Download The Right Ventricular Outflow Tract: The Road to Septal Pacing

The Right Ventricular Outflow Tract:
The Road to Septal Pacing
HARRY G. MOND, M.D., F.R.A.C.P., F.A.C.C., F.C.S.A.N.Z., F.H.R.S.,
RICHARD J. HILLOCK, M.B.CH.B., F.R.A.C.P., IRENE H. STEVENSON, M.B.B.S., F.R.A.C.P., and
ANDREW D. MCGAVIGAN, M.D., M.R.C.P.
From the Department of Cardiology, The Royal Melbourne Hospital, Melbourne, Australia
Background: Pacing from the right ventricular apex is associated with long-term adverse effects on left
ventricular function. This has fuelled interest in alternative pacing sites, especially the septal aspect of
the right ventricular outflow tract (RVOT). However, it is a common perception that septal RVOT pacing
is difficult to achieve.
Methods and Results: In this article, we will review the anatomy of the RVOT and discuss the importance
of standard radiographic views and the 12-lead electrocardiogram in aiding lead placement. We will also
describe a method utilizing a novel stylet shape, whereby a conventional active-fixation, stylet-driven lead
can be easily and reliably deployed onto the RVOT septum. (PACE 2007; 30:482–491)
selective-site pacing, ventricular septal pacing
Introduction
Cardiac pacing from the right ventricular (RV)
apex produces a characteristic wave of depolarization, which results in abnormal ventricular activation from the apex to the base and from right to
left ventricles. This results in an increased total
ventricular activation time with consequent late
activation of the lateral wall of the left ventricle
(LV).1,2 Over time, the sequelae of chronic pacing
from the RV apex are a higher risk of development
of left ventricular dysfunction,3,4 heart failure,5–7
atrial fibrillation,7,8 and death.9 Although the precise mechanisms underpinning the adverse affects
of apical pacing remain unclear, they are likely to
be multifactorial. Animal and human studies have
demonstrated differential muscle stretch10 and
fiber shortening11 in response to RV apical pacing.
This in turn adversely affects cardiac hemodynamics,12,13 induces LV dyssynchrony,4 and increases
myocardial work11 and oxygen consumption.14
The subsequent long-term LV cellular changes,
both at a gross15 and ultrastructural level,16 ultimately lead to LV remodeling and adverse clinical
outcomes. These observations have led to an interest in selective RV pacing sites in order to achieve
a more “physiological” pattern of ventricular activation.17–20
The most studied of these selective sites has
been the right ventricular outflow tract (RVOT),
Dr. Stevenson is supported by a medical postgraduate scholarship from the National Heart Foundation of Australia and the
Cardiac Society of Australia and New Zealand.
Address for reprints: Harry G. Mond, M.D., F.R.A.C.P., F.A.C.C.,
F.C.S.A.N.Z., F.H.R.S., Suite 22, Private Medical Centre, The
Royal Melbourne Hospital, Victoria 3050, Australia. Fax: 613
9347 6760; e-mail: [email protected]
Received November 26, 2006; accepted December 19, 2006.
with increasing focus on the septal aspect of this
structure. RVOT septal pacing, however, is believed to be difficult to achieve and indeed, we
have previously demonstrated that pacing the low
septal aspect is only obtained in 61% of unselected cases undergoing RVOT lead placement using standard implantation techniques.21 On the
other hand, we also demonstrated the clinical utility of the 12-lead electrocardiogram (ECG) in determining pacing site. This, coupled with an increased understanding of the relationship between
fluoroscopy and anatomy of the RVOT,21,22 may aid
in successful lead placement in the low septal aspect of the RVOT.
In this article, we review the anatomy of
the RVOT and discuss the utility of standard radiographic views and the 12-lead ECG in aiding
lead placement. We will also share our experiences with this implant technique and describe
our method utilizing a novel stylet shape enabling
easy deployment of a conventional steroid-eluting
7-French active-fixation lead onto the RVOT septum. Currently, in our institution, we can reach
the RVOT septum in all cases usually at the first
attempt using this technique.
Right Ventricular Outflow Tract
The RVOT has been poorly defined in the
pacing literature, and the term has been used to
describe a variety of pacing sites including the
true outflow tract, the mid septum, and the anterior region above the apex. This confusion persists despite attempts to standardize the nomenclature of nonapical pacing sites.23,24 Consequently,
the resultant acute and chronic studies of RVOT
pacing have produced conflicting results making
interpretation of the published literature difficult.
