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CLINICAL STUDIES

Characteristics of electrograms recorded at reentry circuit sites and bystanders during ventricular tachycardia after myocardial infarction

Dusan Z. Kocovic, MD^*, Tomoo Harada, MD, Peter L. Friedman, MD, PhD and William G. Stevenson, MD

^* Cardiovascular Division, Department of Medicine, Hospital of the University of Pennsylvania, Philadelphia, Pennsylvania, USA
Division of Cardiology, Department of Medicine, Brigham and Women’s Hospital, Harvard Medical School, Boston, Massachusetts, USA

Manuscript received March 24, 1998; revised manuscript received March 8, 1999, accepted April 19, 1999.

Reprint requests and correspondence: Dr. Dusan Z. Kocovic, Hospital of the University of Pennsylvania, Cardiovascular Division, 9 Founders Pavilion, 3400 Spruce Street, Philadelphia, Pennsylvania 19104
kocovic{at}mail.med.upenn.edu

	Abstract

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OBJECTIVES

The purpose of this study was to determine the relation of isolated potentials (IPs) recorded during ventricular tachycardia (VT) to reentry circuit sites identified by entrainment.

BACKGROUND

Reentry circuits causing VT late after myocardial infarction are complex. Both IPs and entrainment have been useful for identifying successful ablation sites, but the relation of IPs to the location in the reentry circuit as determined by entrainment has not been completely defined.

METHODS

Data from catheter mapping of 70 monomorphic VTs in 36 patients with prior myocardial infarction were retrospectively analyzed. Entrainment followed by radiofrequency current (RF) ablation was performed at 384 sites. On the basis of entrainment, sites were classified as reentry circuit exit, central–proximal, inner or outer loop sites. Sites outside the circuit were divided into remote and adjacent bystanders.

RESULTS

Isolated potentials were recorded at 50% (51 of 101) of reentry circuit exit, central and proximal sites as compared with only 8% (11 of 146, p < 0.001) of inner loop and outer loop sites and only 1.8% (2 of 106) of remote bystander sites (p < 0.001). Isolated potentials were also present at 45% of adjacent bystander sites. At central and proximal sites the presence of an IP increased the incidence of tachycardia termination by RF to 47.5% from 24% (p = 0.05). At exit sites tachycardia termination occurred frequently regardless of the presence or absence of IPs (45% vs. 48%, p = NS). Isolated potentials at exit, central and proximal sites had a shorter duration at sites where ablation terminated VT than at sites without termination (20.9 ± 9.6 ms vs. 35.7 ± 15.3 ms, p < 0.001).

CONCLUSIONS

Isolated potentials are a useful guide to sites in the central–proximal region of the reentry circuit, but often fail to identify exit sites where ablation is successful. Entrainment and analysis of electrograms provide complementary information during mapping of VT.

Abbreviations and Acronyms   CL = cycle length   EG-QRS = electrogram to QRS interval   IP = isolated potential   PPI = postpacing interval   RF = radiofrequency current   VT = ventricular tachycardia

Isolated diastolic potentials have also been identified as markers of reentry circuit sites where catheter ablation is often, but not always, effective (5,9–11). These potentials are discrete, low amplitude signals that occur between QRS complexes. Their timing and separation from adjacent larger potentials suggest that they originate from narrow isthmuses in the reentry circuit. This origin is further supported by studies of de Bakker and coworkers in explanted hearts and Downar and Svenson and coworkers during intraoperative mapping (1–3,5,12). We hypothesized that because isolated potentials (IPs) may indicate a narrow portion of the reentry circuit, the amplitude and duration of these potentials may reflect the mass of tissue in the reentry circuit path at that point. Acute termination of tachycardia may be more likely at sites with lower amplitude, shorter duration IPs. The purpose of this study is to determine the relation of IPs and their characteristics to reentry circuit sites identified by entrainment techniques, and to acute VT termination by heating the site during RF catheter ablation.

