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Refinement of CARTO-guided substrate modification in patients with ventricular tachycardia after myocardial infarction

LI, Yi-gang; WANG, Qun-shan; Gerian, Grönefeld; Carsten, Israel; LU, Shang-biao; SHAO, Yun; Ehrlich, Joachim R.; Hohnloser, Stefan H.

Section Editor(s): WANG, Mou-yue; LIU, Huan

Original article
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Background Substrate modification guided by CARTO system has been introduced to facilitate linear ablation of ventricular tachycardia (VT) after myocardial infarction (MI). However, there is no commonly accepted standard approach available for drawing these ablation lines. Therefore, the aim of the present study was to practically refine this time consuming procedure.

Methods Substrate modification was performed in 23 consecutive patients with frequent VTs after MI using the CARTO system. The initial target site (ITS) for ablation was identified by pace mapping (PM) during sinus rhythm and/or entrainment pacing (EM) during VT. According to the initial target site, two approaches were used. The initial target site in approach one has a similar QRS morphology as VT and an interval from the stimulus to the onset of QRS cmplex (S-QRS) of ≥50 ms during PM in sinus rhythm or a difference of the post pacing interval and VT cycle length ≤30 ms during concealed entrainment pacing of VT; The initial target site in approach two has an similar QRS morphology as VT and an S-QRS of <50 ms during PM in sinus rhythm.

Results Overall, 50 lines were performed with a length of (35±11) mm. Procedure time averaged (232±56) minutes, fluoroscopy time (10±8) minutes. Sixteen patients were initially involved into approach one. After completion of 3±1 ablation lines, no further VT was inducible in 13 patients. The remaining 3 patients were switched to use the alternative approach. However, in none of them the alternative approaches were successful. Approach two was initially used in 7 patients. After completion of 3±1 ablation lines, no further VT was inducible in only 2 patients. The remaining 5 patients were switched to approach one, which resulted in noninducibility of VT in 4 of them. The initial successful rate was significantly higher in the group of approach one compared to that in the group of approach two (13/16 patients vs 2/7 patients, P=0.026).

Conclusions The approach for substrate modification of VT after MI can be optimized by identifying the appropriate initial target site with specific characteristics within the zone of slow conduction. The refined approach may facilitate linear ablation of VT, and further reduce the procedure and fluoroscopy time.

(Received June 19, 2007)

Edited by

Department of Cardiology, Xinhua Hospital, School of Medicine, Shanghai Jiaotong University, Shanghai 200092, China (Li YG, Wang QS, Lu SB and Shao Y)

Department of Medicine, Division of Cardiology, J. W. Goethe-University, Frankfurt, Germany (Grönefeld G, Israel C, Ehrlich JR and Hohnloser SH)

Correspondence to: Dr. LI Yi-gang, Department of Cardiology, Xinhua Hospital, School of Medicine, Shanghai Jiao Tong University, Shanghai 200092, China (Tel: 86-21-55964561. Fax: 86-21-55964561. Email: drliyigang@yahoo.com.cn)

This study was supported by grants from the National Natural Science Foundation of China (No. 30670831), and Shanghai Pujiang Program (No. 05PJ14063).

It has been demonstrated that substrate mapping and linear ablation facilitate catheter ablation of stable and unstable ventricular tachycardia (VT) in patients after myocardial infarction.1-13 So far, however, there is no standard approach available for substrate modification of VT after myocardial infarction. Several methods1-8 have been introduced for linear ablation of VT in this situation. For instance, the ablation line can be drawn parallel to, perpendicular to or around the boundary of the scar, or cross the scar area. However, all these approaches are time consuming and associated with potential disadvantages of multiple ablation lines such as further impairment of ventricular function or proarrhythmia. Some parameters of the conventional VT mapping approach may help the substrate mapping and linear ablation.1-4,7,14 Therefore, the aim of the present study was to refine the ablation approach of substrate modification in an attempt to shorten the procedure time, increase the efficacy, and limit the possibility of complications.

