Author + information
- Received February 19, 1998
- Revision received August 5, 1998
- Accepted September 10, 1998
- Published online January 1, 1999.
- Hiroshige Yamabe, MDa,*,
- Ken Okumura, MDa,∗,
- Ikuo Misumi, MDa,
- Hironobu Fukushima, MDa,
- Kazuhiro Ueno, MDa,
- Yoshihiro Kimura, MDa and
- Youichi Hokamura, MDa
- ↵*Address for correspondence: Dr. Hiroshige Yamabe, Division of Cardiology, Kumamoto City Hospital, 1-1-60 Kotoh, Kumamoto, 862-8505 Japan
This study was partly presented at the 70th Scientific Sessions of the American Heart Association. Orlando, FL, November 9–12, 1997.
Objectives. Bipolar electrogram polarity was analyzed to localize the recurrent conduction site in the isthmus between the tricuspid annulus (TA) and inferior vena cava (IVC) in recurrent atrial flutter (AF).
Background. Despite the initial successful linear isthmus ablation, recurrence of transisthmus conduction and AF is not uncommon. It is unclear how the recurrent conduction site can be identified.
Methods. Fourteen patients with recurrent AF were studied: four with late recurrence remote from the first ablation and 10 with early recurrence within 60 minutes after the initial successful ablation. Bipolar electrogram polarity mapping was performed during low lateral right atrium (LLRA) pacing during sinus rhythm while recording bipolar electrograms from the septal portion of the isthmus along the previously ablated line. The septal side of the isthmus from TA to IVC was arbitrarily divided into five sites, and the bipolar electrodes with cathode at the tip and anode at the second was placed at each site. The recurrent conduction site was localized by analyzing the polarity of the bipolar electrogram recorded at each site.
Results. All recurrent AF was due to reentry around TA. During pacing from LLRA, as the mapping electrode was moved from TA to IVC side, the major polarity of the electrogram changed from negative to positive in all patients. A transitional electrogram with the equal amplitudes in positive and negative components was recorded between the sites showing mainly negative and positive electrograms, indicating electrogram polarity reversal at this site. Application of radiofrequency energy to this single site resulted in the elimination of transisthmus conduction in all patients with a single application in 11 patients and 2 or 3 in the remaining 3.
Conclusions. Bipolar electrogram polarity mapping with attention to the polarity reversal point is useful for identifying and ablating the recurrent conduction site.
It is generally accepted that both typical and reverse typical atrial flutters are dependent on conduction through the inferior vena cava-tricuspid annulus isthmus (1,2). Recent reports have demonstrated high cure rates for atrial flutter by the creation of a lesion between the tricuspid annulus and the inferior vena cava. This ablation causes flutter termination, being associated with block of the activation wave front in the isthmus (1–5). It has also been shown that successful catheter ablation is associated with a marked global change in the right atrial activation during atrial pacing in sinus rhythm, suggesting conduction block in the isthmus (6,7). Although the acute success rate of catheter ablation for atrial flutter is fairly high, the recurrence rate is not negligible. Recovery of isthmus conduction after achieving conduction block is commonly observed (8). Adequate localization of the recurrent conduction site in the isthmus provides the benefits of a lower number of radiofrequency energy applications and shorter procedure time.
The technique of vector mapping has been validated as a means of evaluating myocardial activation (9,10). Damle et al. demonstrated the usefulness of vector mapping for localizing accessory atrioventricular pathways in patients with Wolff–Parkinson–White syndrome (11). Fisher et al. found that three-dimensional electrogram mapping based on reversal of electrogram polarity improves left-sided accessory pathway atrial insertion localization (12). In the present study, we localized the conduction site in the inferior vena cava-tricuspid annulus isthmus with the use of a bipolar electrogram polarity mapping technique in patients with recurrent atrial flutter after successful catheter ablation.
