Radiofrequency Catheter Ablation of Atrial Flutter
Further Insights Into the Various Types of Isthmus Block: Application to Ablation During Sinus Rhythm
Background Radiofrequency ablation of type 1 atrial flutter (AFl) has recently evolved toward an anatomically guided procedure directed to isthmuses at the lower part of the right atrium (RA). However, different types of block at these isthmuses may be observed and potentially correlated with different late outcomes. In addition, because the ablation is anatomically guided, ablation should be possible during sinus rhythm.
Methods and Results Forty-four patients underwent ablation of type 1 AFl performed during ongoing tachycardia (33 patients) or sinus rhythm (11 patients). Evidence of inferior vena cava–tricuspid annulus isthmus block was assessed by changes in RA impulse propagation while pacing from both sides of the ablation site. Apparent complete isthmus block was achieved in 43 of 44 patients with 9±7 pulses. However, incomplete block mimicking complete block because of intra-atrial conduction delay but leading to a different low RA activation pattern was individualized. At the end of the procedure, isthmus block was complete in 35 patients and incomplete in 8, but since AFl reinduction was no longer possible, patients were discharged. During a follow-up period of 12.1±5.5 months, 4 patients experienced AFl recurrence; all had shown incomplete or no block.
Conclusions Detailed multiple-point low RA mapping is necessary to differentiate incomplete from complete isthmus block. Complete block is the best marker for long-term success of AFl ablation, although incomplete block may be sufficient to prevent recurrence in a significant number of cases. Isthmus block is achievable during sinus rhythm, and AFl induction is not mandatory.
An initial series of catheter ablation of type 1 AFl was performed with the use of an electrophysiological target, sometimes combined with an anatomic target.1 2 3 4 5 6 Mapping studies had reported that anatomic isthmuses at the low RA and in particular the IVC-TA isthmus represent an obligatory route for type 1 AFl.7 8 9 10 There is now a world trend toward ablation directed at these isthmuses.10 11 12 13 14 15 Search for an electrophysiological target is time consuming and not associated per se with a higher acute and chronic success rate in type 1 AFl ablation. Initial results of these series gave credit to the critical isthmus theory; however, a fairly high recurrence rate (range, 10% to 40%) was observed in almost all series because AFl termination and inability to reinduce were the only markers used to define acute success. We previously reported a limited series showing that freedom from AFl recurrence is best predicted by presence of conduction block at the IVC-TA isthmus.15
Recent observations regarding the circuit of type 1 AFl have further emphasized the importance of this protected isthmus.16 17 However, the IVC-TA isthmus is a wide region to ablate, and intermediate patterns of isthmus block may be observed during the ablation procedure. An incompletely damaged isthmus may be responsible for an intra-atrial conduction delay at the low RA, mimicking an apparent complete block yet permitting late recurrence of type 1 AFl. Therefore, we looked for individualization of different types of block, their stability over time, and their correlation with late success.
In patients with paroxysmal common AFl referred for ablation, clinical AFl may be difficult to induce, and at times atrial fibrillation or uncommon AFl may be the triggered arrhythmias. Because the ablation is anatomically guided and the objective is to create a complete isthmus block, it appeared that the presence of AFl was not mandatory during the ablation procedure. Thus, we decided to test the hypothesis that ablation of common AFl during sinus rhythm is feasible, solely on the basis of isthmus block achievement.18
Classification of Atrial Flutter Pertinent to This Study
This classification essentially was based on intracardiac mapping and derived from previous ECG-based classifications.
Type 1 AFl is defined as AFl with atrial rate range between 240 and 340 bpm that could be influenced by rapid atrial pacing.19
Common right AFl is type 1 AFl exhibiting either a counterclockwise or a clockwise RA rotation and a proximal to distal CS depolarization. This type of AFl is associated with predominantly negative F waves in the inferior ECG leads.
Uncommon right AFl is an AFl with activation mapping (proximal to distal CS activation) and entrainment techniques in favor of a right-sided circuit but with an unusual RA activation pattern corresponding neither to a counterclockwise nor to a clockwise activation. This type of flutter corresponds to various surface ECG F-wave morphologies, including positive F waves in the inferior ECG leads.
