Substrate mapping for Ventricular Tachycardia

Raja Selvaraj JIPMER

Introduction

History

Cassidy … Josephson. The value of catheter mapping during sinus rhythm to localize site of origin of ventricular tachycardia. Circulation 1984;69:1103
Stevenson et al. Identification of Reentry Circuit Sites During Catheter Mapping and Radiofrequency Ablation of Ventricular Tachycardia Late After Myocardial Infarction. Circulation 1993;88:1647

Historically two approaches that shaped VT ablation Josephson - 80s, abnormal electrograms in sinus rhythm that correlated with VT "origin" Stevenson - 90s, VT circuits, entrainment to identify Josephson noted poor positive predictive value Led to early tilt towards mapping VT

Current status

Historical progression
- Obligatory
  - Not inducible
  - Not mappable
  - multiple morphologies
- By choice
Personal / general trend
Evidence / change in scar characteristics

As a general trend in my personal practice, there has been a slow and steady shift towards substrate based strategies Driven by results with such a strategy in many studies and with the VISTA trial specifically looking at this in a randomized fashion. May also be related to change in pathology - earlier reperfusion with primary pci gives more patchy scar which is more likely to have multiple circuits rather than a single large circuit

VISTA

randomised controlled 60 per grp

VISTA

Less recurrence in substrate ablation group

What is the rationale for substrate based ablation?

Zipes et al. Catheter ablation of Arrhythmias. 2nd Ed. 2002

Pros and cons

More lesions
Tackle all potential circuits
Ablation in sinus rhythm - more stable, more effective lesions
Hemodynamic stability of patient better

Electrograms of abnormal substrate

Low voltage zones

Coming to abnormal electrograms, the most important abnormality is reduced voltage

Importance

Substrate usually located within
Safe to ablate inside
Dense scar forms anatomic barrier to anchor lesions

Low voltage zones are important because they contain the substrate and are generally safe to ablate within

Cut-offs

Cut off from normal humans (1), validated in a porcine model (2)

Marchlinsky et al. Linear Ablation Lesions for Control of Unmappable Ventricular Tachycardia in Patients With Ischemic and Nonischemic Cardiomyopathy. Circ 2000;101:1288
Wrobleski … Vivek Reddy. Use of Electrogram Characteristics During Sinus Rhythm to Delineate the Endocardial Scar in a Porcine Model of Healed Myocardial Infarction. JCE 2003;14:524

The cut offs used for bipolar voltage derive from a study in normal humans. Although this was done with a different catheter in a small number of subjects, it has larely stood the test of time.

Voltage cut-offs for endocardial signals 0.5 - 1.5 mV

Apply to both RV and LV
Both cut offs are flexible
Higher voltage cut off (2.5) in patchy scar (early reperfusion)
Patient specific cut offs ?
- Uniform, homogenous lvz with rest of myocardium not changing color (upper cut off)
- See channels / isthmus - lower cut off

The cut-offs are not fixed, can be changed in an individual patient.

High output pacing to identify scar

No capture at 10 mA, 2 ms (1)
May require higher outputs for capture (2)

Apart from just bipolar voltage, high output pacing can help identify dense scar. Especially important when dense scar is used to anchor linear lesions. As described by Stevenson's group, scar is identified by non capture with unipolar pacing at 10mA and 2ms. But higher outputs may sometimes capture areas that are still critical for VT

Soejima / Stevenson
Sarrazin … Morady, Bogun. High-output pacing in mapping of postinfarction ventricular tachycardia. Heart Rhythm 2008;5:1709.

Unipolar voltage

Normal > 8.27 (95th centile) (1)
5.5 for RV (2)
Multiple caveats, especially in RV
- more overlap between normal and abnormal
- adjust for lvh
- 7.5 for RV septum
- 6 - RV septum over aortic root

Hutchinson … Marchlinski. Endocardial unipolar voltage mapping to detect epicardial ventricular tachycardia substrate in patients with nonischemic left ventricular cardiomyopathy. Circ Arrhythm Electrophysiol 2011;4:49
Polin … Marchlinski. Endocardial unipolar voltage mapping to identify epicardial substrate in arrhythmogenic right ventricular cardiomyopathy/dysplasia. Heart Rhythm 2011;8:76

Similar to bipolar voltage, unipolar voltage can also be used to assess scar. Because of its wider field of view, abnormal endocardial unipolar voltage in the presence of normal bipolar voltage can indicate epicardial scar. More variability in cut offs because of this same far field view, especially in the RV.

