(Circulation. 1997;96:2430-2437.)
© 1997 American Heart Association, Inc.
Articles |
Transcutaneous Multielectrode Basket Catheter for Endocardial Mapping and Ablation of Ventricular Tachycardia in the Pig
From Neufeld Cardiac Research Institute, Tel Aviv University (M.E., J.J.G., Z.R.), and the Koret School of Veterinary Medicine, Hebrew University, Jerusalem (D.G.O.), Israel, and the Division of Cardiology, Jefferson University Medical College, Philadelphia, Pa (S.H., A.J.G.) and EP Technologies Inc, Sunnyvale, Calif (D.K.S.). J.J.G. is now at the Division of Cardiology, Department of Medicine, Northwestern University, Chicago, Ill.
Correspondence to Michael Eldar, MD, Neufeld Cardiac Research Institute, Sheba Medical Center, Tel Hashomer 52621, Israel. E-mail meldar{at}post.tau.ac.il
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Abstract |
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Background Endocardial mapping using standard electrode catheters is often technically limited in ventricular tachycardia and constitutes a major obstacle to successful ablation. We wished to examine the utility of a basket-shaped multielectrode mapping catheter (MMC) in the mapping and ablation of ventricular tachycardia.
Methods and Results This study of sustained monomorphic
ventricular tachycardia (SMVT) was conducted in
two phases in the postinfarction pig model. In the first phase, the
utility of the MMC in providing adequate localization of potential
ablation site(s) of SMVT by different techniques (presystolic
potentials, pace mapping, and concealed entrainment) was assessed in 21
pigs. In the second phase, ablation of induced SMVT was attempted in 10
pigs. Mapping of SMVT was performed after percutaneous
introduction of the MMC to the LV. Comprehensive mapping was performed
in 90 episodes of SMVT and required 2.0 to 25 seconds.
Diastolic potentials were recorded during 86 episodes;
good or identical pace maps (
9 of 12 paced surface ECG leads
identical to ventricular tachycardia surface
ECG leads) were obtained in 25 of 31 maps, and entrainment was achieved
during 28 of 42 SMVTs. In 10 pigs, 10 SMVTs were recorded at least
twice and were considered for radiofrequency ablation. An 8-mm tip
ablation catheter was advanced to potential ablation sites with a
specially designed "homing" device, requiring a median time of
120 seconds. In these 10 pigs, either identical pace map (11 of 12, 6
SMVTs) or concealed entrainment (4 SMVTs) guided the ablation
procedure. After ablation, 8 of 10 SMVTs were rendered noninducible,
while 2 pigs died during energy application of degeneration of SMVT to
ventricular fibrillation.
Conclusions The MMC allows rapid, comprehensive, and reliable endocardial mapping during SMVTs, which facilitates successful ablation in the porcine post–myocardial infarction model.
Key Words: tachycardia • endocardium • mapping • ablation
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Introduction |
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Radiofrequency ablation is one of the therapeutic options available to patients suffering from recurrent SMVT after healed myocardial infarction.1 2 3 However, successful ablative therapy depends on localization of the electrophysiological substrate responsible for the maintenance of the presumed reentrant tachycardia.4 5 To date, catheter ablation of SMVTs due to old myocardial infarction is often disappointing, with relatively low rates of definite cure of the arrhythmia.5 6 7 8 9 One of the major obstacles to successful ablation is the difficulty in obtaining comprehensive endocardial maps during the SMVT.10
Current mapping techniques usually use standard electrophysiological catheters with a limited number of electrodes, which are sequentially moved to different endocardial locations during induced VT. Not uncommonly, this is a technically difficult and time-consuming procedure. Moreover, the VTs induced in the electrophysiological laboratory are often either short-lived or hemodynamically unstable, thereby rendering comprehensive endocardial mapping difficult to achieve.3 11 12 Theoretically, the ability to rapidly obtain a comprehensive endocardial mapping during induced VT in the catheterization laboratory could considerably increase the number of patients eligible for ablation. For these reasons, simultaneous multielectrode data acquisition is highly desirable during the electrophysiological study of VT. Currently, high-density endocardial catheter-mapping techniques are still not commonly available to the clinician.
The aim of this study was to evaluate, in an animal model, the feasibility, speed, and performance of a new, transcutaneous basket-shaped mapping catheter carrying 64 electrodes for mapping and guiding radiofrequency ablation of SMVT.
