Sinus Rhythm Electrogram Shape Measurements are Predictive of the Origins and Characteristics of Multiple Reentrant Ventricular Tachycardia Morphologies

Guiding ablation strategies for ventricular tachycardia in patients with structural heart disease by analyzing links and conversion patterns of traceable abnormal late potential zone

BACKGROUND

Substrate-based ablation can treat uninducible or hemodynamically instability scar-related ventricular tachycardia (VT). However, whether a correlation exists between the critical VT isthmus and late activation zone (LAZ) during sinus rhythm (SR) is unknown.

OBJECTIVE

To demonstrate the structural and functional properties of abnormal substrates and analyze the link between the VT circuit and abnormal activity during SR.

METHODS

Thirty-six patients with scar-related VT (age, 50.0 ± 13.7 years and 86.1% men) who underwent VT ablation were reviewed. The automatic rhythmia ultrahigh resolution mapping system was used for electroanatomic substrate mapping. The clinical characteristics and mapping findings, particularly the LAZ characteristics during SR and VT, were analyzed. To determine the association between the LAZ during the SR and VT circuits, the LAZ was defined as five activation patterns: entrance, exit, core, blind alley, and conduction barrier.

RESULTS

Forty-five VTs were induced in 36 patients, 91.1% of which were monomorphic. The LAZ of all patients was mapped during the SR and VT circuits, and the consistency of the anatomical locations of the LAZ and VT circuits was analyzed. Using the ultrahigh resolution mapping system, interconversion patterns, including the bridge, T, puzzle, maze, and multilayer types, were identified. VT ablation enabled precise ablation of abnormal late potential conduction channels.

CONCLUSION

Five interconversion patterns of the LAZ during the SR and VT circuits were summarized. These findings may help formulate more precise substrate-based ablation strategies for scar-related VT and shorter procedure times.

Functional Assessment of Ventricular Tachycardia Circuits and Their Underlying Substrate Using Automated Conduction Velocity Mapping

OBJECTIVES

This study sought to describe the utility of automated conduction velocity mapping (ACVM) in ventricular tachycardia (VT) ablation.

BACKGROUND

Identification of areas of slowed conduction velocity (CV) is critical to our understanding of VT circuits and their underlying substrate. Recently, an ACVM called Coherent Mapping (Biosense Webster Inc) has been developed for atrial mapping. However, its utility in VT mapping has not been described.

METHODS

Patients with paired high-density VT activation and substrate maps were included. ACVM was applied to paired VT activation and substrate maps to assess regional CV and activation patterns. A combination of ACVM, traditional local activation time maps, electrogram analysis, and off-line calculated CV using triangulation were used to characterize zones of slowed conduction during VT and in substrate mapping.

RESULTS

Fifteen patients were included in the study. In all cases, ACVM identified slow CV within the putative VT isthmus, which colocalized to the VT isthmus identified with entrainment. The dimensions of the VT isthmus with local activation time mapping were 37.8 ± 13.7 mm long and 8.7 ± 4.2 mm wide. In comparison, ACVM produced an isthmus that was shorter (length: 25.1 ± 10.6 mm; mean difference: 12.8; 95% CI: 7.5-18.0; P < 0.01) and wider (width: 18.8 ± 8.1 mm; mean difference: 10.1; 95% CI: 6.1-14.2; P < 0.01). In VT, the CV using triangulation at the entrance (8.0 ± 3.6 cm/s) and midisthmus (8.1 ± 4.3 cm/s) was not significantly different (P = 0.92) but was significantly faster at the exit (16.2 ± 9.7 cm/s; P < 0.01). In the paired substrate analysis, traditional local activation time isochronal mapping identified 6.3 ± 2.0 deceleration zones. In contrast, ACVM identified a median of 0 deceleration zones (IQR: 0-1; P < 0.01).

CONCLUSIONS

ACVM is a novel complementary tool that can be used to accurately resolve complex VT circuits and identify slow conduction zones in VT but has limited accuracy in identifying slowed conduction during substrate-based mapping.

