Lateral Boundaries of the Ventricular Tachycardia Circuit Align With Sinus Rhythm Discontinuities

VT Substrate mapping - What's been done and what needs to be done

Substrate mapping is an important component of electrophysiology (EP) study for the treatment of reentrant ventricular tachycardia (VT). It is utilized to detect characteristics of the electrical circuit, and in particular the location and properties of the central common pathway, aka the isthmus, where multiple circuit loops can coincide. Typically, reentrant circuits are single- or double-loop, but as the common pathway size increases, four-loop patterns may emerge, consisting of two parallel isthmuses or a single isthmus with four loops. Arrhythmogenic substrate contains a mixture of scar, calcification, and fibrofatty regions blended with viable ventricular myocytes, which can slow conduction. It is identified in the EP laboratory in part by the presence of low amplitude electrograms and a zone of uniform slow conduction (USC) resulting from a sparsity of remaining viable myocytes and molecular-level remodeling. The electrograms recorded near isthmus boundaries frequently exhibit an abnormal morphology, such as fractionation and late or split deflections, due to the separation of muscle fiber bundles by fibroadipose tissue or calcification, and due to other conduction impediments such as source-sink mismatch, wherein topographic changes to the viable myocardial structure occur. Substrate mapping facilitates the identification of arrhythmogenic regions during sinus rhythm, whereas inducible VT with periods of ongoing reentry, when recordable, can be utilized for further assessment. Substrate modeling augments substrate mapping by seeking to predict electrogram morphology and mapped features and properties to be encountered during EP study based on an accurate depiction of arrhythmogenic tissue. Herein, we elaborate on the details of VT substrate mapping and modeling to the present time.

Wavefront curvature analysis derived from preprocedural imaging can identify the critical isthmus in patients with postinfarcted ventricular tachycardia

BACKGROUND

Where activation wavefront curvature is convexly shaped, functional conduction block can occur.

OBJECTIVE

The purpose of this study was to determine whether left ventricular (LV) wall thickness determined from contrast-enhanced computed tomography (CT) is useful in localizing such areas in clinical postinfarction reentrant ventricular tachycardia (VT).

METHODS

We evaluated data from 6 patients who underwent catheter ablation for postinfarction VT. CT imaging with inHEART processing was conducted 1-3 days before electrophysiological (EP) study to determine LV wall thickness (T). Activation wavefront curvature was approximated as ΔT/T, where ΔT represents wall thickness change. During EP study, bipolar LV VT electrograms were acquired using a high-density mapping catheter, and activation times were determined. Maps of T, ΔT/T, and VT activation were subsequently compared using statistical analyses.

RESULTS

Two of 6 cases exhibited dual circuit morphologies, resulting in a total of 8 VT morphologies analyzed. The LV wall near the VT isthmus location tended to be thin, on the order of a few hundred micrometers. Regions of largest ΔT/T partially coincided with the lateral isthmus boundaries where electrical conduction block occurred during VT. ΔT/T at the boundaries, measured from imaging, was significantly larger compared to values at the isthmus midline and to the global LV mean value (P <.001).

CONCLUSION

Wavefront curvature measured by ΔT/T and caused by source-sink mismatch is dependent on ventricular wall thickness. Areas of high wavefront curvature partly coincide with and may be helpful in locating the VT isthmus in infarct border zones using preprocedural imaging analysis.

Uniform slow conduction during sinus rhythm and low voltage/low voltage gradient ΔV/V characterize the VT isthmus location

BACKGROUND

Reentrant ventricular tachycardia (VT) properties require further elucidation.

OBJECTIVE

To understand circuit mechanisms and improve ablation targeting.

METHODS

In postinfarction VT patients undergoing electrophysiology study and catheter ablation, high-density endocardial electrogram contact mapping data was acquired during sinus rhythm (n = 6) and during VT (n = 12) and annotated by the system. Bipolar endocardial VT voltage was used to compute the voltage gradient, ΔV/V, at isthmus midline and at the lateral boundaries. Voltage was additionally represented as a depth as well as a color change, to better visualize level. Linear regression analysis was implemented to quantitate the sinus rhythm activation gradient along the isthmus long-axis midline, and along 3 other spokes originating from a last activation point.

