Spatial Orientation of the Reentrant Circuit of Idiopathic Left Ventriculap Tachycardia

Yoshifusa Aizawa, MD, Masaomi Chinushi, MD, Hitoshi Kitazawa, MD, Takashi Washizuka, MD, Kazuyoshi Takahashi, MD, Masami Shiba, MD,

Kohji Ohhira, MD, Akira Abe, MD, and Akira Shibata, MD

An ost sustained monomorphic ventricular tachycar- dias (VT) are believed to be due to reentry; the

ability to reset VT in the beat immediately following the fused beat1 or the ability to entrain VP has been con- sidered as evidence of a reentrant mechanism. Howev- er, ‘the major part of the reentrant pathway and, espe- cially, the entrance to the area of slow conduction has rarely been demonstrated in clinical electrophysiologic study in the laboratory. In left ventricular idiopathic VT,3,4 the area of slow conduction is believed to involve the left posterior Purkinje system, but the entrance has not yet been studied. We entrained this type of VT with rapid pacing from the apex and the outflow tract of the right ventricle, and the time interval between the stimu- lus and the entrained electrogram at the exit was com- pared between the 2 sites. The rationale of this study was based on the assumption that if such intervals dif- fer at 2 sites, it would be due to different distances be- tween the pacing site and the entrance to the reentrant circuit.

. . . Six consecutive patients (aged between 28 and 48

years, mean 31 + 11) with idiopathic VT underwent elec- trophysiologic study. All patients had VT with a right bundle branch block and superior axis pattern. In all patients, verapamil was tested and VT was shown to be verapamil responsive: VT was slowed in the rate and/or terminated.

Electrophysiologic study was performed in a drug- free state after obtaining informed consent while the patient was in the nonsedated and postabsorptive state.5,6 In brief, 2 to 3 quadripolar electrode catheters with inter- electrode distance of 5 mm (6Fr multipurpose catheters, USCI, Boston, Massachusetts) were placed against the high right atrium, across the tricuspid valve, and at the apex of the right ventricle. Another electrode catheter was placed at the site of the earliest activation during VT.

Bipolar intracavitary electrograms were filtered at 30 and 500 Hz and recorded on a strip chart at a paper speed of 100 or 200 mm/s (Mingograf 7, Siemens-Elma, Sol- na, Sweden). Surface leads I, II, and V, were recorded simultaneously with intracavitary electrograms.

Induction of VT was attempted using 1 to 3 extrastim- uli at 2 basic cycle lengths of 600 and 400 ms, and every induced VT was recorded on 1Zlead electrocardiogram and used for the analysis of VT morphology.

Stimulation to entrain VT was attempted using the pair of the distal and third electrodes, and recording was

From the First Department of Internal Medicine, Niigata University School of Medicine, Asahimachi, Niigata 951 Japan. Manuscript received January 2 1, 1995; revised manuscript received and accept- ed May 18, 1995.

316 THE AMERICAN JOURNAL OF CARDIOLOGY@ VOL. 76

done by the pair of the second and fourth electrodes of the same catheter.5,6 Rapid pacing was attempted from the apex and the outiow tract of the right ven$cle. In 1 patient, rapid pacing was performed at the outflow tract of the left ventricle that was on the opposite side of the pacing site of the outflow tract of the right ventricle.

The following criteria were used for transient entrain- ment: (1) constant fusion during rapid pacing, (2) pro- gressive fusion at different pacing rates, and (3) advance- ment of the local electrogram at the exit without change in the morpho1ogy.l Rapid pacing was continued for 5 to 10 seconds starting at a cycle length that was 10 to 20 ms shorter than the cycle length of VT

The exit from the area of slow conduction was defined as the earliest site of activation during VT. The electro- gram of the exit should be presystolic.

Conduction time through the area of slow conduction was measured as the time interval between the last stim- ulus artifact and the initial deflection of the entrained elec- trogram at the exit (ST-Exit interval). It was measured at the longest paced cycle and at the same cycle length at 2 pacing sites. Since the conduction time throtigh the area of slow conduction from the entrance to the exit must be common, a different ST-Exit interval would represent dif- ferent distances to the entrance of the reentrant circuit from each pacing site.

Conduction time from the exit to the pacing site (Exit- PS) was measured during VT as the time interval be- tween the initial deflections of the local electrogram at the exit and the pacing site.

When rapid pacing was performed in the left ventri- cle, the ST-Exit interval was determined as the interval from the stimulus to the onset of the QRS complex that was captured by the last stimulus minus the interval between the electrogram at the exit and the onset of the QRS complex of VT.

