Cardiac resynchronization therapy (CRT) is now a well established treatment modality
for adult patients with drug refractory symptomatic congestive heart failure. Multiple
large-scale studies have clearly demonstrated reduction in heart failure-related morbidity
with CRT [1-4]. More recently, a likely independent mortality benefit with CRT has
also been shown . Improvement in quality of life, decrease in left ventricular
diastolic dimension, improved objective assessment of exercise tolerance, and decreased
heart failure-related hospitalizations have all been well demonstrated in the adult
population. Whether or not these benefits occur in younger patients is not clear .
Since none of the major trials for CRT have included children, much is unknown about
the specifics of indication, difficulties with implant, and efficacy in the pediatric
More recently, small case series have shown the potential for benefit for CRT in patients
with congenital heart disease including those with a systemic right ventricle and
single ventricle physiology. Whether long-term benefit for extending CRT therapy to
children with congenital heart disease exists is unclear.
There are several challenges confronting the implanter when considering CRT therapy
in children, and these are summarized in Table 1.
Despite these limitations in our present knowledge as to how indications, implant,
and optimization of CRT devices should be amended in children, evidence is accumulating
that suggests at least a similar benefit in children exists, as has been been demonstrated
In children with left ventricular failure and left bundle-branch block, acute left
ventricular hemodynamics has been shown to improve with biventricular stimulation
. In longer-term studies, CRT has also been demonstrated to improve effort tolerance
and oxygen consumption [9-14]. Isolated reports have shown an average increase in
ejection fraction of 14-18% and typically correlated with a concurrent decrease in
QRS duration of 20-40 ms. Improvement (decrease) in left ventricular end diastolic
diameter with effective resynchronization of up to 6 mm has been reported . Improvement
in heart failure symptoms and New York Heart Association class has also been shown
anecdotally to occur in children as has been well established in adults [14,16,17].
Given this rudimentary but existing evidence of CRT benefit for children with refractory
heart failure symptoms, the implanting physician needs to be familiar with the idiosyncrasies
of pediatric CRT. Not only must the implanter be cognizant of the indications, techniques
for optimization, and problems with implant seen in adults, but understand specifically
the unique challenges of placing optimally a left ventricular pacing lead in children.
In this article, we discuss commonly encountered causes for difficulty in implanting
CRT devices in children with and without congenital heart disease. We suggest potential
solutions and an approach in solving such difficulties. We further present guidelines
for understanding differences in interpreting the standard indications for CRT implantation
and optimization of CRT devices in adults to children .
Difficulties with Implanting Left Ventricular Leads in Children
Implanting left ventricular leads in children is generally more challenging than in
adults. The reasons for this, as expected, are largely a result of the smaller size
of the vessels and cardiac chambers as well as the increased risk of cardiac perforation
as with most pediatric cardiac procedures. Beyond this, however, there are specific
challenges that are addressed in this section involving challenges often unique to
the pediatric population.
Cannulating the Coronary Sinus
Entering the coronary sinus in children requires an accurate understanding of the
standard fluoroscopic views and important anatomic landmarks in these views. Vagaries
of the coronary ostium itself in the young and the immediately adjacent atrial myocardium
is important to appreciate, particularly when selecting sheaths and manipulating the
pacing lead. A prominent thebesian valve at the ostium of the coronary sinus likely
occurs more frequently in this age group as well.
Because the ventricular apex is to the left of the body, the fluoroscopic views must
take this into consideration to obtain anatomic accuracy. The use of the left anterior
oblique (LAO) and right anterior oblique (RAO) projections is highly recommended and
can be used to avoid common complications (Figure 1). In the LAO view, the coronary
sinus extends from the right atrium to the operator's right of the screen. This characteristic
movement to our right (left atrium) is an important indicator that the coronary sinus
had been entered. It should be remembered, however, that when there is severe cardiac
enlargement, the heart may be rotated, and the right ventricular apex may be very
"leftward" appearing. By looking at the orthogonal view (RAO), it will be easy to
differentiate between deep ventricular engagement and that of the coronary sinus.