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Methods |
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The experimental protocol conformed to the position of the American Heart Association on research animal use13 and was approved by the ethical committees of the Neufeld Cardiac Research Institute and Jefferson Medical College.
The study consisted of two phases. The aim of the first phase was to assess whether the basket catheter provides adequate information to identify potential ablation site(s). Criteria considered as guides for ablation were presystolic and diastolic potentials, pace mapping, and concealed entrainment (see below). In the second phase of the study, the ability to perform catheter ablation of VT with the information provided by the MMC and the above-mentioned techniques was examined.
The mapping data from both phases of the study have been combined to provide an assessment of the ability to localize VT using the MMC.
VT Model Preparation
Seventy-eight normal Landrace female pigs, weighing 29 to 72 kg
(39±11 kg), were used in this study. After premedication with 0.1
mg/kg droperidol and 2 mg/kg pethidine HCl IM, diazepam
0.5 mg/kg IV, followed by thiopental 10 to 12 mg/kg IV,
was administered to induce general anesthesia. Myocardial
infarction was then induced under halothane-maintained
anesthesia (1.5% to 2.5% in oxygen) by the closed-chest
technique previously described.14 In short, by the
percutaneous Seldinger technique, an 8F left
coronary Amplatz catheter was advanced to engage the left main
coronary artery ostium. Through this catheter, a 2.5- to 3.5-mm
balloon angioplasty catheter was placed in the proximal or medial part
of the left anterior descending coronary artery. After
inflation of the balloon, 100 µL of 75- to 150-µm agarose gel beads
was injected through the lumen of the angioplasty catheter into the
coronary artery. The balloon was deflated 30 seconds later, and
the animal was allowed up to 3 hours of recovery.
Three to 6 weeks later, the surviving animals were anesthetized again, and SMVT was induced by apical right ventricular endocardial pacing (Medtronic 5328), with programmed stimulation at a basic cycle length of 350 ms with one to four extrastimuli or overdrive pacing at rates of 240 to 360 bpm, as previously described.14 When a rapid SMVT (>300 bpm) was induced, either amiodarone 3 to 12 mg/kg or procainamide 15 to 20 mg/kg IV was administered to slow the VT, and ventricular stimulation was repeated. Sustained monomorphic VT was defined as monomorphic VT lasting >30 seconds.
MMC Deployment
Before MMC deployment, heparin sodium was administered at a
loading dose of 200 IU/kg IV, followed by a maintenance dose to
maintain an activated clotting time >300 seconds. Procedure
time averaged 4.0±2.7 hours (range, 1 to 9 hours).
An MMC (Constellation catheter, EP Technologies) carrying 64 electrodes mounted on eight flexible nitinol splines (eight ring electrodes per spline) was used. Each spline is identified by a letter (A to H) and each electrode by a number, 1 being the most distal and 8 the most proximal on the spline. The electrodes are equally spaced at 3, 4, or 5 mm apart in three different available MMC sizes, having a deployed diameter of 38, 48, or 60 mm, respectively. The MMC was chosen from the three available sizes according to prior measurements of left (or right) ventricular dimensions by transthoracic echocardiography. A 9F guiding sheath was introduced into a femoral artery, placed retrogradely through the aortic valve, and the MMC was delivered inside it and deployed in the LV. The MMC was placed in the right ventricle via a femoral vein.
Signals from each of the bipolar electrode pairs of the MMC were displayed (EP Lab, Quinton Electrophysiology Corp, or BARD LabSystem, Bard Electrophysiology), amplified and filtered (30 to 250 Hz), digitized at 1 kHz, and recorded directly onto an optical disk. The EP Lab recording system permitted simultaneous acquisition of 16 bipolar electrograms along with a surface 12-lead ECG. Therefore, acquisition of all 32 bipolar endocardial electrograms during VT required two sequential recordings from each half of the basket (four splines) at a time. The LabSystem allowed simultaneous acquisition of all 32 bipolar electrograms and the 12-lead ECG.
Mapping During Sinus Rhythm
1. During normal sinus rhythm, unipolar pacing was performed at
3 mA and 10 mA from each of the 64 endocardial electrodes of the MMC to
confirm electrode-tissue contact. A switch box (EP Technologies)
allowed rapid unipolar pacing of consecutive electrodes.