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METHODS

Patient characteristics

The subjects of this study were 23 consecutive patients (22 men, mean age (67 ± 7) years) referred for radio-frequency ablation of recurrent VT. All patients had a history of ≥1 myocardial infarction (anterior in ten, inferior in eleven and both anterior and inferior in two) with a mean ejection fraction of (29±9)%. Catheter ablation was performed due to frequent implantable cardioverter-defibrillator (ICD) discharges in 18 patients and recurrent VTs in 5 patients. All but 2 patients had implanted ICDs after ablation. Altogether 21 patients received ICDs before and after ablation. All patients had failed therapy with ≥1 antiarrhythmic drug, including amiodarone in 19 of 23 patients.

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Electrophysiological study and mapping

After informed consent was obtained, the electrophysiological study was performed. Quadripolar electrodes were positioned in the high right atrium, the His bundle position and the right ventricular apex. The latter was used for programmed ventricular stimulation with the application of up to 3 extrasystoli at 3 basic ventricular drive cycle lengths. Pacing was performed at twice the diastolic threshold (pulse width 2 ms) with a programmable stimulator (Biotronic UHS 20, Erlangen, Germany). An electroanatomical magnetic mapping system (CARTO, NAVI-STAR, Cordis-Webster, Johnson and Johnson, USA) with the Navistar ablation catheter (Cordis-Webster, Johnson and Johnson, USA) was used.15,16 The catheter was advanced via the femoral artery retrogradely across the aortic valve into the left ventricle for stimulation and mapping. The surface ECG and bipolar intracardiac electrograms were recorded utilizing a computerized multichannel system (BARD LabsystemTM 2.56, USA). Electrograms were filtered at 30 to 500 Hz and displayed at 100 mm/s; peak-to-peak amplitude was measured automatically. The left ventricle was mapped during sinus rhythm or during incessant tolerable VT to identify the low-voltage (<1.5 mV) area of infarction.1 The center of the scar was defined as a region where failure to capture at maximum output was observed (10 mA at 2-ms stimulus width).2-4 Scar boundaries were defined as regions adjacent to the scar where the bipolar voltage electrogram was 1.0-1.5 mV.3

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Refined approach of substrate modification

The sequence of mapping and ablation is as described before.3 Briefly, in patients presenting in sinus rhythm at the beginning of the study, VT was initiated by programmed ventricular stimulation from the right ventricle in order to compare the VT QRS morphology to that obtained during pace mapping (PM). Subsequently, substrate mapping of left ventricle was performed during sinus rhythm to identify the low-voltage (<1.5 mV) area of infarct scars.2 The initial target site for ablation was identified by PM during sinus rhythm and/or entrainment pacing during VT. Linear ablation was done from this site to the center of the scar and perpendicular to the boundary of the scar or to the mitral annulus if the initial starting site was within 2-3 cm of the mitral annulus.3 After completion of each line, VT inducibility was reassessed at two different sites of right ventricle and additional ablation lines added as needed. In patients presenting with incessant stable VT at the beginning of the study, voltage/activation mapping was performed and the initial site was identified during VT. The subsequent linear ablation procedures were identical to those described above.

In patients with different QRS morphologies of VT, all of these QRS morphologies were used as references for PM. The site with best match of one of the various VT morphologies was taken as the initial ablation target site.

The endpoint of this study was noninducibility of ventricular tachycardia by programmed right ventricular stimulation at 2 sites with the application of up to 3 extrasystoli at 3 basic ventricular drive cycle lengths, or an optimal initial ablation target site could not be found.

According to the initial target site, patients were divided into two groups. Patients with an initial target site of the following characteristics were included in approach one: a similar QRS morphology as VT and a stimulus to QRS interval (S-QRS) of ≥50 ms during PM in sinus rhythm and a difference of the post pacing interval and VT cycle length ≤30 ms during concealed entrainment pacing of VT.17 Patients with an initial target site of the following characteristics were included in approach two: a similar QRS morphology as VT and an S-QRS of <50 ms during PM in sinus rhythm.

If VT was still inducible after completion of 3 ablation lines using one approach, or the initial target site for each approach was not found within 20 minutes, the subject would be switched to the alternative approach. Then, maximal three additional ablation lines would be drawn.

