss of
the normal, midline ventricular activation pattern for a dysynchronous activation pattern.
There have been small reports of this chronic ventricular dysynchrony potential playing a
role in depressed myocardial performance.(15) Standard therapy for patients with WPW and
clinical SVT includes invasive intracardiac electrophysiology study (EPS) with possible
radiofrequency ablation (RFA) in order to eliminate the AC. These patients are then left
with normal conduction over the AV node, presumably restoring the midline activation
sequence.

The second group of patients is those children with SVT and no evidence of ventricular
preexcitation (i.e., no WPW). These patients have a normal, midline ventricular activation
pattern in sinus rhythm. Standard treatment for children afflicted with these variants of
SVT would also include EPS and RFA. As part of the standard EPS, these patients receive
intermittent atrial and ventricular pacing. The result of the transient ventricular pacing
is presumably a dysynchronous activation pattern that could be evaluated with TSI. With
pacing off, the normal activation pattern should be restored. Therefore, these patients
will also serve as their own controls.

The third group is essentially normal (normal 4 chamber anatomy) pediatric patients who have
been referred for an echocardiogram to evaluate a possible heart murmur, syncope or chest
pain. Their data will be used to establish normal pediatric values for TSI.

The fourth group is normal (normal 4 chamber anatomy) young adult subjects who will be
recruited for participation in this study. Their data will be used to establish normal
adult values for TSI. These values or results will not be compared to the other three
pediatric groups.