Furthermore, even when the true RVOT is considered, the anatomy is complex and many studies do
C 2007, The Authors. Journal compilation C 2007, Blackwell Publishing, Inc.
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Figure 1. Illustration of the heart highlighting the RV septal anatomy. The RVOT is bordered by
the pulmonary valve above and the superior aspect of the tricuspid apparatus below (both dotted
lines). The upper part of the septal wall is the conus arteriosus, bordered below by the supraventricular crest. To the anatomical left of the septomarginal trabeculation, which continues into the
moderator band, are the septoparietal trabeculations. These structures have been emphasized in
this illustration.
not distinguish between different sites within this
structure,19 which may be important, as activation
patterns and wavefront propagation will depend
on the pacing site within this large area of the right
ventricle.
Anatomy of the RVOT and
Relevance to Pacing Site
For purposes of cardiac pacing, the RVOT is
bounded by the pulmonary valve superiorly and
the superior aspect of the tricuspid apparatus inferiorly (Fig. 1).25 Although the RVOT is often considered as having four quadrants—superior (high)
and inferior (low), both with free wall and septal
aspects,24 this is probably an oversimplification.
In fact, the septal RVOT is a misnomer, as much
of it abuts the proximal ascending aorta and thus
the superior and maybe part of the inferior RVOT
“septum” lies above the aortic valve.21,24 Therefore, using the strict anatomical definition of a septum as a structure that can be removed without exiting the heart, only part of the inferior portion of
the RVOT septum can be considered as truly septal. As demonstrated in Fig. 1, the “septal” component of the conus arteriosus is high and smooth
walled and its position makes it both anatomically
and probably electrophysiologically unsuitable for
the attachment of a pacing lead. Below the level
of the supraventricular crest (crista supraventricu-
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laris) lies the inferior portion of the septum, which,
to the left of the septomarginal trabeculation, is a
cul-de-sac filled with the septoparietal trabeculations and is ideal for pacing lead attachment as it
is truly septal.
From an electrophysiologist’s perspective, the
walls of the RVOT can be divided into four segments; septal, anterior, posterior, and free walls
(Fig. 2), and is a useful concept in the catheter ablation of focal ventricular tachycardia. Although this
Figure 2. Schematic of a cross-section of the chest to
demonstrate the relationship of the four areas of the
RVOT to surrounding structures. Depending on the level
of the RVOT, behind the septum lies either the left ventricle (LV) or ascending aorta (Ao).
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MOND, ET AL.
Figure 3. Above: 40◦ LAO, PA, and 40◦ RAO fluoroscopic images demonstrating a dual chamber
implant with the ventricular lead in the RVOT septum (arrows). A multipolar catheter lies at the
position of the His bundle with a line above demarcating the lower margin of the RVOT. Below:
PA fluoroscopic image demonstrating a dual chamber implant with the ventricular lead below
the RVOT (arrow).
arrhythmia commonly originates high in the septal aspect of the RVOT, just below the pulmonary
valve, it can arise from a focus anywhere within
the outflow tract. These foci produce characteristic
patterns on the ECG depending on the site of origin within the RVOT. These patterns can be reproduced by pace-mapping at these sites, and studies
of catheter ablation have demonstrated the utility
of the 12-lead ECG in localizing the focus, which
may aid in successful ablation.26,27 Lessons from
these studies and the fluoroscopic appearances of
successful sites of ablation22 can be applied to the
pacing arena as they have demonstrated characteristic paced ECG and radiographic appearances
from two sites important to RVOT pacing—the septum and free wall.
Within the pacing literature, pacing at septal
RVOT sites is associated with a reduced QRS duration21,28–32 which represents a shorter total ventricular activation time and may therefore have
less impact on ventricular dyssynchrony. In addition, chronic pacing from the septal aspect may
not be associated with the cellular disarray and
ultrastructural changes associated with pacing at
the apex,16 which may in turn translate into clinical benefit. Indeed, if one examines RVOT pacing
studies, where a septal RVOT pacing site was specified, improved outcomes both acutely33–35 and
in the medium-term29,30,32 have been consistently
demonstrated in the RVOT group compared to apical pacing sites. This is in contrast to the heterogeneous results of studies which did not specify
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RVOT pacing site. Therefore, the true septal aspect of the RVOT is the ideal target for placing a
ventricular lead.
