The “earliest point of origin”. QRS waves usually appear when the impulse reaches the exit along the scar border area and then conducts to the surrounding myocardium. pre-QRS wave electrograms are typically recorded from the exit area.30,219-221 Non-contact scalar systems can also identify the exit area where the depolarizing wave travels from the scar.62,128 In the presence of a foldback loop channel adjacent to the exit, the potential is usually earlier, between the QRS waves that are diastolic potentials. There are also often isolated potentials or split potentials.30,219,222,223 Outside the isthmus, in the outer ring or in areas away from the scar, depolarization usually occurs within the QRS wave. However, points in the proximal isthmus of the folded loop can be depolarized at the terminal part of the QRS wave.30,224 Sites outside the folded loop are bystanders (Figure 2). Extensive conduction abnormalities in the scar region can lead to depolarization of the parasternal region at diastolic potentials, which can be easily confused with the isthmus. Therefore recording early potentials at individual sites is not a reliable indicator of the ideal ablation target. Drag band methods are useful for selecting ablation targets (see below).
For stable ventricular tachycardia, the complete sequence of excitation is determined and combined with drag band labeling to guide ablation. More commonly, agonistic labeling is limited, and ablation targets are identified by marking areas of presystolic excitation and isolated potentials, combined with drag band labeling.
2.3 Pacemakers
Pace marking refers to the pacing of the ventricular tachycardia in the absence of an attack, and the sequence of excitation is usually evaluated by 12-lead electrocardiography at the marker site. When the QRS pattern at pacing is consistent with the pattern of ventricular tachycardia onset, the pacing site is likely to be close to the point of origin of focal ventricular tachycardia or the exit of the scar-associated folding loop51,225-228.
Either bipolar or unipolar pacing is indicated. Bipolar pacing produces small stimulation artifacts, possibly capturing the apical electrode while also capturing the proximal segment annular electrode, and the recorded signal will be less accurate, especially when relatively large pole spacing (8-10 mm) and high intensity currents (>8 mA) are applied.227 Pacing slightly above the pacing threshold may improve accuracy. However repeated threshold tests are usually not practical. Some laboratories initially apply 10 mA, 2-ms stimulation, sometimes decreasing the stimulation intensity if the area of pacing capture is considered large, or increasing the stimulation intensity (e.g., increasing to 20 mA) if tissue capture is required in scarred areas.53,230 Whether body surface labeling and automatic quantitative comparison of ventricular tachycardia during an episode and QRS morphology during pacing labeling help improve the clinical application of pacing labeling is unclear.231,232 It is not clear231,232.
In focal ventricular tachycardia, pacing scaling can be useful in suggesting the location of the point of origin, although sequential excitation scaling may be more accurate.10,227,233,234 The ideal target point should be in general agreement with the QRS wave during tachycardia, including each tangent and general obliquity. Pacing markers are particularly useful when tachycardia is difficult to induce when ablation is required.
For scar-associated ventricular tachycardia, pacing in the exit region will match the QRS wave during ventricular tachycardia. However, there are some limitations and caveats due to the potential for large folding loops and the presence of abnormal conduction. Pacing in normal areas outside the exit can also produce QRS morphological changes similar to ventricular tachycardia. Therefore, pacing specimens are usually combined with voltage specimens in the stromal specimen to identify possible exits.73,235 Although pacing specimens near the exits may result in QRS wave patterns similar to those of ventricular tachycardia, pacing at the proximal part of the isthmus of the folding loop in sinus rhythm may produce significantly different QRS wave patterns because the stimulus waves generated by pacing at this site are conducted along the block path of ventricular tachycardia.225,228,235 226,228,235 Therefore, when pacing in the scar region, the site that does not match the QRS pattern of ventricular tachycardia at pacing does not necessarily lie outside the foldback loop.
