Pre-Procedural Imaging to Direct Catheter Ablation of Atrial Fibrillation: Anatomy and Ablation Strategy
Credits:Brief Title:imaging of the LA in AF.E. Kevin Heist MD PhD*, Godtfred Holmvang MD^, Suhny Abbara MD#, Jeremy N. Ruskin MD*, and Moussa Mansour MD*
Cardiac Arrhythmia Service*, Cardiology Division^, and Cardiovascular Imaging Section#, Massachusetts General Hospital, Harvard Medical School, Boston, MA USA
Address for Correspondence/Reprints:Moussa Mansour MD ,Cardiac Arrhythmia
Service and Cardiac Unit,Massachusetts General Hospital,Gray 109,55 Fruit St, Boston, MA 02114
doi : 10.4022/jafib.v1i1.398
catheter ablation of atrial fibrillation (AF) requires a detailed understanding
of left atrial anatomy in order to maximize the safety and efficacy of the
procedure. Common and rare variants of
left atrial and pulmonary venous anatomy have been described which can affect
the optimal ablation strategy for each individual patient. These variants include the presence of a
right or left middle pulmonary vein, a left or right common pulmonary vein, a
common inferior pulmonary vein, a right top pulmonary vein, and other rare
forms of anomalous pulmonary venous drainage.
There are also important patient-specific differences in pulmonary
venous ridges and left atrial roof morphology.
Pre-procedural CT or MR imaging can define these anatomic variants in
exquisite detail and be used with image-integration strategies to direct the
ablation procedure. In this review, we
describe common and uncommon variants that can be identified by pre-procedural
imaging, and suggest ablation strategies tailored to these anatomic variants.
Catheter-based pulmonary vein isolation has become an accepted
treatment for atrial fibrillation (AF)1, based on
the observation that electrical activity in the pulmonary veins may serve to
To perform this procedure successfully and safely requires knowledge of
the anatomy of the left atrium (LA) and pulmonary veins (PVs). Early ablation procedures utilized
fluoroscopy (at times aided by contrast injection), providing limited anatomical
detail in regard to the anatomy of relevant structures. As catheter ablation for AF has evolved, it
has become increasingly clear that a detailed understanding of a patients’ LA
and PV anatomy can facilitate the ablation procedure and reduce the potential
for complications such as pulmonary vein stenosis. Magnetic Resonance Angiography (MRA) and
Computed Tomography (CT) can produce highly precise images delineating the
anatomy of the LA, PVs and surrounding structures. Recent advances in image integration have
allowed the merging of LA/PV images from pre-procedural MRA or CT with
real-time electroanatomic maps, allowing for direct visualization of catheter
position within the MRA or CT images3. Anatomical variants of the LA and PVs have
been identified by these imaging studies that have implications for the
catheter ablation procedure. In this
review, we summarize common and uncommon LA and PV anatomy, and also describe
ablation strategies that can be utilized when these anatomic variants are
The most common PV anatomy involves 4 pulmonary veins: 2 left
and 2 right. (Figure
1) This pattern has been reported in various series in: 56% of 105
patients with AF imaged by MRA4, 62% of 55
patients (approximately 1/2 with AF) imaged by MRA5,
and 81% of 58 patients with AF imaged by CT6. Typically the superior pulmonary veins (both
left and right) are more anteriorly directed compared to the inferior pulmonary
Figure 1 Pulmonary Vein anatomy is shown in this posterior MRI view of the left
atrium, including single discrete ostia for each of the four pulmonary veins.
Most studies of PV size have reported ostial diameters ranging
from approximately 7-25 mm in the setting of typical PV anatomy 4-10.
Some studies have reported that the superior veins have a larger ostial
diameter than the inferior veins4,6-8, while other studies have reported similar ostial size
for the superior versus inferior vein ostia5,9. When the PV
anatomy is typical, the right vein ostia have been reported to be slightly
larger than the left in most studies4,7-9, and comparable in another study5,
although rigorous statistical comparison was not performed between left and
right vein ostial diameters in all studies.
