Effect of recording site on postpacing interval measurement during catheter mapping and entrainment of postinfarction ventricular tachycardia

State of the Art: Mapping Strategies to Guide Ablation in Ischemic Heart Disease

Catheter ablation to prevent ventricular tachycardia (VT) that emerges late after a myocardial infarction aims to interrupt the re-entry substrate. Interruption of potential channels and regions of slow conduction that can be identified during stable sinus or paced rhythm is often effective and a number of substrate markers for guiding this approach have been described. While there is substantial agreement with different markers in some patients, the different markers select different regions for ablation in others. Mapping during VT to identify critical re-entry circuit isthmuses is likely more specific, and most useful when VT is incessant or frequent during the procedure or when sinus rhythm substrate ablation fails. Both approaches are often combined. These methods for identifying and characterizing post-infarct-related arrhythmia substrate and the re-entry circuits are reviewed.

Entrainment Mapping

Mapping during ventricular tachycardia (VT) aims to elucidate mechanism, describe myocardial propagation, and identify the origin and critical regions of VT that can be targeted for ablation, most commonly with radiofrequency ablation. Most VTs in structural heart disease are due to macro-reentry in and around scar. A combination of mapping techniques, including mapping to identify the arrhythmia substrate, activation sequence mapping, pace-mapping, and entrainment mapping, may be used to identify putative ablation targets. This review describes the principles of entrainment mapping as it pertains to catheter ablation of scar-related VT.

Novel use of postpacing interval mapping to guide radiofrequency ablation of focal atrial tachycardia with long intra-atrial conduction time

If the intra-atrial conduction time of a focal tachycardia is prolonged and exceeds the tachycardia cycle length, isochronal electroanatomical maps may be confusing by not showing the classical centrifugal pattern of focal activation. In addressing the hypothesis that by mapping post pacing intervals (PPI), the shortest PPI (ideally identical to tachycardia cycle length) would clearly display the site of origin of a focal tachycardia; this technique was used successfully to identify the origin and ablate the atrial tachycardia of a patient with previously corrected Tetralogy of Fallot.

Recognition of far-field electrograms during entrainment mapping of ventricular tachycardia

OBJECTIVES

The goal of this study was to assess entrainment for distinguishing far-field potentials (FFP) due to depolarization of tissue at a distance from the mapping catheter from the local potential (LP) due to depolarization of tissue at the catheter electrode during mapping of ventricular tachycardia (VT).

BACKGROUND

Electrograms with multiple peaks commonly complicate mapping and identification of catheter ablation targets in infarcts.

METHODS

Retrospective analysis of catheter mapping data from eight patients with prior infarction was performed to evaluate multipotential electrograms at sites where pacing entrained VT. Potentials that were visible and not altered during pacing were defined as FFP. Potentials obscured by the pacing stimulus were designated possible LPs. The criteria for FFP were then assessed in a second cohort of five patients.

RESULTS

At 32 of 39 (82%) sites with multiple potentials, entrainment identified one of the potentials as an FFP. Radiofrequency ablation, assessed at 15 sites, reduced the amplitude of LPs by 62%, without significant effect on FFP amplitude. At 56% of sites with multiple potentials, measuring the postpacing interval to an FFP would lead to erroneous classification of the site location relative to the reentry circuit. In prospective evaluation, double potentials were identified at 77 sites in infarcts; entrainment demonstrated an FFP at 66 (86%) sites.

CONCLUSIONS

Far-field potentials are common during mapping in infarcts. Many can be distinguished from local potentials by entrainment, improving the accuracy of mapping.

Estimation of entrainment response using electrograms from remote sites: validation in animal and computer models of reentrant tachycardia

INTRODUCTION

Studies suggest that entrainment response (ER) of reentrant tachycardia to overdrive pacing can be estimated using signals from sites other than the paced site.

METHODS AND RESULTS

A formula for estimation of ER using remote sites against the difference between the postpacing interval (PPI) and tachycardia cycle length (TCL) determined solely from the paced site signal was validated in experimental data and using a simple two-dimensional cellular automata model of reentry. The model also was used to study the behavior and features of entrained surfaces, including the resetting of tachycardia phase by single premature paced stimuli. Experimental results from 1,484 remote sites in 115 pacing sequences showed the average of the median ER estimate error at each pacing site was -2 +/- 5 msec, and the median ER estimate was within 10 msec of PPI-TCL for 94% of pacing sites. From simulation results, ER at the paced site was accurately estimated from >99.8% of 20,764 remote sites during pacing at 24 sites and three paced cycle lengths. Intervals measured from remote electrograms revealed whether the site was activated orthodromically or nonorthodromically during pacing, and results of simulations illustrated that the portion of the surface activated nonorthodromically during pacing increased with distance from the pacing site to the circuit. The phenomenon of nonorthodromic activation of reentrant circuits predicted by modeling was discernible in measurements taken from the animal model of reentrant tachycardia. Results also showed that, for single premature stimuli that penetrated the tachycardia circuit, phase reset of the tachycardia was linearly related to distance between the central obstacle and the paced site.

