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The Multicenter Thermo°Cool Ventricular Tachycardia Ablation Trial
Abstract & Commentary
By John P. DiMarco, MD, PhD
Source: Stevenson WG, et al. Irrigated radiofrequency catheter ablation guided by electroanatomic mapping for recurrent ventricular tachycardia after myocardial infarction. Circulation. 2008;118:2773-2782.
In this paper, stevenson et al, from 18 centers with expertise in ventricular tachycardia (VT) ablation, report the results of VT ablation using the Thermo°Cool-irrigated, tipped-radiofrequency ablation catheter. Patients were eligible for the study if they had multiple episodes of sustained monomorphic VT that had required either cardioversion or antiarrhythmic drug administration in the previous six months. Patients with incessant ventricular tachycardia were also eligible for inclusion. Patients with multiple VT morphologies, unmappable VT, and prior failed VT ablations could be included. The study period was between 1999 and 2003. Mapping was performed during ventricular tachycardia that had been initiated by programmed stimulation in the electrophysiology laboratory. Ablation sites were selected based on several factors: isolated mid-diastolic potentials during VT, entrainment with concealed fusion, a post-pacing interval within 30 m/sec of the VT cycle length, or VT termination in response to catheter pressure. If VT could not be mapped due to hemodynamic instability or inconsistent inducibility of a stable morphology, ablation sites were selected if they had abnormal low-signal electrograms with double potentials, wide fractionated potentials, or isolated late potentials.
The prespecified primary endpoint was freedom from recurrence of sustained monomorphic VT at six months. If the patient had incessant VT as the reason for study, absence of incessant VT was the endpoint. A total of 231 patients with VT after prior myocardial infarction were enrolled in the trial. They were predominantly male, with advanced cardiac disease, including markedly depressed left-ventricular ejection fractions (median 0.25), histories of congestive heart failure (62%), and prior revascularization attempts (CABBG - 55%; PCI - 33%). The median number of VT episodes in the preceding six months was 11. All patients had failed one or more antiarrhythmic drug trials, and 70% had experienced recurrent VT despite therapy with amiodarone. An implantable cardioverter defibrillator (ICD) was present in 94% of the patients before ablation, and at least one prior VT ablation had been attempted in 37%.
During electrophysiologic study, a total of 864 different ventricular tachycardias (median, three per patient) were inducible. At least one VT was considered mappable in 154 patients (67%), but 69% of the patients also had unmappable VTs. VT was considered unmappable if it was not hemodynamically tolerated or if it was unstable and shifted to a different morphology during study. A single-ablation procedure was performed in 207 patients; two procedures were performed in 18 patients, and one patient had three procedures before hospital discharge. The ablation lesions were applied in a linear pattern in 39% of the cases; there were focal lesions in 30% and in multiple areas in 28%. The median procedure time was 315 minutes, with a fluoroscopy time of 45 minutes. At the end of the case, at least one monomorphic VT was still inducible in 99 patients (43%). The cycle length of the ventricular tachycardia that was inducible after ablation was usually faster than the initially targeted VT.
Seven patients died within seven days of the procedure, with uncontrollable VT and progressive hypotension the most common cause. Four patients died in the electrophysiology laboratory and two after completion of the initial ablation procedure. There was one death secondary to cardiac perforation that resulted in cardiogenic shock. Non-fatal complications were seen in 27 patients. Worsened heart failure occurred in six patients during, or shortly after, the procedure. This may have been related to the volume of fluid administered with the irrigation catheter. Vascular complications occurred in 10 patients. No patient had a thromboembolic complication or stroke immediately related to the procedure.
After six months follow-up, 123 patients were free from recurrent VT or incessant VT (53%). In the subgroup of patients with incessant VT, nine of 37 had recurrent incessant VT, and nine others had recurrent, intermittent VT that could now be managed by their ICD. Ablations successfully reduced the frequency of VT, with a reduction of ≥ 75% in 67% of patients. However, 20% of the patients had an increase in the number of VT episodes. At last follow-up, 28% of the patients were no longer taking antiarrhythmic drugs.
Patients who had recurrent VT after ablation were older, had more heart failure, more frequently had concomitant atrial fibrillation, and more often had multiple prior myocardial infarctions. They also had more inducible VTs, received more radiofrequency lesions, and more often had VT inducible after ablation. At one year after study entry, 40 of 231 patients had died and six were lost to follow-up. The one-year actuarial mortality was 18%. Of the 40 post-ablation deaths, 14 (35%) were due to recurrent ventricular arrhythmias and 35% due to progressive heart failure.
Stevenson et al concluded that catheter ablation, using an irrigated-tipped catheter, can reduce the episodes of recurrent VT in patients with prior myocardial infarction. Although intermediate-term mortality and morbidity remain high in this population, catheter ablation has a significant role.
This paper presents an accurate description of the role of ventricular tachycardia mapping and ablation in patients with prior myocardial infarctions during the period covered by the study. Although the frequency of VT episodes can be reduced in many patients, total elimination of recurrent VT often is not possible. Most patients will still require either antiarrhythmic drugs and/or continued ICD therapy after the procedure. The reasons for this are well outlined by the authors. Most patients referred for a VT ablation have extensive scarring and multiple potential circuits that can support VT. Mapping during tachycardia may be impossible due to either the hemodynamic effects of the VT or instability of the VT circuit. Most patients will have advanced disease; thus, prolonged mapping and ablation procedures may be associated with significant risk.
Since this study was performed, there have been several significant advances in techniques for VT ablation. It is now recognized that epicardial sites of VT origin are present in many individual patients, and techniques for epicardial VT mapping and ablation have been developed. Percutaneous techniques for hemodynamic support can be used to support patients with unstable VTs during the procedure. Mapping systems also have evolved, and now permit more rapid and accurate definition of myocardial scarring and potential VT circuits.
Even with these advances, however, VT ablation in patients with prior myocardial infarction or non-ischemic cardiomyopathies continues to play mostly an adjunctive role to ICD-based therapy.