	Methods

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Data from 36 consecutive patients (Table 1) undergoing endocardial mapping and RF ablation were reviewed retrospectively. All patients were referred for therapy of recurrent and inducible VT late after myocardial infarction (range 2 months to 20 years). For the purposes of this analysis we included data from sites where: 1) pacing from the mapping catheter entrained VT, 2) endocardial electrograms were of sufficient quality to allow analysis and 3) RF energy was applied to the site during VT to determine if heating the site would terminate VT.

In the initial 18 patients four to six surface electrocardiogram leads were recorded simultaneously with intracardiac electrograms at paper speeds of 100 mm/s for mapping (VR-16, PPG Medical Systems, Pleasantville, New York, and Bloom, Reading, Pennsylvania). A 1-mV calibration signal was recorded with intracardiac electrograms. In the subsequent 18 patients all data were digitally recorded along with a continuous 12-lead electrocardiogram (Quinton, Toronto, Canada and Prucka Engineering, Houston, Texas). Bipolar and unipolar intracardiac electrograms were recorded from the distal and proximal electrodes of the mapping catheter. Electrograms were filtered at 30 to 500 Hz in 20 patients or at 40 to 400 Hz in 18 patients as dictated by the recording system. Catheter position was assessed by fluoroscopy in two planes and in five patients also by transesophageal echocardiography (14).

Mapping and RF application.   Mapping focused on areas of ventricular akinesis or dyskinesis. If VT was not incessant, fractionated electrograms or late potentials were sought during sinus rhythm and pace-mapping was performed at these sites (13,15). Ventricular tachycardia was then initiated by programmed stimulation. Unipolar pacing from the distal electrode of the mapping catheter was used to entrain or reset the tachycardia (5). Stimuli had an amplitude of 5 to 10 mA and pulse width of 2 ms, which was increased to 9 ms if consistent capture was not achieved. On the basis of entrainment mapping, sites were classified as described previously (Fig. 1) (5,8).

View larger version (34K): [in this window] [in a new window]   Figure 1 (Top) A flow chart of the mapping scheme is shown. If pacing entrains tachycardia with concealed fusion the postpacing interval (PPI) or S-QRS interval is assessed to determine if the site is in the reentry circuit. If the site is not in the circuit it is classified as an adjacent bystander. If the site is in the circuit the S-QRS interval expressed as a percentage of the ventricular tachycardia cycle length (VTCL) is used to classify the site relative to the circuit exit. Sites with S-QRS <70% of the VTCL (exit, central and proximal sites) have the highest incidence of tachycardia termination and are referred to collectively as isthmus sites. If pacing entrains tachycardia with QRS fusion and the PPI is consistent with a reentry circuit site, the site is classified as an outer loop site. Sites where pacing entrains VT with QRS fusion and the PPI exceeds the VTCL are remote bystanders. EG-QRS = the interval from the electrogram recorded at the pacing site during VT to the QRS onset. See text for discussion. (Bottom) A theoretical reentry circuit is shown. The reentry wave fronts are indicated by the black arrows. The gray arrows indicate excitation wave fronts from the circuit that depolarize tissue that is not in the circuit (bystanders). The gray stippled areas represent inexcitable regions in the chronic infarct. The circuit contains two loops and a common pathway through which conduction is slowed. The common pathway is a relatively small mass of tissue in the chronic infarct, depolarization of which generates low amplitude signals that are not detectable in the standard body surface electrocardiogram. The QRS complex is inscribed after the excitation wave front leaves the common pathway at the exit, and begins propagating around the border of the scar through the outer loop. The excitation wave fronts then return back to the entrance of the common pathway. Several regions that are in the chronic infarct but do not participate in the circuit are labeled as bystanders.

Definitions.   Entrainment with QRS fusion: continuous resetting of tachycardia with constant QRS fusion (5,6).

Entrainment with concealed fusion: continuous resetting of tachycardia without a change in the QRS morphology (Fig. 2) (5,6).

