REVIEW
Ablation of Isthmus Dependent Atrial Flutter: When
to Call for the Next Patient
MEVAN WIJETUNGA, ALEX GONZAGA, and S. ADAM STRICKBERGER
From the Division of Cardiology, Washington Hospital Center, Washington, D.C.
Introduction
Typical atrial flutter is a common reentrant
atrial arrhythmia. The critical portion of the reentrant circuit is the rim of tissue between the tricuspid valve (TV) annulus and the inferior vena cava
(IVC) (Fig.1).1,2 The reentrant circuit can operate
in either clockwise or counterclockwise directions
(Figs. 2–3). Elimination of conduction across the
TV-IVC isthmus eliminates the tachycardia.3 The
endpoint of ablation is to achieve bidirectional TVIVC isthmus block. Successful ablation of TV-IVC
Address for reprints: S. Adam Strickberger, M.D., 110 Irving
Street, NW, #5A-12,Washington, DC 20010-2975. Fax : 202-8773018; e-mail: [email protected]
Received April 14, 2004; revised June 1, 2004; accepted
July 6, 2004.
isthmus dependent atrial flutter approaches 99%,
has a 2%–3% recurrence rate, and rare serious
complications.4 The purpose of this review is to
define the various endpoints for ablation of TVIVC isthmus dependent atrial flutter, assess some
of the common pitfalls associated with each, and
provide a practical approach to determine when
the ablation line is complete.
Diagnosis and General Approach
From a clinical and therapeutic perspective
it is advantageous to classify atrial flutter as
either TV-IVC isthmus dependent or independent. An ablation line across the TV-IVC isthmus will only cure a patient of atrial flutter if
the TV-IVC isthmus is a critical component of the
tachycardia circuit. Therefore, prior to ablation,
Figure 1. A schematic illustration of the right lateral view of the heart with the lateral wall of the right atrium pealed
away. The arrows depict the reentrant circuit of TV-IVC isthmus dependent atrial flutter in the more common counterclockwise direction. The circuit is adjacent to the crista terminalis (CT). The rim of tissue that is critical for atrial flutter
is between the tricuspid valve (TV) and the inferior vena cava (IVC). Ablation that results in complete bidirectional
block anywhere across the TV-IVC isthmus will eliminate atrial flutter. A septal ablation location is illustrated by
the speckled bar across the TV-IVC isthmus. (Adapted from Atlas of Human Anatomy, Frank Netter, MD 1997, and
published with permission from Medical Education and Publications, Novartis Pharmaceuticals Corporation.)
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ABLATION OF ISTHMUS DEPENDENT ATRIAL FLUTTER
Figure 2. Standard 12 lead surface electrocardiograms from a
patient with counter-clockwise
(left panel) and clockwise (right
panel) TV-IVC isthmus dependent atrial flutter. (Courtesy of
Bradley P. Knight, MD, University
of Chicago, Chicago, IL.)
Figure 3. Intracardiac electrograms of TV-IVC isthmus dependent atrial flutter are shown.
Counter-clockwise and clockwise TV-IVC isthmus dependent atrial flutter is present in the left
and right panels, respectively. A duodecapolar catheter is used for recording right atrial activation. The distal pair of electrodes (RAD ) are positioned in the coronary sinus. Electrode pairs 2-7
(RA2−7 ) are generally on the TV-IVC isthmus, while the more proximal pairs of electrodes are
usually located on the lateral wall of the right atrium (RA8−9 ), and either near or on the interatrial
septum (RAp ). During counter-clockwise flutter, the activation proceeds sequentially up the atrial
septum (RAp ) over the roof of the right atrium to the lateral free wall of the right atrium (RA9,8 ),
and then across the TV-IVC isthmus (RA7,6,5,4,3,2 ), Past the coronary sinus (RA1 ), and back to the
atrial septum. The atrial activation sequence is reversed with clockwise atrial flutter. The ablation
catheter is positioned medially on the TV-IVC isthmus (ABLD,P ). (Courtesy of Bradley P. Knight,
MD, University of Chicago, Chicago, IL.)
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WIJETUNGA, ET AL.