CONCLUSION

The ER is a complex but predictable perturbation of the global activation sequence of reentrant tachycardias. This predictability allows calculations of the response from anywhere on the perturbed surface. These findings suggest new techniques for measurement of the ER, which may lend themselves to computer-based methods for accurate and rapid mapping of reentrant circuits.

Static relationship of cycle length to reentrant circuit geometry

BACKGROUND

Knowledge of the pathway common to both wave fronts in figure-8 reentrant circuits (ie, the isthmus) is of importance for catheter ablation to stop reentrant ventricular tachycardia. It was hypothesized that quantitative measures of reentry isthmus geometry were interrelated and could be correlated with tachycardia cycle length.

METHODS AND RESULTS

A canine infarct model of reentrant ventricular tachycardia in the epicardial border zone with a figure-8 pattern of conduction was used for initial analysis (experiments in 20 canine hearts with monomorphic reentry). Sinus-rhythm and reentry activation maps were constructed, and quantitative (skeletonized) geometric parameters of the isthmus and border zone were measured from the maps. Regression equations were used to determine significant correlation relationships between skeletonized variables, which can be described as follows. Tachycardia cycle length, measured from the ECG R-R interval, increases with increasing isthmus length, width, narrowest width, angle with respect to muscle fibers, and circuit path length determined by use of sinus-rhythm measurements. After this procedure, in 5 test-set experiments, tachycardia cycle length measured from the R-R interval, in combination with regression coefficients calculated from initial experiments, correctly predicted isthmus geometry (mean estimated/actual isthmus overlap 70.5%). Also, the circuit path length determined with sinus-rhythm measurements correctly estimated the tachycardia cycle length (mean error 6.2+/-2.5 ms).

CONCLUSIONS

Correlation relationships derived from measurements using reentry and sinus-rhythm activation maps are useful to assess isthmus geometry on the basis of tachycardia cycle length. Such estimates may improve catheter ablation site targeting during clinical electrophysiological study.

Electroanatomic mapping of entrained and exit zones in patients with repaired congenital heart disease and intra-atrial reentrant tachycardia

BACKGROUND

Characterization of reentrant circuits and targeting ablation sites remains difficult for intra-atrial reentrant tachycardias (IART) in congenital heart disease (CHD).

METHODS AND RESULTS

Electroanatomic mapping and entrainment pacing were performed before successful ablation of 18 IART circuits in 15 patients with CHD. Principal features of IART circuits were atrial septal defect (4 patients), atriotomy (3 patients), other atrial scar (3 patients), crista terminalis (3 patients), and right atrioventricular valve (5 patients). A median of 176 sites (range, 96 to 317 sites) was mapped for activation and 13 sites (range, 9 to 28 sites) for entrainment response. Postpacing intervals within 20 ms of tachycardia cycle length and stimulus-to-P-wave intervals of 0 to 90 ms (exit zones) were mapped to atrial surfaces generated by electroanatomic mapping. Criteria for entrainment were met over a median of 21 cm2 of atrial surface (range, 2 to 75 cm2), 19% (range, 1% to 81%) of total area tested. Using integrated data, relations between activation sequence and protected corridor of conduction could be inferred for 16 of 17 LARTs. Successful ablation was achieved at a site distant from the putative protected corridor in 9 of 18 (50%) circuits.

CONCLUSIONS

The right atrium in CHD supports a variety of IART mechanisms. Fusion of activation and entrainment data provided insight into specific IART mechanisms relevant to ablation.

The N + 1 difference: a new measure for entrainment mapping

OBJECTIVES

The purpose of this study was to develop and test a new entrainment mapping measurement, the N + 1 difference.

BACKGROUND

Entrainment mapping is useful for identifying re-entry circuit sites but is often limited by difficulty in assessing: 1) changes in QRS complexes or P-waves that indicate fusion, and 2) the postpacing interval (PPI) recorded directly from the stimulation site.