The ability to entrain VT was checked according to the criteria previously mentioned, and if VT was en- tiained, the ST-Exit and Exit-PS intervals were mea- sured at the longest paced cycle length and compared between the pacing sites.

Values are presented as mean + SD. The t test was used for comparisons and a p value co.05 was consid- ered significant.

In 6 patients, monomorphic VT was induced and the cycle length was 338 +: 12 ms. The exit was mapped in the left ventricle at the apicoposterior site of the inter- ventricular septum and it was presystolic: 29 + 6 ms be- fore the onset of the QRS complex of VT.

During rapid pacing at the apex of the right ventricle, constant fusion was confirmed in 3 patients, and at short- er cycle lengths, the QRS complex changed into the ful- ly paced morphology. In the other 3 patients, constant

AUGUST 1, 1995

fusion was not observed during rapid pacing, but the local electrogram was advanced by pacing without change in the morphology (Figure I). After the cessation of rapid pacing, VT resumed and the return cycle showed a fre- quency-dependent prolongation when rapid pacing was performed at pro- gressively higher rates and interrupted at 298 + 39 ms. During rapid pacing from the outflow tract of the right ven- tricle, constant fusion was demonstrat- ed in all patients in 21 paced cycle length, and the ST-Exit interval was constant when the surface electrocar- diogram showed constant fusion, which was established in the initial 4 to 6 beats of pacing. As rapid pacing was

TABLE I Conduction Times (ms) Through the Slow Pathway During Entrainment of Ventricular Tachycardia

Patient Number VTCL PCL RVA

ST-Exit

RVO IV RVA

Exit-PS

RVO LV

1 340 320 325 325 - 30 75 - 2 330 310 350 310 - 60 70 - 3 320 310 360 350 - 20 40 - 4 350 290 360 335 40 65 - 5 340 300 370 320 1 60 80 6 350 340 400 360 305* 40 50 ii

338 312 361 333 38 63 +12 +17 225+ *17 +18+ *15

*First measured to the onset of the QRS complex of ventricular tachycardio (340 ms) and correct- ed by the Exit QRS [interval 35 ms).

tp co.02 compared with value at outflow tract. Exit-PS = exit site to pacing site interval; LV = outflow tract of the left ventricle; PCL = paced cycle

length; RVA = right ventricular apex; RVO = right ventricular outflow; ST-Exit = stimulus to exit site interval; VTCL = ventricular tachycardia cycle length.

repeated at progressively shorter cycle lengths, VT was interrupted at 300 + 40 ms. In 1 patient, VT was en- trained from the outflow tract of the left interventricular septum and constant fusion was confirmed during pac- ing.

At the paced cycle length of 312 + 17 ms, the mean ST-Exit interval was 361 + 25 ms at the apex and 333 f 17 ms at the outflow tract of the right ventricle (Table I); the mean ST-Exit interval was significantly shorter at the outflow tract (p ~0.02). A typical recording is shown in Figure 2. The mean Exit-PS interval was shorter at the apex than at the outflow tract of the right ventricle: 38 + 18 versus 63 + 15 ms (p ~0.02). During rapid pac- ing from the left ventricle, the St-QRS interval was 340 ms, and the corrected ST-Exit interval was 305 ms. The Exit-PS interval was 50 ms and the return cycle at the

pacing site was 355 ms, which was identical to the cycle length of VT (Figure 2C).

. . . In reentrant VT, the exit from the area of slow con-

duction or in some cases, me critical reentrant path- way7,8 might be detected at laboratory, but the entrance to the reentrant circuit has not yet been studied. This type of VT treated in the present study is characterized by a QRS configuration and a responsiveness to verapa- mil, and can be entrained with rapid pacing.4,5 The mech- anism is therefore considered to be reentry with an excitable gap.

The Purkinje system is considered to be actively en- gaged in forming the reentrant circuit. The exit from the zone of slow conduction can be demonstrated in the left ventricle at the apicoposterior area of the interventricu-

II

Vl

RVO

HBE

RVA

1000 ms

FIGURE 1. Rapid pacing at the right ventricular a x p”

(RVA) was attempted to entrain ventricular tachycardia (cycle length [CL1 360 ms). The local electrogram at the exit in the eft ventricle (LV) was advanced without change in morphology, as shown by the asterisk (the preceding stimulus resulted in a fusion beat’). The last 2 beats shown on the right demonstrate total complexes. HBE = His-bundle electrogram recording site; RVO = right ventricular outflow tract; I, II, and V, = su r?

paced QRS ace electrocor-

diographic leads.