Figure 4. Concealed entrainment during
atrial flutter is demonstrated. Atrial pacing (220 ms) is performed from the ablation catheter which is positioned on
the TV-IVC isthmus and during atrial
flutter. The last two pacing stimuli (S1 )
are shown, and atrial capture (A1 −A1 )
is present. Concealed entrainment is
demonstrated by observing that the postpacing interval (PPI) equals the tachycardia cycle length (TCL). The presence of
concealed entrainment confirms that the
TV-IVC isthmus is a critical portion of the
reentrant circuit. Abbreviations are as in
Figure 3.
confirmation that the tachycardia is dependent
upon the TV-IVC isthmus should be obtained,
i.e., demonstrate that concealed entrainment is
present.5 This is achieved by pacing from the TVIVC isthmus during atrial flutter and observing the
postpacing interval, the atrial activation sequence,
and the P wave morphologies. When the TV-IVC
isthmus is a critical portion of the reentrant circuit, then concealed entrainment is demonstrable,
and the postpacing interval is equal to the tachycardia cycle length (Fig. 4). Furthermore, the atrial
activation sequence, and the P wave morphology
observed during pacing are identical to those observed during the tachycardia. The morphology of
the P wave, however, is often difficult to assess
during this maneuver.
After confirmation that the atrial flutter is dependent upon the TV-IVC isthmus, ablation can
proceed either during atrial flutter, sinus rhythm,
or sinus rhythm with pacing from the coronary sinus (Fig. 5). This is true for any atrial flutter that
is dependent upon the TV-IVC isthmus, even unusual forms.6,7
After the TV-IVC isthmus linear lesion has
been created, demonstration of complete bidirectional block is required. The key issue is how one
differentiates between slow conduction and no
conduction across the TV-IVC isthmus. All of the
techniques which are used to assess for complete
bidirectional TV-IVC isthmus block assess the rel1430
ative conduction times of the right atrium and the
TV-IVC isthmus. As the block across the TV-IVC
isthmus becomes more complete, less atrial tissue
is depolarized via conduction across the TV-IVC
isthmus.
Because differentiation between slow conduction and no conduction can be difficult, it is
important to use a variety of techniques to demonstrate the presence of complete bidirectional TVIVC isthmus block. Unidirectional block is quite
unusual; however, demonstration of bidirectional
TV-IVC block is the generally accepted endpoint.
With each of the techniques discussed, bidirectional conduction block across the TV-IVC isthmus
is demonstrated by pacing on the lateral wall of
the right atrium and observing counterclockwise
block, and by pacing from the coronary sinus and
demonstrating clockwise block. Complete bidirectional TV-IVC isthmus block may be rate related.
Hence, when evaluating for TV-IVC isthmus conduction, pacing should be performed with a long
drive cycle length.
Techniques to Identify Bidirectional TV-IVC
Isthmus Block
Double Potentials
Double potentials along the ablation line during ablation for TV-IVC isthmus dependent atrial
flutter is generally considered the gold standard for
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ABLATION OF ISTHMUS DEPENDENT ATRIAL FLUTTER
Figure 5. A 45◦ left anterior oblique fluoroscopic view of the heart with the catheters positioned for ablation of TV-IVC
isthmus dependent atrial flutter. A duodecapolar catheter is positioned as described in Figure 3. The distal electrode
pair is in the coronary sinus (E D ), electrode pairs E 2−7 are on the TV-IVC isthmus, electrode pairs E 8−9 are on the lateral
wall of the right atrium, and the proximal electrode pair (E P ) is on the roof of the right atrium near the interatrial
septum. The circle indicates the ostium of the coronary sinus (CS). Electrode pairs E D to E 4 are positioned medial to
the intended ablation line.
determining complete bidirectional block.2 Double potentials exist on both sides of a line of block.
When there is a gap in a line of block, the isoelectric period between the double potentials shortens the nearer the electrograms are to the gap.
At the gap in the line of block, double potentials are no longer present, and the electrogram
is typically long and fractionated, but can also be
discreet.
The data to date suggest that when the interval between double potentials is > 110 ms,
then bidirectional TV-IVC isthmus block is present
(Fig. 6).8 When the interval between double potentials is < 9O ms, then bidirectional block is not
present. There may or may not be complete bidi-
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rectional TV-IVC isthmus block when the interval
between double potentials is 90-110 ms. However,
if there is complete bidirectional TV-IVC isthmus
block, then the interval between the double potentials will be constant along the length of the
ablation line and > 90 ms. However, theoretically
at least, a patient may have an interval > 110 ms
between double potentials and not have complete
bidirectional block. This is an issue worth considering particularly in the setting of an extremely
slow TV-IVC isthmus dependent atrial flutter.
Sometimes the amplitude of double potentials
is too small to assess. When this occurs, other techniques are required to assess for complete bidirectional TV-IVC isthmus block.
