Author + information
- Received June 24, 1999
- Revision received November 30, 1999
- Accepted January 17, 2000
- Published online May 1, 2000.
- Dipen C. Shah, MDa,*,
- Atsushi Takahashi, MDa,
- Pierre Jaı̈s, MDa,
- Meleze Hocini, MDa,
- Jing Tian Peng, MDa,
- Jacques Clementy, MDa and
- Michel Haı̈ssaguerre, MDa
- ↵*Reprint requests and correspondence: Dr. Dipen C. Shah, Hôpital Cardiologique du Haut-Lévêque, Avenue de Magellan, 33604 Bordeaux-Pessac Cedex, France
We sought to assess the dynamic temporal course of conduction recovery during and after radiofrequency (RF) catheter ablation of the cavotricuspid isthmus.
Although cavotricuspid isthmus block is accepted as the best end point of ablation for typical flutter, conduction recovery is thought to underlie many eventual recurrences. Its time course and frequency have not been determined.
In a prospective group of 30 patients (26 men and 4 women, age 64 ± 12 years) undergoing ablation of typical flutter in the cavotricuspid isthmus, the morphology of the P wave during pacing from the low lateral right atrium after achievement of complete isthmus block was identified as a reference. Regression of this morphologic P wave change was confirmed to be associated with intracardiac evidence of the recovery of cavotricuspid isthmus conduction and was observed throughout the procedure both during ablation in sinus rhythm (n = 15, group B) and just after flutter termination (n = 15, group A).
Stable complete isthmus block was achieved in all patients; 29 had a terminal positivity of the paced P wave. Flutter termination resulted in stable block and terminal P wave positivity in three patients, transient terminal P wave positivity and transient block despite continuing RF at the same site in five patients and no block in the remaining seven patients. Conduction recovery identified by recovery of P wave changes was nearly as common (48%) during ablation in sinus rhythm. Multiple recoveries were noted in some patients, and 72% of all recoveries occurred within 1 min. Conduction recovery was only rarely associated with coagulum, impedance elevation or pops.
Conduction recovery in the cavotricuspid isthmus is common during and after ablation and can be accurately, dynamically and continuously observed by monitoring the recovery of the low lateral right atrial paced P wave change.
Typical atrial flutter, which has a well-known right atrial (RA) circuit including the critical cavotricuspid isthmus, has now become routinely amenable to curative treatment by catheter ablation (1–3). During ablation, though, flutter termination by radiofrequency (RF) application in the cavotricuspid isthmus is no longer considered a suitable end point because of both high recurrence rates and a lack of correlation with isthmus block. Cavotricuspid isthmus block is accepted to be the best end point for ablation, although conduction recovery after achievement of block may occur and underlies recurrences (4–6). The timing and frequency of dynamic conduction changes after cavotricuspid isthmus ablation have not been examined. We assessed a prospective cohort of patients undergoing ablation of typical flutter on the basis of the hypothesis that the occurrence and exact timing of conduction recovery could be recognized by continuous monitoring of changes in the paced P wave morphology after the identification of the P wave morphology corresponding to isthmus block.
Thirty patients (26 men and 4 women, mean age 64 ± 12 years) undergoing ablation of typical flutter (cycle length 254 ± 24 ms) were studied. Eleven patients had structural heart disease, including two with coronary heart disease, four with both dilated cardiomyopathy and valvular heart disease and one with congenital heart disease.
1.2 Electrophysiologic study and ablation
The details of the procedure have been previously described (7,8). All antiarrhythmic drugs were stopped at least 48 h in advance, and the procedure was performed after 4 to 6 h of fasting. Informed consent was obtained from all patients. Two catheters were introduced from a femoral approach. A 7F Cordis Webster catheter (Baldwin Park, California) with a thermocouple-equipped 4-mm tip electrode was used for ablation and rove mapping in the RA. A 6F quadripolar 2-mm electrode catheter (Xtrem, Medicorp) placed low down in the lateral RA as near as possible to the anticipated ablation line (typically at 7 o’clock in the left anterior oblique view), so that all four electrodes were in good contact, was used for stimulation from its distal bipole and recording from its proximal bipole. In all cases, the paced P wave was confirmed to be dominantly or completely negative in leads II, III, aVF and V6. Bipolar electrograms were recorded at high gains (0.1 mV/cm through a bandpass of 30 to 500 Hz) and at a paper speed of 100 mm/s while bipolar stimulation was performed at four times the diastolic threshold, with a 2-ms output pulse width from a programmable stimulator.
