(Circulation. 2000;102:3080.)
© 2000 American Heart Association, Inc.
Clinical Investigation and Reports |
Usefulness of Unipolar Electrograms to Detect Isthmus Block After Radiofrequency Ablation of Typical Atrial Flutter
From the Department of Cardiology, Hospital General Universitario Gregorio Marañón, Madrid, Spain.
Correspondence to Julian Villacastin, Laboratorio de Electrofisiología, Departamento de Cardiología, Hospital General Universitario Gregorio Marañón, C/Dr. Esquerdo 46, 28007 Madrid, Spain. E-mail jvillacastin{at}secardiologia.es
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Abstract |
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Background—RS morphology of the unipolar electrogram is associated with propagation of the wave front through the exploring electrode, whereas positive uniphasic (R) unipolar electrograms are characteristic of the end of activation.
Methods and Results—Unipolar electrograms were recorded in 45 consecutive patients with atrial flutter who were undergoing radiofrequency ablation (RFA). Bidirectional cavotricuspid isthmus (CTI) block was achieved in 44 patients. The unipolar electrogram obtained before RFA at the low anterolateral right atrium during coronary sinus pacing changed from RS, rS, or QS to R or Rs in all patients after clockwise CTI block was obtained. The morphology of unipolar electrograms recorded close to the coronary sinus during pacing from the low anterolateral right atrium changed from RS or rS to R or Rs in all but 4 patients after counterclockwise CTI block. In the patient in whom CTI block was not achieved, the RS morphology of the unipolar electrogram remained unchanged. In 18 patients, the results of the RFA were assessed with only the unipolar electrogram. The unipolar electrogram correctly predicted 100% and 89% of the cases of clockwise and counterclockwise CTI block, respectively.
Conclusions—The creation of CTI block is associated with an easily detectable loss of negative components and development of an R or Rs pattern of the unipolar electrogram recorded close to the ablation line while pacing at the opposite side of the CTI.
Key Words: atrial flutter • electrophysiology • ablation
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Introduction |
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The aim of radiofrequency catheter ablation (RFA) for atrial flutter (AFl) is to create a complete and stable bidirectional cavotricuspid isthmus (CTI) block.1 2 3 4 5 6 7 8 9 Although other methods have been proposed to detect the creation of CTI block,10 meticulous mapping of the tricuspid annulus (TA) before and after RFA is still the technique of choice.11 The purpose of our study was to investigate the usefulness of unipolar recordings to assess bidirectional CTI block.
Spach et al12 13 showed that conduction along the longitudinal axis of the fibers in cardiac muscle produces a characteristic biphasic (RS) unipolar electrogram, whereas positive unipolar (R) electrograms are characteristic of termination of the activation wave front.
We hypothesized that an RS unipolar electrogram recorded in
the CTI area would be predictive of conduction, whereas a dominant R
unipolar electrogram could be indicative of CTI block
(Figure 1
).
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Methods |
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Study Population
The study population consisted of 45 consecutive patients (mean age 63±10 years, 76% males) referred for catheter ablation of typical AFl. Structural heart disease was present in 26 patients: hypertensive cardiomyopathy (5 patients), ischemic heart disease (7 patients), cor pulmonale (9 patients), valvular heart disease (4 patients), and congenital heart disease (1 patient). Two patients had previously undergone an unsuccessful attempt at catheter ablation of the AFl.
Electrophysiological Evaluation and
Recordings
All patients gave informed written consent.
Antiarrhythmic drug therapy was discontinued for at least 5 half-lives
in all but 7 patients (amiodarone in 4 patients and a I-C–class
antiarrhythmic drug in 3 patients).
Catheters were positioned around the TA (20 electrodes, Halo, Cordis Webster, Inc), at the coronary sinus ostium (CSos; Marinr [Medtronic], with a 4-mm-tip electrode, and 2-5-2– or 2-2-250–mm interelectrode spacing), and at the His region (Josephson, Bard Angiomed).