2. Bipolar pace mapping was performed during sinus rhythm from the
electrode pairs recording the earliest middiastolic
or presystolic potentials or continuous diastolic
activity during VT (see below). Pacing was performed at a rate similar
to the VT rate. In several animals, pace mapping was also performed
from adjacent bipolar pair electrodes. These pacing sites were selected
because they were in close proximity to the bipole recording
the earliest electrogram on the same or adjacent splines. Pacing was
performed at a rate identical to that of the SMVT from the
"earliest" site. Pacing started at 3 mA and was increased up to
20 mA to allow ventricular capture. "Good" pace maps
were defined as showing surface ECGs very similar to those during VT in
9 to 10 leads, and "identical" pace maps as being similar in 11
to 12 leads. Pace maps similar in 8 or fewer leads were defined as
"poor." In addition, the distance from the pacing artifact to the
onset of the paced QRS complex was compared with the
presystolic potential–to–surface QRS interval. A prolonged
stimulus-to-QRS interval was defined as being 40 ms.
Mapping During Induced VT
1. For identification of presystolic,
middiastolic, or continuous diastolic
potentials, a presystolic potential was defined as any part of
a bipolar electrogram that preceded the earliest Q wave on the surface
ECG; a middiastolic potential was defined as an isolated
diastolic potential that was separated from the adjacent
ventricular electrograms; and continuous
diastolic electrical activity was defined as activity that
bridged the interval between two adjacent ventricular
electrograms.
2. For transient entrainment, pacing (3 to 20 mA) from bipolar electrode pairs and/or from the ablation catheter tip recording presystolic, middiastolic, or continuous diastolic activity was performed during VT at cycle lengths 7 to 60 ms shorter than the VT cycle length. Transient entrainment was defined according to standard criteria and was further classified as either manifest or concealed entrainment.5 15 16 17 The stimulus-to-QRS interval during concealed entrainment was measured from the last pacing artifact to the next beat.
Ablation Procedure
Many animals had multiple VT morphologies induced. To adequately
assess the effects of the ablation procedure, a VT was targeted for
ablation if it was induced or occurred spontaneously at least
twice.
Potential ablation sites were identified by demonstrating early, middiastolic, or continuous diastolic activation plus either identical pace maps with long stimulus-to-QRS interval and/or demonstration of concealed entrainment, achieved by pacing from the tip of the ablation catheter.
By the Seldinger technique, an 8F, 8-mm thermistor-embedded deflectable-tip ablation catheter (Blazer T, EP Technologies) was inserted transaortically into the LV. The ablation catheter was directed to the desired target with a homing device. The homing device includes a display of 64 miniature light-emitting diodes arranged in an array similar to the arrangement of the 64 MMC electrodes. A low-level alternate current is transmitted from the ablation electrode, and the resultant potentials are sensed at each MMC electrode. On the basis of the sensed voltages at each of the MMC electrodes, the homing device determines whether the ablation electrode is in the near proximity of one (or more) MMC electrode and lights the corresponding diode(s). The ablation catheter is thus moved inside the ventricle under fluoroscopic guidance and directed toward the target with the aid of the homing device.
The power source used for catheter ablation was a commercially available generator with temperature feedback capabilities (EP Technologies) that delivered continuous unmodulated radiofrequency current at a frequency of 500 kHz. Radiofrequency energy was delivered between the catheter tip and a skin patch for a maximal duration of 120 seconds per application to achieve a tip-tissue interface temperature of 70°C to 80°C.
After each RF energy application, induction of SMVT was attempted again by the induction protocol previously described. In some pigs, when SMVT was noninducible, isoproterenol was administered intravenously, with the end point being a 20% increase in the basal sinus rate, and the induction protocol was repeated. Successful ablation was defined as the inability to reinduce the same SMVT after ablation. If SMVT was still inducible, the above procedure was repeated until noninducibility was achieved.
At the end of the experiment, the pigs were killed under general anesthesia by injection of 20 mL KCl 15% IV, and their hearts were subjected to macroscopic examination.
Statistical Analysis
Values are given as a range and mean±SD or as median and
interquartile (25% to 75%) range. The statistical tests include (1)
the two-tailed (two-sample, unequal variance) Student's t
test and (2) the 
2 test. Significance was
prospectively defined as P<.05.