Pacing in the isthmus of the folding loop produces a long S-QRS interval and a paced QRS wave that is consistent with ventricular tachycardia.51,53,235
2.4 Drag band labeling
Drag band labeling is useful in stable ventricular tachycardia to confirm the site of the folding loop and to identify the parasternal site. Drag banding requires pacing slightly faster than the ventricular tachycardia frequency.30,219,222,223,225,236-243 In fibrillatory ventricular tachycardia, tachycardia resumes after pacing has stopped, QRS wave morphology is stable suggesting that each stimulus wave is reformed into a fibrillatory loop, and the tachycardia is dragged. Drag banding can be confirmed by the presence of stable fusion, progressive fusion, or termination of tachycardia when conduction block occurs.241
Drag banding is measured assuming that pacing in the fold loop does not alter conduction. The pacing circumference is only slightly shorter than the ventricular tachycardia circumference (10-30 ms shorter) to reduce the probability of termination or alteration of ventricular tachycardia. A pacing output slightly above the pacing threshold avoids capture of the distal myocardium, but repeated determination of the pacing threshold is not practical. 10 mA, 2 ms pacing intensity is feasible and can be reduced if necessary. In the scar region, higher-intensity pacing is usually required to seize and confirm the folding loop site.230 Although some investigators have applied unipolar pacing to reduce the possibility of anodal seizure, many centers apply bipolar pacing.30,219,236-244
In intra-fold loop pacing, the postpacing interval at the stimulation site is approximately the fold loop runtime, i.e., the tachycardia circumference. The postpacing interval increases with increasing conduction time between the pacing site and the folding loop. There are several possible reasons for this error. The postpacing interval is measured to local potentials. In scar areas where fragmentation and splitting potentials are present, it is difficult to discriminate between local potentials and far-field potentials generated by tissue depolarization away from the pacing site.216 Noise and stimulation artifacts during drag banding can interfere with the electrogram.245,246 If pacing slows conduction within the folding loop, the postpacing interval is prolonged.
QRS wave patterns during drag banding suggest whether the pacing site is located in the isthmus. When paced distally outside the ventricular tachycardia loop, pacing changes QRS morphology, resulting in QRS fusion or unlike ventricular tachycardia seizure maps. Pacing in the isthmus of the folding ring, pacing to drag ventricular tachycardia does not alter ventricular excitation distal to the folding ring, because the paced wavefront applies ventricular excitation to the exit of the ventricular tachycardia. The QRS pattern during dragging is consistent with the onset of ventricular tachycardia, defined as occult dragging, i.e., dragging with occult fusion and true dragging. In these isthmus sites, the S-QRS interval, which indicates the conduction time from the pacing site to the exit, is equivalent to the local electrogram to QRS interval. In contrast, at parasternal sites adjacent to the folding loop isthmus, drag banding occurs without QRS wave fusion, but the postpacing interval exceeds the ventricular tachycardia perimeter and the S-QRS exceeds the local electrogram to QRS wave interval. Occult drag band occurs during pacing with a post-pacing interval suggestive of a foldback loop, but with a long S-QRS interval (>70% of ventricular tachycardia circumference), often proximal to the isthmus region (inner loop), where radiofrequency ablation is less likely to terminate ventricular tachycardia.
Pacing at the outer ring of the folding loop, where excitation is conducted along the scar border, has a postpacing interval approximately equal to the ventricular tachycardia circumference and has QRS wave fusion during towing. Because QRS wave fusion is sometimes difficult to identify on the surface ECG, a careful analysis of the 12-lead ECG during pacing and comparison of QRS patterns during pacing and ventricular tachycardia episodes should be performed. QRS wave fusion is sometimes difficult to detect if <22% of the QRS interval is due to a reverse wavefront.248 Therefore, some outer-loop sites are mistaken for exit areas or inner-loop sites.
In hemodynamically stable ventricular tachycardia, ablation of the isthmus confirmed by drag band markers usually terminates the ventricular tachycardia.219,222,223,243,249 The vast majority of ventricular tachycardia can be terminated in the isthmus region with an isolated mid-diastolic potential and a low recurrence rate at follow-up.219,223,238 For unstable ventricular tachycardia, limited drag band markers combined with stromal markers ablate the scar region target 26,59.