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WIJETUNGA, ET AL.
Figure 6. This figure illustrates a variety of endpoints for ablation of TV-IVC isthmus dependent atrial flutter and
their pitfalls. In the left panel before any applications of radiofrequency energy were delivered, the transisthmus
conduction interval is 35 ms, and an obvious chevron activation sequence is observed. After delivery of several
applications of radiofrequency energy (middle panel), the transisthmus conduction interval is 120 ms, a chevron
pattern is present but the wavefronts collide more inferiorly in the right atrium (RA5 as opposed to RA8 , in the
left panel). Complete bidirectional TV-IVC isthmus block is observed in the right panel. Widely split first (DP 1 ) and
second (DP 2 ) components of the double potentials (140 ms) are recorded from the ablation catheter. Note that atrial
activation occurs sequentially from the proximal electrode pair to the most distal electrode pair which is just lateral
to the ablation line. The transisthmus conduction interval is 170 ms. The middle panel demonstrates a transisthmus
conduction interval of 120 ms which can be associated with complete bidirectional TV-IVC isthmus conduction block,
but is not in this instance. The atrial activation sequence changed gradually. Careful inspection reveals a chevron
pattern, indicating some activation across the TV-IVC isthmus, though clearly occurring more slowly than in the left
panel and more rapidly than in the right panel. In the right two panels, coronary sinus pacing is performed at a rapid
rate. In this instance, the 2:1 atrioventricular conduction block facilitated identification of the double potentials, and
allowed easy differentiation of the second component of the split potentials from the ventricular electrogram. If not
noted, all units are in ms. All abbreviations are as previously noted.
Termination and Induction of Atrial Flutter
Atrial flutter can terminate during an
application of radiofrequency energy. However,
termination during an application of radiofrequency energy is rarely associated with complete
bidirectional TV-IVC isthmus block, and should
not be considered a reliable ablation endpoint
(Fig. 7).9 Unlike with ablation of paroxysmal
supraventricular tachycardia, the inability to
induce atrial flutter is not a reliable endpoint for
ablation.10 Additionally, incremental atrial pacing
1432
may lead to the inadvertent induction of atrial
fibrillation.
Atrial Activation Sequence Associated
with Bidirectional Block
The right atrial activation sequence during counterclockwise TV-IVC isthmus dependent
atrial flutter occur sequentially down the lateral wall of the right atrium and adjacent to the
crista terminalis, across the TV-IVC isthmus, past
the coronary sinus, up the atrial septum, over the
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ABLATION OF ISTHMUS DEPENDENT ATRIAL FLUTTER
Figure 7. TV-IVC isthmus dependent atrial flutter terminates during an application of radiofrequency energy (left
panel). Coronary sinus pacing is instituted (right panel). Transisthmus conduction is still present (right panel), and
the wavefronts collide in the high lateral right atrial wall. All abbreviations are as previously noted.
roof of the right atrium, and back to the lateral free
wall of the right atrium (Fig. 1).11 The atrial activation sequence is reversed with clockwise atrial
flutter, but of course the TV-IVC isthmus is still the
critical component of the reentrant circuit. When
conduction block is incomplete, pacing from the
coronary sinus during sinus rhythm generates an
atrial activation sequence with a chevron pattern,
i.e., atrial depolarization occurs across the TV-IVC
isthmus, and at the same time depolarization proceeds up the atrial septum, and across the roof of
the right atrium. The latest site of right atrial activation is along the lateral aspect of the right atrium,
and usually arrives there simultaneously from the
TV-IVC isthmus and the roof of the right atrium
(Figs. 6 and 7). The wavefronts can collide, theoretically at least, anywhere in this area and the
exact location depends upon the relative conduction velocities of the right atrium and the TVIVC isthmus. Prior to ablation and with appropriate relative conduction velocities, the atrial
activation sequence can theoretically be consistent with complete bidirectional TV-IVC isthmus
block.
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Complete bidirectional block is demonstrated
by pacing from the coronary sinus and the lateral
wall of the right atrium, and observing sequential atrial activation that terminates at the ablation
line, on the contralateral side from the pacing site
(Figs. 6 and 8).
A variety of subtleties in the atrial activation sequence can make identification of complete
bidirectional TV-IVC isthmus block difficult. First,
during applications of radiofrequency energy,
gradual delay in the atrial activation sequence
may be observed prior to achieving complete bidirectional TV-IVC isthmus block (Fig. 6). Second,
when there is very slow conduction across the
ablation line, activation of both the low lateral
right atrium and the TV-IVC isthmus may be delayed, but relative to each other, may occur simultaneously and not sequentially (Fig. 6). Slow
conduction across the TV-IVC isthmus may also
be consistent with bidirectional TV-IVC isthmus
block. Finally, both during and after applications
of radiofrequency energy, intermittent or transient complete bidirectional block may also be
observed.