Left AFl is diagnosed by a distal to proximal CS depolarization and by a postpacing interval shorter during distal CS entrainment than during entrainment from any RA sites.
Fifty-one patients were referred to our department at Rouen University Hospital for RF catheter ablation of AFl from November 10, 1993, to September 21, 1995. Seven patients showed either a left AFl (n=5) or uncommon right AFl (n=2). Therefore, our study population comprised only 44 patients (37 men, 7 women; mean age, 57±12 years) exhibiting common AFl with inverted F waves in the inferior leads, with either counterclockwise or clockwise rotation. Five patients had previously undergone ablation of AFl (RF ablation in 4 patients, DC ablation in 1), but recurrence occurred after a mean period of 2.7 months (range, 0.5 to 7). Thirteen of 44 patients had had previously documented episodes of atrial fibrillation but by far, the majority of the recorded arrhythmias were AFl. All 44 patients had suffered from frequent recurrent episodes of AFl for a mean duration of 42±42 months despite a mean number of 2.6±1.2 antiarrhythmic medications. A structural heart disease was observed in 26 of 44 patients (Table⇓). Two patients had Steinert dystrophic myopathy. In our population, mean left atrial diameter and ejection fraction (measured by echocardiography) were 36±6 mm and 59±12%, respectively. Antiarrhythmic drugs had been stopped before the ablation procedure for at least five half-lives except for 14 patients receiving amiodarone.
Ablation was performed in all patients after informed consent had been obtained. Patients were moderately sedated with flunitrazepam 1 or 2 mg, according to the body weight, 12 hours and 1 hour, respectively, before the ablation procedure. Additional sedation (nalbuphine 10 to 20 mg) was given during the procedure according to perception of pain or discomfort. Electrograms from the CS were recorded by either a decapolar or a hexapolar catheter introduced by left subclavian puncture. A Duodecapolar catheter “Halo” (Webster Laboratories) with 10 bipoles (2-mm paired spacing) separated by 1-cm distance was used in 40 patients to record activation sequence around the TA. This catheter preferentially recorded the activation of the atrium adjacent to the tricuspid ring including its anterosuperior portion (proximal bipole H19-20), the anterolateral RA, and the lateral part of IVC-TA isthmus (distal bipole H1-2) (Fig 1⇓). Attention was paid to carefully position bipole H1-2 in the immediate vicinity of the lateral side of the ablation site. A decapolar catheter was used in 4 other patients for detailed mapping of the lateral RA and the lateral part of the IVC-TA isthmus. An additional multipolar catheter was introduced to obtain electrograms from the His bundle region. Electrograms were filtered between 30 and 250 Hz and recorded on computerized multichannel systems (Midas, PPG Hellige Biomedical, Lenexa KS, and Lab 24 BARD medical system), allowing up to 32 simultaneous recordings and analysis at a paper speed of 100 to 200 mm/s. Electrical stimulation was delivered through a programmable stimulator (Bloom Associates); caution was observed to pace no higher than twice the diastolic threshold with a 2-ms pulse duration to prevent initial activation of remote areas that may affect the data. If the patient was in sinus rhythm at the beginning of the electrophysiological study, incremental atrial pacing (to 180 to 200 ms) or programmed atrial stimulation (1 to 3 extrastimuli at two different cycle lengths) were performed from the PCS to induce AFl.