Of the 31 consecutive patients with typical atrial flutter referred to our hospital from July 1996 to October 1997 for radiofrequency catheter ablation, 14 had the recurrence of atrial flutter after successful ablation in the acute or chronic phase. These 14 patients (11 men and 3 women; mean age: 68 years, range: 39 to 84 years) were included in this study (Table 1). Recurrence of atrial flutter was observed 10, 1, 2, and 12 months after the successful initial ablation procedure in patients 1, 2, 3, and 10, respectively. In the other patients, recurrence of typical atrial flutter was noted within 60 min after the initial successful ablation procedure. One patient had a previous history of surgical closure of atrial septal defect. Two patients had dual-chamber pacemakers implanted for coexisting sinus node dysfunction. Hypertrophic cardiomyopathy was detected in one patient. All drugs were discontinued three days before the ablation procedure. Written informed consent was obtained from all of the patients before the procedure according to the protocol approved by the Hospital Human Research Committee.
Two 6-Fr quadripolar electrode catheters (USCI, Billerica) were percutaneously inserted into the right atrium and placed in the coronary sinus ostium and the His bundle region. Two 7-Fr large-tip (4 mm in length), deflectable quadripolar electrode catheters with a 2 mm interelectrode distance (Cordis Webster, Baldwin Park, California) were also inserted into the right atrium for right atrial mapping and pacing. An additional “halo” catheter consisting of 10 bipoles (2 mm interelectrode distance, 10 mm interbipole distance; Cordis Webster, Baldwin Park, California) was also inserted in the right atrium and used for mapping the area adjacent to the tricuspid annulus. This catheter was maintained in a plane approximately parallel to the tricuspid annulus and in proximity to the annulus.
Bipolar electrograms from the coronary sinus ostium, his bundle region, and sequential right atrial sites were recorded along with the surface electrocardiogram (leads II and V1) with the use of a polygraph (RMC-2000, Nihon Kohden, Tokyo, Japan). The intracardiac recordings were filtered with a bandpass of 50 to 600 Hz. The right atrium was paced with an output of 2- to 3-fold of the diastolic threshold and a pulse width of 2 ms using a cardiac stimulator (SEC-3102, Nihon Kohden, Tokyo, Japan).
Typical atrial flutter was defined as a reentrant tachycardia with counterclockwise right atrial activation in the anterior view. The presence of reentry was confirmed by the criteria of entrainment (13–15). The inferior vena cava-tricuspid annulus isthmus was an integral limb of the reentry circuit in all patients since concealed entrainment was demonstrated during pacing from the isthmus (2).
Initial catheter ablation procedure
After heparin was intravenously administered (bolus injection of 3000 IU followed by an hourly injection of 1000 IU), radiofrequency catheter ablation of the medial portion of the isthmus was performed during atrial flutter using a 7-Fr large-tip (4 mm in length), deflectable quadripolar electrode catheter with a 2 mm interelectrode distance (Cordis Webster, Baldwin Park, California). The radiofrequency energy generator (CAT 500, Central Inc., Japan) delivered a continuous, unmodulated sine waveform at 500 kHz in an unipolar mode between the tip of the ablation catheter and a large skin electrode placed under the patient’s back. The energy delivery was automatically stopped if an impedance rise of >30 ohms occurred. The initial ablation procedure was performed irrespectively of the electrographic characteristics noted in the isthmus region (1,16). The ablation catheter was positioned inferiorly at the 6 o’clock position in the isthmus in the left anterior oblique view (60°).
Radiofrequency energy (20 W for 30 s at each site) was applied to five to eight successive sites from the tricuspid annulus to the inferior vena cava so as to ablate the isthmus linearly. The number of energy application to scan the isthmus from the tricuspid annulus to the inferior vena cava depended on the length and the shape of the isthmus. Using a biplane fluoroscopy and a halo and coronary sinus catheters as a reference, we took care to deliver the radiofrequency energy on the same line. If the atrial flutter was not interrupted by linear ablation of the isthmus, the procedure was repeated in the same region. Energy application was continued until the atrial flutter was terminated.
After atrial flutter was interrupted, the patterns of activation around the tricuspid annulus during pacing from the lateral and septal portions of the isthmus were analyzed in order to confirm the presence of bidirectional conduction block in the isthmus region. During pacing from the septal and lateral portions of the isthmus at a rate 10 beats/min faster than the sinus rate, nine points around the tricuspid annulus were mapped in the right atrium before and after the ablation procedure (Fig. 1A). Block of the impulse propagation in the medial portion of the isthmus was determined by a unidirectional activation pattern around the tricuspid annulus (6,7). If either bidirectional or unidirectional conduction through the isthmus was observed, energy application to the isthmus was repeated. Successful ablation was defined as the bidirectional elimination of conduction through the isthmus and the inability to induce atrial flutter. The mean number of linear ablation lesions required for the initial successful ablation was 4 ± 2 (Table 1). After successful ablation, the conduction pattern in the isthmus was examined by pacing at an appropriate interval at least for a 60-min period.