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Radiofrequency catheter ablation

Unmodulated radiofrequency current was delivered in unipolar fashion from a generator (Stockert EpshuttleTM, Germany). An 8-French cooled radiofrequency ablation catheter with an external irrigation system (Thermocool, Biosense Webster, Johnson and Johnson, USA) was used. The Navistar ablation catheter consists of a 4-mm irrigated tip electrode and a 2-mm ring electrode separated by 1 mm of spacing with impedance and temperature monitoring. This catheter has a tip with 6 holes through which saline flows at 30 ml/min during radiofrequency application. Initial power was 20 to 30 W, and the power was gradually increased to achieve a fall in impedance of 5 to 10 Ohms or a maximal measured electrode tip temperature of 45°C. Energy application was continued for a minimum of 30 seconds and a maximum of 2 minutes. Radiofrequency current application was discontinued if measured impedance increased by >10 Ohms, or the catheter changed position.

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Follow-up

Following the procedure, patients were continuously monitored for 24 hours. Echocardiography was performed within one day after the procedure to exclude valvular damage and pericardial effusion. All patients were followed up at 3-month intervals in the outpatient clinic and instructed to contact our outpatient clinic immediately for any recurrence of arrhythmia symptoms. VT recurrence was documented via ICD interrogation. Echocardiography was repeated 3 months after ablation.

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Statistical analysis

All data are displayed as mean ± standard deviation (SD). Variables were compared using Student’s t test or by Fischer’s exact test as appropriate. A P value <0.05 was considered statistically significant.

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RESULTS

A total of 90 different VTs with a mean cycle length of (377±94) ms were induced at baseline and during the procedure. Among them, 35 VTs were stable, 55 unstable. The average was 4±2 VTs per patient. All the VT morphologies had been used as references for identifying the initial target site. Electroanatomical maps of the left ventricle were defined from an average of 268±69 points (range 196 to 414) per patient.

Overall, 50 lines were created with a mean length of (35±11) mm and 14±7 radiofrequency pulses per line. Procedure time averaged (232±56) minutes, fluoroscopy time (10±8) minutes.

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Refined approach for substrate modification

Approach one was initially used in 16 patients. After completion of 3±1 ablation lines, no further VT was inducible in 13 patients. Other 3 patients were switched to use the alternative approach, because the ideal initial target site was not found within 20 minutes in 1 patient, VT was still inducible after completion of 3 ablation lines in 2 patients. However, none of them were successful after completion of 3 additional ablation line using approach two afterwards. Approach two was initially used in 7 patients. After completion of 3±1 ablation lines, no further VT was inducible in only 2 patients. Other 5 patients were switched to use the approach one, which resulted in noninducibility of VT in 4 of them. The successful rate was significantly higher in the group of approach one initially used compared to that of approach two initially used (13/16 patients vs 2/7 patients, P=0.026). Either VT or ventricular fibrillation was still inducible at the end of the procedure in 4 patients. Figures 1 to 3 show an example of substrate mapping and modification.

Figure 1. A:

Figure 1. A:

Figure 2.

Figure 2.

Figure 3. A:

Figure 3. A:

During a mean follow-up of (11±8) months, sustained VT recurred in 5 patients. A second procedure was performed in 4 of them. An average of 3±1 additional lines were drawn using approach one. Subsequently, VT was no longer inducible in all but one patient, in whom approach two was used afterwards and was also unsuccessful. VT recurred in this patient again 3 months after the second unsuccessful ablation procedure. A third substrate mapping procedure was performed during incessant VT. A critical area of VT circuit was not found during endocardial mapping. Activation mapping indicated the critical area of VT might be located in epicardial site of the left ventricle. However, after the third ablation procedure the patient could be converted into sinus rhythm, and remained in sinus rhythm under amiodarone.

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DISCUSSION

The present study demonstrates that identifying an optimal initial target sites can refine the approach for substrate modification of VT after myocardial infarction. These initial target sites should have a stimulus to QRS interval (S-QRS) of ≥50 ms during PM in sinus rhythm or a difference of the post pacing interval and VT cycle length ≤30 ms during concealed entrainment pacing of VT. This strategy results in an initial noninducibility of VT in 13 of 16 patients (81%) within a limited number of ablation lines.

