Substrate modification of ventricular tachycardia: Can Ripple Mapping help improve success rates by identifying critical channels?

Ablation of ventricular tachycardia (VT) has been shown to reduce VT recurrence more favourably than drug therapy in a number of trials. The approach to VT ablation in many of these studies has been left to operator discretion. Termination of induced VT during ablation reassures the operator that the critical pathway for the VT has been identified. However, this is only possible when the VT is either haemodynamically stable or mechanical support is used. Substrate modification during sinus rhythm is an established, alternative, therapy for patients as many do not tolerate their VT to allow mapping, or may have VT's that are not easily inducible or sustained long enough to map. The VISTA study elegantly showed this in a randomised trial of patients with ischaemic cardiomyopathy, having better outcomes with extensive substrate modification compared to conventional VT mapping and ablation.1 Patients with nonischaemic cardiomyopathy (NICMO) have historically been less well represented in the literature regarding outcomes after substrate modification. Nonetheless there is an appreciable body of evidence in favour of substrate modification in NICMO too. However, despite the evidence in favour of substrate modification to treat recurrent, scar-related, VT there is no consensus on how to best perform the procedure and VT noninducibility remains the only endpoint that can be applied objectively. Inevitably, the approaches to substrate modification vary with the common aim of “ablating until VT is non-inducible.” Scar homogenisation has been proposed to avoid sequential VT simulation and instead continuing to ablate until every single abnormal electrogram, near or within scar, has been abolished with the expectation that VT has to be non-inducible at this stage.2 Despite trials of scar homogenisation showing benefit, there is still an intellectual attraction to the concept of functional substrate modification endpoints, to restrict ablation to areas of scar considered to be critical to re-entry. Multiple approaches have been proposed; arbitrarily changing the voltage threshold that defines immutable scar, deploying linear mapping catheters within scar to visualise the direction of activation, pacing manoeuvres to map or evoke late potentials and more recently even 3D activation mapping to identify areas of conduction deceleration within scar. Promising substrate modification results have also been achieved using scar dechanneling, where slow conduction channels, considered critical to re-entry, are targeted for ablation.3 3D electroanatomic mapping has been used to guide substrate modification in all of these approaches. However, identifying the critical substrate, such as slow conduction channels, within the scar is challenging and time-consuming. The presence of far-field components on the bipolar electrogram limits accurate annotation of both bipolar voltage and activation time. The most common use of 3D mapping to find these channels has, so far, been to manually tag electrograms on the bipolar voltage map, using the conventional 0.5−1.50 mV limit for the endocardium. It is important to appreciate that the seminal studies which defined these limits used large electrode tip catheters on earlier versions of the 3D mapping systems. It is not clear if there is a consistent bipolar voltage range that defines “dense” or “immutable” scar in the ventricle, even in patients with ICM. Ripple Mapping was designed to overcome limitations related to electrogram annotation.4, 5 A Ripple Map shows each bipolar electrogram as a 3D bar that oscillates at its electroanatomic location, on the geometry, according to the electrograms voltage amplitude. When multiple points are collected, the Ripple Map is played dynamically showing each electrogram activating relative to a fiducial reference signal. The Ripple Map is played overlying a bipolar voltage map, allowing simultaneous interpretation of scar and propagation in 3D. Ripple Mapping been shown to be superior to conventional activation mapping in atrial tachycardias.6 In this issue of the Journal of Cardiovascular Electrophysiology, Gigle et al. use Ripple Mapping to retrospectively analyse the 3D electroanatomic maps of patients with nonischemic cardiomyopathy (NICMO) following catheter ablation for VT. The authors superimposed a bipolar voltage map using conventional limits of 0.5−1.50 mV to define healthy, border zone and scar tissue. The Ripple Map was then played to show the activation during sinus rhythm. Areas of tissue < 0.50 mV that had ongoing Ripple bar activation were considered to still be conducting, and the voltage limits sequentially lowered. Eventually, the lower voltage limit that define dense scar was defined based on an absence of Ripple bars, and therefore conduction. This allowed the authors to identify approximately two channels per substrate map. The mean lower limit of bipolar voltage that defined the nonconducting boundaries of a channel was 0.66 mV. However, there was a wide confidence interval with voltages as low as 0.2 mV also demonstrated. The authors then compared the shortest and longest distances between these channels and the position of ablation tags. Patients who had successful outcomes from their ablations had tags that colocated with these ripple channels, unlike those with recurrences. The study supports the concept of Ripple Mapping guided substrate ablation in NICMO but it remains to be determined whether this approach can be applied during cases.7-10 This will be essential to assess prospective outcomes of Ripple Mapping guided substrate ablation in NICMO. Ripple Mapping has been used to guide substrate modification of postinfarct VT. Conduction channels within scar were identified by following the activation path of Ripple bars corresponding to late potentials. The benefit of using Ripple Mapping was that the wavefront arising from far-field electrogram components could be distinguished from the late activation potentials within scar. Ripple Mapping conduction channels were seen in all 15 patients studied in one prospective study and ablation of the entire channel led to 85% acute noninducibility and 77% free of VT at follow-up.11 More recently, the Ripple Map was viewed in reverse sequence to sequentially group Ripple bars of scar potentials into clusters activating concurrently. This approach allows ablation to be directed to the channel entrance and demonstrating downstream loss of the scar potentials as an endpoint. The importance of such functional endpoint for substrate modification cannot be overstated.12 The paper by Gigle et al. paves the way for prospective studies of Ripple Mapping guided substrate ablation of NICMO and to determine whether similar functional endpoints can be applied in NICMO scar.