Localization of the recurrent conduction site using bipolar electrogram polarity mapping and catheter ablation
Bipolar electrogram polarity mapping was performed after documentation of the recurrent isthmus conduction and the induction of atrial flutter by rapid atrial burst pacing. Bipolar electrogram polarity mapping was performed during low lateral right atrial pacing at a rate that was 10 beats/min faster than the sinus rate by recording bipolar electrograms from the septal portion of the isthmus adjacent to the previously performed ablation line (Fig. 1B). The bipolar electrograms recorded with cathode at the tip and anode at the second electrode were filtered between 0.05 and 300 Hz to retain the local low frequency components and thus analyze the atrial electrogram vectorial content. Conventional bandpass filtering (50 to 600 Hz) was found to cause significant electrogram distortion and amplitude attenuation. The previously ablated line was identified by the catheter position recorded on the radiographs in the right and left anterior oblique views during initial successful ablation procedure and was also identified by attenuated atrial electrograms. The septal portion of the isthmus extending from the tricuspid annulus to the inferior vena cava was arbitrarily divided into five regions: from site 1 (tricuspid valve side) to site 5 (inferior vena cava side). Mapping of these sites was performed with the same catheter used for ablation. The catheter was positioned at the half past five o’clock position in the isthmus in the left anterior oblique view (60°) and positioned parallel to the previously ablated line. The catheter bipole orientation was maintained during bipolar electrogram polarity mapping using biplane fluoroscopy. To avoid a displacement of the mapping catheter, bipolar electrogram polarity mapping was performed under a condition in which the patient was asked to breath shallowly.
The polarity of the bipolar electrogram recorded at each site was analyzed. To exclude the influence of repolarization, the duration of the electrogram used for polarity analysis was defined by referring the duration of the electrogram recorded at the same mapping site with a filter setting of 50 to 600 Hz. The electrogram was defined as a positive electrogram when the ratio of the positive component of the electrogram to the negative one was more than 1.50 and was defined as a negative electrogram when this ratio was less than 0.66. When the ratio of the positive component of the electrogram to the negative one was between 0.66 and 1.50, then the electrogram was defined as a transitional electrogram. The recurrent conduction site was localized based on this polarity change. Bipolar electrogram polarity mapping was also performed at the same sites before ablation in patients 4, 5, 6, 7, 8, 9, 11, 12, 13, and 14 to examine the polarity change of the electrograms following ablation (Fig. 1C).
Radiofrequency energy (20 w for 30 s) was applied to the site 1 to 2 mm toward the inferior vena cava from the transitional electrogram recording site so as to ablate the midportion of the mapping bipole.
Values are expressed as the mean ± SD. Differences between electrophysiologic parameters were analyzed using the Student’s ttest for paired data. A value of p < 0.05 was considered statistically significant.
Characteristics of recurrent atrial flutter
Atrial mapping of the recurrent typical atrial flutter revealed the same activation pattern as that observed before ablation. However, the mean tachycardia cycle length of the typical atrial flutter was longer than that before the initial successful ablation (256 ± 30 vs. 283 ± 35 milliseconds) (Table 1).
Bipolar electrogram polarity mapping of the isthmus
During the analysis of recurrent isthmus conduction, the transitional electrogram was observed at one of the five mapped sites: it was at site 2 in two patients (patient 5 and 11), at site 3 in nine patients (patients 1, 3, 4, 6, 7, 8, 9, 10, and 14), at site 4 in two patients (patients 12 and 13) and at site 5 in patient 2 (Fig. 2). The ratios of the positive component of the electrogram to the negative one of the positive, transitional and negative electrograms were 3.6 ± 1.8 (range: 2.2 to 8.5), 1.0 ± 0.1 (range: 0.8 to 1.3) and 0.3 ± 0.1 (range: 0.1 to 0.5), respectively. Anterior movement of the recording bipole toward the tricuspid valve side from the transitional electrogram recording site resulted in negative electrogram recording, while posterior movement to the inferior vena cava side resulted in positive electrogram recording in all of the patients (Fig. 2). Thus, a site with transitional electrogram was sandwiched between sites with positive and negative electrograms except for patient 2 in whom the transitional electrogram was recorded at site 5.