Structure and function of the ventricular tachycardia isthmus

Catheter ablation of postinfarction reentrant ventricular tachycardia (VT) has received renewed interest owing to the increased availability of high-resolution electroanatomic mapping systems that can describe the VT circuits in greater detail, and the emergence and need to target noninvasive external beam radioablation. These recent advancements provide optimism for improving the clinical outcome of VT ablation in patients with postinfarction and potentially other scar-related VTs. The combination of analyses gleaned from studies in swine and canine models of postinfarction reentrant VT, and in human studies, suggests the existence of common electroanatomic properties for reentrant VT circuits. Characterizing these properties may be useful for increasing the specificity of substrate mapping techniques and for noninvasive identification to guide ablation. Herein, we describe properties of reentrant VT circuits that may assist in elucidating the mechanisms of onset and maintenance, as well as a means to localize and delineate optimal catheter ablation targets.

Limitations and Pitfalls of Substrate Mapping for Ventricular Tachycardia

The fundamental hypothesis of substrate mapping for scar-mediated ventricular tachycardia is that surrogates of the isthmus can be identified and targeted with ablation during sinus rhythm. These surrogates include electrocardiographic indications for electric discontinuity such as fractionation, split, late, and long potentials, also evident as sites displaying activation slowing. However, ablation strategies targeting these surrogates during sinus rhythm have resulted in unacceptably high rates of clinical failures, promoting the idea that a more widespread ablation may be required. High-resolution mapping technologies provide an opportunity to examine the substrate at greater detail; however, their use has not yet translated into improved clinical outcomes. This may be related to ongoing efforts to examine the same surrogates at higher resolution instead of using high-resolution technologies for discovering new and potentially more specific surrogates. This article reviews common limitations and pitfalls of substrate mapping and discusses new opportunities for high-resolution mapping to increase the accuracy of substrate mapping: 1) multielectrode mapping catheters provide an opportunity to rapidly examine the substrate during electrophysiological conditions that more closely simulate ventricular tachycardia by means of activation from different directions and coupling intervals; 2) electrogram annotation methods based on the maximal negative derivative of the extracellular potential or maximal voltage are often inaccurate in nonuniform anisotropic tissue. The use of multielectrode catheters may improve the accuracy of electrogram annotation by using spatiotemporal dispersion of single-beat acquisitions and a localized indifferent reference; and 3) resetting and entrainment remain important methods for studying re-entry for and guiding ablation.

Slow uniform electrical activation during sinus rhythm is an indicator of reentrant VT isthmus location and orientation in an experimental model of myocardial infarction

BACKGROUND

To validate the predictability of reentrant circuit isthmus locations without ventricular tachycardia (VT) induction during high-definition mapping, we used computer methods to analyse sinus rhythm activation in experiments where isthmus location was subsequently verified by mapping reentrant VT circuits.

METHOD

In 21 experiments using a canine postinfarction model, bipolar electrograms were obtained from 196-312 recordings with 4mm spacing in the epicardial border zone during sinus rhythm and during VT. From computerized electrical activation maps of the reentrant circuit, areas of conduction block were determined and the isthmus was localized. A linear regression was computed at three different locations about the reentry isthmus using sinus rhythm electrogram activation data. From the regression analysis, the uniformity, a measure of the constancy at which the wavefront propagates, and the activation gradient, a measure that may approximate wavefront speed, were computed. The purpose was to test the hypothesis that the isthmus locates in a region of slow uniform activation bounded by areas of electrical discontinuity.

RESULTS

Based on the regression parameters, sinus rhythm activation along the isthmus near its exit proceeded uniformly (mean r²= 0.95±0.05) and with a low magnitude gradient (mean 0.37±0.10mm/ms). Perpendicular to the isthmus long-axis across its boundaries, the activation wavefront propagated much less uniformly (mean r²= 0.76±0.24) although of similar gradient (mean 0.38±0.23mm/ms). In the opposite direction from the exit, at the isthmus entrance, there was also less uniformity (mean r²= 0.80±0.22) but a larger magnitude gradient (mean 0.50±0.25mm/ms). A theoretical ablation line drawn perpendicular to the last sinus rhythm activation site along the isthmus long-axis was predicted to prevent VT reinduction. Anatomical conduction block occurred in 7/21 experiments, but comprised only small portions of the isthmus lateral boundaries; thus detection of sinus rhythm conduction block alone was insufficient to entirely define the VT isthmus.

CONCLUSIONS

Uniform activation with a low magnitude gradient during sinus rhythm is present at the VT isthmus exit location but there is less uniformity across the isthmus lateral boundaries and at isthmus entrance locations. These factors may be useful to verify any proposed VT isthmus location, reducing the need for VT induction to ablate the isthmus. Measured computerized values similar to those determined herein could therefore be assistive to sharpen specificity when applying sinus rhythm mapping to localize EP catheter ablation sites.