Radiographic Anatomy of the RVOT
As stated, the superior margin of the RVOT
is the pulmonary valve. The inferior margin can
be considered as a line drawn from the superior
tricuspid annulus (at the level of the His bundle)
to the interventricular septum. This can be represented fluoroscopically as a multipolar catheter
passing though the summit of the tricuspid valve,
with recording of a His potential, with the line then
projected toward the left cardiac border (Fig. 3).23
For RVOT lead placement, three views are
essential:
The postero-anterior (PA) aspect is the best for
determining if the lead lies within the low RVOT
(Fig. 3). The view, however, gives little help as to
which RVOT segment the lead is actually attached
to. When two leads lie in the RVOT, one septal and
one free wall, the latter is usually pointing upward
or superior, although this finding is not consistent
(Fig. 4).
The 40◦ right anterior oblique (RAO) helps
confirm RVOT positioning. It is not unusual for
a lead to enter the coronary sinus and lie within
the great cardiac vein. As seen in Fig. 5, a lead
in the coronary sinus/great cardiac vein mimics
the RVOT in the PA view. The RAO confirms the
posterior aspect of the lead in the cardiac venous
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Figure 4. The 40◦ LAO, PA, and 40◦ RAO fluoroscopic images demonstrating two leads in the
RVOT. The PA and RAO views were not helpful in determining where the leads were secured. The
LAO view confirms that the lead passes posterior (to the right) (A) into the septum, whereas the
other passes anterior (to the left) (B) into the free wall.
system. The same principle can be applied to RV
apical pacing and inadvertent positioning of the
lead in the middle or lateral cardiac vein.
The septal and free wall aspects of the RVOT
are best defined by the 40◦ left anterior oblique
(LAO) view. This is demonstrated in Figs. 3
and 4, with the septal position being characterized
by a posterior orientation of the lead tip, which
faces toward the right and the free wall site by a
leftward anterior orientation.
A fourth view, the 90◦ left lateral (LL), is also
very valuable, but almost impossible to achieve
during an implant using single plane fluoroscopy,
because of sterile drapes, lead shields, arm boards,
and monitoring equipment. A posterior projection
of the lead tip indicates septal placement and is
100% specific (Fig. 6).21 In comparison, a lead on
the free wall passes anteriorly toward the sternum.
The anterior segment of the RVOT, particularly the junction with the septum, where the anterior descending coronary artery lies, is a transition
zone. Not surprisingly the appearances lie between
the other two described lead tip positions (Fig. 7).
In both LAO and LL projections, the lead tip points
superiorly.
ECG Correlates of Lead Position
The septal RVOT is a more posterior and leftward structure than the free wall25 and this is
reflected in the typical ECG patterns of pacing
at these sites (Fig. 8). Septal pacing produces a
shorter QRS duration21,29–31 and typically a negative or isoelectric vector in lead I. Indeed, this feature has a 90% positive predictive value for septal
placement.21 Conversely, free wall sites are associated with prolonged QRS duration, notching of
the inferior leads (lead III), and a positive vector
in lead I.21 The anterior wall varies between these
two appearances and frequently has an almost isoelectric vector in lead I.
Lead Placement in the RVOT Septum
We have developed a novel technique to reliably attach a conventional 7-French active-fixation
Figure 5. The 40◦ LAO, PA, and 40◦ RAO fluoroscopic images demonstrating a dual chamber
pacing system with the ventricular lead in the coronary sinus and great cardiac vein. The lead
lies a little higher than usual in the PA view. In the RAO and LAO views, the lead lies posterior at
the back of the heart rather than anterior. The broken line delineates where the lead should lie
on the RVOT septum. At the bottom of each view, an ECG cable is present.
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Figure 6. LL fluoroscopic images of a dual chamber pacing system displaying a ventricular lead
in the RVOT septum (A) projecting posteriorly (arrow) and in the RVOT free wall (B) projecting
anterior (arrow) toward the sternum.
Figure 7. PA, 40◦ RAO, LL, and 40◦ LAO fluoroscopic images of a dual chamber pacing system
to demonstrate a ventricular lead in the anterior wall of the RVOT. The LL view shows the lead
pointing upward (arrow) with a slight bend to the right in the LAO view (arrow). There is also an
old nonfunctioning lead at the RV apex.