At least 10 bipolar or unipolar simultaneous intracardiac electrograms were recorded with 1 surface ECG lead (II or aVF), at paper speeds of 100 and 200 mm/s (Midas, Hellige Biomedical). Unipolar electrograms were recorded with the exploring electrode connected to the positive pole and a catheter electrode located at the inferior vena cavae connected to the negative pole. The gain was set at 1 to 2 mV/cm and the filtering at 0.05/2500 Hz. Bipolar electrograms were recorded with a gain amplification of 0.5 to 1 mV/cm and filtered at 30/500 Hz. The ablation procedure was always performed during sinus rhythm. In all patients, the conduction status of the CTI was assessed during pacing at cycle lengths of 500, 400, and 300 ms from the CSos and from the low anterolateral right atrium (LALRA).
Radiofrequency Ablation
A quadripolar 8-mm-tip electrode ablation catheter
with temperature control (Blazer T, EP Technologies) was inserted
through a right femoral sheath. The CTI linear lesion was made
sequentially during CSos pacing, with point-by-point ablation from the
ventricular aspect of the TA to the inferior venae cava. Radiofrequency
(RF) energy (550 kHz unmodulated sine wave output up to 100 W)
was delivered through a generator (EP Technologies) with a temperature
setting of 70° for 30 seconds at each point with no movement of the
catheter.
The end point was bidirectional CTI block and noninducibility of isthmus-dependent AFl by programmed stimulation that persisted for at least 30 minutes. In case of failure, additional RF applications were made over the previous line, with RF applied at the sites where large single bipolar electrograms were found.
Unipolar Electrogram Evaluation
Protocol
We analyzed the morphology of unipolar electrograms
before and after the first RF ablation line and after each additional
RF application using the distal electrode of the ablation catheter. We
tried to position the exploring catheter as close as possible to the
ablation line but where it could still record a clearly visible atrial
electrogram, before pacing from the opposite side. We were extremely
careful in positioning the exploring electrode at the same place each
time. This was verified by the morphology of the local ventricular
electrogram and by anatomic landmarks in the same fluoroscopic (left
anterior oblique, right anterior oblique, and
posterior-anterior) projections, together with the relationship
of the exploring catheter with the more stable catheters located at the
His and CSos.
To evaluate the ability of unipolar electrogram morphology to guide the ablation procedure, for the last 18 patients, the bipolar recordings from the TA were initially hidden, and only the unipolar electrograms recorded from the distal electrode of the ablation catheter were used to assess CTI block. Once CTI conduction status was predicted by unipolar electrograms, the presence or absence of CTI block was confirmed by conventional TA mapping.
Definitions
The definitions used of clockwise CTI block,
rate-dependent clockwise CTI block, counterclockwise CTI block, and
rate-dependent counterclockwise CTI block have been described
elsewhere.14
We defined conduction delay as the persistence of ascending
septal activation during LALRA pacing or of ascending lateral free-wall
activation during CSos pacing, but with an ablation-related increase
10 ms of the interval between the stimulation spike and the atrial
deflections obtained at the other side of the RF ablation
line.
Unipolar Waveforms
The following definitions of unipolar waveforms were
used:
RS: One positive deflection followed by a negative
deflection with an R/S ratio 1/3 and 
3/1. The magnitude of the S
wave was calculated with the line between the atrial and ventricular
unipolar electrograms used as a reference. In case of doubt about the
unipolar electrogram morphology due to overlapping of the atrial and
ventricular unipolar electrograms, several maneuvers (carotid sinus
massage, adenosine infusion, or atrial pacing) were performed to
separate atrial from ventricular activity.
rS: One small positive deflection (0.1 mV) followed by a
negative deflection with an r/S ratio <1/3.
RS: One large positive deflection followed by a small
negative deflection (0.1 mV) with an R/s ratio >3/1.
QS: Only 1 large negative deflection. Small positive deflections <0.1 mV were not considered.
R: Only 1 large positive deflection. Small negative deflections <0.1 mV were not considered.