View larger version (42K): [in this window] [in a new window]   Figure 2 Pacing at a proximal site in the reentry circuit where an isolated potential (IP) is present is shown. From the top of the figure are 10-ms time lines, 12 surface electrogram leads from I to V6 and intracardiac, bipolar recordings from the distal and proximal electrodes of the mapping catheter (A-BI-D and A-BI-P) and the right ventricular apex (RVA). Sustained monomorphic ventricular tachycardia is present with a cycle length (CL) of 490 ms. The last three stimuli (S) of a stimulus train at a CL of 450 ms are shown. Pacing accelerates the QRS complexes to the paced CL of 450 ms without altering their morphology as compared with that of the QRS complexes during tachycardia (entrainment with concealed fusion). The postpacing interval (PPI) is 510 ms. The S-QRS complex interval is 300 ms, which is less than 70% and more than 50% of the tachycardia CL and therefore consistent with proximal site. The electrogram recorded from the distal electrode pair of mapping catheter has an IP (arrowhead) with amplitude of 0.1 mV and duration of 10 ms. The main component of the electrogram (arrows) is also abnormal and is 90 ms in duration with an amplitude of 0.4 mV.

S-QRS interval: the interval from the stimulus to the onset of the following QRS complex during entrainment.

EG-QRS interval: during VT, the interval from the electrogram onset to the following QRS onset.

S-QRS–EG-QRS difference: the difference between the S-QRS interval during entrainment with concealed fusion and the EG-QRS interval during VT (5). A difference less than 20 ms is associated with a PPI–VTCL difference consistent with a reentry circuit site (5).

Isthmus sites: defined by entrainment as those where pacing entrains tachycardia with concealed fusion with a PPI or S-QRS interval indicating that the site is in the reentry circuit and a S-QRS interval of less than 70% of the tachycardia CL. These findings identify exit, central and proximal sites (Fig. 1), sites where acute termination of VT by ablation is most frequent (5,8).

Isolated potential: a discrete potential preceding the QRS onset and separated from other potentials by an isoelectric interval. At sites with IPs, a second larger electrogram was often inscribed, usually coincident with the QRS. This signal is designated as the major component of the electrogram (Fig. 2).

Statistical analysis.   Continuous data are expressed as mean ± 1 SD. Statistical analysis was performed with logistic regression implemented using the general estimating equation approach in the GENMOD procedure of SAS to adjust for possible correlation of sites within patients (SAS/STAT Software: Changes and Enhancement Through Release 6.12, 1997:247–348, SAS Institute, Cary, North Carolina) (13). A value of p < 0.05 was considered significant.

	Results

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A total of 384 sites evaluated during 70 VTs met inclusion criteria. Ventricular tachycardias had CLs ranging from 720 to 250 ms (mean 419 ± 89 ms). Isolated potentials were present at 78 of the 384 sites (20%) (Table 2, Fig. 2 and 3). Pacing entrained VT with concealed fusion at 171 sites during 52 VTs and entrained VT with QRS fusion at 213 sites in 70 VTs. By entrainment criteria 101 sites were isthmus sites (exit, central or proximal) (Fig. 2 and 4) in the reentry circuit, 146 were sites in the circuit outside the isthmus sites (inner and outer loop sites) and 137 sites were bystanders (Fig. 5). As shown in Figure 3, IPs were more frequent at isthmus sites (51 of 101, 50%) compared with other types of sites in the reentry circuit (11 of 146, 8%, p < 0.001) or bystanders (16 of 137, 12% p < 0.001). Isolated potentials were uncommon at remote bystanders (1.8%), but were relatively frequent at adjacent bystanders (45%, p < 0.001). Isolated potentials recorded from nonisthmus reentry circuit sites (outer and inner loops) had longer duration (43 ± 17 vs. 29 ± 15 ms, p = 0.004) and tended to have greater amplitude (0.51 ± 0.29 vs. 0.38 ± 0.29 mV, p = 0.21) than those recorded from reentry circuit sites (Table 2). The major component of the electrogram at isthmus sites was smaller than at nonisthmus reentry circuit sites (p < 0.001) and at bystander sites (p < 0.001) (Table 2).