Radiofrequency ablation was performed with a generator Osypka H.A.T. 200S (Osypka Gmbh) that delivered continuous unmodulated current at 500 kHz. RF current was delivered in a monopolar fashion either in the power mode (preset power output of 50 W) when using an 8-mm tip electrode catheter (Blazer, EP Technology) or in a temperature-guided mode (preset temperature of 60° to 70°C) with a 4- or 6-mm tip electrode catheter (Osypka). Choice of a thermistor ablation catheter was not randomized but was dependent on multiple parameters such as a high probability of catheter instability because of atrial enlargement, personal choice of the physician, and catheter availability. Ablation was anatomically guided and directed to the IVC-TA isthmus. The ablation catheter was introduced into the RA by a right femoral approach in 43 patients and by the left subclavian approach in 1. Initial position of the ablation catheter within the isthmus near the TA was such that a large ventricular potential and a small atrial electrogram were recorded (Fig 2⇓). The preset duration of the pulse was 90 seconds. The ablation catheter was progressively withdrawn under fluoroscopic guidance during RF delivery, with sequential stops, in order to reach the junction between the RA and the IVC. RF application was repeated until AFl termination (Fig 3⇓) and achievement of bidirectional isthmus block. Subsequent pulses were delivered either on the same line (when application was prematurely stopped because of catheter instability and/or impedance rise) or immediately more laterally or septally. In the case of ablation failure at the IVC-TA isthmus or ablation catheter instability at that site, ablation was performed at the TA–CS ostium–Eustachian crest isthmus. This isthmus is located at the interatrial septum and associates the CS ostium–TA isthmus and 1 to 2 cm posterior to the CS ostium (Fig 4⇓). More recently, in 6 patients we used a special long venous sheath (Daig Minnesota) to increase ablation catheter stability at the lower part of the RA.
RF energy was delivered during either ongoing AFl or sinus rhythm during PCS pacing. In the latter case, at least one previous episode of type 1 AFl had to be documented on 12-lead ECG. The PCS site was chosen because of the excellent stability of the catheter and because it allowed immediate visualization of the creation of block at the lower part of the RA when it was combined with the TA mapping. In the first 12 patients of our series, after tachycardia termination, we attempted to reinduce AFl by pacing from different RA sites (including the PCS and the lateral RA). These pacing maneuvers consisted of rapid incremental pacing (from 350 to 180 ms) or programmed atrial stimulation. For the subsequent 21 patients, the activation pattern at the lateral RA during PCS pacing was the main criterion used to pursue the ablation. However, induction was performed in 3 more patients to further differentiate incomplete block from complete block when low RA mapping could not. When evidence of clockwise isthmus block was obtained, LLRA pacing (from bipole H1-2, at the lateral side of the ablation site) was performed to assess the bidirectional characteristic of the block in 36 of 44 patients. Induction was then performed in all patients at the end of the procedure. Ablation was considered successful when AFl was no longer inducible and evidence of block at the IVC-TA isthmus was observed.
Definitions of the Different Types of Isthmus Block Observed
During PCS Pacing (600 ms)
Baseline RA activation (Fig 5⇓)
Two RA wave fronts of impulse propagation are noted. One wave front propagates from the PCS pacing site in a clockwise direction toward the IVC-TA isthmus to the LLRA. The other wave front from the PCS ascends in the septum to the high RA in a counterclockwise direction, with resulting collision of wave fronts at the upper part of the lateral RA.15
Complete clockwise isthmus block (Fig 5⇑)
This is defined by the observation of a purely descending wave front at the lateral wall down to the IVC-TA isthmus when pacing from the PCS, ie, the septal side of the ablation site. This block is associated with a marked prolongation of the PCS–H1-2 interval duration.
Incomplete clockwise isthmus block (Fig 5⇑)
This is said to occur when a descending wave at the lateral wall still allows the lateral part of the isthmus to be activated from the PCS, resulting in displacement of collision at the lower part of lateral RA. The distal bipole of the Halo catheter (H1-2) at the lateral part of the IVC-TA isthmus is activated slightly before or at the same time as bipole H3-4, situated more laterally.
During LLRA Pacing (600 ms)
Baseline RA activation (Fig 6⇓)
Two (septal and lateral) ascending wave fronts of RA activation are observed leading to impulse collision at the high lateral wall, suggesting the absence of counterclockwise conduction block at the lower part of the RA.
Counterclockwise isthmus block (Fig 6⇑)
This is defined by the observation of one single ascending wave front at the lateral wall followed by a completely descending wave front at the septum to reach the CS ostium. Compared with baseline, counterclockwise isthmus block is associated with the inversion of the direction of the septal activation from ascending to descending. The PCS electrogram, close to the low lateral pacing site, is activated after the high RA and the His bundle region.