Interobserver Agreement
Two electrophysiologists analyzed 90 random and
isolated unipolar electrograms and were asked to define unipolar
electrograms and to predict the presence of CTI
block.
Statistical Analysis
Statistical analysis was performed with JMP 3.0.1
statistical software (SAS Institute Inc, 1994). Data are reported as
distributions and mean±SD values. Statistical comparisons for 2 groups
were performed with the paired t test. A probability
value <0.05 was considered significant. To investigate the agreement
among observers, the 
-statistic was calculated with the statistical
program Systat 5.0 for Macintosh (Systat
Inc).
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Results |
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After a median of 12±8 RF applications, bidirectional block of the CTI was achieved in 44 patients. In 1 patient, bidirectional conduction through the CTI was still present after 35 RF applications. In this patient, no further applications of RF current were delivered.
Unipolar Morphology and Clockwise CTI
Conduction
At the beginning of the study, clockwise CTI conduction
was observed in all patients. The unipolar electrogram obtained at the
LALRA during pacing from the CSos was RS in 22 patients, rS in 20
patients, and QS in 3 patients.
After clockwise CTI conduction block was achieved, the
unipolar electrogram recorded at the LALRA adjacent to the ablation
line changed to R in 31 patients and to Rs in 13 patients
(Figure 2). The interval between the stimulus artifact and
the local atrial electrogram increased from 66±17 to 146±23 ms
(P<0.001). In the single patient without CTI block at
the end of the procedure, the unipolar electrogram remained
unchanged.
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In patients with Rs at the chosen site after CTI block, we mapped the adjacent sites. In all patients, a monophasic R unipolar electrogram could be obtained by positioning the catheter close enough to the line of block. However, in most cases, the R wave was of low amplitude, presumably because of its proximity to the endocardial lesion. Conversely, by moving the exploring catheter away from the line of RF applications, we observed an increase in the amplitude of the negative component of the unipolar electrogram in all patients.
Unipolar Morphology and Counterclockwise
CTI Conduction
At the beginning of the study, counterclockwise CTI
conduction was present in all patients during LALRA pacing. The
unipolar electrogram recorded at the septal aspect of the CTI during
pacing from the LALRA was RS in 21 patients, rS in 23 patients, and QS
in 1 patient.
After ablation, counterclockwise conduction block through the CTI was observed in all but 1 patient. The unipolar electrogram recorded at the septal aspect of the right atrium adjacent to the line of block converted to R in 21 patients, to Rs in 21 patients, and to RS in 4 patients. The local activation time at that site increased from 55±13 to 129±18 ms (P<0.001). In the patient in whom conduction block was not achieved, no changes in local activation time were observed, and the unipolar electrogram remained nearly identical to that observed before RF delivery. In the 4 patients with successful CTI block and an RS unipolar electrogram, a marked increase in the amplitude of the R component of the unipolar electrogram was observed. However, a clear R or Rs morphology was not seen even when we explored adjacent areas.
Unipolar Morphology During Rate-Dependent
Block
In 11 patients, we observed transient rate-dependent
CTI block (clockwise CTI block in 11 patients and bidirectional CTI
block in 3 patients) before persistent CTI block was obtained. These
patients provided an opportunity for us to study the changes in
unipolar electrogram morphology during a variable conduction status
while maintaining the exploring catheter at exactly the same site.
Clockwise CTI transient block was observed at a mean cycle of 375±55
ms after RF attempts
(Figures 3 and 4). In all these patients, we observed perfect
concordance between the unipolar electrogram morphology and the
appearance/disappearance of the CTI block. The unipolar electrogram was
QS, rS, or RS before CTI block and always changed to R or Rs after CTI
block. Although in most cases, the change in the unipolar electrogram
morphology was sudden, we could observe a gradual change in 2 patients,
with a progressive decrease of the S wave during the change from an RS
to an R unipolar electrogram.