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Figure 8. Typical ECG appearances of pacing from the RVOT free wall and septum. Lead 1 has
been enlarged to show the differences between these sites with a QS wave for septal pacing and
an R wave for the free wall. Notching of the QRS in lead III (enlarged) is often seen with free wall
pacing. The fusion beat (F) highlights the importance of forced ventricular pacing.
lead to the RVOT septum, with either the stylet or
loaded lead being shaped in a characteristic fashion (Fig. 9). To prevent damage to the lead, stylet
shaping is preferred to lead shaping. Firstly, a generous distal curve is created with the terminal
Figure 9. The prepared stylet with the generous curve
and bent distal 2 cm with posterior angulation.
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2 cm of wire bent to create a swan neck deformity
similar to the design suggested by Vlay.36,37 This
design, however, will only place the pacing electrode on the septum in 61% of cases.21 Therefore,
the ideal stylet shape must reliably position the tip
of the lead more posteriorly to allow advancement
to the septal aspect. This can be achieved by modifying the curved stylet shape. Holding the curved
end of the stylet with the summit upward, the terminal straight bend is shaped forward. If the stylet
loaded lead is shaped, care must be taken to ensure that the anode ring lies in the terminal straight
portion. If the stylet is then reversed, ready to be
inserted into the lead, the terminal bend faces posteriorly (Fig. 9). Leads can be inserted from both
the left and right sides, using either cephalic or
subclavian venous access. When cephalic vein cut
down is preferred, it is desirable to cannulate the
vein with an appropriate sized introducer to aid in
directing the tortuous lead body directly into the
superior vena cava.
The lead is advanced into the pulmonary
artery and withdrawn into the RVOT. This excludes inadvertent cannulation of the coronary sinus. Furthermore, in our experience, when the
lead is placed directly into the RVOT and not
withdrawn from the pulmonary artery, the final
position may be too low or too high and can be
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MOND, ET AL.
Figure 10. PA fluoroscopic images to demonstrate passage of the lead to the pulmonary artery.
Left: The distal part of the lead is prolapsed through the tricuspid valve toward the RVOT. Right:
Once beyond the pulmonary valve, the tip of the lead lies in the left pulmonary artery. The lead
is then retracted back to the RVOT.
surprisingly difficult to attach to the wall as it may
not lie in the trabeculated area to the right of the
septomarginal trabeculation (Fig. 1). Although advancement of the lead into the pulmonary artery
could be achieved using a conventional curve and
then the prepared stylet inserted, this has the disadvantage of requiring the distal portion of the
prepared stylet to negotiate the tricuspid valve
and curvature toward the outflow tract. Not infrequently, during this stylet insertion maneuver, the
lead tip is prolapsed into the inflow tract of the
right ventricle and possibly atrium.
The preferred and easier method of passing
the fully loaded lead into the pulmonary artery is
to catch the tip on low atrial structures, the tricuspid apparatus or the floor of the right ventricle, and
then insert more lead, creating a loop arching into
the body of the right ventricle (Fig. 10). Occasionally, the stylet may need to be slightly withdrawn
to facilitate lead tip prolapse into the pulmonary
artery. With the shaped stylet, the lead almost always enters the left pulmonary artery. The lead
is again fully loaded with the stylet and the lead
retracted slowly. With experience, the movement
of the tip through the pulmonary valve and in
particular over the supraventricular crest and
septomarginal trabeculation can be felt. The lead is
then gently pushed forward into the pocket where
the septoparietal trabeculations lie until resistance
is felt with the lead tip making contact with the
septal wall. The screw is then deployed.
Confirming Septal Lead Placement
The 40◦ RAO and LAO views are used to confirm septal positioning. The lead is positioned in
a similar position almost every time, irrespective
of the size of the RV chamber or orientation of the
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heart. The described stylet shape with the posterior angulation is critical in achieving this, allowing low septal RVOT pacing site in 100% of cases.
Indeed, using this design, we have been unable to
position the lead in any other area of the RVOT.
Conversely, preparing a stylet with an anterior angulation only allows attachment to the free wall of
the RVOT (Figs. 4 and 11).