In the RAO view, the coronary sinus catheter or pacing lead advances neither anteriorly
(ventricle) nor posteriorly (atrium) but appears to come right out at the operator.
A common and usually successful maneuver is to take a soft bidirectionally deflectable
catheter that has been placed through a guiding sheath. The catheter is first placed
in the right ventricle posteriorly. Strong counterclockwise torque is then applied
to force the catheter towards the intraventricular septum. Now, holding this same
torque, the catheter is gently pulled back until a characteristic to-and-fro movement
is noted, suggesting cannulation of the coronary sinus. This maneuver is done in the
RAO projection, and as soon as the change in movement is noted, the fluoroscopic camera
can be moved to the LAO view to be certain that the deflectable catheter is advancing
to the operator's right (patient's left) as is typical of the coronary sinus. Once
entered, the guiding sheath is then moved over the catheter to gain access to the
proximal portion of the coronary sinus, preferably about 1-2 cm distal to the coronary
Performing this maneuver in every case in the standard fluoroscopic projections will
significantly enhance the ease in which this important first step for CRT delivery
is performed. As seen in Figure 1, at times, the ostium and proximal portion of the
coronary sinus can be fairly tortuous, but again, noting that the catheter when being
advanced does in fact go to the operator's right in the LAO projection and neither
anteriorly or posteriorly in the RAO projection and using gentle force, the CS can
be safely negotiated. An additional fluoroscopic landmark is that in the RAO projection
at the junction of the diaphragm and cardiac silhouette a lucency just posterior to
the coronary sinus called the epicardial "fat pad" can be appreciated. This is an
approximate landmark for the operator to know as to when to expect entering the coronary
sinus when pulling back the catheter torqued onto the septum as explained above.
Prominent Thebesian Valve
Rarely, a completely occlusive imperforate thebesian valve may preclude standard cannulation
of the coronary sinus. Even with near occlusive valves, typically the inferior-ventricular
quadrant of the coronary sinus ostium will not be covered. This is one of the reasons
why the maneuver described above starting from the ventricle as inferiorly as possible
and pulling back to the coronary sinus is more likely to be successful in the difficult
case. In the rare patient with a near occlusive valve, gentle force with a blunt catheter
being certain that one is in the plane of the coronary sinus and using intracardiac
or transesophageal ultrasound guidance is successful. In very rare circumstances,
radiofrequency energy posteriorly applied (to avoid AV nodal damage) can be used to
create a controlled perforation of the thebesian valve and allow entrance of the coronary
The Subeustachian Pouch
An important anatomical fact should be appreciated when implanting left ventricular
leads in children. The atrial myocardium between the eustachian ridge and the tricuspid
valve immediately adjacent to the coronary sinus ostium is often aneurysmal or pouch-like
in young children. This subeustachian pouch (Figure 2) can make placement of a guiding
sheath in an appropriate plane to cannulate the CS difficult. When pulling back the
catheter or sheath with counterclockwise torque being applied, the sheath will appear
to suddenly jump. At this point, gentle injection of contrast dye will show swirling
or stagnation in the subeustachian pouch. The operator may mistakenly think that further
counterclockwise torque is required, and then the guiding sheath or catheter will
move behind the eustachian ridge, making entering the coronary sinus impossible. Understanding
in the RAO projection when the subeustachian region has been entered can make negotiating
the CS easier. When this difficulty is noted, it is best to use guiding sheaths that
do not require stability on the inferior atrial wall such as relatively straight guiding
sheaths or sheaths with very large secondary curvatures that "balance" against the
lateral right atrial wall (straight Attain™ or Worley™ sheaths).
Difficulty with Advancing within the CS
Once the coronary sinus has been entered and the guiding sheath stabilized in its
proximal portion, the next task that may be difficult in children is advancing the
pacing lead within the coronary sinus to a ventricular vein that drains the lateral
wall of the left ventricle.