RESULTS

The mean voltage along the isthmus long-axis was 0.234 ± 0.137 mV, vs 0.383 ± 0.290 mV aside boundaries (P < .001). The gradient ΔV/V along the isthmus long-axis was 0.425 ± 0.324, vs 0.823 ± 0.550 at boundaries (P < .001). Sinus rhythm activation was uniform (mean r2 = 0.93 ± 0.05) and slow (∇ = 0.16 ± 0.03 mm/msec) along the spoke coinciding with isthmus long-axis midline, vs less uniform (mean r2 = 0.32 ± 0.25) and rapid (∇ = 0.73 ± 0.62 mm/msec) along the other spokes (P < .001 and P = .003, respectively). Plotting r2 vs ∇, parameters of isthmus vs nonisthmus spokes were clearly separable.

CONCLUSION

A low-voltage trench coincides with the VT isthmus, vs abrupt voltage increase at the lateral boundaries, which may contravene prior definitions of conducting channels. Sinus rhythm uniform slow conduction occurs at the VT isthmus location, preventing circuit disruption while enabling the formation of an excitable gap to perpetuate reentry.

Sinus rhythm activation signature indicates reentrant ventricular tachycardia inducibility and approximate isthmus location

BACKGROUND

Sinus rhythm activation time is useful to assess infarct border zone substrate.

OBJECTIVE

We sought to further investigate sinus activation in ventricular tachycardia (VT).

METHODS

Canine postinfarction data were analyzed retrospectively. In each experiment, an infarct was created in the left ventricular wall by left anterior descending coronary artery ligation. At 3 to 5 days after ligation, 196-312 bipolar electrograms were recorded from the anterior left ventricular epicardium overlapping the infarct border zone. Sustained monomorphic VT was induced by premature electrical stimulation in 50 experiments and was noninducible in 43 experiments. Acquired sinus rhythm and VT electrograms were marked for electrical activation time, and activation maps of representative sinus rhythm and VT cycles were constructed. The sinus rhythm activation signature was defined as the cumulative number of multielectrode recording sites that had activated per time epoch, and its derivative was used to predict VT inducibility and to define the sinus rhythm slow/late activation sequence.

RESULTS

Plotting mean activation signature derivative, a best cutoff value was useful to separate experiments with reentrant VT inducibility (sensitivity, 42/50) vs noninducibility (specificity, 39/43), with an accuracy of 81 of 93. For the 50 experiments with inducible VT, recording sites overlying a segment of isochrone encompassing the sinus rhythm slow/late activation sequence spanned the VT isthmus location in 32 cases (64%), partially spanned it in 15 cases (30%), but did not span it in 3 cases (6%).

CONCLUSION

The sinus rhythm activation signature derivative is assistive to differentiate substrate supporting reentrant VT inducibility vs noninducibility and to identify slow/late activation for targeting isthmus location.

Role of activation signatures in re-entrant ventricular tachycardia circuits

INTRODUCTION

Development of a rapid means to verify the ventricular tachycardia (VT) isthmus location from heart surface electrogram recordings would be a helpful tool for the electrophysiologist.

METHOD

Myocardial infarction was induced in 22 canines by left anterior descending coronary artery ligation under general anesthesia. After 3-5 days, VT was inducible via programmed electrical stimulation at the anterior left ventricular epicardial surface. Bipolar VT electrograms were acquired from 196 to 312 recording sites using a multielectrode array. Electrograms were marked for activation time, and activation maps were constructed. The activation signal, or signature, is defined as the cumulative number of recording sites that have activated per millisecond, and it was utilized to segment each circuit into inner and outer circuit pathways, and as an estimate of best ablation lesion location to prevent VT.

RESULTS

VT circuit components were differentiable by activation signals as: inner pathway (mean: 0.30 sites activating/ms) and outer pathway (mean: 2.68 sites activating/ms). These variables were linearly related (p < .001). Activation signal characteristics were dependent in part upon the isthmus exit site. The inner circuit pathway determined by the activation signal overlapped and often extended beyond the activation map isthmus location for each circuit. The best lesion location estimated by the activation signal would likely block an electrical impulse traveling through the isthmus, to prevent VT in all circuits.

CONCLUSIONS

The activation signal algorithm, simple to implement for real-time computer display, approximates the VT isthmus location and shape as determined from activation marking, and best ablation lesion location to prevent reinduction.