All patients underwent ECG monitoring for 1 or 2 days before hospital discharge. All patients had a close follow-up control at periodic intervals by us and by their own physicians and at least one 24-hour ambulatory ECG. Patients and cardiologists were contacted by telephone at the end of January 1996 for updated clinical outcome. A control electrophysiological study was systematically performed during follow-up in 20 of 44 patients.
The PCS–H1-2 interval durations during PCS pacing or LLRA pacing were measured and expressed as mean and standard deviation. PCS–H1-2 interval durations of patients with complete isthmus block were compared with those of patients with incomplete isthmus block through the use of the Mann-Whitney nonparametric test. Recurrence rate of type 1 AFl in the two groups of patients with incomplete and complete isthmus block was compared by the χ2 test. A value of P<.05 was considered statistically significant.
In the electrophysiological laboratory, 28 patients were initially in common AFl, whereas 16 others were in sinus rhythm. Twenty-three patients presented a counterclockwise rotation AFl with a descending wave front at the anterolateral RA and an ascending wave front at the septum. Five other patients showed a spontaneously clockwise rotation AFl. In both counterclockwise and clockwise AFl, the CS catheter was depolarized from the proximal to the distal bipole, suggesting a passive activation of the left atrium. The induction of sustained AFl was possible by incremental atrial pacing from the PCS (up to 200 ms) in only 6 of 16 patients (counterclockwise rotation in 3 and clockwise rotation in 3). Nonsustained episodes of AFl were triggered in 2 others, whereas in 3 cases sustained (n=1) and nonsustained (n=2) episodes of atrial fibrillation were induced. Mean AFl cycle length in our population was 246±28 ms.
Isthmus block was successfully achieved in one session in 43 of 44 patients (98%), with a mean of 9±7 pulses (median, 6). Mean procedure duration was 197±59 minutes, with a mean fluoroscopic time of 41±18 minutes. Among the 34 patients who presented with a sustained episode of AFl, the ablation procedure was started during tachycardia in 33 patients (26 and 7 patients with a counterclockwise and clockwise rotation AFl, respectively) because restoration of sinus rhythm induced by catheter motion was observed in 1 patient. On the other hand, ablation was strictly performed during sinus rhythm in 11 patients after failure of AFl induction. See Table.
Ablation During Flutter
A mean of 4.6±3.8 RF pulses (median, 3) was necessary to terminate AFl in the 33 patients. After restoration of sinus rhythm, evidence of absence of isthmus conduction block was observed in 21 patients (64%) with persistent collision of counterclockwise and clockwise wave fronts at the lateral RA during PCS pacing. Reinduction of AFl was achieved by PCS pacing in 6 of 21 patients. In the remaining 15 patients, ablation was pursued in sinus rhythm during PCS pacing in a search for a completely descending wave front at the lateral RA related to a clockwise isthmus block (Fig 7⇓). Clockwise block induced a prolongation of the PCS–H1-2 interval duration to a mean of 163±26 ms during PCS pacing. Bipole H1-2 was positioned immediately at the lateral side of the ablation site at the isthmus. When clockwise block was obtained, LLRA pacing was performed to ensure the bidirectional characteristic of the block in 25 of 33 patients. The mean H1-2–PCS interval duration was 158±28 ms during H1-2 pacing. Finally, a mean of 6.3±6.2 additional pulses (median, 4) was necessary not only to prevent inducibility of AFl but also to obtain bidirectional isthmus block, leading to a total number of 9±8 pulses to achieve the final end point. Isthmus block could not be achieved in 1 patient who had severe back pain, and the procedure had to be stopped.