Some of the discrepancies might be explained by the finding that there
is substantial ovality to the pulmonary vein ostia8,10, which could affect the measurement of ostial diameter
depending on the axis utilized for measurement.
It was also found that there is greater ovality to the left veins
compared to the right, with ovality ratios of 1.4-1.5 on the left versus 1.2 on
the right8, possibly related to a “compressive”
effect of the ridge separating the left pulmonary veins from the left atrial
appendage (LAA), and also to compression of the left pulmonary veins
(particularly the left inferior pulmonary vein) by the aorta.
There is also
substantial variation in the distance between the right and left pulmonary
veins. (Figure 2) The distance from the os of the RSPV to the
LSPV averages 33 + 11mm, but with great variation between AF patients,
ranging from 10 to 66 mm. The distance
from the os of the RIPV to LIPV is similar ( 39 + 12 mm), and also shows
great inter-patient variability, ranging from 17 to 71 mm8.
Figure 2: patient with a narrow distance between the right and left pulmonary veins is shown in posterior (2A) and anterior cutaway endoluminal (2B) MRI views. Note the small distance between the antra of the left and right veins (particularly the inferior veins).
With typical PV anatomy, encirclements are typically made
around the antra of each set of pulmonary veins (Figure 1),
with the goal of achieving electrical isolation of the veins. If the ridges separating the superior and
inferior pulmonary veins are sufficiently wide, further linear ablation may be
performed along these ridges to electrically separate the superior and inferior
veins on either the left and/or the right side.
Alternatively, a “box lesion” can be performed around all 4 pulmonary
veins, isolating them en-bloc from the remainder of the left atrium11 (Figure 3). This “box” lesion has potential advantages in
that a larger region of the posteror left atrium is isolated compared to the
traditional lesion set, and fewer ablation lesions are delivered along the
posterior left atrium in regions which may be adjacent to the esophagus. The box lesion may be particularly desirable
when there is a relatively small distance separating the right and left
pulmonary veins, whereas it may be more difficult to achieve electrical
isolation using the box lesion when this distance is very large.
Figure 3: A “box” ablation lesion set is shown in this posterior MRI view of the left atrium (the view and anatomical structures are similar to figure 1).
Middle Pulmonary Vein (RMPV)
A single RMPV (3 total right sided veins) is the most commonly
encountered PV variant in most reported series, with incidence ranging from
9.5%10 to 27%4. When present, this vein is nearly always
smaller than typical pulmonary veins, with a reported ostial diameter ranging
from 7.5 + 2.1 mm4 to 9.9 + 1.9 mm6. The RMPV may project either anteriorly or
posteriorly, depending on which segment of lung it is draining4. The ridges separating the right pulmonary
veins are typically quite narrow when a RMPV is encountered (<2mm in
narrowest width in > 50% of patients) 12. (Figure 4) Because these ridges are generally narrow
in the presence of a RMPV, we typically avoid catheter ablation along these
ridges when this variant is encountered to reduce the risk of inadvertent
ablation within the pulmonary veins, as catheter stability along these narrow
ridges is typically poor. Of note, a
left middle pulmonary vein has been reported only rarely (0-3% in one study8 and not described at all in many other similar
Figure 4: A right middle pulmonary vein is shown in these views. 4A shows an endoluminal view demonstrating the ridges between the right pulmonary veins.
Right Middle Pulmonary Veins
Multiple (>2) right middle
pulmonary veins are also encountered in the LA.
The incidence of this variant has been reported in various series at
2%5, at 4-5%8, and 4%13. These variants
can be sub-classified based on whether there are 2 or 3+ right middle pulmonary
veins and on the branching pattern13, but
currently reported series are too small to accurately estimate the relative
frequency of these sub-variants.
Although no published reports are available on dimensions of the ridges
separating multiple right pulmonary veins, it is our experience that the widths
of these ridges (as well as the diameters of the vein ostia) tend to be quite
Based on concern for pulmonary vein stenosis if ablation is performed
inadvertently in these small veins, we typically avoid ablation along the
ridges separating multiple right middle pulmonary veins.
Figure 5: Multiple right pulmonary veins are shown in this anterior MRI view.