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Results |
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Of the 78 pigs, 46 survived the induction of MI and were included in the study. In phase 1, myocardial infarction was induced in 45 pigs. Of these, 18 died. Sustained monomorphic VT (n=72 episodes) was induced in 21 of the 27 surviving pigs. In an additional 2 pigs, only nonsustained episodes of VT (n=6) could be induced. During phase 2, myocardial infarction was induced in 33 pigs, of which 14 died. Of the 19 surviving pigs, VT (n=25 episodes) was induced in 17. Ablation of VT was attempted in 10 of the pigs (see below). Sustained monomorphic VT of a single morphology was not reproducibly induced in the other 7 pigs. Unipolar pacing at 3 mA resulted in capture of 42±11 electrodes (66±17%), and at 10 mA, capture occurred in 56±4 electrodes (88±7%).
Mapping of SMVT
A total of 97 episodes of SMVT were induced, 90 of which were
successfully mapped. Seven episodes were not completely mapped because
of technical limitations of the recording system (see "Study
Limitations" section). The mapped SMVT episodes had a mean cycle
length of 221±61 ms (range, 130 to 370 ms). Episodes induced after
administration of antiarrhythmic drugs had a significantly longer mean
cycle length (250±74 ms, n=46) than those induced without drugs
(196±56 ms, n=51, P=.04). Of the SMVT episodes, 59% had a
left bundle-branch block morphology in lead V1 and 41% a
right bundle-branch block morphology.
Endocardial mapping of all 32 bipolar electrode pairs during episodes of SMVT performed by two sequential recordings of 16 bipolar pairs (EP Lab) required between 2.5 and 25.0 seconds (8.2±5.5 seconds) to complete, while simultaneous mapping of all 32 bipoles (LabSystem) was completed within 2 seconds. The basket maintained a stable intracardiac position, generally providing good-quality electrograms.18
Presystolic or middiastolic activity was detected during 86 episodes of SMVT by at least 1 electrode pair. The time interval between the earliest diastolic potential and the surface QRS onset ranged between 5 and 97 ms (33±22 ms). Continuous diastolic activity during SMVT was detected during 13 episodes of SMVT. In most SMVTs in which middiastolic potentials were recorded, presystolic or diastolic potentials were also identified in electrograms from several adjacent bipolar pairs.
Fifty-one pace maps were obtained and compared with the respective episodes of SMVT. Pace maps obtained by pacing from 31 sites manifesting the earliest diastolic potentials were good or identical in 25 (81%) and poor in 6 (19%). Conversely, of 16 pace maps from other than earliest sites, only 5 (31%) were either good or identical, whereas 11 (69%) were poor (P=.0001).
Pacing during SMVT to achieve transient entrainment was attempted during 42 SMVTs. In 6 episodes, capture was not obtained. Another 28 episodes showed manifest entrainment with surface QRS fusion. During 8 other episodes, concealed entrainment was demonstrated
Ablation of SMVTs
Identification of Ablation Site
(Table
)
Ten morphologies of SMVT in 10 pigs were induced at least twice
and were thus considered for ablation. Their cycle lengths ranged from
200 to 280 ms (262±17 ms); 3 had left and 7 right bundle-branch block
morphology. Presystolic and/or middiastolic
potentials were recorded in all SMVTs, with an earliest
endocardial-to-QRS interval of 18 to 98 ms. Pace mapping was performed
in 6 pigs. Identical pace maps were obtained from the bipolar electrode
pair manifesting the earliest diastolic activity (n=3)
and/or from the ablation catheter tip directed toward and touching one
electrode of the same electrode pair (n=5). The pacing
stimulus–to–surface QRS interval of these pace maps ranged between 28
and 65 ms and was very similar to the diastolic
electrogram–to–surface QRS interval from the same electrodes during
the respective SMVT in 4 of the 6 animals (pigs 1, 2, 8, and 9)
(Table). Ventricular pacing from the ablation catheter tip
was performed during 5 episodes of SMVT and produced concealed
entrainment in 4. The diagnosis of entrainment was based on an
identical morphology of the paced QRS complexes relative to those of
the respective SMVT in all 12 surface leads and entrainment of the last
paced beat to the pacing rate followed by subsequent return of the
original SMVT. The stimulus-to-QRS interval during concealed
entrainment ranged between 28 and 98 ms and was similar to the
endocardial diastolic-to-QRS interval during SMVT in 3 of 4
pigs (pigs 2, 3, and 5) (Table).