Ablation During Sinus Rhythm
The ablation was performed while monitoring the lateral RA activation pattern during constant PCS pacing at 600 ms. We observed, as the result of RF applications, a progressive delay of conduction at the isthmus associated with a downward displacement of the collision site at the lateral wall during PCS pacing (Fig 5⇑). Creation of an isthmus block resulted in a completely descending wave front at the lateral RA and final prolongation of the PCS–H1-2 interval duration by a mean of 87 ms (from 58±18 ms at baseline to 145±30 ms). Bidirectional block was obtained in all 11 patients with a mean of 7±5 (median, 6) RF applications. The H1-2–PCS interval duration was equal to 147±30 ms during H1-2 pacing.
RF applications were exclusively applied at the IVC-TA isthmus in 36 patients. In 4 other patients, failure of previous pulses to achieve the end point (ie, interruption of AFl and bidirectional block of impulse propagation at the IVC-TA isthmus) led us to change our anatomic target toward the TA–CS ostium–Eustachian crest isthmus. Last, this septal isthmus was used as first target in 4 patients because in these patients the ablation catheter was stable at this isthmus, whereas it was very unstable at the IVC-TA isthmus.
Utility of the Large-Tip Catheter
A large-tip (8 mm) catheter was used as the catheter of initial choice for ablation in 22 patients, whereas a thermistor catheter (4 or 6 mm) was initially chosen in 22 others. In 12 patients (54%), a change of catheter to the large tip was required after a mean of 10.7 pulses with the thermistor-tip catheter had failed to achieve IVC-TA isthmus block. A change of catheter to thermistor tip was necessary in 5 of 22 patients (22%) after failure of a mean of 7.4 pulses with a large-tip catheter used as first choice. In the total population, therefore, the catheter that was finally used for achieving isthmus block was the large tip in 28 cases (64%), whereas it was the standard catheter in only 16 cases (36%).
Individualization of Different Types of Isthmus Block
Complete Versus Incomplete Block
Despite the achievement of a significant prolongation of the interval duration between the PCS and the lateral portion of the IVC-TA isthmus in each case of apparent isthmus block, two very similar patterns of isthmus block were individualized. In both incomplete and complete clockwise block, a descending wave front at the lateral wall was recorded during PCS pacing. However, block was defined as incomplete when this descending wave front still allowed the lateral part of the isthmus to be activated from the PCS, resulting in displacement of collision at the lower part of lateral RA. When this occurred, the distal bipole of the Halo catheter (H1-2) lying over the lateral part of the IVC-TA isthmus was activated slightly before H3-4 and H5-6 bipoles, situated more laterally and superiorly along the RA wall. This suggested a residual (though weak and very slow) conduction through the isthmus. At the end of a 30-minute observation period, out of 43 patients with isthmus block, a complete clockwise block during PCS pacing was seen in 35 patients and an incomplete block was seen in 8 others. Yet in the latter cases, AFl reinduction was no longer possible. The PCS–H1-2 interval duration was significantly shorter in patients with incomplete block (132±22 versus 164±26 ms, P=.002). However, overlap of values did not allow individualization of a critical value to differentiate complete from incomplete block. This distinction required careful sequential activation mapping on both sides of the ablation site. A PCS–H1-2 interval duration >160 ms was observed only with complete block, but only 55% of patients with complete block showed a PCS–H1-2 interval duration >160 ms. The importance of careful mapping of the lateral part of the IVC-TA isthmus is underlined by the fact that residual isthmus conduction could not have been diagnosed in 7 of our 8 patients with incomplete clockwise block if analysis of the activation sequence of bipoles H1-2 and H3-4 had not been compared with bipole H5-6 at the low lateral wall (Fig 5⇑). Finally, the completeness of the counterclockwise block was less easy to diagnose because careful mapping of the low posteroseptal RA was technically more difficult and was not always performed during LLRA pacing.
Rate-Dependent Isthumus Blocks
Rate-dependent isthmus blocks have been observed in some patients (either during PCS or LLRA pacing) with a sudden change of the direction of impulse propagation and a sudden increase of conduction time at the low RA when the pacing rate increased. Fig 8⇓ represents the sudden occurrence of a counterclockwise block of conduction at the isthmus associated with a change of the septal activation wave front from ascending to descending, a positivation of the P wave, and a PR interval prolongation during LLRA incremental pacing. During PCS pacing, a 2-to-1 clockwise complete isthmus block was also observed in 2 patients during RF application.