Septal pacing site can also be confirmed with
the LL fluoroscopic view, but this may be very difficult to obtain during the procedure. Analysis of
the lead I vector during forced ventricular pacing
(Fig. 8) also will aid in confirming lead placement
with a negative or isoelectric vector in the majority
of cases, although on occasion a small R wave may
be present.21
Occasionally, the screw may be deployed
while the lead lies in the conus arteriosus and the
final fluoroscopic appearance position suggests too
high an attachment (Fig. 12). In this situation, it is
not unusual to find an elevated stimulation threshold or failure of ventricular capture at high voltage
outputs. The screw should be retracted and a lower
position found. With experience, lead positioning
below the RVOT can be readily recognized (Fig. 3).
Although the inferior boundary of the RVOT can be
delineated with a multipolar catheter, this should
not be required in routine practice.
Discussion
The method described reliably places a conventional active-fixation lead onto the true RVOT
septum. Using this novel stylet shape, the procedure is easy to perform without specific maneuvers
apart from simple retraction of the lead into the
RVOT from the pulmonary artery. This is a technique familiar to most implanters who position
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Figure 11. PA and 40◦ LAO fluoroscopic images of a ventricular lead in the RVOT. Above: The
terminal bend in the stylet is fashioned posterior and thus the lead is directed into the septal
position. Below: The terminal bend in the stylet is fashioned anterior and thus the lead is directed
into the free wall position.
leads at the RV apex using a similar technique
and a straight or slightly curved stylet. It is highly
reproducible and provided the stylet is shaped
correctly, is usually attached at the first attempt
and cannot be positioned anywhere other than the
RVOT septum. The final fluoroscopic appearances
are also very similar for the vast majority of cases.
Thus for the first time a reproducible site within
the RVOT septum can be reached which will facilitate the design of trials to examine whether pacing
from this area is physiologically preferable to the
RV apex.
Figure 12. PA fluoroscopic images of the ventricular lead in the RVOT. Left: The lead is attached
high in the RVOT. There was no ventricular pacing at 10 V. Right: The lead has now been placed
in the low RVOT septum and the stimulation threshold was 0.5 V.
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In more than 100 cases of septal positioning in
our institution, no pacing complications have been
encountered. In particular, unacceptable elevated
stimulation thresholds have not been documented.
Given the reproducibility of achieving this site and
understanding its anatomical relationship, we believe that lead perforation is not possible in this
position. Consequently, pericarditis and pericardial effusions should not occur. Similarly, due to
the anatomy of this site, diaphragmatic or intercostal stimulation cannot occur. Furthermore, in
our last 500 cases with leads attached to the septum, anterior, or free wall within the RVOT, no
dislodgements have occurred, which is similar to
the experience of other operators.37,38
Study Limitations
A potential limitation is that because of the
reproducibility of lead placement using our described technique, the novel stylet shape allows
only one area of the RVOT septum to be reached.
This is in contrast to the recently described methods of lead placement via a catheter delivery
system whereby thin, lumenless leads are positioned using a directional, deflectable catheter
(Medtronic Inc., Minneapolis, MN, USA). However, to date the only data published are concerned
with implant safety and electrical performance.39
Although this new modality potentially allows a
wider choice of pacing site, its clinical utility is as
yet unproven, and as the technique differs significantly from the standard implant, there is likely
to be a significant learning curve with potential
attendant risks.
In comparison, our technique is a modification of standard implantation practice, easy to perform, free of significant complications, and reliably and safely achieves lead placement in the low
RVOT septum. Furthermore, pacing at this reproducible site produces a narrower QRS duration
compared to other areas in the RVOT suggesting
a more physiologic approach to RV pacing,21 and
theoretically, this may be the optimal “sweet spot”
for pacing. Whether this translates into clinical
benefit remains to be proven. It may be possible
to alter the stylet shape or implantation technique
to reach other areas if the zone described does not
fulfill physiologic expectations.
Finally, this manuscript did not intend to give
implant data which has already been published
from our center.21 Long-term follow-up data will
also be prepared for publication in due course.
Conclusion
A novel and simple technique is described
using a novel curved stylet shape with a posteriorly directed secondary curve to allow rapid and
easy placement in the low RVOT septum. The
technique and final positioning are highly reproducible with no operative and postoperative complications. By standardizing the RVOT pacing site,
meaningful trials can now be easily conducted to
determine if this is the physiologic “sweet spot”
for pacing the right ventricle.
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