The conventional ablation approach for the VT in patients after myocardial infarction has several important limitations. About two thirds of VTs induced during electrophysiological study or spontaneously occurred after myocardial infarction are hemodynamically unstable.2,3 VT can be deteriorated into faster VT or ventricular fibrillation by entrainment pacing or other mapping maneuvers. On the other hand, it is difficult for the patients who had significant impairment of left ventricular function already at the time of initial presentation with VT to tolerate a long procedure. All of these factors preclude VT mapping rendering catheter ablation impossible. The substrate mapping approach guided by three-dimensional system is much helpful in this situation as described before.1-13 With this approach VT can be ablated not only during VT, but also in sinus rhythm with potential possibility to shorten the procedure time and fluoroscopy exposure. To focus on this point, however, the results from the initial reports were not very optimistic. Namely procedure times were averaged more than 7 hours with fluoroscopy times of longer than 28 minutes.1,2 Although the procedure and fluoroscopy times were remarkably reduced by our stepwise approach,3 it still needs to be further refined. In our previous study, we demonstrated the importance of choosing initial target site in order to facilitate the ablation procedure. In present study, we further investigate the characteristics of the initial target site. The result from this study extends the stepwise approach3 by providing a refined approach for substrate modification, which may help to increase the efficacy of this modality and shorten procedure time.

PM during sinus rhythm provides a measure of slow conduction indicated by the interval between the stimulus-QRS interval exceeding 40 ms.5,17 The exit of VT re-entry circuit is more likely to be at the border of the infarct and close to the normal infarct myocardium, and often has no or less delay during PM in sinus rhythm. However, the center part of VT re-entry circuit is more likely to be within the infarct area. If pacing at those sites, it often shows a long stimulus-QRS delays which indicate a potential isthmus, adjacent to regions of conduction block.6

It has been demonstrated by the mapping during heart operation18,19 that the time interval between the exits of VT to the cardiac muscle which form the QRS complex may need 50 ms, therefore, some investigators do not choose the site with a <50 ms of presystolic potential as the target site for energy delivery.20 However, a site with an interval between the presystolic potentials to QRS complex or the stimulus to QRS complex of >40 ms was taken as the ablation target by other investigators.21,22 The incidence of VT termination during energy delivery at this point was shown up to 50%.21 The optimal time interval from the presystolic potential to QRS complex or the stimulus to QRS complex for VT ablation in this situation is still unclear. Especially, the evidence is even less during substrate mapping and linear ablation of VT. Therefore, it is important to identify the initial target site for the substrate modification due to blindness to VT during energy delivery in sinus rhythm. The direction of ablation line during this approach is mainly dependent on the initial target site during substrate modification. An nonspecific initial target site may lead the line to departure of the critical area for the VT origin, and result in ineffective ablation or less damage to the VT related substrate tissue. Our results support the concept to start the linear ablation from an initial target site with an interval from the stimulus to QRS complex of >50 ms.

A difference of the post pacing interval and VT cycle length ≤30 ms during concealed entrainment pacing of VT was a frequently used criterion to define whether a site is within the circuit of VT or not.11,12,21-23 In the present study, we also demonstrate that the initial target site with this characteristic facilitates to the ablation procedure. Our early study24 and others6 may explain this result. In that early study, we have demonstrated that in postinfarction patients with pleiomorphic, hemody-namically stable VT, a shared isthmus may be present in approximately 40% of VTs. In accordance with ours, Brunckhorst et al6 recently showed that only one isthmus per infarct region may be due in part to the use of that isthmus for multiple VTs with different exit, such that ablation of that isthmus abolished more than one VT. It is likely that the shared isthmus more often goes through the site with concealed entrainment during VT, or with a long stimulus to QRS interval during PM in sinus rhythm. Alternatively, different isthmus is likely to accumulate in one infarct area in a relatively clustered way, extending the ablation line from that site may easily damage more VTs. Thus, the limited ablation line can cure most of VTs as demonstrated in the present study.

Although the novel approach is effective and safe, ICD implantation is still necessary due to the potential recurrence of VT, which may result from new myocardial ischaemia, or recovery of the ablated tissue.

The present study evaluated such a new refined approach proved to be effective in most patients during follow-up. This therapeutic success was accomplished with a shorter procedure time, less fluoroscopy exposure, and with markedly less number of ablation lines compared to previous studies.1,2

The results in the present study could be adjusted due to an increase in the number of patients evaluated. In addition, the characteristics of the myocardial infarction in different position could not be compared. Further randomized studies with a large number of patients need to be performed for the establishment of an optical approach of substrate modification in this subgroup of patients.

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Keywords:

ventricular tachycardia; mapping; ablation; substrate modification

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