Ablation of Reentry-Vulnerable Zones Determined by Left Ventricular Activation From Multiple Directions: A Novel Approach for Ventricular Tachycardia Ablation: A Multicenter Study (PHYSIO-VT)

BACKGROUND

The optimal method to identify the arrhythmogenic substrate of scar-related ventricular tachycardia (VT) is unknown. Sites of activation slowing during sinus rhythm (SR) often colocalize with the VT circuit. However, the utility and limitations of such approach for guiding ablation are unknown.

METHODS

We conducted a multicenter study in patients with infarct-related VT. The left ventricular (LV) was mapped during activation from 3 directions: SR (or atrial pacing), right ventricular, and LV pacing at 600 ms. Ablation was applied selectively to the cumulative area of slow activation, defined as the sum of all regions with activation times of ≥40 ms per 10 mm. Hemodynamically tolerated VTs were mapped with activation or entrainment. The primary outcome was a composite of appropriate implanted cardioverter-defibrillator therapies and cardiovascular death.

RESULTS

In 85 patients, the LV was mapped during activation from 2.4±0.6 directions. The direction of LV activation influenced the location and magnitude of activation slowing. The spatial overlap of activation slowing between SR and right ventricular pacing was 84.2±7.1%, between SR and LV pacing was 61.4±8.8%, and between right ventricular and LV pacing was 71.3±9.6% (P<0.05 between all comparisons). Mapping during SR identified only 66.2±8.2% of the entire area of activation slowing and 58% critical isthmus sites. Activation from other directions by right ventricular and LV stimulation unmasked an additional 33% of slowly conducting zones and 25% critical isthmus sites. The area of maximal activation slowing often corresponded to the site where the wavefront first interacted with the infarct. During a follow-up period of 3.6 years, the primary end point occurred in 14 out of 85 (16.5%) patients.

CONCLUSIONS

The spatial distribution of activation slowing is dependent on the direction of LV activation with the area of maximal slowing corresponding to the site where the wavefront first interacts with the infarct. This data may have implications for VT substrate mapping strategies.

Activation During Sinus Rhythm in Ventricles With Healed Infarction: Differentiation Between Arrhythmogenic and Nonarrhythmogenic Scar

BACKGROUND

In infarct-related ventricular tachycardia (VT), the circuit often corresponds to a location characterized by activation slowing during sinus rhythm (SR). However, the relationship between activation slowing during SR and vulnerability for reentry and correlation to components of the VT circuit are unknown. This study examined the relationship between activation slowing during SR and vulnerability for reentry and correlated these areas with components of the circuit.

METHODS

In a porcine model of healed infarction, the spatial distribution of endocardial activation velocity was compared between SR and VT. Isthmus sites were defined using activation and entrainment mapping as areas exhibiting diastolic activity within the circuit while bystanders were defined as areas displaying diastolic activity outside the circuit.

RESULTS

Of 15 swine, 9 had inducible VT (5.2±3.0 per animal) while in 6 swine VT could not be induced despite stimulation from 4 RV and LV sites at 2 drive trains with 6 extra-stimuli down to refractoriness. Infarcts with VT had a greater magnitude of activation slowing during SR. A minimal endocardial activation velocity cutoff ≤0.1 m/s differentiated inducible from noninducible infarctions (P=0.015). Regions of maximal endocardial slowing during SR corresponded to the VT isthmus (area under curve=0.84 95% CI, 0.78-0.90) while bystander sites exhibited near-normal activation during SR. VT circuits were complex with 41.7% exhibiting discontinuous propagation with intramural bridges of slow conduction and delayed quasi-simultaneous endocardial activation. Regions forming the VT isthmus borders had faster activation during SR while regions forming the inner isthmus were activated faster during VT.

CONCLUSIONS

Endocardial activation slowing during SR may differentiate infarctions vulnerable for VT from those less vulnerable for VT. Sites of slow activation during SR correspond to sites forming the VT isthmus but not to bystander sites.