Coronary Sinus Dissection
Particularly in the very young, the coronary vein can be quite friable, and even a
relatively moderate manipulation can result in coronary venous dissection. Although
this is well tolerated by patients, once dissection occurs, further manipulation in
the coronary sinus or placing the lead may be precluded. Perhaps the most important
method to avoid coronary dissection in addition to applying very gentle force is to
never advance an oversized sheath (inner diameter of the sheath larger than the outer
diameter of the wire or guiding catheter within the sheath) in tandem into the coronary
sinus. Once a guiding catheter has been placed relatively deep into the coronary vein,
it should be pulled back as the sheath is being advanced. Similarly, when advancing
the pacing lead once the guide wire has advanced to a vein of interest, when trying
to advance the lead, the wire should be gently pulled back as the lead is advanced.
After advancing about a centimeter, the wire can then be re-advanced into the ventricular
vein of interest and the maneuver repeated until the lead is in the required position.
This maneuver (pulling back on wire while pushing the lead) is particularly important
to execute when negotiating tortuous vessels or at the point of branching in the ventricular
veins and tributaries.
A common reason for difficulty in advancing the pacing lead or causing venous damage
is inadvertent sub-selection of an atrial or ventricular branch of the coronary sinus.
If the operator is unfamiliar with the expected course of the coronary sinus in the
RAO and LAO projections, an atrial branch may have been entered after engaging the
coronary sinus. With continued forward force application or further counterclockwise
torque, the lead/catheter will not only fail to advance but dissection may occur.
Similarly, soon after engaging the coronory sinus, a posterior ventricular vein may
have been entered. Again, without familiarity with this possibility, the operator
may think that the coronary sinus has not been entered and the lead/catheter is in
the right ventricle and pull out of the vein when simply advancing the lead or sheath
over the catheter to allow lead placement may have given a satisfactory result.
Just as the thebesian valve "guards" the ostium of the coronary sinus, various valves
may be present, particularly in children, throughout the course of the coronary sinus
including the junction with the great cardiac vein and at the ostia of the various
ventricular venous branches. The most consistent of these valves is the so-called
Vieussens valve located at the ostium of the posterolateral vein, typically at the
same location of the vein of Marshall (oblique atrial vein) approximately 2-3 cm from
the coronary sinus ostium in children. This valve, if small, does not cause much
difficulty once a wire has crossed the valve and entered either the posterolateral
ventricular vein or great cardiac vein. When nearly circumferential, however, this
may preclude entering the posterolateral vein, a typical target for left ventricular
lead implantation (Figure 3). When this occurs, either a more anterior lateral or
a more posterior vein can be entered. A tributary of one of these veins draining
the lateral wall can be used for satisfactory left ventricular pacing .
The ideal target to place the left ventricular pacing lead in most patients is the
midportion of the left ventricular free wall. There is typically a lateral cardiac
vein draining this location into either the great cardiac vein or coronary sinus .
Sometimes, however, the takeoff of this vein may be very tortuous, or the vein itself
may be too small to safely cannulate. It is important for operators to be aware of
extensive collaterals that exist between the anterior, posterior, and lateral venous
circulation on the free wall of the left ventricle. These collaterals are usually
of sufficient size in children to take some of today's smaller diameter over-the-wire
left ventricular pacing leads. The typical maneuver required is to first cannulate
the posterior or anterior cardiac vein with the lead. With the lead placed in this
vein, then using a soft-tip guide wire (Whisper™) the collateral is entered. Once
entered, gently pulling back on the guide wire while advancing the lead will sub-select
the collateral. In the LAO projection, the lead should be seen to be on the lateral
wall. In addition, when pacing from a true lateral wall location, a QS wave (negative
deflection) in lead I of the electrocardiogram will be noted (see below). A dictum
worth remembering for operators is that the left ventricular free wall site and not
the vein draining it directly is the target for pacing the lead (Figure 4).