Unipolar voltage

Delayed potentials

Principle

Bundles of surviving muscle fibers within scar
Slow conduction because of poor coupling
Far field early signal and near field late signal

Late potential - How late is late ?

EGM extending beyond surface QRS (1)
Remnant from SAECG
More likely to identify in late activated regions
2 or more components, separated by isoelectric segment >50 ms (sinus) or 150 ms (RV pacing) (2)
High frequency signals distinct from far-field electrogram (3)

Cassidy … Josephson. Endocardial catheter mapping in patients in sinus rhythm: relationship to underlying heart disease and ventricular arrhythmias. Circulation. 1986;73:645-652
Arenal et al. Ablation of electrograms with an isolated, delayed component as treatment of unmappable monomorphic ventricular tachycardias in patients with structural heart disease. JACC 2003;41:81.
Jais P … Haissaguerre M. Elimination of local abnormal ventricular activities: a new end point for substrate modification in patients with scar-related ventricular tachycardia. Circulation 2012;125:2184

Traditional definition of late as extending beyond QRS comes from an extension of late potentials in SAECG and is not logical. Delay from near field potential is perhaps a better definition

Variables

Catheter tip
Sinus rhythm / pacing, pacing site
Gain
Window
Density
Contact force

Many variables affect why late potentials may or may not be seen

Ablation approaches

Different approaches

Linear ablation (1)
Late potentials (2)
LAVA (3)
Scar homogenization (4)
Scar dechanneling (5)
Core isolation (6)

Marchlinski et al. Linear ablation lesions for control of unmappable ventricular tachycardia in patients with ischemic and nonischemic cardiomyopathy. Circulation 2000;101:1288
Arenal A et al. Ablation of electrograms with an isolated, delayed component as treatment of unmappable monomorphic ventricular tachycardias in patients with structural heart disease. J Am Coll Cardiol 2003;41:81
Jais P … Haissaguerre M. Elimination of local abnormal ventricular activities: a new end point for substrate modification in patients with scar-related ventricular tachycardia. Circulation 2012;125:2184
De Biase … Natale A. Endo-epicardial homogenization of the scar versus limited substrate ablation for the treatment of electrical storms in patients with ischemic cardiomyopathy. J Am Coll Cardiol 2012;60:132–141.
Berruezo A … Brugada J. Combined endocardial and epicardial catheter ablation in arrhythmogenic right ventricular dysplasia incorporating scar dechanneling technique. Circ Arrhythm Electrophysiol 2012;5:111–121.
Tzou WS … Marchlinski FE. Core isolation of critical arrhythmia elements for treatment of multiple scar-based ventricular tachycardias. Circ Arrhythm Electrophysiol 2015;8:353–361.

In practice

Discuss the choices one has to make in practice and my protocol

VT induction before mapping

Not essential for substrate mapping
Useful
- Provides baseline inducibility
- Pacemaps during mapping

May or may not induce VT before the procedure I do to provide baseline inducibility information and to provide morphology for pacemap

Sinus rhythm or pacing ?

Bipolar voltage comparable in sinus rhythm / ventricular pacing (1)
RV pacing better than sinus to identify late potentials (2)

Brunckhorst … Stevenson. Impact of Changing Activation Sequence on Bipolar Electrogram Amplitude for Voltage Mapping of Left Ventricular Infarcts Causing Ventricular Tachycardia. JCE 2005;12:137.
Arenal et al. Ablation of electrograms with an isolated, delayed component as treatment of unmappable monomorphic ventricular tachycardias in patients with structural heart disease. JACC 2003;41:81.

Although voltage may not differ in sinus rhythm vs pacing, late potentials better seen with pacing.

Sinus rhythm or pacing

Arenal et al. Ablation of electrograms with an isolated, delayed component as treatment of unmappable monomorphic ventricular tachycardias in patients with structural heart disease. JACC 2003;41:81.

In each example, late potential merges with far field in sinus rhythm, but is separated with pacing.