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Ablation Procedures
Steering of the ablation catheter to the target site
(measured in 4 animals) required between 15 and 660 seconds (median,
120 seconds; interquartile range, 60 to 240 seconds), and fluoroscopic
time needed to achieve this was 17 to 540 seconds (median, 84 seconds;
interquartile range, 58 to 150 seconds). After identification of the
ablation site (see above), 24 applications of radiofrequency energy
were performed in 10 pigs. In each pig, only 1 morphology of SMVT was
targeted except for 1 pig (pig 7), which died during mapping of a
second SMVT.
In pig 1, SMVT with a left bundle-branch block morphology and a cycle
length of 240 ms occurred both spontaneously during sinus rhythm and
after rapid ventricular pacing (Fig 1A). No early diastolic
potentials were detected by the 32 bipolar MMC recordings in
the LV (Fig 1B). The earliest electrogram was recorded at F3-4, and
radiofrequency energy was delivered once at electrode F4. However, the
SMVT recurred spontaneously. An MMC catheter was then deployed in the
right ventricle. Middiastolic potentials were clearly
recorded during SMVT from bipoles C7-8 and D5-6 and a
presystolic potential from bipole D1-2 (Fig 1C). A pace map
from D5-6 was identical to the induced SMVT (Fig 1D), whereas pace maps
from the other two bipolar pairs were both poor. The
tachycardia could not be entrained, despite pacing with
good ventricular capture. An ablation catheter was then
inserted to the right ventricle and directed toward D5. An identical
pace map was obtained at this location. After radiofrequency energy
delivery adjacent to D5 (n=3) during sinus rhythm, the SMVT did not
recur, either spontaneously or after programmed stimulation, with or
without isoproterenol. Examination of the heart revealed confluent
ablation lesions on opposite sides of the interventricular
septum: left side, 0.9x0.6 cm; right side, 0.6x0.4 cm.
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In pig 2, 1 of 5 different morphologies of SMVT was induced repeatedly,
with a right bundle-branch block morphology and a cycle length of 278
ms. Middiastolic potentials were recorded from the MMC
bipolar pair G1-2 (Fig 2A), and concealed
entrainment was achieved by pacing from this bipole (Fig 2B). After
radiofrequency energy application adjacent to G1 (n=2), the same SMVT
could not be induced. At autopsy, a 1.2x0.5 ablation lesion was found
at the LV apex. A small nonprotruding thrombus was found attached to
the basket catheter electrode G1.
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In pig 3, 1 of 4 different morphologies of SMVT was repeatedly induced,
with a right bundle-branch block morphology at a cycle length of 260
ms. The earliest middiastolic potentials were recorded
in MMC bipole C1-2 (Fig 3A). A pace map
from this bipole was identical (Fig 3B). An ablation catheter was
advanced to MMC electrode C2, and pacing during sinus rhythm at this
site produced a poor pace map. The SMVT was reinduced, being similar
(but not identical) to the previous one (Fig 3C). Pacing from the
ablation catheter tip adjacent to C2 during SMVT produced concealed
entrainment (Fig 3C), and after radiofrequency energy application
(n=2), this SMVT could not be induced. At autopsy, a 1.0x0.8-cm
ablation lesion was found at the inferior part of the LV
aspect of the interventricular septum. A thrombus
(
1x2 mm) was found attached to the basket catheter electrode
C2.
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In pig 5, 1 of 3 SMVTs was induced twice. Endocardial mapping with the MMC revealed a middiastolic potential. Pacing from the bipole recording the diastolic potential during SMVT produced concealed entrainment. Delivery of radiofrequency energy at this location resulted in termination of the SMVT and subsequent inability to induce it.
In pig 7, 1 of 2 SMVTs was successfully ablated, and the pig died
during endocardial mapping of a second SMVT. In pigs 8, 9, and 10, pace
maps from sites of earliest systolic or
middiastolic electrograms were identical and served as the
guide to ablation. The 3 VTs were successfully terminated during the
first RF application in each and could not be reinduced later. Pigs 4
and 6 died during delivery of radiofrequency energy to ablation target
sites (Table
) due to acceleration of VT to ventricular
fibrillation and inability to resuscitate.
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Discussion |
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To the best of our knowledge, this is the first report of successful ablation of SMVT in a post–myocardial infarction pig model. This goal was achieved by the combined use of a multielectrode basket-shaped mapping catheter and a homing device to guide the ablation catheter to the target.