No Unidirectional Complete Block
No unidirectional complete block was observed in the 36 patients in whom both PCS and LLRA pacing were performed. No counterclockwise AFl was induced when complete clockwise block was present. Conversely, a counterclockwise AFl was sometimes reinduced in the presence of an incomplete clockwise block during PCS pacing.
Observation of double potentials at the ablation site was frequent but not constant when pacing from either the septal or the lateral side of the targeted site. These double potentials represent sequential activation of both sides of a local line of conduction block as demonstrated by Shimizu et al20 and Feld et al.21 They were associated with complete isthmus block (Fig 9⇓) but were also found in some patients with an incomplete isthmus block revealed by a persistent collision of two wave fronts at the lateral RA during PCS pacing (Fig 9⇓). During constant PCS pacing, slight lateral movements of the ablation catheter (from the septal to the lateral side of the ablation site) were frequently performed to look for a sudden dramatic prolongation of the stimulus to local A interval (Fig 10⇓). This further denoted the presence of conduction block at the isthmus yet was not enough per se to differentiate incomplete and complete block.
Transient complete atrioventricular block was seen in one patient when the CS-TA isthmus was targeted, but atrioventricular conduction rapidly recovered, with no late conduction disturbances. One patient experienced a pneumothorax related to the left subclavian puncture that required evacuation by a chest tube. Frequent symptoms of minor chest pain were noted when the large-tip electrode catheter was used. However, no significant pericardial effusion was observed after the procedure.
During a mean follow-up of 12.1±5.5 months (11.7±3 months for patients ablated in sinus rhythm) 4 of 44 patients (9%) experienced common AFl recurrence. Retrospective analysis revealed that all had shown incomplete block (n=3) or no block at all (n=1). No patient with complete bidirectional isthmus block presented recurrence of common AFl (P=.0009). However, 10 patients complained of persistent palpitations. Twelve patients had paroxysmal episodes of atrial fibrillation (5 with a previous history of fibrillation) with associated uncommon AFl in 3. Three patients underwent a second ablation procedure after having experienced recurrence of common AFl. At the beginning of the procedure, previously known incomplete block was observed in 2 patients and absence of block was observed in 1 patient. Two patients underwent a second successful ablation with creation of a complete bidirectional isthmus block. Achievement of complete isthmus block could not be obtained in the remaining patient. Finally, antiarrhythmic medications were prescribed in 13 patients during the follow-up period.
A systematic control electrophysiological study was performed at a mean of 4 months (range, 1 to 23) after ablation in 20 of 44 patients. In 16 of 20 patients, the degree of isthmus block remained stable at electrophysiological control (complete block in 15 patients and incomplete in 1). In 3 patients, conduction through the IVC-TA isthmus partially recovered, with regression of complete to incomplete block with persistence of an intra-atrial conduction delay at the isthmus. An acute result progressed during follow-up in the last patient, leading to a complete isthmus block. During this electrophysiological control, reinduction of AFl was systematically attempted by incremental PCS pacing down to 180-ms cycle length. No episode of common AFl could be induced in any patient, but uncommon AFl and atrial fibrillation were induced in 2 and 6 patients, respectively.
The present series supports the concept that anatomically guided RF catheter ablation is highly effective to treat type 1 AFl. Moreover, this study defines new end points for AFl ablation on the basis of electrophysiological criteria of isthmus block, which necessitate careful and detailed low RA mapping.
Acute and Long-term Results
High acute success defined by noninducibility of AFl and isthmus block creation was obtained when targeting isthmuses at the low RA. The ablation was anatomically guided to the IVC-TA isthmus, but in some cases the ablation was delivered at the TA–CS ostium–Eustachian crest isthmus because the catheter was more stable at that site. Theses isthmuses are wide areas, commonly measuring 20 to 25 mm in width, and this may explain acute failure and/or even inability to terminate AFl when a standard ablation catheter with a 4-mm tip electrode is used. Previous small series that used larger-tip electrode (8 mm) ablation catheters had suggested that they could be of help for RF ablation of AFl.4 15 22 The very large-tip (8 mm) catheter appears in our series to be safe and seems more useful than the standard ablation catheter for ablation of AFl, leading to an acute success rate comparable to those observed for supraventricular tachycardia.