Common Pulmonary Vein (LCPV)
The second most common pulmonary vein variant reported in most
studies is a LCPV, which has a single ostium from the LA, and then typically
divides distally into superior and inferior branches. (Figure 6) The incidence of LCPV has been reported
to be as rare as 3.4% of patients6 whereas
another study reported this variant in 32% of patients with AF5. Other studies have reported a LCPV incidence
of 9.5%10, 14%13,
17%4,9, and 6% in patients with
AF and 20% in patients without AF8. Of note, in
the study describing a 32% incidence of LCPV in AF patients, 7% were described
as a LCPV with a “long trunk” and the remaining 25% with a “short trunk” 5. It is possible
that some investigators might describe a particular anatomy as comprising
separate left superior and inferior pulmonary veins, while others might
classify this same anatomy as a LCPV with a “short trunk”, and this may
contribute to the reported discrepancies in the incidence of LCPV. Given that this single vein must drain the
entire left lung, it is not surprising that it generally has an ostium that is
substantially larger than the ostia of typical pulmonary veins. The average ostial diameter of the LCPV has
been reported at 19.4 + 5.3 mm9, 26.0 + 4.0 mm4
and 32.5 + 0.5 mm6.
Studies which measured the LCPV ostia in 2 dimensions have noted it to
have significantly greater ovality than other pulmonary veins8,10, and so axis of measurement may explain the
discrepancies in reported diameter of this vein.
Typically we attempt to isolate a
LCPV by encirclement around the entire antrum of the common vein. Of note, commercially available “lasso”
catheters are often smaller than the os of a typical LCPV, and will often slide
deep into this vein (Figure 6). Effective imaging of this vein, with
pre-procedural CT/MR and/or intra-procedural ICE, can help to prevent
inadvertent ablation within a LCPV.
Ablation within the LCPV (along the ridge between superior and inferior
branches) is generally avoided given the risk of pulmonary vein stenosis when
ablating within a vein. It is our
experience that it is generally more difficult to isolate a LCPV with this
approach compared to isolation of separate LSPV and LIPV. In some cases, especially when a LCPV has a
very “short trunk” before branching, limited ablation is performed carefully
between superior and inferior branches in order to achieve LCPV isolation when
isolation can’t be achieved by ablation around the ostium of the LCPV.
Figure 6: A left common pulmonary vein (LCPV) is shown.
Common Pulmonary Vein (RCPV)
A RCPV (a single right pulmonary vein ostia, typically then
separating into superior and inferior branches) is less commonly encountered
than a LCPV. This variant has been
reported in 0-2%8 and 2%13
of patients, while other studies with similar numbers of patients have not
reported this variant4,6,9,10. As with a
LCPV, there may be a certain degree of observer discretion in identifying a
RCPV with a “short neck” versus typical anatomy including separate RSPV and
RIPV. We typically plan an ablation
strategy for a RCPV similar to a LCPV, with attempts to isolate the entire vein
en bloc with antral encirclement. We
reserve ablation at the ridge between the superior and inferior branches of a
RCPV only for cases in which the ridge is wide, catheter stability on the ridge
is good, and if ablation at this site is necessary to completely isolate the
vein after full antral ablation has been performed.
Inferior Pulmonary Vein (CIPV)
A rare variant of drainage for the
inferior lobes of both lungs is an CIPV, which typically empties via a common
ostium into the central region of the infero-posterior LA. (Figure 7) It is not clear what the true incidence is of
this rare variant, but it appears to be encountered in less than 1% of
patients, relegating its description to single case reports15,16. Like a LCPV or
RCPV, it is possible to isolate an CIPV en bloc by a single encirclement around
the common ostia, reducing the risk of pulmonary vein stenosis which would
exist with ablation deeper into each vein.
Figure 7: A common inferior pulmonary vein (CIPV) is shown in this superior MRI view.