View larger version (28K): [in this window] [in a new window]   Figure 3 (A) The incidence of isolated potentials (IPs) at the different types of reentry circuit sites is shown. (B) The incidence of VT termination during RF current application at sites with and without IPs is shown. Adj = adjacent; RF = radiofrequency current.

View larger version (40K): [in this window] [in a new window]   Figure 4 Pacing at an exit site where an isolated potential (IP) is present is shown. From the top of the figure are 10-ms time lines, 12 surface electrogram leads from I to V6 and intracardiac, bipolar recordings from the distal and proximal electrodes of the mapping catheter (M-DIS and M-PR) and the right ventricular apex (RVA). Sustained monomorphic ventricular tachycardia (VT) is present with a cycle length (CL) of 420 ms. The last three stimuli (S1) of a stimulus train at a CL of 400 ms are shown. Pacing accelerates the QRS complexes to the paced CL of 400 ms without alternating their morphology as compared with that of the QRS complexes during tachycardia (entrainment with concealed fusion). After the last stimulus, the next electrogram recorded at the pacing site is an IP, which occurs after 420 ms. The postpacing interval (PPI) therefore matches the VTCL consistent with a reentry circuit site. The S-QRS complex interval is 80 ms, which is less than 30% of the VTCL and therefore consistent with an exit site. The electrogram recorded from the distal electrode pair of the mapping catheter contains an IP with an amplitude of 0.2 mV and duration of 30 ms (arrowheads). The main component of the electrogram (arrows) is also abnormal with a duration of 160 ms and an amplitude of 0.3 mV. All measurements are in milliseconds.

View larger version (43K): [in this window] [in a new window]   Figure 5 Pacing at a remote bystander site where an isolated potential (IP) is present is shown. From the top of the figure are 10-ms time lines, surface electrocardiogram leads I to V6 and bipolar recordings from the distal (A-BI-D) and proximal (A-BI-P) electrode pair of the left ventricular mapping catheter and the right ventricular apex (RVA). Sustained monomorphic ventricular tachycardia with a cycle length (CL) of 320 ms is present. The last three stimuli (S1) of a stimulus train at the mapping site are shown. Pacing at a CL of 310 ms entrains tachycardia with QRS fusion. In the left ventricular recording the postpacing interval (PPI) is 380 ms and therefore exceeds the tachycardia CL by 60 ms, consistent with a bystander site. An IP is present marked with arrowheads and the main component of the electrogram is marked with arrows. The amplitude of the IP is 0.7 mV and duration is 40 ms. All measurements are in milliseconds.

For central, proximal and outer loop sites the presence of an IP markedly increased the likelihood that ablation would terminate VT. Ablation terminated tachycardia at 19 of 40 (47.5%) central and proximal sites with IPs compared with seven of 29 (24%) of central and proximal sites without IPs (p = 0.05). Radiofrequency current terminated tachycardia at five of 10 (50%) outer loop sites with IPs compared with three of 94 (3%) outer loop sites without IPs (p < 0.001). At exit sites, however, ablation frequently terminated tachycardia regardless of the presence or absence of IPs (45% vs. 48%, p = NS) (Fig. 3).

For isthmus sites IP duration was shorter where ablation acutely terminated tachycardia as compared with those isthmus sites without acute termination (Table 3) (p = 0.001); IP amplitude was not statistically different (p = 0.7).

	Discussion

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The present study further clarifies the relation of IPs to different types of reentry circuit sites identified by entrainment, and to tachycardia termination by RF ablation. Isolated potentials were associated with sites at the exit and proximal to the exit of the reentry circuit (Fig. 3). At exit sites the presence of an IP per se did not increase the likelihood that ablation would interrupt VT, possibly because a narrow portion of the circuit joins a larger mass of adjacent myocardium in these regions. Thus, a low amplitude signal from a narrow path may merge with and be obscured by the signal from the larger mass of adjacent myocardium. Alternatively, the exit region may be susceptible to ablation despite the absence of a narrow isthmus. At central and proximal sites, however, the presence of an IP increased the likelihood of VT termination by RF ablation.