As previously reported, ablation of clockwise rotation AFl was possible by creating RF lesion at the same site as for counterclockwise rotation AFl, suggesting the critical role of these isthmuses in the circuit of this type of AFl.11 14 23 24
Despite satisfactory acute results, a fairly high recurrence rate of AFl has been reported by all the authors during follow-up because acute success was defined by AFl termination and noninducibility. We have reported in a limited series that achievement of bidirectional IVC-TA isthmus block was an excellent marker of late success.15 This series further confirms that recurrence of AFl is lowered by acute achievement of bidirectional isthmus block at the low RA. No patient with previous history of multirecurrent AFl experienced common AFl at a 1-year follow-up, when a complete block was obtained. Thus, this criterion of isthmus block definitely should be added to the noninducibility criteria for termination of the ablation procedure.
Ablation in Sinus Rhythm
This study also shows that anatomically guided ablation of common AFl is feasible during sinus rhythm without the need of AFl induction. In addition, it points out that termination and lack of reinduction is not correlated with isthmus block and therefore is not the final end point of the procedure. This methodology allows ablation to be completed without the need to perform time-consuming reinduction maneuvers between RF pulses. It also prevents the unwanted induction of atrial fibrillation episodes resulting from AFl reinduction attempts. However, type 1 AFl with inverted F waves in the inferior leads must be previously documented in each patient. Performing the ablation during PCS pacing combined with mapping of the lateral RA was possible in all of our patients. Similar to what is observed during accessory pathway ablation (with the delta wave disappearance), it allows a quick (from beat to beat) visualization of the effect of RF application at the target site. This complete change of RA activation pattern during PCS pacing suggests a preferential RA conduction in the caudo-cranial and cranio-caudal directions. Interestingly, Tabuchi et al17 observed an identical RA activation pattern in the same conditions (after isthmus ablation and during low posteroseptal RA pacing) in a canine experimental model of AFl based on the creation of an intercaval lesion.25 26 This may suggest the existence of enhanced anisotropic conduction and/or transverse conduction block at the RA posterior wall and in particular at the crista terminalis that would favor the emergence of AFl in these patients.27
Different Types of Isthmus Block
Several patterns of isthmus block were observed, but we individualized two very similar patterns that correlated with different outcomes. Creation of intra-atrial conduction delay at the lower part of the RA after RF applications did not per se warrant absence of conduction through this region. In addition to the achievement of marked prolongation of conduction at the lower part of the RA and a drastic change of the RA activation sequence when pacing from one side of the ablation site, a careful mapping of the isthmus was mandatory to distinguish complete from incomplete isthmus block. Indeed, to attest absence of impulse propagation at the isthmus crossing the ablation site during PCS pacing (complete clockwise block), the activation of the immediate lateral side of the ablation site depends on the descending wave front at the lateral wall. When the lateral side of the ablation site at the isthmus was depolarized before the LLRA during PCS pacing, this revealed residual conduction (although altered) at the isthmus that would authorize later AFl recurrence. In this situation, clockwise block was still incomplete, with an activation pattern at the IVC–TA isthmus showing an eccentric activation (from septal to lateral) during PCS pacing. This incomplete block mimicked the activation pattern related to complete block, producing intra-atrial conduction delay at the low RA. Only a careful mapping of the lateral part of the isthmus allowed distinction between the two types of block. Seven of 8 cases of incomplete block would not have been diagnosed had comparison of activation of the lateral part of the isthmus and LLRA not been performed. Thus, a multipolar catheter at the lateral RA would not have been able to differentiate both types of block in the majority of our patients. Detailed mapping of the IVC-TA isthmus was provided by the Halo catheter that gave stable signals.