2.5 Stromal calibrations
Matrix labeling is used to identify areas that may support foldback during sinus rhythm or paced rhythm based on anatomical and electrophysiological features. This approach facilitates ablation of multiple ventricular tachycardias, polymorphic ventricular tachycardias, because hemodynamically unstable ventricular tachycardias cannot be marked or reliably induced.50,53,55,59,73,95,99,101,250-255 Even in hemodynamically stable ventricular tachycardias, substrate marking facilitates reduction of agonistic sequential marking or dragging marking of the region of interest. In these cases, voltage labeling is confirmed in the scarred region at the onset of ventricular tachycardia104.
Substrate labeling usually begins with the identification of the scar region based on electroanatomic parameters (usually voltage) in the electroanatomic map of the ventricle of interest. Marking the fold exit, channel (isthmus), and slow conduction sites can be confirmed by pacing markers and electrogram features, and then localized for ablation. Given the less precise localization of the folding loop, a more extensive ablation strategy is often applied to ablate larger areas within the scar.
Identification of the scar
Scar tissue is identified based on the bipolar electrogram electrogram. Applying a 4 mm apical electrode, 1 mm annular pole spacing, and 2 mm ring (1 mm pole spacing) filtering (10-400 Hz, 95% of normal left intraventricular electrograms have positive and negative fronts between >1.53-1.55 mV.73 Low voltage areas correlate well with scar areas on animal models.256 Particularly low scar areas ( < 0.5 mV or less) are defined as dense scars. However, it is important to recognize that these regions may contain surviving myocardium or the isthmus of the folding loop.53 The low-voltage region between 0.5 and 1.5 mV is defined as the border zone and is also a region of special interest because surgical endocardial resection in this region can successfully eliminate ventricular tachycardia.73
Outlet and access
The scar area is usually large, with a mean circumference of 21 cm in a study of postinfarction patients.59 To narrow the extent of ablation within the scar, the target ablation area needs to be referenced to pacing markers and additional electrogram parameters. The ablation line is generally in the low voltage region to minimize damage to the functional myocardium.
The exit can be determined by pacing calibrations at the scar border, with the ablation line at the infarct border (bipolar electrogram voltage usually at 0.5-1.0 mV) approximately parallel to the infarct border. Additional ablation injury lines were perpendicular to the border. An extension line through the outlet to the dense myocardium may also be required.73,101,252,253 It is unclear whether pacing marker measurements of QRS waves and ventricular velocity are perfectly consistent than essentially consistent.
Several characteristics of potential isthmuses have been described.50,51,55,95,99,250,250a Sites where pacing produces long S-QRS but matches the pattern of ventricular tachycardia QRS waves are seen in some folding-loop isthmuses and can be targets for ablation.51 Areas of late potentials or split potentials with isolated diastolic components are observed in some folding-loop isthmuses during sinus rhythm and pacing rhythm and can be targets for ablation 50,95,99,263. Potential channels within low-voltage scars were identified by setting color coding to expose relatively higher-voltage areas on either side of the low-voltage dense scar area. Ablation of these channels in a single-center study showed good results55,250.
The extent of nonexcitable scars can be determined and marked by high pacing thresholds (>10 mA, 2 ms unipolar pacing).53 In some cases, these nonexcitable regions can be designated as target channels for ablation.
Stroma-guided ablation is a reasonable strategy for ablation of ventricular tachycardia, but labeling cannot be performed in stable ventricular tachycardia. It can directly guide the ablation of scar-associated ventricular tachycardia. Whether it can be applied to inducible ventricular tachycardia when clinical ventricular tachycardia is ablated is not known. Different criteria for confirming exit and setting ablation injury have not been directly controlled.
3. Ablation endpoints
The ablation endpoint of scar-associated SMVT, the immediate success of the procedure is overwhelmingly assessed by programmed electrical stimulation, usually applying three extra-period stimuli in the right ventricle (occasionally in the left) to one or more stimuli. At the beginning of the study, the endpoint was subjective due to the deficiency of programmed stimulation on repetitive spontaneous ventricular tachycardia.