I read with interest this week's lead article in the New England Journal of Medicine that evaluated whether prophylactic catheter ablation of ventricular tissue would reduce the incidence of later implantable cardioverter-defibrillator (ICD) shocks called "SMASH-VT:" Substrate Mapping and Ablation in Sinus Rhythm to Halt Ventricular Tachycardia.
"Substrate modification," for those unfamiliar, is a term used by electrophysiologists to change the underlying milleu of the heart to prevent arrhythmias. This is performed by using a three-dimentional mapping system (Biosense-Webster's Carto system was used in this study exclusively) to define where small-amplitude (i.e, less than 1.5 mV in amplitude and implicitely thought to be abnormal) electrogram signals were detected within the heart. These areas were labeled as "scar" within the interior of the heart. By ablating, or burning, areas within the scar, it is hoped to disrupt the potentially life-threatening reentrant circuits that cause the abnormal heart rhythms that cause ICDs to fire. Many ablation lesions are used to create the "lines" that disrupt these circuits.
But the process of determining where to ablate in these cases is laborious and time-consuming. First, not all ablation was performed in sinus rhythm, as the pseudonym of the trial would suggest. The authors did try to induce ventricular tachycardia in the patients, and even tried to have rapid arrhythmias slowed with a class Ic antiarrhythmic to define the predominant clinical ventricular arrhythmia to target. Adding to the complexity of the studies performed, fully 74 percent of the patients underwent both a transseptal (crossing a catheter from the right to the left atrium to access the left ventricle) and simultaneous retrograde aortic approach (reaching the left ventricle by passing the ablation catheter up the femoral artery backward through the aortic valve and into the left ventricle).
It is an impressive tour d' force to perform these procedures. All in the name of reducing ICD shocks in the patient with an indwelling ICD and a history of ventricular tachycardia. Electrophysiologists reading this study know this.
So I found it interesting that I could not estimate my own Ankle Swelling Index (ASI) accurately from this study. That's the index of how large my ankles will become after standing at the patient's bedside to perform this procedure. A one-hour procedure gives an Ankle Swelling Index of 1, a two-hour procedure gives an ASI of 2, and so on. You see the reason I could not estimate this accurately is because no ablation times or radiation exposure times were mentioned in this study. Why? Were they excessive?
Instead, I must estimate. So given the complexity of the ablation procedure described in this study, I estimate the procedure times for each of each of these patients to exceed four hours each: an ASI greater than 4. And I would not be surprised if several indexes were over 5.
Man. That's huge.
It's huge because, like it or not, to ever apply prophylactic ventricular ablation to prevent ICD shocks in a patient who has already received (or will receive) an ICD, the time spent for both the patient and the doctor has to be worth the risks involved in long procedures. The complexities of this ablation approach, while shown to be feasible and safe (at least in this small group of carefully-selected patients), it was by no means without risk (three of the ablation patients had some complications). Further, not reporting the time required to implement this approach has serious implications for recommending the type of anesthesia and estimating whether these patients with limited ventricular function can lie recumbent long enough to undergo the procedure.
So while there appears to be a role for substrate-based catheter ablation to prevent ICD shocks, careful evaluation for the Ankle Swelling Index as well as the risks inherent to the ablation approach used in this sick patient population should be considered carefully before applying this approach to every patient receiving an ICD for the indiction of ventricular tachycardia or ventricular fibrillation.
Reddy VY, Reynolds MR, Neuzil P, Richardson AW, Taborsky M, Jongnarangsin K, Kralovec S, Sediva L, Ruskin JN, Josephson ME. "Prophylactic Catheter Ablation for the Prevention of Defibrillator Therapy." N Engl J Med Dec 27, 2007: 357(26): 2657-2665.
Editorial: Estes NAM. "Ablation after ICD Implantation — Bridging the Gap between Promise and Practice." N Engl J Med Dec 27, 2007; 357(26): 2717-2719.