Source-Sink Mismatch Causing Functional Conduction Block in Re-Entrant Ventricular Tachycardia

Ventricular tachycardia (VT) caused by a re-entrant circuit is a life-threatening arrhythmia that at present cannot always be treated adequately. A realistic model of re-entry would be helpful to accurately guide catheter ablation for interruption of the circuit. In this review, models of electrical activation wavefront propagation during onset and maintenance of re-entrant VT are discussed. In particular, the relationship between activation mapping and maps of transition in infarct border zone thickness, which results in source-sink mismatch, is considered in detail and supplemented with additional data. Based on source-sink mismatch, the re-entry isthmus can be modeled from its boundary properties. Isthmus boundary segments with large transitions in infarct border zone thickness have large source-sink mismatch, and functional block forms there during VT. These alternate with segments having lesser thickness change and therefore lesser source-sink mismatch, which act as gaps, or entrance and exit points, to the isthmus during VT. Besides post-infarction substrates, the source-sink model is likely applicable to other types of volumetric changes in the myocardial conducting medium, such as when there is presence of fibrosis or dissociation of muscle fibers.

Detection of the diastolic pathway, circuit morphology, and inducibility of human postinfarction ventricular tachycardia from mapping in sinus rhythm

OBJECTIVE

The purpose of this study was to determine whether sinus rhythm activation maps could be used to detect the origin and characteristics of reentrant ventricular tachycardia in postinfarction patients.

METHODS

In each of 11 post-myocardial infarction patients, unipolar electrograms were acquired at 256 virtual endocardial sites using noncontact mapping. Electrograms were marked for activation time and mapped on a three-dimensional grid. Spatial differences in sinus rhythm activation time were correlated to isthmus characteristics and to activation through the diastolic pathway during tachycardia on the basis of the presence of contiguous lines of slow conduction and block.

RESULTS

Twenty tachycardia morphologies were analyzed. Fourteen sustained reentrant circuit morphologies occurred in nine patients, with dual morphologies having a shared isthmus occurring in five of nine patients. Dual morphologies were caused by changes in entrance-exit point location about a common isthmus. One transient circuit morphology of <10 beats occurred in three of nine patients also having sustained reentry. The estimated isthmus determined from sinus rhythm activation overlapped the diastolic pathway determined from tachycardia maps with 83.8% sensitivity and 89.2% specificity. The mean difference in sinus rhythm activation time across the isthmus border was larger in transient compared with sustained morphologies (32.8 +/- 9.5 ms vs. 22.8 +/- 1.8 ms), with smaller isthmus size (4.8 +/- 1.1 cm(2) vs. 10.0 +/- 1.1 cm(2); P < .05), narrower entrance-exit points (7.0 +/- 1.5 mm vs. 9.3 +/- 0.8 mm; P < .05), and greater activation time difference across them (16.3 +/- 3.5 ms vs. 10.1 +/- 1.0 ms; P < .05).

CONCLUSION

In post-myocardial infarction patients, the reentry isthmus can be localized in the endocardial border zone from sinus rhythm activation maps. Nonsustained reentry occurs when isthmus size is small and entrance-exit points are narrow and more electrically discontinuous.

Model of reentrant ventricular tachycardia based on infarct border zone geometry predicts reentrant circuit features as determined by activation mapping

BACKGROUND

Infarct border zone (IBZ) geometry likely affects inducibility and characteristics of postinfarction reentrant ventricular tachycardia, but the connection has not been established.

OBJECTIVE

The purpose of this study was to determine characteristics of postinfarction ventricular tachycardia in the IBZ.

METHODS

A geometric model describing the relationship between IBZ geometry and wavefront propagation in reentrant circuits was developed. Based on the formulation, slow conduction and block were expected to coincide with areas where IBZ thickness (T) is minimal and the local spatial gradient in thickness (DeltaT) is maximal, so that the degree of wavefront curvature rho proportional, variant DeltaT/T is maximal. Regions of fastest conduction velocity were predicted to coincide with areas of minimum DeltaT. In seven arrhythmogenic postinfarction canine heart experiments, tachycardia was induced by programmed stimulation, and activation maps were constructed from multichannel recordings. IBZ thickness was measured in excised hearts from histologic analysis or magnetic resonance imaging. Reentrant circuit properties were predicted from IBZ geometry and compared with ventricular activation maps after tachycardia induction.