The duration and amplitude of an IP probably depends on the mass of tissue, conduction velocity in the path, distance from the recording electrode pair and the angle of depolarization wave front relative to the recording electrodes. For isthmus sites (exit, central and proximal sites), IPs had a shorter duration and tended to have lower amplitude where ablation interrupted reentry. These IPs may originate from a smaller mass of tissue, which may therefore be a narrower path more susceptible to interruption.

Isolated potentials are infrequent at outer and inner loop sites. Outer loop sites may exist along the border of the infarct. Entrainment at these sites produces an excitation wave front that propagates away from the infarct, altering the sequence of ventricular activation distant from the scar. These sites may be in relatively broad regions of the reentry path, explaining the low incidence of tachycardia termination. A broad path is further supported by the greater amplitude of the major electrogram component at outer loop sites as compared with isthmus sites. Ablation is less effective at outer loop sites, but the presence of an IP at such sites increases the likelihood of success. One possible explanation is that outer loop sites with an IP are near a narrower portion of the circuit, from which the IP is recorded.

Limitations.   All studies of mapping and ablation in humans are subject to potential sampling biases because ablation is not performed at sites that are unlikely to be in the reentry circuit. Previous studies that selected ablation sites for the presence of IPs do not provide a good estimate of the efficacy of ablation at reentry circuit sites that do not have an IP (9–11). Studies that evaluated IPs only at sites where pacing entrained VT with concealed fusion (10) do not provide data on ablation at remote bystander sites and outer loop sites where IPs are sometimes recorded. By including, in this retrospective analysis, data from all sites where both pacing was performed for entrainment and then RF heating was applied, this study provides unique information on the relation of IPs to different types of reentry circuit sites. Data from the first 18 patients were obtained before the entrainment mapping criteria had been validated. Pacing was, however, performed at these sites allowing the relation of the site to the reentry circuit to be determined retrospectively. Although sites with normal electrograms were not assessed, sites with abnormal electrograms indicative of the infarct region were evaluated with entrainment and ablation. Thus a large number of sites that are now recognized as outer loops and bystander sites were included, as well as exit, central–proximal and inner loop sites. In our later patients mapping specifically sought exit and central–proximal sites. Ablation was performed at outer loop and adjacent bystander sites only if more desirable target sites were not identified. Although selection of sites for ablation was not arbitrary, this series likely contains the only reasonably large number of outer loop and bystander sites that will be studied in this manner now that these sites can be recognized by entrainment.

The limitations of entrainment mapping have been extensively discussed (5,8). Analysis of entrainment, the PPI and S-QRS interval assumes that pacing does not alter conduction velocities and conduction paths in the reentry circuit (5). The slowest stimulus trains and the most recently captured stimuli were used to avoid altering the reentry circuit during entrainment. Analysis of the PPI and electrogram–QRS interval assumes that local depolarization can be inferred from the electrogram timing, which is of limited accuracy in regions where the signal is fractionated. Termination of tachycardia by RF ablation requires adequate tissue contact and energy delivery for heating. We included only sites where pacing stimuli were captured, suggesting that contact was likely to be adequate.

The majority of our patients (69%) were receiving amiodarone, which likely had some effect on the arrhythmia substrate as it often slows inducible VT. It seems unlikely, however, that amiodarone would affect the relationships of isolated potentials within reentry circuit.

Clinical implications.   Isolated potentials often originate from apparent isthmuses in tachycardia reentry circuits that are desirable targets for catheter ablation. Isolated potentials should not be the sole mapping criteria to select sites for ablation because they can also originate from bystander regions and are absent at many reentry circuit sites, particularly exit sites, where ablation is successful.

	Acknowledgments

 
The authors wish to express their appreciation to John Orov, PhD for his help with statistical analysis, and Mrs. Susan Henry and Ms. Carol Stuart for their secretarial help.

	References

Top Abstract Methods Results Discussion References

 

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This Article Abstract Figures Only Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Kocovic, D. Z. Articles by Stevenson, W. G. Search for Related Content PubMed PubMed Citation Articles by Kocovic, D. Z. Articles by Stevenson, W. G.