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Unipolar Morphology and Conduction
Delay
In 8 patients, conduction times through the CTI
increased 10 ms in either the clockwise (6 patients) or
counterclockwise (2 patients) direction during the course of the
procedure. In these cases, changes in the unipolar electrograms
remained predictive of conduction through the CTI despite the presence
of a conduction delay
(Figure 5).
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Blinded Prospective Study
In the group of patients in whom the TA bipolar
electrograms were initially hidden, the sensitivity, specificity, and
positive predictive values of recording an R or Rs unipolar electrogram
at the LALRA for predicting clockwise CTI conduction block were 100%.
The sensitivity, specificity, and positive predictive values of
recording an R or Rs unipolar electrogram at the CSos were 89%, 100%,
and 100%, respectively, for predicting counterclockwise CTI conduction
block.
Interobserver Agreement
Agreement between observers was good when unipolar
electrogram morphology was analyzed (-statistic 0.78) and very
good when the observers had to decide, using only the unipolar
electrogram, whether the CTI was blocked (-statistic
0.95).
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Discussion |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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The present study shows the usefulness of recording unipolar electrograms to assess CTI conduction during RFA of AFl. In particular, CTI block is associated with an easily recognizable change in the morphology of unipolar electrograms when recorded close to the ablation line on the opposite side of the pacing site.
Significance of Unipolar Electrogram
Morphologies
Spach et
al12 13
found that conduction along the longitudinal axis of the fibers in
cardiac muscle produces a characteristic biphasic, smooth, unipolar
waveform RS. In contrast, positive uniphasic unipolar R electrograms
are characteristic of the end of propagation. Uniphasic unipolar R
electrograms can also be observed at the point of collision of 2 wave
fronts, and negative waveforms QS are recorded in the vicinity of the
site of excitation
onset.12 13
Since Spach et al described their findings, several studies have
demonstrated the value of unipolar electrograms in localizing the site
of origin of cardiac activation in patients with atrial or ventricular
tachycardias and accessory AV
pathways.15 16 17 18
In patients with AFl, unipolar recordings have only been used to
characterize zones of slow
conduction.19
Methods to Confirm CTI Block
In recent years, confirmation of CTI conduction block
during pacing from the LALRA and CSos has become the preferred end
point in RFA of
AFl.4 5 6 7 8 9
This method requires detailed mapping of the CTI, tricuspid ring, and
interatrial septum. However, even after meticulous mapping, doubts may
still exist about the presence of complete CTI block. Because RF
applications may slow the conduction velocity at the CTI, the site of
collision can vary along the lateral wall of the right atrium (pacing
from the CSos), mimicking CTI
block.14 In
addition, in some patients, conduction through the crista terminalis
may produce simultaneous activation of the distal electrodes of the
mapping catheter, despite CTI
block.14 20
Thus, even when perfect stability of the catheters is achieved during
the entire procedure, which is not easy due to the anatomy of the CTI,
the classic TA mapping approach may have some
limitations.21
Other investigators have tried to simplify the confirmation of CTI block. Double potentials separated by an isoelectric interval have been recognized as markers of local block.15 This method also requires meticulous mapping of the CTI, and difficulties in interpretation may exist because of the decrease in amplitude as a result of the RFA.
Unipolar Recordings and CTI Block
The rationale of our study was to assess conduction
across the CTI without needing a detailed mapping of this region, given
that unipolar electrograms can detect not only local but also distant
activity. In accordance with our hypothesis, pacing from each side of
the CTI before ablation resulted in an RS, rS, or QS pattern on the
opposite side of the CTI. An S wave is expected, because the propagated
wave front passes through the CTI and moves away from the exploring
electrode. After CTI block, the paced wave front has to go around the
TA before it reaches the exploring electrode, located at the other side
of the line of block. Therefore, a clear change in the unipolar
electrogram is expected (from QS, rS, or RS to R or Rs) because the
wave front now ends at or close to the exploring electrode. The
relative voltage of the R wave is likely to depend on the amount of
tissue that depolarizes within the CTI, as well as the distance of the
recording electrode from this
area.13 However, to
observe the expected unipolar electrogram changes, the exploring
catheter should be located in close proximity to the line of block. We
observed that if the exploring catheter was separated from the line of
block, activation of the area of tissue between the exploring electrode
and the line of block could generate an S wave of sufficient voltage to
result in an RS unipolar electrogram .