RESULTS

Mean IBZ thickness was 231 +/- 140 microm at the reentry isthmus and 1440 +/- 770 microm in the outer pathway (P <0.001). Mean curvature rho was 1.63 +/- 0.45 mm(-1) at functional block line locations, 0.71 +/- 0.18 mm(-1) at isthmus entrance-exit points, and 0.33 +/- 0.13 mm(-1) in the outer reentrant circuit pathway. The mean conduction velocity about the circuit during reentrant tachycardia was 0.32 +/- 0.04 mm/ms at entrance-exit points, 0.42 +/- 0.13 mm/ms for the entire outer pathway, and 0.64 +/- 0.16 mm/ms at outer pathway regions with minimum DeltaT. Model sensitivity and specificity to detect isthmus location was 75.0% and 97.2%.

CONCLUSIONS

Reentrant circuit features as determined by activation mapping can be predicted on the basis of IBZ geometrical relationships.

Method to Predict Isthmus Location in Ventricular Tachycardia Caused by Reentry with a Double-Loop Pattern

BACKGROUND

During electrophysiologic study, induction and mapping of clinical reentrant ventricular tachycardia can be difficult. Hence, analysis of sinus-rhythm electrograms for reentry localization is of potential clinical relevance. Herein is described a method of sinus-rhythm electrogram shape analysis, that does not require arbitrary threshold values, for localization of double-loop reentrant circuits that drive clinical tachycardias.

METHODS AND RESULTS

Reentrant ventricular tachycardia was induced by premature stimulation in 23 postinfarction canine hearts 4-5 days after left anterior descending (LAD) ligation. Sinus-rhythm activation maps were constructed from bipolar electrograms acquired at 196-312 sites in the epicardial border zone. The timing of all electrogram peak deflections during one cycle of sinus rhythm was determined, and the mean (MPD) and deviation (DPD) were taken, respectively, as estimates of the time of activation wavefront crossing, and the duration of local electrical activity. These variables were then mapped on a computerized grid. The line of most uniform and sharp MPD gradient predicted the propagation direction through the double-loop reentrant circuit isthmus that would occur upon tachycardia induction. The sharpest transitions in DPD bounding this line predicted the positions of arcs of block that would border the reentrant circuit isthmus during tachycardia. The actual and estimated isthmuses overlapped by a mean of 84.1 +/- 3.8%. In six experiments lacking inducible reentrant tachycardia, no line of sharp MPD gradient was present in the MPD maps.

CONCLUSIONS

Analysis of multiple sinus-rhythm deflections can localize the reentrant ventricular tachycardia isthmus without introduction of arbitrary threshold points and peak choices that may lead to error.

Ventricular Tachycardia Duration and Form Are Associated with Electrical Discontinuities Bounding the Core of the Reentrant Circuit

BACKGROUND

Successful prediction of reentrant ventricular tachycardia duration and form from sinus-rhythm electrogram signals in canine hearts is relevant to clinical studies, to potentially improve catheter ablation treatment during EP study.

METHODS/RESULTS

Following LAD ligation of canine hearts, activation maps were constructed from 312 border zone sites 4-5 days postinfarction. When reentrant ventricular tachycardia was inducible via programmed stimulation, the core of the circuit was defined on the maps as the enclosed area formed by the adjoining lines of slowest conduction and block bounding the protected region of the reentrant circuit. The number, location, and width of points of entrance or exit of the activation wavefront about the core were determined. Core perimeter location was then marked on the sinus-rhythm activation map, and the difference in activation time at opposite recording sites along the core perimeter was measured. Mean sinus-rhythm activation was highly discontinuous along the core perimeter in 10 transient reentry experiments (30.1 +/- 4.4 ms), moderately discontinuous in 13 sustained experiments (16.7 +/- 1.8 ms), and only slightly discontinuous in 5 noninducible experiments (9.7 +/- 1.7 ms). For transient versus sustained experiments, the entrance/exit points were narrower (mean: 6.5 +/- 1.0 mm vs 9.5 +/- 1.8 mm) with larger sinus-rhythm discontinuity across them (mean: 23.8 +/- 6.0 ms vs 11.8 +/- 2.1 ms). As core size increased, so did the number of entrance/exits present during reentry (P < 0.001). With increasing core size, four-loop (quatrefoil) reentry was frequently observed.

CONCLUSIONS

Whether reentrant ventricular tachycardia will be inducible in the canine infarct border zone, and its duration and form, is associated with the characteristics of electrical discontinuities present about the core perimeter.