“Top” Pulmonary Vein (RTPV)
Another uncommon pulmonary vein variant is the RTPV. This is characterized by a single, typically small pulmonary vein draining the upper
regions of the right lung and emptying into the roof of the right atrium,
superior to the typically larger RSPV. (Figure 8) Its incidence has been reported as 3 out
of 91 subjects in one series17, although other
similar series have not reported it, and so its true incidence is somewhat
uncertain. It is important to identify
this variant in pre-procedural imaging, as its ostium is superior to the antrum
of the RSPV, at a region that is typically ablated when performing
circumferential ablation of the right pulmonary veins. It is possible that a RTPV could be
identified by electroanatomic mapping or intracardiac echocardiography (if
pre-procedural imaging was not performed), but there would be far greater risk
of failing to identify this vein by these modalities, especially if its
presence were not being actively sought.
Given the generally small size of the RTPV, the risk of PV stenosis is
presumed to be high if ablation is performed inadvertently within this
vein. When this variant is encountered,
modifying the ablation line to avoid ablating within this vein (possibly by
extending the ablation line further onto the LA roof to incorporate the RTPV
into the encirclement of the other RPVs) seems to be a prudent choice.
Figure 8: A right “top” pulmonary vein (RTPV) is shown, marked by the arrow in all 3 figures.
Pulmonary Vein Drainage
Partial anomalous pulmonary venous
return (PAPVR) has been extensively reported in the cardiac surgical literature18, and these variants may be defined in great detail by
pre-procedural CT or MR imaging19. One example we encountered is a patient
presenting for catheter ablation of paroxysmal AF with an anomalous RSPV
draining directly into the SVC (Figure 9). This
abnormality is often associated with the presence of sinus venosus
ASDatrial septal defect. Other
variants in the PAPVR category include the scimitar syndrome (drainage of right
lower lobe pulmonary veins into the IVC at or below the level of the
diaphragm), and left sided PAPVR (drainage of left superior pulmonary vein into
a vertical vein that connects to the left innominate vein, therefore causing a
left to right shunt). A separate entity in the spectrum of pulmonary venous return
abnormalities is the cor triatriatum, where all four pulmonary veins drain into
a posterior receiving chamber that is separated by an abnormal septum from an
anterior left atrial chamber20.
There is, however, a paucity of published information about
ablation strategies in patients with PAPVR.
Without pre-procedural imaging or detailed intra-procedural ICE imaging,
it seems likely that many of these variants would be missed in a typical anatomical
ablation strategy. When these variants are identified during pre-procedural
imaging, however, alternative ablation strategies for these patients, such as
electrical isolation of the SVC (thereby electrically disconnecting the
anomalous RSPV from the SVC and right atrium) in the patient shown in Figure 9, can be tailored to an individual patients anatomy.
A patient with an anomalous right superior pulmonary vein (RSPV) connection to the superior vena cava (SVC) is shown.
The dimensions of the ridges
separating the pulmonary veins, and separating the left pulmonary veins from
the left atrial appendage (LAA), are highly relevant to the ablation procedure (Figure 10).
Ablation along these ridges can facilitate isolation of the veins, and
wider ridges typically allow for greater catheter stability with less risk of
inadvertent catheter slippage into the pulmonary veins. It is difficult to accurately assess
these ridges even with highly detailed electro-anatomic mapping alone. The dimensions of these ridges can be
assessed with high accuracy with pre-procedural MR or CT imaging, however, and
can also be assessed with intracardiac echocardiography. Intracardiac echocardiography (possibly used
in conjunction with pre-procedural imaging) can also provide important real-time
information on catheter position in relation to these pulmonary vein ridges.
Figure 10: The ridge separating the left atrial appendage (LAA) from the left superior and inferior pulmonary veins (LSPV and LIPV) is shown.