Radiofrequency energy in the form of an unmodified 550-kHz sine wave from a Stockert Cordis generator was delivered sequentially point by point between the catheter tip and a large backplate. The target temperature was set to 60 to 70°C, and RF was applied for a duration of 60 to 90 s without moving the catheter during energy delivery. The catheter position was monitored flouroscopically, and RF delivery was interrupted prematurely in the event of displacement, an audible pop or a significant rise in impedance (>25 ohms as compared with the initial value). In the case of one of the last two events or intermittently at random, the catheter was withdrawn and the tip was examined for adherent coagulum. Ablation was begun in 15 patients (group A) during typical flutter (13 with counterclockwise flutter and 2 with clockwise flutter) and in the other 15 in sinus rhythm (group B). Ablation during flutter was performed by targeting electrograms coinciding with the center of the surface electrocardiographic (ECG) plateau (for counterclockwise flutter) and the initial downslope of the flutter wave (for clockwise flutter) until termination. The ablation line was then located by double potentials during pacing from the low lateral RA, and gaps in its continuity were identified by previously described electrogram markers and ablated (Fig. 1)(9). When ablation was begun during low lateral RA pacing, atrial potentials at the same stimulus-to-electrogram interval were targeted in the isthmus from the tricuspid to the inferior vena cava edge, followed by identification of the line on the basis of double potentials and completion of the line by ablation of gap electrograms. The end point of isthmus block was defined by the stable achievement of a complete parallel corridor of double potentials in the isthmus from the tricuspid to the inferior vena cava edge. Descending septal activation during lateral RA pacing and a descending lateral RA activation sequence during coronary sinus ostial pacing were also verified (7,8).
1.3 Paced P wave monitoring
Low lateral RA pacing from the distal bipole was performed at a cycle length of 681 ± 66 ms from the beginning of the procedure in the 15 patients in sinus rhythm and from just after termination of flutter in the remaining 15 patients during continuing delivery of RF energy at the same point. The 12-lead surface ECG was monitored continuously during RF delivery, whereas leads I, II, III and V1 were displayed continuously during the whole procedure. Throughout the procedure, a 12-lead ECG was recorded on optical disk media for retrieval when necessary. The paced P wave morphology could therefore be continuously monitored to detect beat-to-beat changes. In case of an activation delay >40 ms to the first spike of double potentials, the pacing position was considered suboptimal and moved nearer to the ablation line to maximize the P wave change. After achievement of complete isthmus block (verified by intracardiac mapping), the resulting morphology of the paced P wave was used in each patient as the reference. Any regression of the paced P wave morphologic changes in comparison to this reference morphology was considered an indicator of recovery of isthmus conduction; its timing was noted and intracardiac mapping was performed for confirmation. On the basis of published data on paced P wave changes during achievement of isthmus block (5,10), the changes produced by cavotricuspid isthmus block were defined by the appearance of de novo terminal positivity (prominently seen in leads II, III, aVF and V6) and classified by two of the authors as none or significant. Every significant P wave change was correlated with the presence or absence of isthmus conduction, and ablation was continued in the presence of persisting conduction or conduction recovery. After the initial achievement of complete block, recovery of the P wave morphology was used to time the exact instant of conduction recovery, just as the final P wave change timed the achievement of block; thus, monitoring of the paced P wave morphology allowed dynamic and continuing assessment of isthmus conduction.
1.4 Statistical methods
Continuous variables are presented as the mean value ± SD. Statistical comparison was performed using the Fisher exact test. A p value <0.05 was considered significant.
After the delivery of 11 ± 7 RF applications, stable isthmus block was achieved in all patients; 29 of 30 exhibited a final paced P wave morphology with terminal P wave positivity, whereas one patient (no. 18) did not develop significant terminal positivity. The morphologic change was complete from one beat to the next in 17 patients (Figs. 1 and 2), ⇓ but occurred progressively in two steps in nine patients, indicating that isthmus conduction could persist despite an initial P wave change. A dynamic change in the P wave morphology was not documented in the three patients who had stable block and a terminally positive P wave on flutter termination.