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WIJETUNGA, ET AL.
Figure 8. Schematic explanation for the changes in
electrogram polarity changes observed during coronary
sinus (CS) pacing after creation of complete bidirectional TV-IVC isthmus block. Two pairs of electrodes
(E1 and E2) of a duodecapolar electrode catheter are
positioned along the TV-IVC isthmus, just lateral to the
ablation line. Wavefronts that result from pacing stimuli
(S) within the coronary sinus are designated by arrows
that exit the coronary sinus and travel around the tricuspid annulus (TA). Before ablation (panel A), there
is clockwise activation across the TV-IVC isthmus, resulting in positive initial polarity at E1 and E2. With an
ablation line that contains a large gap (panel B), there
is still clockwise activation across the TV-IVC isthmus,
and the initial polarity at E1 and E2 remains positive.
The wavefronts now collide lower in the right atrium
than before ablation (panel A). After an additional applications of radiofrequency energy (panel C), the gap is
still present but smaller. Conduction across the ablation
line has slowed, and there is collision of clockwise and
counter-clockwise wavefronts in the isthmus, between
E1 and E2. This results in reversal of the initial polarity
at E2, but not at E1. After complete line of block has
been created (Panel C), E1 and E2 are activated only in
the counter-clockwise direction, resulting in an initial
polarity that is reversed and negative at both recording sites. The circle denotes the coronary sinus ostium.
(Figure and legend are reproduced from J Cardiovasc
Electrophysiol 2001; 12:393-399, with permission from
Blackwell Futura Publishing Inc.)
Transisthmus Conduction Interval
The transisthmus conduction interval is measured while pacing from the coronary sinus, and
the lateral wall of the right atrium during sinus
rhythm. With pacing from either location the transisthmus conduction interval is measured from the
stimulus artefact to the atrial electrogram on the
contralateral side of the pacing site and the ablation line. Before ablation and while pacing from
the coronary sinus, depolarization proceeds in the
1434
Figure 9. An example of a change in electrogram polarity before (left panel) and after (right panel) complete bidirectional TV-IVC isthmus block is achieved.
The recordings are obtained during coronary sinus pacing. The ablation line was created medial to RA2 . After
successful ablation (right panel), the electrogram polarity reversed in all the leads where the atrial activation
is now occurring in the opposite, or counter-clockwise
direction, i.e., RA6 through RA2 , as opposed to RA2
through RA6 , before ablation. The electrogram polarity
did not change in the proximal electrode pairs because
the atrial tissue in that area had consistently been depolarized in the counter-clockwise direction. Complete
bidirectional TV-IVC isthmus block was confirmed by
the presence of widely split double potentials along the
length of the ablation line (right panel). All abbreviations are as in the previous figures.
clockwise direction across the TV-IVC isthmus.
When bidirectional block is obtained, the atrial tissue lateral to the ablation line is activated from the
counter-clockwise direction, and the transisthmus
conduction interval increases(Figs. 6 and 8).12,13
When pacing from the low lateral wall of the right
atrium, the transisthmus conduction interval is
measured from the stimulus artefact to the atrial
electrogram just medial to the ablation line. An increase of the transisthmus conduction interval of
at least 50%, and to an absolute minimum value of
150 ms is associated with a sensitivity, specificity,
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ABLATION OF ISTHMUS DEPENDENT ATRIAL FLUTTER
Table I.
Sensitivity, Specificity, and Predictive Accuracy for Complete Bidirectional TV-IVC Isthmus Block
Criteria
DP1−2 ≥ 90 ms
DP1−2 ≥ 110 ms
DP1−2 ≥ 90 ms + isoelectric interval
DP1−2 ≥ 90 ms + negative DP2
DP1−2 ≥ 90 ms + isoelectric
interval + negative DP2
Sensitivity (%)
Specificity (%)
PPV (%)
NPV (%)
100
83
83
90
77
80
100
96
88
100
86
100
96
90
100
100
83
83
88
78
DP1−2 = interval separating the two components of the double potentials; DP2 = second component of a double potential; PPV = positive
predictive value; NPV = negative predictive value. (Modified from J Am Coll Cardiol 2001; 38:750–755, with permission from the
American College of Cardiology Foundation.)
positive predictive value, and negative predictive value for identifying complete bidirectional
block of 100%, 80%, 89%, and 100% respectively.14 The primary advantage of this technique
is its simplicity, and its major limitations are the
relatively low specificity and positive predictive
value.