The main findings of this study include (1) the demonstration of comprehensive endocardial mapping of rapid SMVTs with a multielectrode basket-shaped catheter; (2) the feasibility of identification of potential ablation sites by the combined use of diastolic potential activation maps, pace maps, and/or concealed entrainment; (3) rapid intraventricular guidance of an ablation catheter with the aid of a special homing device toward the MMC electrode identified as a potential ablation site; and (4) successful radiofrequency catheter ablation of SMVT in the post–myocardial infarction pig model.
We have previously shown the feasibility of recording activation maps during SMVT in the pig model by use of the MMC.18 In the present study, we used MMCs of different sizes to best match ventricular dimensions. In addition, we redeployed the basket catheter in the LV if capture at 10 mA was obtained in <48 electrodes or if presystolic activity was not recorded by any of the bipoles. Finally, we obtained right ventricular recordings, if necessary. This may explain the higher rates of presystolic electrograms identified in this study compared with the previous one (89% versus 58%).18
In one case (pig 1), no presystolic or diastolic
potentials were recorded by the MMC in the LV during SMVT. However,
middiastolic potentials were subsequently identified (Fig 1) after deployment of the MMC in the right ventricle. Because most VTs
occurring after myocardial infarction arise from the LV, it seems that
the MMC could be useful to rapidly rule out an LV origin of SMVT in
selected patients. Importantly, comprehensive mapping was quickly
completed. One recording system (EP Lab) imposed the
performance of sequential recording of 16 bipolar
electrograms at a time, which considerably prolonged the
recording time. Despite this limitation, a comprehensive map
during SMVT required <25 seconds to complete, including the time spent
on manually disconnecting and reconnecting the recording cable
and the recovery of the electrograms after reconnection. However,
simultaneous recording of all 32 bipolar
electrograms allowed comprehensive endocardial mapping within 2
seconds. This activation mapping time is considerably shorter than
projected time with a conventional mapping catheter carrying 2 to 8
electrodes and was independent of the SMVT morphology.
One of the problems encountered during standard roving catheter mapping of SMVT is the distinction of true diastolic potentials from artifacts.19 Mapping with the MMC often allows detection of the progression of the diastolic potentials through adjacent electrodes, suggesting that they reflect true activation rather than artifacts.
Conventional methods (including detection of diastolic potentials, activation maps, pace maps, and transient entrainment) were used in this study to identify potential ablation sites.20 The MMC proved to be extremely efficient in rapidly recording diastolic potentials and supplying endocardial activation times during SMVT. Pace maps from electrode pairs showing the earliest diastolic potentials were usually either good or identical. Pace maps from adjacent electrode pairs were always worse. Thus, in this model of SMVT, pace maps proved to be accurate and specific in identification of arrhythmia origin. In humans, pace maps are particularly useful for locating arrhythmia foci in structurally normal hearts21 22 but much less useful in guiding ablation of post–myocardial infarction SMVT.9 23 24 25 The reason for the apparently better performance of pace maps in this pig model of SMVT is not clear. It could, however, indicate a less complex anatomy of the infarcted area in this model, so that pacing from or proximal to the presumed endocardial exit point encounters fewer obstacles to conduction and produces ventricular activation similar to that during SMVT. In addition, the Purkinje fiber network is more extensive and penetrates deeper from the endocardium to the epicardium in the pig than in primates.26 27 This may contribute to rapid intramyocardial conduction that may provide for a more homogeneous ventricular depolarization during pacing and SMVT.27 28 Finally, pace maps from adjacent sites were not performed systematically. It is conceivable that good or identical maps could also have been obtained from additional sites. A long stimulus-to-QRS interval during pace mapping is consistent with slow conduction away from the pacing site and seems to be useful in targeting potential endocardial ablation sites.23 29 Similarity in duration between endocardial diastolic potential–to-QRS interval during SMVT and pacing stimulus–to-QRS interval during identical pace maps suggests that the diastolic potentials are recorded from an area of slow conduction that is an integral part of the reentrant circuit.5 In fact, this combination was found in 4 of the SMVTs successfully ablated in our study (pigs 1, 2, 8, and 9). In the other pigs, the stimulus-to-QRS interval during pace map was either shorter (pigs 3 and 10) or considerably longer (pig 4) than the diastolic potential–to-QRS interval during the SMVT at the same sites. The reason for the difference is not clear but could be partly related to the differences in the pacing and SMVT cycle lengths or represent different tissue conduction characteristics during SMVT and pacing. In addition, even a small distance between the recording MMC bipoles and the pacing catheter tip could result in different conduction characteristics during SMVT and pacing.