The pulmonary vein ridges relevant
to the ablation procedure have average minimal widths in the range of 2-8 mm12. The average width of the ridges separating
the RPVs (in the presence of a RMPV) are significantly less than average width
of the ridges separating the LSPV from the LIPV. The width of the ridge separating the LPVs
from the LAA tends to narrow as the ridge ascends from the LIPV to the LSPV. As a LCPV is relatively uncommon, the width
of the ridge separating the LCPV from the LAA has not been described. These generalities cannot fully account for
patient-specific differences in ridge dimensions, however, and so accurate
imaging to delineate the ridges in each patient is useful to direct the ablation
Atrial Roof Morphology
A variety of morphologies of the LA
roof have been described, including flat, convex and concave21. Although the ablation strategy at the LA roof
is not substantially different for these variants, identification of an
individual patient’s LA roof morphology can facilitate catheter contact along
the LA roof and minimize risk of perforation. Pouches arising from the LA roof
have also been described, which may increase the difficulty of creating
complete electrical isolation along a “roof line” connecting antral ablation
lines from the left and right superior pulmonary veins. Of note, the right pulmonary artery often
passes very close to the LA roof, and so potentially could be injured by
ablation at this site21. Pulmonary artery injury has not yet been
reported as a complication of catheter ablation of AF, however, and this may
relate to the cooling effect of high blood flow in this structure.
The esophagus runs in close proximity to the left atrium, and
injury to the esophagus during AF ablation has resulted in the rare but
potentially fatal complication of atrio-esophageal fistula formation22,23.
The relationship between the esophagus and the left atrium can be
identified by pre-procedural cardiac CT scanning24
and by MRI, providing a potential tool to identify LA sites that are in close
proximity to the esophagus. It has been
noted, however, that the esophagus can migrate from a site adjacent to the left
PVs to a site adjacent to the right PVs during an ablation procedure25. For this
reason, real-time assessment of esophageal location by methods such as
esophageal temperature monitoring26, barium
swallow27 or intracardiac echocardiography28 may be more useful in minimizing the risk of
esophageal injury than pre-procedural assessment of esophageal location.
Atrial Appendage (LAA)
The LAA lies in close proximity to the left pulmonary
veins. It is rare to target ablation
within the LAA during typical ablation procedures for AF, although atrial
tachycardias have been reported which were successfully ablated within or at
the antrum of the LAA29. The ridge separating the LAA from the left
pulmonary veins is a target for ablation during isolation of the left pulmonary
veins, and catheter stability allowing effective lesion formation without
inadvertent ablation within the LAA or left-sided veins can be difficult at
this site (Figure 10). Pre-procedural cardiac imaging can be useful
to define this ridge (including length, width, and orientation), and
integration of electro-anatomic mapping with the resulting images can
facilitate ablation along this ridge12. Pre-procedural imaging can also define the
size and dimensions of the LAA in great detail, with substantial inter-patient
differences documented in LAA size30, and this
may prove useful in directing device occlusion of the LAA, which is currently
an investigational strategy intended to reduce strokes resulting from AF31,32.
There are several limitations in the
ability of pre-procedural imaging to accurately direct catheter ablation of
AF. It is possible for volume shifts to
occur in the atria between the time of imaging and the time of ablation that
may lead to differences in pre-acquired images compared to real-time
mapping. Three dimensional rotational LA
angiography has been described which allows for LA imaging immediately prior to
catheter mapping33, but this technology currently
lacks the spatial resolution possible with CT or MR imaging. Concordance of cardiac cycle gating between
pre-procedural CT or MR imaging and real-time mapping has not been universally
performed, possibly leading to error in image registration. Additionally, cardiac deformation by catheter
contact can create distortions during real-time catheter mapping which are not
identified by pre-procedural imaging.
Real-time imaging strategies, such as (currently available) intracardiac
echocardiography or (currently investigational) interventional MR imaging
during the ablation procedure can provide even more precise detail of catheter
position in relation to complex LA anatomy.
The LA is a complex structure, with
great inter-patient variability. A
detailed understanding of an individual patient’s LA anatomy may improve the
ability to effectively isolate the pulmonary veins and to produce other desired
ablation lines and lesions within the LA.
This may also reduce the risk of complications such as cardiac
perforation and damage to surrounding structures, including the esophagus and
the pulmonary veins. Pre-procedural
cardiac CT or MR imaging can produce highly detailed representations of each
individual patient’s anatomy within the LA and relative to surrounding
structures. This allows an ablation
strategy to be tailored for each patient to maximize both efficacy and safety.
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