The electrograms obtained before the ablation procedure were transitional at all sites from 1 to 5 (Fig. 3). The ratio of the positive component of the electrogram to the negative one was 1.1 ± 0.1 (range: 0.8 to 1.4). In addition, electrogram polarity reversal from negative to positive electrograms while the mapping catheter was pulled from the tricuspid annulus to the inferior vena cava was not observed in any of the patients.
Results of re-ablation
A single application of radiofrequency energy to the recurrent conduction site determined by bipolar electrogram polarity mapping, i.e., the electrogram polarity reversal site, resulted in the elimination of isthmus conduction in all of the patients. In patients 2, 3, and 7, one or two more energy applications to the same site was required since transisthmus conduction again resumed. The number of energy application required was 1 ± 1 per patient (range: 1 to 3) and the total energy delivered was 1434 ± 917 joules (range: 467 to 3406 joules, Table 1). All patients were free of symptoms and had no recurrence of atrial flutter during an average follow-up period of 9 ± 3 months (range: 5 to 15 months). No complication was noted during the ablation procedure or during the follow-up period.
Figure 4illustrates the activation patterns around the tricuspid annulus before and after ablation of recurrent atrial flutter in patient 9. As shown in Fig. 2 and 3, electrogram polarity reversal was not observed before the initial ablation procedure, but it was after the recurrence of initially successfully ablated atrial flutter. When the isthmus conduction recurred, the impulse propagated in a clockwise direction toward the lateral portion of the isthmus and in a counterclockwise direction with upward activation in the septum to the anterior portion of the tricuspid annulus during atrial pacing from the septal portion of the isthmus (Fig. 4, panel A). After ablation of the recurrent conduction site (site 3 in Fig. 2), only counterclockwise activation around the tricuspid annulus was observed during pacing from the septal portion of the isthmus (Fig. 4, panel B). Also, only clockwise activation around the tricuspid annulus was observed during pacing from the lateral portion of the isthmus (Fig. 4, panel C). During pacing from the septal and lateral portions of the isthmus after ablation, double potentials were recorded in the medial portion of the isthmus where the ablation was performed (site M-IS in Fig. 4, panels B and C). The timing of the latter potential was the latest around the tricuspid annulus, further confirming conduction block in the isthmus.
Anatomically guided ablation of the inferior vena cava-tricuspid annulus isthmus has been shown to be effective for the treatment of typical and reverse typical atrial flutters (1–8). Although the success rate for atrial flutter ablation is relatively high, recurrence of atrial flutter is not infrequent. Schwartzman et al. first documented recovery of isthmus conduction following ablation (8), and suggested that the factors associated with recovery of conduction include inadequate tissue heating and transient effects of radiofrequency energy application to the isthmus myocardium. Although flutter recurrence may be minimized by ensuring isthmus conduction block during pacing from the low lateral right atrium and coronary sinus ostium, it is still difficult to prevent all recurrence of isthmus conduction (7). For recurrent atrial flutter, it may be unnecessary to perform radiofrequency energy application to the isthmus extending from the inferior vena cava to the tricuspid annulus. If the recurrent conduction site in the isthmus could be identified, conduction block in the isthmus could be created with a minimum number of energy application. In the present study, we attempted to localize the recurrent conduction site in the isthmus using a bipolar electrogram polarity mapping technique.