Electrogram Polarity Changes Associated with
Complete Bidirectional TV-IVC Isthmus Block
Before complete bidirectional TV-IVC isthmus
block is present and coronary sinus pacing is performed during sinus rhythm, atrial depolarization
of the TV-IVC isthmus occurs sequentially in the
clockwise direction. Hence, the polarity of the
initial depolarization is the same from each pair
of recording electrodes that are on the TV-IVC
isthmus (Figs. 8 and 9). When complete bidirectional block of the TV-IVC isthmus is achieved,
depolarization of the electrode pair just medial
to the line of block retains its original polarity.
However, the atrial tissue lateral to the line of
block is depolarized from the counter-clockwise
direction. This is opposite to the original direction of depolarization, and accordingly, the polarity of the atrial electrograms on the TV-IVC isthmus that are lateral to the ablation line reverse
(Figs. 8 and 9).15,17
Common Pitfalls
The different techniques utilized to identify complete bidirectional TV-IVC isthmus
block during ablation for atrial flutter described
herein are easy to use, complimentary, and are
typically determined from the same tracing
(Table I; Figs. 6 and 8). This is helpful when
there is transient complete bidirectional TV-IVC
isthmus block. With transient complete block, the
associated atrial activation sequence, transisthmus
conduction interval, and the width of the double
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potentials associated with complete bidirectional
block are known. The use of multiple techniques
to assess complete bidirectional TV-IVC isthmus
block is also helpful when a technique does
not provide a clear answer regarding complete
bitdirectional block. For instance, if the double
potentials are separated by 95 ms, then careful
examination of the atrial activation sequence, or
the use of other techniques to assess for bidirectional block, are called for. Finally, unusual forms
of atrial flutter that utilize the TV-IVC isthmus
exist.6,7 The approach to ablation of these tachycardias follows the same routine: demonstrate
that the tachycardia is dependent on the TV-IVC
isthmus, create a linear lesion across the isthmus,
and then with the techniques described above,
demonstrate that complete bidirectional block is
present.
Complications
The overall incidence of serious complications associated with radiofrequency ablation of
TV-IVC isthmus dependent atrial flutter has been
estimated at 0.4% (Table II).4,18 The most common
Table II.
Procedural Complications Associated with Ablation of
Atrial Flutter
Atrioventricular block
Significant tricuspid regurgitation
Cardiac tamponade
Groin hematoma
Transient ST-segment elevations
in the inferior leads
Acute occlusion of the right coronary artery
Thromboembolic complications
October 2004
< 1%
1%
< 1%
1%
< 1%
1%
1%
1435
WIJETUNGA, ET AL.
of these is complete atrioventricular block that occurs in 0.2% of patients.4
Newer Ablation Technologies
and Three-Dimensional Mapping
Radiofrequency energy delivered via 8 and
10 mm ablation electrodes or via irrigatedtip catheters are now used for treating atrial
flutter.19,20 The endpoints for ablation of TVIVC isthmus dependent atrial flutter have been
defined during ablation with “standard” 4
mm ablation electrodes. These endpoints probably still apply when ablating atrial flutter with larger electrode catheters, irrigated-tip
catheters and with nonradiofrequency energy
sources.
The specialized three-dimensional, noncontact, nonfluoroscopic mapping systems with intrinsic navigation systems have been utilized to
identify the reentrant flutter circuit and achieve
complete bidirectional TV-IVC isthmus block.21−25
The primary advantage of these systems is the navigation tools which provide assistance in creating
continuous, linear ablation lesions, and reduce fluoroscopic exposure.
Conclusions
A variety of useful endpoints for ablation of
TV-IVC isthmus dependent atrial flutter have been
described. These include double potentials separated by 110 ms, the atrial activation sequence
with pacing from the coronary sinus during sinus
rhythm, transisthmus conduction interval, and
electrogram polarity changes. Each of these techniques has limitations. Being familiar with all of
these techniques allows the opportunity of using a
variety of techniques to confirm when complete
bidirectional TV-IVC isthmus conduction block
has been achieved. This should be associated with
a lower risk of recurrent atrial flutter. Furthermore,
it allows the electrophysiologist to objectively determine when to “call for the next patient.”
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