METHODS

In computer simulations of re-entry circuits, the interval from a stimulus that reset tachycardia to a timing reference during the second beat after the stimulus was compared with the timing of local activation at the site during tachycardia to define an interval designated the N + 1 difference. The N + 1 difference was compared with the PPI-tachycardia cycle length (TCL) difference in simulations and at 65 sites in 10 consecutive patients with ventricular tachycardia (VT) after myocardial infarction and at 45 sites in 10 consecutive patients with atrial flutter.

RESULTS

In simulations, the N + 1 difference was equal to the PPI-TCL difference. During mapping of VT and atrial flutter, the N + 1 difference correlated well with the PPI-TCL difference (r > or = 0.91, p < 0.0001), identifying re-entry circuit sites with sensitivity of > or = 86% and specificity of > or = 90%. Accuracy was similar using either the surface electrocardiogram or an intracardiac electrogram (Eg) as the timing reference.

CONCLUSIONS

The N + 1 difference allows entrainment mapping to be used to identify re-entry circuit sites when it is difficult to evaluate Egs at the mapping site or fusion in the surface electrocardiogram.

Radiofrequency ablation of haemodynamically unstable ventricular tachycardia after myocardial infarction

OBJECTIVE

To determine whether radiofrequency (RF) ablation might have a role in haemodynamically unstable ventricular tachycardia.

METHODS

10 patients with a history of ventricular tachycardia producing haemodynamic collapse in whom drug treatment had failed and device therapy was rejected underwent RF ablation of ventricular tachycardia in sinus rhythm. The arrhythmogenic zone was defined on the basis of abnormal systolic movement, the presence of fragmentation (low amplitude, prolonged multiphasic electrograms), and pace mapping. RF lesions were delivered in power mode in linear fashion within the defined arrhythmogenic zone.

RESULTS

Success (no ventricular tachycardia inducible postablation or at retest) was achieved in six patients, possible success (a different ventricular tachycardia inducible at more aggressive stimulation) in three. In one patient, the procedure was abandoned because of poor catheter stability. There were no clinical events during a mean (SD) follow up period of 23 (10) months in any of the nine patients defined as definite or possible successes.

CONCLUSIONS

RF ablation for addressing haemodynamically unstable ventricular tachycardia opens the door for the wider use of catheter ablation for treating this arrhythmia.

Clinical value of the postpacing interval for mapping of ventricular tachycardia in patients with prior myocardial infarction

INTRODUCTION

The postpacing interval (PPI) has been used to discriminate bystander sites from critical sites within a ventricular tachycardia (VT) reentry circuit, with a PPI that is similar to the VT cycle length (CL) being indicative of a site within the reentry circuit. The purpose of this study was to assess the clinical value of the PPI for identifying effective target sites for ablation of VT at sites of concealed entrainment in patients with prior myocardial infarction.

METHODS AND RESULTS

In 24 patients with coronary artery disease and a past history of myocardial infarction, 36 VTs with a mean CL of 483+/-80 msec (+/- SD) were mapped and targeted for radiofrequency (RF) ablation. The only criterion used to select target sites for ablation was concealed entrainment. In a post hoc analysis, the PPI was measured at 47 ineffective and 26 effective ablation sites. The mean PPI-VTCL difference at the 26 effective sites (114+/-137 msec) did not differ significantly from the mean at the 47 ineffective sites (177+/-161 msec; P = 0.1). The sensitivity of a PPI-VTCL difference < or = 30 msec for identifying an effective ablation site was 46%, the specificity 64%, the positive predictive value 41%, and the negative predictive value 68%.

CONCLUSION

The PPI-VTCL difference is not useful for discriminating between sites of concealed entrainment that are within or outside of a VT reentry circuit in patients with prior infarction. Therefore, in patients with prior infarction, the PPI is not clinically useful for identifying sites of concealed entrainment at which RF ablation should or should not be attempted.

Radiofrequency catheter ablation for postinfarct ventricular tachycardia

Catheter mapping and radiofrequency ablation of postinfarct sustained ventricular tachycardia (VT) remain one of the greatest challenges for the electrophysiologist. Although there were no major breakthroughs during the past year, several refinements and clarifications of existing mapping criteria were published. In addition, initial reports appeared describing new mapping systems and ablation technologies that may significantly impact the way ablation studies are performed as well as the way in which they affect success rates. Uncertainties remain as to how effective catheter ablation will be as a longterm cure for this type of VT. For the foreseeable future, catheter ablation in postinfarct VT will remain adjunctive rather than primary therapy.