Left Ventricular Lead Implantation in Congenital Heart Disease
Anecdotal and in relatively small case series [19,20], CRT has been shown to be beneficial
in improving cardiac function and quality of life in various congenital heart diseases.
The implanter, however, must have an accurate knowledge of the relevant cardiac anatomy,
location of the coronary sinus, site of emptying of the ostium of the coronary sinus,
and whether or not right-to-left shunting exists that would preclude endovascular
implantation. In this section, we will review common causes of difficulty and a general
approach for implanting leads in patients with congenital heart disease.
Persistent Left Superior Vena Cava
Several congenital anomalies may have an associated persistent left superior vena
cava. However, this anomaly often exists as a separate entity. The right superior
vena cava may exist in addition to the persistent left superior vena cava, and one
or more anastomotic veins between the vena cavae may be present. During initial experience
with CRT, operators assumed that using the left superior vena cava would be easier
for accessing the coronary sinus and a ventricular vein since this structure drains
into the coronary sinus (Figure 5). However, lead stability is not optimal in these
cases since the coronary sinus and ventricular veins are often grossly enlarged. The
author's preference when the right superior vena cava cannot be accessed is to cannulate
the left subclavian vein and place a lead into the main body of the coronary sinus
via the left SVC. Once the lead reaches this location, counterclockwise torque is
applied so that the guide wires, lead, and sheath will travel anterolaterally to the
junction of the left SVC with the coronary sinus into a more anterior vein. An anterolateral
or lateral branch of an anterior vein is then sub-selected and the left ventricular
pacing lead placed.
When the left superior vena cava is not entirely patent, it may remain as a smaller
oblique vein of Marshall (Figure 5). In certain children requiring biventricular pacing,
to obtain ideal left atrial-left ventricular synchrony, a left atrial pacing lead
also requires to be placed. For those patients, the coronary sinus is entered in a
manner described above. However, a deflectable catheter is advanced about 2-3 cm into
the coronary sinus and then a small posterior deflection is made and the catheter
further rotated in a counterclockwise direction, observing catheter movement in the
RAO projection. Atrial vein engagement is noted by posterior movement in the RAO projection
and the recording of large atrial electrogram from the pacing lead or mapping catheter.
In the setting of more complex congenital heart disease, the left superior vena cava
may offer the best option to access both ventricles (endocardially and via the coronary
venous system). Angiography from the vein and from the left superior vena cava is
sometimes useful to understand the best target, guiding sheath, and catheter torque
to obtain optimal results .
Where is the Coronary Sinus?
A critical question that must be asked by the implanter and thoroughly researched
with review of prior admitting data is where the coronary sinus drains in the patient
being considered for CRT and with congenital heart disease . For example, in a
patient with an older-type of Fontan procedure, wherein the right atrial appendage
is connected to the right ventricular outflow tract, the coronary sinus will drain
normally into the right atrium. Thus, even in a patient with tricuspid atresia, ventricular
(specifically left ventricular) pacing can be effected via an endocardial route utilizing
the coronary sinus .
Exact surgical details and recent imaging data are essential before proceeding. In
patients, for example, with AV canal defects, the closure of the septum primum defect
and/or placement of a prosthetic tricuspid valve may be done in such a way that the
coronary sinus actually drains into the right ventricle, distal to the tricuspid valve
placement. For such patients, the catheter that is being used in an attempt to engage
the coronary sinus must first access the ventricle and then appropriately maneuvered
to cannulate the coronary sinus. Right ventricular as well as right atrial angiography
may be useful, for even when the coronary sinus has been excluded from the right-sided
structures accessible through the vena cavae, large thebesian veins may be draining
into the basal right ventricle that can be cannulated and a left ventricular pacing
lead placed either directly or via anastomoses to the coronary veins.
Where is the Apex and Which Ventricle is Anterior?
In addition to clearly understanding the detailed anatomy of a patient's congenital
defect, the operator should be aware of where the apex is located and the relative
positions of the right and left ventricles.