Concealed entrainment was obtained in 4 SMVTs in our study and was associated with successful ablation in all. This is consistent with data from human SMVT, in which concealed entrainment is believed to represent pacing from within the area of slow conduction.5 29 30 However, we did not systematically record the endocardial postpacing interval, which may indicate whether pacing was performed from an area of slow conduction or from a bystander region.5
The use of a homing device made maneuvering of the ablation catheter to the target relatively easy and rapid. The procedure in the animal laboratory was performed with a fluoroscopy system limited to imaging in the posteroanterior and right anterior oblique positions. This limited the ability of fluoroscopic guidance of the ablation catheter. Under these conditions, the homing device was felt to be essential for the correct localization of the ablation catheter. The role of the homing device in the human catheterization laboratory still remains to be defined.
Ablation was judged successful for 8 morphologies of SMVT in 8 pigs by the inability to reinduce VT after RF application, and 2 animals died during energy application. In addition, in 5 pigs (pigs 5, 7, 8, 9, and 10), VT terminated during delivery of radiofrequency energy, which supports the accuracy of the identification of the ablation target. Interestingly, in one case (pig 1), ablation of the left side of the interventricular septum was not effective, and right-sided septal ablation was required. This could indicate a right-sided exit point of the SMVT or a deep intramyocardial origin requiring deeper penetration of the radiofrequency lesion.
Study Limitations
The study has several limitations. The MMC is capable of
recording 32 bipolar electrograms. It is conceivable that more
accurate activation maps would be obtained by a larger number of
electrodes. However, in this model the electrode density seemed to
yield enough data to guide the ablation procedure.
Seven episodes of SMVT were not fully mapped because the EP Lab system allowed the simultaneous recording of only 16 bipolar electrograms and required disconnection and reconnection of an extension cable. This limitation was not observed in any SMVT recorded by the Lab System, which allows simultaneous recording of all 32 bipolar electrograms.
The ability to localize a potential ablation site for all 10 SMVTs is encouraging. We believe that this reflects the short mapping time by the MMC combined with the relatively comprehensive and accurate mapping it provides. No attempt was made to perform endocardial mapping of SMVT with a standard mapping catheter, precluding the conclusion that the methods used in this study are more efficient than conventional ones for the mapping and ablation of SMVT. However, it is generally accepted that mapping time for SMVT with a standard mapping catheter is at least an order of magnitude longer than that required in the present study.
Electrode-tissue contact, assessed by the ability to capture underlying myocardium, was achieved for only 66% to 88% of the MMC electrodes. This could limit the accuracy of the recorded electrograms.
Success of the ablation procedure was defined as the inability to reinduce the same morphology of the SMVT and was achieved in 8 pigs. However, the inability to reinduce the ablated form of SMVT could have been due to chance, to postablation predominance of another form of SMVT, or to other unidentified causes. Furthermore, the pigs were killed at the end of the acute experiment, and thus long-term success could not be tested.
The pig heart is smaller than the human heart. Moreover, no significant aneurysm developed in this model. Dilated human hearts, particularly if aneurysmal, might be less amenable to comprehensive mapping with the basket catheter.
One investigator found thrombus on the MMC electrode that was in contact with the ablation catheter tip during energy delivery in 2 pigs. This investigator ablated at a higher set temperatures (80°C) directly over an MMC electrode. The other investigator ablated with lower set temperatures (70°C) and did not require contact with the MMC electrode. On the basis of these experiences, we recommend that when ablation is done with the MMC in place, direct contact with an MMC electrode should be minimized and ablation set temperatures should be <80°C.
Conclusions
The MMC allows rapid, comprehensive, and apparently accurate
endocardial mapping during SMVT. The use of the homing device assists
in the direction of an ablation catheter to the presumed target, and
the combination of the MMC and homing device seems to facilitate the
achievement of successful ablation of SMVT in the post–myocardial
infarction pig model. Exploration of the clinical utility of the MMC
for SMVT mapping and ablation in humans seems warranted.
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Selected Abbreviations and Acronyms |
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Acknowledgments |
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This study was supported in part by EP Technologies, Sunnyvale, Calif. Dr Goldberger was supported by the James Loughman Travelling Fellowship of the North American Society of Pacing and Electrophysiology. The excellent secretarial assistance of Elaine Finkelstein is highly appreciated.
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Footnotes |
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The first two authors contributed equally to this article.
Received January 22, 1997; revision received May 23, 1997; accepted May 28, 1997.
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References |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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