2.1 Flutter termination and conduction recovery (group A)
Low lateral RA pacing was started 4 ± 5 s after termination of flutter during continuing application of RF energy at the same site. Termination occurred 22 ± 9 s after beginning delivery of the 5 ± 4th RF application in the isthmus; in no case was termination due to occurrence of ectopy/ectopics. After termination, three patients had stable terminal P wave positivity, whereas five patients had a terminal positivity of the initial paced P waves that disappeared after 27 ± 30 s, despite ongoing RF delivery at the same site (Fig. 3). The terminal positivity correlated with isthmus block and recovery of the P wave change with persisting isthmus conduction (Fig. 1). In six patients, the paced P wave after termination did not show terminal positivity. The last patient had terminal P wave positivity despite persisting conduction through the isthmus. Thus, eight patients had isthmus block on flutter termination, but five of them recovered conduction after complete block (i.e., in 5 [62.5%] of 8 RF applications, conduction recovered after complete block had produced flutter termination). The remaining seven patients had no evidence of isthmus block despite flutter termination. Subsequent RF applications during pacing targeting remnant gaps were required in 12 patients to produce stable terminal P wave positivity and stable block (see subsequent text).
2.2 Conduction recovery in sinus rhythm
The 15 patients in group B required 11 ± 6 RF applications in sinus rhythm for complete, stable isthmus block. In all patients but one (no. 18), a significant P wave change was documented in the form of terminal positivity (Fig. 1 and 2). However, eight patients developed 14 episodes of conduction and P wave recovery after transient block (each episode lasting 153 ± 228 s), requiring further RF applications for stable block. In addition, of the 12 patients in group A (see subsequent text) who required further ablation during pacing for persisting isthmus conduction, eight also displayed 11 episodes of conduction recovery and concomitant P wave recovery after transient block, each episode lasting 49 ± 107 s (Fig. 4).
Therefore, in groups A and B combined, conduction recovery during pacing (i.e., without a change in rhythm) was documented in a total of 25 times in 16 patients (1.56 times/patient) (Fig. 4). Recovery occurred nine times during continuing RF delivery at a gap site (labile changes were also noted) (Fig. 1) and in 16 after completion of RF delivery. For the 27 patients who underwent ablation in sinus rhythm (i.e., groups A and B, except for the three patients with complete, stable isthmus block on flutter termination), 48% of RF applications producing block during pacing demonstrated recovery (vs. 62.5% during ablation in flutter; p = NS). The mean time from block to recovery of each episode was 107 ± 185 s (range 1 to 840).
P wave recovery was always associated with evidence of isthmus conduction, and no instance of conduction recovery without a P wave recovery was observed. The morphology of the recovered P wave was intermediate between the baseline and the final P wave in five instances and returned to the baseline morphology in all others. After the initial achievement of complete isthmus block, the time course of recovery followed an exponential decay (Fig. 5), with 18 instances (72%) occurring within 1 min.
2.3 Other factors affecting efficacy of RF delivery
In this study including 346 RF deliveries in the cavotricuspid isthmus, adherent coagulum was found on the withdrawn catheter tip in 41 instances, although a significant impedance rise was observed in only three instances. Audible “pops” prematurely forced termination of 11 applications, whereas catheter displacement was noted in seven instances. However, of the total of 30 RF applications that resulted in transient block, only three were associated with adherent coagulum—one with a “pop” and one with catheter tip displacement.
After further ablation resulting in block, the stability of the P wave change and intracardiac evidence of block were confirmed during a monitoring period of 33 ± 29 min (range 6 to 150; in 11 patients, >30 min). After a follow-up of 23 ± 2 months, there were no recurrences.
This study shows that the dynamic temporal course of recovery of isthmus conduction can be accurately identified by monitoring the paced P wave morphology, and thus demonstrates the high incidence of recovery of isthmus conduction before the achievement of stable block.
The time course of conduction recovery has not been assessed, chiefly because isthmus conduction has been only intermittently monitored after the achievement of isthmus block (6). Surface ECG monitoring during low lateral RA pacing from near the ablation line has been described as an indicator of the achievement of block (5,10). In this study, however, each patient’s paced P wave morphology acted as his or her control, allowing simple and continuous real-time monitoring of recovery of isthmus conduction (as the reverse of the changes resulting in block). This eliminated the problem of inter individual variability of the forward change in the paced P wave morphology due to nonidentical pacing sites as well as varying atrial activation patterns.
3.1 High incidence of conduction recovery
This study indicates a high incidence of conduction recovery. The present technique of producing linear isthmus block relies on the coalescence of multiple, in-line punctuate lesions. In much the same way that an accessory pathway ablated by a point lesion recovers conduction in up to 10% (11), any of the individual lesions making up the linear isthmus block can recover, resulting in a higher incidence than is the case with accessory pathways. The temporal course with 72% of the recoveries occurring within the first minute after the achievement of block reflects the similar underlying tissue changes mediating electrophysiologic block and recovery, thus mandating at least 14 min (840 s) of surveillance after the achievement of complete block, although later recovery has been reported without specifying the exact timing of the change in conduction (6).
