Tracking dynamic conduction recovery across the cavotricuspid isthmus

1.1 Patients

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 In the case of one of the last two events or intermittently at random, the catheter was withdrawn and the tip was examined for adherent coagulum.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).

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Figure 1

An example of labile intermittent conduction through a discrete “gap” in the ablation line, reflected both in the local intracardiac electrograms and recovery of the P wave change on the surface ECG. Lead II is shown here, and for the first two paced atrial beats, the P wave shows terminal positivity correlating with a late (second) potential (at 160 ms after the stimulus artifact) on the ablation line and an “r” wave on the unipolar electrograms. The local bipolar electrograms are abruptly transformed into a fractionated continuum with an activation time of 85 ms for the next two beats (stars), accompanied by a QS pattern on the unipolar electrograms and recovery of the P wave with a double negative deflection. This example provides a clear demonstration of the dynamic association between the recovery of cavotricuspid isthmus conduction and P wave recovery.

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.

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).

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Figure 3

Tracking of dynamic changes in isthmus conduction immediately after termination of clockwise flutter by RF delivery in the isthmus. Top panel (A), pacing from the low lateral RA was initiated 1.5 s after flutter termination and the initial eight paced P waves show a terminal positivity similar to the flutter morphology, but which changes thereafter (star) to a negative deflection. The paced P wave morphology later changed (star) to the same characteristic morphology seen immediately after flutter termination during additional RF applications, as shown in the bottom panel (B), and intracardiac mapping confirmed the achievement of complete isthmus block. This indicates recovery of transient isthmus conduction block after flutter termination despite the continuing delivery of RF energy at the same site. In both panels, arrows indicate the pacing stimulus.

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).

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Figure 4

A summary of the results of P wave tracking of dynamic changes in isthmus conduction as a result of RF ablation. After termination of flutter as a result of ablation, 12 of 15 patients had no block. Further ablation was required to produce stable isthmus block. Conduction recovery after the achievement of complete block across the isthmus occurred in 16 patients, all requiring further ablation. See text for details.

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.

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Figure 5

Time course of P wave and isthmus conduction recovery after initial achievement of block. Twenty-five instances occurred in 16 patients, with the majority occurring in up to 60 s.

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.

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.

3.5 Conclusions

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.