Catheter Ablation of VT Effectively Prevents Subsequent ICD Shocks in Post-MI Secondary Prevention Patients: Results of the SMASH VT Trial

June 05, 2006

"... although the results are promising, a large randomized trial involving more centers is needed to better assess the efficacy of prophylactic substrate-based VT ablation in high-risk post-MI patients."

Substrate-based catheter ablation aimed at eradicating ventricular tachycardia (VT) can significantly benefit post-myocardial infarction (MI) patients who are at high risk for life-threatening arrhythmias, and is an effective adjunctive therapy to implantable cardioverter defibrillators (ICDs) for secondary prevention in these patients. Presented at the Heart Rhythm Society 2006 Annual Scientific Sessions in Boston, Massachusetts, the results of the late-breaking trial suggest that ablation therapy in these patients prevents further episodes of VT and ventricular fibrillation (VF), decreases subsequent ICD therapies, and may reduce mortality.

Vivek Reddy, MD, Massachusetts General Hospital (Boston), who presented the results of the Substrate Mapping & Ablation in Sinus Rhythm to Halt Ventricular Tachycardia (SMASH VT) study, [1] told meeting attendees that although the results are promising, a large randomized trial involving more centers is needed to better assess the efficacy of prophylactic substrate-based VT ablation in high-risk post-MI patients.

According to Dr. Reddy, ICDs are the mainstay therapy in patients with a history of MI who are at high risk for VT/VF. However, he noted that ICDs are far from perfect. The shocks can be painful, causing anxiety or depression in up to one third of patients; concomitant drug therapy is required in up to 70% of ICD patients; and patients with ICDs can still die of sudden death, since the relative risk reduction with the devices is about 60%.

A minimally invasive, catheter-based approach to eradicate VT would be the "ideal" therapy in secondary prevention patients, Dr. Reddy said. However, standard catheter-based mapping techniques, performed during hemodynamically stable VT, are tolerated in only 5% to 10% of cases, and patients may have other, unstable VTs not detected during mapping. Better results have been obtained with surgical ablation techniques (mapping during sinus rhythm and surface-based ablation), which may be applicable to catheter-based approaches, he stated. Although substrate-based catheter ablation is usually reserved for patients who experience multiple ICD therapies, the SMASH VT trial investigated the technique as an adjunctive therapy for secondary prevention in a larger group of post-MI ICD patients who have had as few as 1 VT/VF episode.

Trial Design

SMASH VT was a prospective, randomized, controlled trial conducted at 2 centers in the United States and 1 in the Czech Republic. The study enrolled 127 patients with a history of MI and either a plan for ICD implantation or recent (within the past 6 months) ICD implantation for one of the following indications: VF arrest, hemodynamically unstable VT, or syncope and inducible VT at electrophysiologic study. MI patients with an ICD implanted for any reason who had experienced a recent single, appropriate ICD shock were also eligible for enrollment. Of importance, patients were not allowed to be on antiarrhythmic drugs for the control of VT or VF. Patients with multiple ICD therapies necessitating antiarrhythmic drugs or ablation therapy, ongoing cardiac ischemia, stroke within the past 6 months, or any condition precluding the use of anticoagulant drugs were excluded from enrollment.

Patients included in the trial (mean age 66 years) had significant left ventricular dysfunction (ejection fraction 31.7%), were predominately New York Heart Association class II, 67% had undergone prior revascularization, and approximately 50% of patients presented with unstable VT (9% of patients had a prior ICD with single appropriate shock). Beta-blocker and angiotensin converting enzyme inhibitor therapy was used in > 90% of patients at baseline.

Investigators randomized patients to ICD only (control; n = 64) or to ICD plus ablation (n = 63). This latter group underwent electroanatomic mapping with the CARTO system (Biosense-Webster; Diamond Bar, California), and substrate modification was performed by targeting the exit sites of all induced VTs and/or late potentials within the infarct zone, using either a standard 4-mm tip radiofrequency (RF) ablation catheter or, more frequently (in 79% of patients), an irrigated 3.5-mm tip RF catheter. Follow-up continued for 2 years, with echo studies performed at 3 and 12 months. The study's primary endpoint was appropriate ICD therapy, defined as shocks or antitachycardia pacing (ATP).

Results

During the 2-year follow-up period, the primary endpoint, appropriate ICD therapy, was significantly reduced in the ablation group compared with control (15% vs 33%; P = .022) (Figure). A Kaplan-Meier plot of freedom from appropriate ICD therapy showed that the curves diverged early in the follow-up period.

Ablation therapy was also associated with a significant reduction in the rate of appropriate shocks (10% vs 31%, respectively; P = .004). Although all-cause mortality was lower in the ablation arm compared with control, the difference was not statistically significant (7% vs 17%; P = .098).By Kaplan-Meier analysis, freedom from all-cause mortality showed a slight trend favoring ablation ( P = .073). Complications of the ablation procedure were minimal and included 1 pericardial effusion, 1 heart failure exacerbation, and 1 deep venous thrombosis.

Figure. SMASH VT: outcomes at 2-year follow-up.
*Appropriate ICD therapy = shocks or antitachycardia pacing (primary endpoint)
Discussion

Dr. Reddy concluded that the results of the SMASH VT trial demonstrate that substrate-based VT ablation is a safe and feasible procedure that can decrease subsequent ICD therapies in post-MI secondary prevention patients and may reduce mortality. However, he did point out several study limitations, including the small number of centers involved, the evolution of ablation therapy that occurred over the course of the study, the fact that prophylactic use of antiarrhythmic drugs and statin drugs was not assessed, and the lack of data collection on other outcome parameters, such as quality of life and the economic impact of reducing ICD therapy.

Reference
  1. Reddy VY, Neuzil P, Richardson AW, et al. Final Results from the Substrate Mapping and Ablation in Sinus Rhythm to Halt Ventricular Tachycardia (SMASH VT) Trial. Program and abstracts from the Heart Rhythm 2006, the 27th Annual Scientific Sessions of the Heart Rhythm Society; May 17-20, 2006; Boston, Massachusetts.

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