Any of the multiple lesions required to make up the linear ablation line of block can recover conduction, with much less evident consequences in the early stages of formation of linear lesions (i.e., before the line is complete or nearly so). This suggests that the incidence of lesion recovery, as opposed to isthmus conduction recovery, may be even higher. Similarly, although the temporal kinetics indicate the highest incidence of recovery soon after RF delivery, conduction recovery does not always indicate recovery of the last delivered lesion.
After the termination of flutter by the delivery of RF energy, the institution of low lateral RA pacing immediately after flutter termination allowed an assessment of dynamic conduction changes. In the majority of cases, termination was the result of complete isthmus block, which in some cases recovered despite continuing RF delivery at the same site. In others, conduction slowing alone may have been sufficient to terminate flutter, or a rate-dependent block may have occurred at the flutter rate with persisting conduction at the slower stimulation rate in sinus rhythm, or transient block may have recovered within the few seconds before the institution of pacing. Apparent conduction recovery within 1.5 s after termination in two patients favors conduction slowing or rate-dependent block as the mechanism of flutter termination in these patients.
P wave recovery was always associated with the return of isthmus conduction; however, the occurrence of intermediate degrees of P wave changes correlating with persistent conduction in 30% of patients in this study makes the use of a forward P wave change unreliable as a surrogate marker for block, because further and individually varying P wave changes correlating with complete isthmus block were documented.
3.2 Mechanisms of recovery
Recovery of electrical function occurs because of reversible changes produced by the application of RF, characteristically at the edge of the lesion (12). Thus, while conduction recovery after cessation of RF delivery typically reflects the occurrence of this phenomenon in thick myocardium, the reason for recovery, despite continuing RF delivery at the same site, is not clear but could be related to minor changes in position due to a change in the rhythm. This may be difficult to appreciate on flouroscopy, although the artifacts of ongoing RF delivery preclude the assessment of local electrograms. Evident catheter displacement was only rarely associated with conduction recovery in this study. Another possibility is that a change in the rhythm alters local convective cooling, and thus delivered energy unfavorably. This is supported by the somewhat higher incidence of transient block during ablation associated with a change in atrial rhythm (62.5% with flutter termination vs. 48% during continuous pacing in sinus rhythm; this was statistically nonsignificant). However, the occurrence of significant conduction recovery during continuing low lateral RA pacing also suggests that other factors, such as a reduced effectiveness of RF as a sequel to ongoing RF delivery (because of local coagulum or edema), are also a likely possibility. In this study, however, adherent coagulum or audible pops were only infrequently associated with conduction recovery. Studies using magnetic resonance imaging or intracardiac echocardiography have documented significant local edema during and after RF delivery, with a resulting doubling of wall thickness (13,14), thus reducing the effective depth of penetration of RF and possibly producing both recovery as well as resistance to conventionally delivered RF energy.
3.3 Clinical relevance
The findings of this study suggest that, in practice, during a period of at least 15 min after the achievement of complete isthmus block, continuous monitoring of the low lateral RA paced P wave morphology for evidence of regression of the morphologic change of terminal positivity permits the immediate recognition of nearly all episodes of recovery of isthmus conduction and allows reablation to minimize recurrences.
3.4 Study limitations
Tracking the low lateral RA paced P wave provides an index of unidirectional conduction recovery, although this by itself has been shown to increase the risk of recurrence (4,5). The rare patients without any forward P wave change require intracardiac recording for monitoring, but this can be minimized by the appropriate selection of pacing sites. Moreover, the implications of adherent coagulum are limited by the fact that the catheter tip was not systematically examined after every RF application, but do underline the rarity of the association of impedance rise, “pops” and adherent coagulum with conduction recovery.
Tracking the paced P wave morphology during stimulation from the low lateral RA is a simple and effective monitoring index for the assessment of conduction once isthmus block has been achieved and reveals a high incidence of conduction recovery during RF ablation performed both during flutter and in sinus rhythm.
- electrocardiogram or electrocardiographic
- right atrium or atrial
- Received June 24, 1999.
- Revision received November 30, 1999.
- Accepted January 17, 2000.
- American College of Cardiology
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