Knowledge of where the apex is located will allow the operator to adjust the fluoroscopic
views accordingly. For example, in a patient with dextrocardia, the RAO angle is lined
up along the axis of the apex and will in effect be used like the LAO projection to
know when the coronary sinus is cannulated and whether the ventricular free wall is
being accessed for pacing. The orthogonal view, in this case the LAO projection, will
be used similarly to the RAO projection in the normal heart to follow the typical
course of the coronary sinus and ventricular veins. If a patient has mesocardia, then
the AP becomes the effective LAO view (Figure 6).
Once the preimplant knowledge of the apex and coronary sinus drainage is known, it
can be very useful to know the relative positions of the two ventricles. If the right
ventricle is anterior, generally, coronary sinus lead placement is not distinctly
more difficult than with normal anatomy. However, when the right ventricle is the
posterior ventricle, careful venography, possible coronary arteriography to view the
venous phase, and placement of a lead in the right ventricle prior to attempting coronary
sinus cannulation can be useful.
In patients with L-TGA and morphological right ventricular failure, CRT may be beneficial
[24,25]. The coronary venous anatomy is unusual in these patients . The coronary
sinus itself goes with the atrium, that is, the left atrium is normally located in
these patients, and the coronary sinus follows the usual radiographic course. However,
the ventricular veins are more similar to the typical right ventricular veins. Thus,
a distinct lateral, anterolateral, or posterolateral vein is usually not present.
The veins, however, may be large enough to place a pacing lead, otherwise the middle
cardiac or anterior intraventricular vein are cannulated and lateral branches used
for pacing. The morphological left ventricle (systemic venous circulation) drains
primarily through the septal veins (anterior intraventricular and middle cardiac vein)
and large thebesian veins that directly empty into the right atrium.
ICD Leads in the Coronary Sinus and Ventricular Veins
In patients with a prosthetic tricuspid valve, it may not be possible (mechanical
prosthesis) or inadvisable (bioprosthesis) to cross the valve to place an ICD lead
in the right ventricle. In these patients, using similar techniques (Figure 7) described
above to cannulate and place a lead in the coronary veins, a guiding sheath can be
used to sub-select a posterior ventricular vein. Because of the relatively shorter
length of the ICD leads, standard guiding sheaths may need to be cut to allow passage
of the ICD lead into the vein of interest. Both tined as well as active fixation leads
have been used in this situation. When an active fixation mechanism is used, care
must be taken in using the RAO projection to see that the screw is being deployed
towards the ventricular myocardium and not in the free wall of the middle or posterior
Epicardial Placement of the Left Ventricular Lead
Patients with intracardiac shunts have a contraindication to placement of an endocardial
system. The majority of experience with epicardial pacing leads for left ventricular
stimulation comes from pediatric experience. Epicardial leads are frequently used
in this situation because of the smaller ventricular veins in infants and small children.
The use of epicardial leads is documented to have shorter longevity and higher incidence
of associated exit block in comparison to endocardial steroid-eluting leads. Recently,
however, the creation of steroid-eluting epicardial leads and longer duration follow-up
have shown that acceptable results can be obtained with these pacing systems [26-28].
The steroid-eluting tip reduces inflammation, decreasing exit block, and keeps electrical
thresholds in an acceptable range, an important consideration given the long duration
of lead requirement when placed in younger children. Most successful cases of CRT
in congenital heart disease in infants have been with an epicardial pacing system
in Europe and the United States [22,29]. A suggestion from these studies  is that
in children weighing less than 55 pounds, an epicardial biventricular pacing lead
should be considered as a first option because of the small veins. The author's preference,
however, is to attempt endocardial pacing as the first option and resort to epicardial
implant when this fails.
Bridge to Cardiac Transplant
In recent years, CRT along with assist devices has been used as a bridge to heart
transplant. Patients on heart transplant waiting lists have established criteria for
CRT . Some patients  have been taken off the transplant list based on unmistakable
improvement in ejection fraction and clinical status following CRT .