In 4 patients, counterclockwise block did not result in a lack of S wave at the septal side of the CTI. The reason for this is not totally clear to us. Theoretically, an oblique ablation line could produce an RS complex in the presence of CTI block. It is also conceivable that if the CTI line of block is made close to the septum, when the exploring electrode is positioned at the septal side of the line of block, it could record left atrial activation propagating away from it, thus inscribing a significant S wave.
Unipolar Recordings and Incomplete CTI
Block
The unipolar electrogram did not change significantly
in our study with the presence of incomplete CTI block (after a failed
RF line) compared with the unipolar electrogram recorded before the
ablation attempt. This was not unexpected, because in case of
incomplete block, independently of the location of the gap, the wave
front crossing the CTI (sometimes with a prolonged conduction time)
should pass through the exploring electrode, generating a propagation
wave front that moves away from it. This seems to be an important
advantage of unipolar electrograms, because they record relatively
distant activity. Thus, unipolar electrograms seem to be able to
distinguish between CTI block and slow conduction through the
CTI.
Simplification of the Procedure
A possible role of unipolar electrogram recordings
could be simplification of the ablation procedure in patients with AFl.
We have demonstrated in 18 patients that the presence or absence of
bidirectional CTI block can be diagnosed solely on the basis of the
unipolar electrogram. This represents an initial attempt to simplify
the CTI ablation procedure to enable just 2 catheters to be used (one
for stimulation and the ablation catheter for unipolar electrogram
recording). It is our experience that it is extremely unusual to
produce isolated unidirectional counterclockwise block during the
ablation procedure. Thus, a possible strategy could be to test
conduction routinely in the clockwise direction with unipolar
electrograms, because they are completely reliable for this purpose.
Once counterclockwise block has been achieved, clockwise conduction
could be tested with unipolar electrograms. If R or Rs morphology is
observed, no further testing would be required, because this
observation is a reliable predictor of block. However, in the 10% with
RS morphology, assessment of clockwise conduction in the conventional
fashion with multiple bipolar recording would be
required.
Study Limitations
One limitation of our study is that we cannot be
completely sure that the sites where we recorded the unipolar
electrograms before and after CTI block were exactly the same. Thus,
could minor modifications in the electrode location justify the changes
in unipolar electrogram morphology? Two findings in our study make this
unlikely. Before CTI block, despite exploration of multiple sites, we
were not able to observe unipolar electrograms with R or Rs morphology
during pacing from the LALRA or from the CSos in any patient. In
addition, the observation of a rate-dependent block of the CTI gave us
an extraordinary opportunity to appreciate the changes in morphology of
the unipolar recordings at the same site when CTI block
developed.
Theoretically, conduction velocity through the CTI could be so slow as to result in collision of the 2 wave fronts close to the RF line. In such a situation, the unipolar electrogram would be misleading, because collision of wave fronts produces an R-wave pattern. However, this situation would not be recognized by the bipolar recordings either, because activation of the lateral right atrial wall would be craniocaudal.
We used an 8-mm electrode to record unipolar electrograms. Because we have not systematically studied other electrode sizes, we are not sure whether our results can be applied to the use of different ablation catheters.
Clinical Implications
The present study demonstrates for the first time the
usefulness of unipolar electrograms to assess CTI block. The changes
observed in unipolar electrograms on the other side of the line of RF
application during pacing from the CSos and from the LALRA may
differentiate the presence of conduction block from slow conduction
through the CTI. Thus, unipolar electrograms could be used to simplify
the procedure to estimate conduction status through the CTI by using
only 2 catheters for RF ablation of common
AFl.
Received May 11, 2000; revision received July 26, 2000; accepted July 28, 2000.
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