Bipolar electrogram polarity mapping in localizing the recurrent conduction site
Previous reports have suggested that the information provided by the morphology of the recorded electrogram can be used to determine the direction of activation relative to an electrode (9–11). Fisher et al. have localized the atrial insertion site of the accessory pathway using the electrogram polarity reversal phenomenon, a phenomenon caused by reversal of local activation vectors (12). In this study, we have assumed that the recurrent conduction site in the isthmus is a point of source for radial propagation of the activation front. Recently, Shah et al. demonstrated the activation along the ablation line at the isthmus in patients with recurrent atrial flutter (17). As demonstrated in their report, the atrial activation at sites adjacent to the previously ablated line occurred as a simple unidirectional activation from the recurrent conduction site (Fig. 5). Thus, the electrograms recorded at sites adjacent to the previously ablated line showed positive or negative electrogram, and the electrogram recorded at the recurrent conduction site showed transitional one. Furthermore, electrogram polarity reversal was noted along the previously ablated line in all patients but one in whom the transitional electrogram was recorded at site 5. Although the activation mapping on the previously ablated line was not performed in the present study, these findings imply that the transitional electrogram recording site was the site from which the activation wave front spread during low lateral right atrial pacing. This was confirmed by the fact that a single energy application to this transitional electrogram site eliminated transisthmus conduction in most patients.
In order to obtain the accurate wave front vector analysis, the mapping bipole must be maintained to be parallel to the ablation line in the isthmus. The electrograms obtained before ablation in 10 of the patients support the accuracy of bipolar electrogram polarity mapping. In these patients, the electrograms obtained before ablation showed different morphologic characteristics compared with those after ablation. Before ablation, the electrograms were transitional at all mapping sites although they were morphologically different between patients. These findings imply that the wave front propagated from the low lateral right atrium through the isthmus crossing the mapping catheter at all mapping sites perpendicularly before ablation.
Relevance to the previous study
Shah et al. reported that the recurrent conduction site in the isthmus can be identified by analyzing the electrograms obtained along the previously performed ablation line (17). They demonstrated that the recovered gap within the initial ablation line is represented by a single or a fractionated potential spanning the isoelectric interval of adjacent double potentials. A major difference between the study by Shah et al. and the present bipolar electrogram polarity mapping is the mapping site. Mapping was performed along the ablation line in their method, while the bipolar electrogram polarity mapping was performed at the exit of activation downstream of ablation line and not at the site of recurrent conduction in the ablation line. The method by Shah et al. is useful to identify the recovered gap when the initial ablation line was performed in a single straight form. However, as they indicated, it may not be as efficacious for previous wide and irregularly targeted ablation lesions. The efficacy of the present bipolar electrogram polarity mapping method to such wide and irregularly targeted ablation lesions remains to be elucidated although only a single application resulted in the elimination of transisthmus conduction in most of the present cases.
Catheter ablation of recurrent atrial flutter
In the present study, electrogram polarity reversal was observed in a single site in each patient and application of radiofrequency energy to this site resulted in the elimination of the isthmus conduction in all of the patients. These results indicate that the recurrence of atrial flutter after initially successful linear ablation of the isthmus is usually due to a single point source. Shah et al. also have reported similar results (17). Our results may also be applied for localizing the defect in the ablated line of the isthmus during the primary ablation procedure for atrial flutter, especially when atrial flutter could not be terminated even with multiple energy applications but atrial flutter cycle length was increased.
The present study suggested that a single recurrent conduction site in the isthmus can be easily identified by bipolar electrogram polarity mapping. However, the ability to discriminate multiple recurrent conduction sites is limited by the size of the catheter used for mapping. It is difficult to distinguish multiple gaps from single ones when the gaps are small and closely located. To identify small multiple recurrent conduction sites in the isthmus using bipolar electrogram polarity mapping, simultaneous mapping of the entire isthmus using a multipolar electrode catheter with a small tip and short interelectrode distance may be helpful. This limitation of the identification of multiple gaps may also be applied to the method described by Shah et al. (17). Although we suggest the usefulness of bipolar electrogram polarity mapping in identifying the recurrent conduction site, the analysis of the polarity is based on the presupposition that the mapping bipole is placed in parallel with the previously ablated line. Furthermore, the previously ablated line may not necessarily be single or linear especially in the patient in whom multiple radiofrequency energy has been delivered. Thus, it should be emphasized that the mapping catheter not only has to be parallel to, but in close proximity to the previously ablated line.
Conventional bipolar electrogram polarity mapping is a valid technique for the identification of recurrent conduction site in the isthmus in the setting of recurrent atrial flutter. A complete new linear ablation line is not required for the treatment of recurrent atrial flutter. The recurrent conduction site can be selectively ablated with a minimum number of radiofrequency energy application by identifying a critical recurrent conduction site.
- Received February 19, 1998.
- Revision received August 5, 1998.
- Accepted September 10, 1998.
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