Selection Criteria: Challenges for the Pediatric Population
Even in the adult population, intensive study is presently being done to understand
which exceptions to the standard criteria for recommending CRT therapy should be made.
In the pediatric population, however, these issues are paramount to consider. Very
few pediatric patients, both in the literature and in practice, meet the adult criteria
for CRT, yet have benefited from this form of therapy .
Present guidelines include having patients in New York Heart Association (NYHA) class
III-IV, normal sinus rhythm, electrical evidence of ventricular dyssynchrony with
QRS duration greater than 120 ms, and left ventricular ejection fraction less than
35% [31,32]. Further, there is evidence that when standard pacemakers are placed in
young patients, they tend to require pacing for many years, and CRT may offset RV
pacing-induced cardiomyopathy . The present indications for CRT, future considerations,
and specific issues related to children are summarized in Table 2 and elaborated on
Electrical dyssynchrony, as evidenced by bundle-branch block or wide QRS duration,
is likely associated with mechanical dyssynchrony. This is not, however, a consistent
relationship. This discordance (between QRS duration and likelihood of CRT benefit)
is more pronounced in children since their normal QRS duration and intraventricular
conduction time tend to be smaller [18,31,34,35]. Thus, children with highly symptomatic
heart failure despite optimal medical therapy should not be precluded from consideration
of CRT based on QRS duration alone. Therapy must be individualized in these cases
Ejection Fraction and New York Heart Association Functional Class
Investigations are ongoing as to whether earlier intervention with CRT in adults (NYHA
class II) is beneficial. In children, considerations for earlier intervention may
be important prior to adverse remodeling given the duration in which therapy will
be required for heart failure and the higher relative incidence of transplant need
when severe heart failure has its onset early in life .
Chronic Right Ventricular Pacing
In adult patients who have drug refractory heart failure and ejection fraction less
than 35% but are right ventricular paced (and as a result have a wide QRS), generally
receive CRT therapy despite this situation not being specifically supported in randomized
The issue is more complex when considering CRT in children. Should the implanter wait
until severe cardiomyopathy develops? The answer to this question is presently undefined.
Some studies suggest that children with complete AV block and constantly paced from
the right ventricle developed cardiomyopathy, and this can be prevented or treated
by upgrading their devices to include left ventricular pacing [33,37]. However, it
remains unclear whether the right ventricular pacing alone produced cardiomyopathy
or an underlying disease process was responsible or AV block and cardiomyopathy as
The major randomized trials performed for CRT recruited adults in sinus rhythm. Whether
or not a benefit exists for patients in atrial fibrillation or other atrial arrhythmias
with CRT is being investigated. In children, the issues are even more complex. Cardiomyopathy
in the setting of congenital heart disease is often complicated by scar-related atrial
flutter that may be persistent in later stages. Clinical judgment must be individualized;
however, in general, if ventricular rates can be well controlled (so as to allow biventricular
pacing most of the time), then the atrial flutter alone should not preclude CRT recommendation.
Even when in sinus rhythm, children often have faster sinus rates and shorter atrioventricular
conduction times. When sinus tachycardia is present, biventricular pacing that tracks
the atrium may be rapid, and an element of tachycardia-related dysfunction may offset
the benefits from resynchronization.
Optimization of CRT Devices: Challenges in Children
Approximately a third of patients who receive CRT devices with appropriate and established
indications for their placement do not respond to therapy. In certain cases, either
the myocardial disease process is so far advanced or the substrate is worsening (new
infarctions) that CRT benefit is far outweighed by these factors. In others, however,
the left ventricular lead's position, function, and device programming may be suboptimal
and result in the failure of benefit. A variety of techniques to try and optimize
resynchronization devices in adults have been attempted, but none have yielded consistent
and reproducible results.
Since the expected result of CRT is mechanical resynchronization, echocardiographic
parameters have been developed and tried to assess the extent of continuing dyssynchrony
and then by reprogramming the device (offset between right and left ventricular stimulation)
or repositioning the lead to check if optimal synchrony is occurring. A major issue
with using these techniques in children is that of inadequate frame rates with presently
existing systems. Conduction velocity, even in the diseased child's heart, is relatively
rapid, and thus when frame rates are low, predicting the most dyssynchronous or latest
to activate site can have unreliable spatial resolution.