Prefer pacing unless

Ventricular pacing not tolerated
RV being mapped

My personal preference is to map during pacing. Exceptions are when patient doesnt tolerate the adverse hemodynamic effects of pacing and RV mapping

Pacing for RV mapping

Example of how LP better seen in sinus rhythm compared to pacing in a patient with ARVC

Unknown

multiple pacing vectors at each point
LV pacing for RV substrate ?
Extrastimuli ?

There remain various unknowns in rhythm while mapping for LPs

Choosing a window

Late potentials can be very late! I use a window extending 400 ms from the reference

Setting gain

When mapping within low voltage zone, set the gain so that the noise floor is just seen

Identifying abnormal electrograms

Setting right gain
Operator supervised annotation
'Beat averaging'

Very important for operator to participate in collection and annotation of each point

Differentiate from artifact

Mentally the operator assesses a series of beats at each location, to identify small potentials

Annotating points - binary / continuous

Broadly two methods to annotate late potentials.

Can tag each point with late potentials
Annotate latest sharp activation as local timing and late activated region can be seen on LAT map. Allows gradient of activation to be seen, but needs simultaneous display of bipolar voltage and LAT map to visualise substrate

Pacing from abnormal sites

Non capture to identify scar
Latency - correlates with late potential - may be discordant in 40% (1)
Differentiate far field from near field (1)
Paced morphology
Putative isthmus (2)

Baldinger … Stevenson. Electrogram Analysis and Pacing Are Complimentary for Recognition of Abnormal Conduction and Far-Field Potentials During Substrate Mapping of Infarct-Related Ventricular Tachycardia. Circulation: Arrhythmia and Electrophysiology. 2015;8:874-881
de Chillou C … Marchlinski FE. Localizing the critical isthmus of postinfarct ventricular tachycardia: the value of pace-mapping during sinus rhythm. Heart Rhythm 2014;11:175

High output pacing from site can identify scar. Pacing also differentiates near field from far field and provides pacemaps

Pacemap

Often approximate matches are sufficient during pacemapping. Unlike entrainment, perfect matches are rare because of antidromic wavefront

Multiple exits with pacing

Sometimes changes in morphology seen with pacing and indicated exits from multiple channels

Ablation - Individualized strategy

Scar with pacemap - linear abl along scar border
Isthmus within scar / between scar and annulus - linear ablation
Late potentials - target late potentials
Channel seen - dechanneling
Activation map / entrainment - Partial maps, correlation with substrate

Single approach in all patients as described in studies is artificial. Typically a combination of approaches required and is chosen based on the substrate in an individual patient.

Rapid, partial activation map

A strategy of substrate mapping does not preclude activation mapping. Activation map can be focused and quick keeping in mind the substrate. In this patient, the substrate clearly indicates a 'channel' which forms the isthmus during VT Activation map during VT confirms this forms a critical isthmus

Ablating channels to eliminate LP

Gradient of late potentials indicates activation proceeding from the lateral end.

Ablating channels to eliminate LP

After linear ablation at lateral end to close the "channel", voltage map shows closing of the gap while the late potentials more distally have disappeared.

Isthmus - inferior wall MI

Sometimes isthmus bordered by anatomic obstacle. In this patient with previous inferior wall MI, an isthmus between the scar and the mitral valve is seen

Isthmus - inferior wall MI

EGMs at this isthmus show late potentials during sinus rhythm and mid diastolic activation during VT

LP ablation

May not always eliminate
Ablate at entrance
Post ablation re-mapping

All studies show less than 100% elimination of late potentials with ablation. Late potentials distant to the ablation site may be abolished if ablation is performed at the entrance.

Power settings

safety of high power in scar
30W / 40 W
longer duration (1)

Tzou WS … Marchlinski FE. Core isolation of critical arrhythmia elements for treatment of multiple scar-based ventricular tachycardias. Circ Arrhythm Electrophysiol 2015;8:353–361.

Generally areas of scar are safe to ablate in. While I use 30W, going up to 40W at sites with resistant potentials, higher powers and long ablation times have been descirbed.

Conclusion

Summary

"Substrate mapping" - common term for a collection of approaches
Characterisation of locations are based on rhythm other than tachycardia
Tailored mix of approaches for an individual patient
Principally substrate based strategy for mapping and ablation remains the cornerstone

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Substrate mapping for Ventricular TachycardiaRaja Selvaraj JIPMER