The PCS-LLRA interval duration was not a discriminant parameter to differentiate incomplete from complete block in our population, even though in the presence of incomplete and complete blocks this interval duration does not correspond to activation of the same structures. After complete block achievement, this interval duration represents intra-atrial conduction time around the TA between the PCS and the LLRA. Interindividual variation of this interval duration in the case of complete block is explained by the interval duration dependence on individual intra-atrial conduction properties and RA dimension. In patients with incomplete block, this interval duration represents conduction time for the impulse to cross a partially damaged isthmus. This interval duration depends in particular on the respective positions of the distal bipole of the Halo catheter (H1-2) at the low RA and the proximal bipole of CS catheter relative to the ostium. During follow-up, AFl recurrence was noted in 3 patients with an incomplete isthmus block despite absence of inducibility at the end of the procedure. This differentiation of two types of block is of interest because no AFl recurrence occurred when the block was complete.
Even though counterclockwise block was less easily diagnosed by our mapping technique, no unidirectional complete block was observed in our population. No type 1 counterclockwise AFl was induced when a complete clockwise block was present. Conversely, a counterclockwise AFl was sometimes reinduced in the presence of an incomplete clockwise block. Finally, during the procedure, some patients with a partially damaged isthmus exhibited a rate-dependent or a 2-to-1 block.
Double potentials have been reported to be markers of conduction block at the isthmus.17 They were occasionally recorded at the ablation site, but we did not systematically look for them and try to validate them by pacing maneuvers. These double potentials are related to activation of both sides of the ablation site by different activation wave fronts traveling in opposite directions.20 21 28 29 However, they are not ideal markers of complete isthmus block because (a) despite careful isthmus mapping, they were not constantly observed, and (b) they were also found in some patients with incomplete block. Because the area for ablation is so large, their presence does not necessarily indicate that block from the entire area has been accomplished. Therefore, the mere presence of double potentials is not sufficient, and only a careful mapping of the activation sequence at the low RA is able to differentiate complete from incomplete isthmus block.
Stability of Isthmus Block During Follow-up
As previously reported,14 changes in conduction at the isthmus can occur during follow-up with either progression or regression of the block. In our series, the degree of isthmus block remained stable in the majority of the patients who underwent a systematic electrophysiological control during follow-up. Partial regression of the block was observed in 9% of the patients but without recurrence.
In some cases, intra-atrial conduction delay at the caudal part of the RA may be difficult to differentiate from complete conduction block even with a multiple-point mapping of the IVC–TA isthmus. This residual conduction through the isthmus may be theoretically observed in the presence of a complete clockwise block pattern as defined. However, intra-atrial conduction delay at the isthmus may be sufficient in that situation to prevent late recurrence because no type 1 AFl was observed during follow-up in patients with complete isthmus block. Finally, the counterclockwise characteristic of isthmus block has not been assessed in all patients, and even during LLRA pacing, completeness of counterclockwise block was less easy to diagnose by our mapping technique. Therefore, unidirectional complete clockwise block with incomplete counterclockwise block may have been misdiagnosed as bidirectional complete block.
Achievement of conduction block at the caudal part of the RA is almost always obtained and is different from AFl interruption. Although incomplete isthmus block may be sufficient to prevent late recurrence in a significant number of cases, complete isthmus block is the best marker for long-term success of AFl ablation and should be the end point of this type of procedure. This underlines the importance of detailed multiple-point mapping of this region. Achievement of block is feasible during sinus rhythm, and this should allow the ablation procedure to be performed in sinus rhythm in patients presenting this rhythm in the electrophysiological laboratory. The degree of conduction block persists in the vast majority of the patients during follow-up, and it is further confirmed that acute complete isthmus block is associated with lack of recurrence.
Selected Abbreviations and Acronyms
|IVC||=||inferior vena cava|
|LLRA||=||low lateral right atrium (atrial)|
|RA||=||right atrium (atrial)|
We gratefully acknowledge the technical assistance of Gerard Pontier.
- Received March 4, 1996.
- Revision received July 16, 1996.
- Accepted July 31, 1996.
- Copyright © 1996 by American Heart Association
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