The effects of exercise and posture may also be more relative in children than in
adults. Despite relatively advanced ventricular dysfunction, children may be more
active and prone to changing their position. In these circumstances, the intracardiac
conduction velocities and pacing vectors may change appreciably (asymmetric decremental
conduction), making any attempt at optimization at rest in the supine position unreliable.
Children may also have substantial changes in their underlying substrate, particularly
the population waiting for transplantation. New surgical scars, atrial arrhythmias,
and the effects of conduction slowing membrane active medications once again will
not be helped with attempts at optimization prior to these changes.
Since electrical stimulation must first occur prior to contraction of the myocardium,
electrical synchrony has been used as a surrogate for mechanical synchrony, and attempts
at using the 12-lead electrocardiogram to optimize CRT devices have arisen . The
QRS duration, fusion of electrocardiographic vectors, and recognition of anodal stimulation
or inordinate capture latency have been used in adults, but their limitations must
be understood when applying to the pediatric population .
Normalization of the QRS duration with biventricular stimulation is an inexact but
usable surrogate for synchronization. As noted above, in children, however, the QRS
is often in the normal range (as compared to adults) even with advanced cardiomyopathy
and anecdotally shown benefit with resynchronization.
QRS Vector Fusion
Each pacing site gives rise to a characteristic vector that can be deduced with analysis
of the 12-lead electrocardiogram. For example, right ventricular pacing from the apex
results in a left bundle-branch block pattern with negative QRS complexes in leads
II, III, aVF, and a tall R wave in lead I. On the other hand, left ventricular pacing
from the midlateral wall results in a right bundle-branch block pattern and a negative
QRS in lead I. It follows, therefore, that biventricular stimulation when optimally
resynchronizing the heart (equal contributions from both pacing sites), the electrocardiogram
should be fused or intermediate between RV and LV pacing vectors (Figure 8). If, however,
biventricular stimulation results in a pacing vector very similar to right ventricular
pacing alone, then either the left ventricular lead has been placed too close to the
right ventricle (anterior intraventricular vein) and should be repositioned or there
is significant capture latency and exit delay from the pacing site to the rest of
the ventricle. In the later situation, if recognized on the electrocardiogram, programming
an offset so that the left ventricular lead output is delivered prior to the right
ventricular lead output may better synchronize the heart and decrease symptoms.
Another cause for the electrocardiogram to appear like right ventricular pacing alone
even with biventricular stimulation is the phenomenon of anodal stimulation .
This problem may be more frequent in children given the small size of their hearts.
Here, despite the left ventricular pacing threshold being adequate when stimulated
between the left ventricular lead tip and the RV lead (coil or ring electrode), the
stimulation wave front is from the anode (RV lead) and thus negating any benefit from
biventricular stimulation. The implanter should be aware of and look for this phenomenon
with the electrocardiogram and if found, consider bipolar stimulation with the LV
lead or programming the output or pacing vector differently.
Just as with adults, however, when CRT devices are placed in children, every effort
to ensure close to 100% biventricular stimulation should be made. Causes that may
be decreasing the amount of ventricular pacing including frequent PVCs, PACs that
fall in the PVARP, atrial arrhythmias with rapid ventricular conduction, etc., should
be sought and remedied.
Cardiac resynchronization therapy has made a major impact in the lives of many adults
with drug refractory congestive heart failure. While applying this therapy to children
has been shown to be beneficial, distinct differences in how the physician should
approach care in children exist. The standard indications often have to be modified
with clinical judgment and techniques for optimization should be exactly understood
prior to applying in the pediatric population.Specific difficulties associated with
implanting a left ventricular pacing lead in children, and suggestions on how to overcome
these difficulties have been reviewed in this paper.