BRIEF REPORTS 317

lar septum, and catheter ablation at this site has been apex of the right ventricle. The time interval consisted highly effective in curing VT9 However, the details of of the conduction time between the pacing site and the the reentrant circuit are not yet known. entrance to the reentrant circuit, and that between the en-

In the present study, the conduction time was mea- trance and the exit. Because the conduction time through sured as the time interval between the stimulus and the the reentrant pathway is common, the shorter ST-Exit entrained electrogram at the exit. The ST-Exit interval interval at the outflow tract would imply that the pacing was significantly shorter at the outflow tract than at the site is closer to the entrance than that in the apex. On

1000 ms

B Pacing

‘---v---V 340 355

:,I

C Pacing

I ---JL’ j-M 340 355

340

RVA-+ A__nr a, 4, y II

HBE~?---I-+~- / *- 4. +

1000 Ills

FIGURE 2. A, Entrainment of ventricular tachycardia (VT) (c from tit

de length 355 ms) e right ventricular

apex (RVA). The configu- rations of the QRS com- plexes were very similar

~5u~~~~~~e~~ gram at the exit in the left ventricle (LV) showed a fusion (F) between VT and rapid pacing during sinus rhythm, as shown on the right. The stimulus to exit site interval (ST-Exit) was 390 ms and the exit site to pacing site interval (Exit-PS) was 35 ms. B, entrainment of VT from the right ventricular out- flow tract (RVO). During constant fusion, ST-Exit was 365 ms; Exit-PS was 50 ms. C, entrainment from the left ventricular outflow tract. Although slight, fusion was evident. The ST-Exit should be 305 ms (340 (stimulus to the onset of the entrained QRS complex) minus 35 ms (ExiCPS before QRS onset)). The Exit-PS was 50 ms. HBE = His-bundle electr other a “b

ram recording site; breviations as in

Figure 1.

318 THE AMERICAN JOURNAL OF CARDIOLOGY@ VOL. 76 AUGUST 1, 1995

the other hand, the conduction time from the exit to the pacing site-the Exit-PS interval-was longer at the out- flow tract than at the apex of the right ventricle.

In 1 patient, rapid pacing from the base of the inter- ventricular septum of the left ventricle resulted in con- stant fusion, and the postpacing return cycle was iden- tical to the cycle length of VT. These findings suggest that the rapid pacing was performed outside the area of slow conduction, but the finding that the return cycle was identical to the cycle length of VT would imply that the pacing site is at or very close to the entrance of the area of slow conduction. Stevenson et allo demonstrated such a possibility by computer simulation-that pacing at the entrance can result in a fused complex because the wave- fronts might propagate antidromic directions in addition to the orthodromic conduction. From these observations, the reentrant circuit of idiopathic VT can be described as in Figure 3: The entrance to the area of slow con- duction is localized at the base of the left interventricu- lar septum and the exit at the apicoposterior site of the left interventricular septum.

The small number of patients and the technical limi- tation of mapping would be limitations of the study, but the findings were uniform and the interpretation should not be altered when more data are accumulated on a larg- er number of patients. Because this particular VT is effec- tively and safely cured with catheter ablation, analysis of responses to each radiofrequency lesion would provide further clues for understanding the reentrant circuit.

In 6 patients with idiopathic left ventricular VT, the spatial orientation of the reentrant circuit was estimated from the results of transient entrainment of VT with rapid pacing at different sites. The en- trance to the area of slow conduction was located toward the outflow tract, whereas the exit was locat- ed at the apicoposterior area of the left interventric- ular septum.

1. Ahnendral JM, Stamato NJ, Rosenthal MF, Mxchlinski FE, Miller JM, Joseph- son ME. Resetting response patterns during sustained ventricular tachycardia: rela- tionship to the excitable gap. Circulation 1986;74:722-730. 2. Waldo AL, Henthorn RW, Plumb VJ, McLean WAH. Demonstration of the mechanism of transient entrainment and interruption of ventricular tachycardia with rapid atria1 pacing. .I Am Co11 Cardiol 1984;3:422-430. 3. Belhassen B, Shapira I, Pellege A, Laniado S. Idiopathic recurrent sustained yen- tricular tachycardia responsive to verapamil: an ECG-elecuophysiologic entity. Am

Tro Tro + Ts =

333+17 !,

..I

FIGURE 3. Scheme of the reentrant circuit of idiopathic left ven- tricular tachycardia. The stimulus to exit site interval was Ion cle 9

er at the apex than at the outflow tract of the right ventri- RV). Because the conduction time through the area of slow

conduction (Ts) is common to both pacing sites, the difference is due to the conduction time through the right ventricular apex (Tra) (solid circle), and the right ventricular oufflow tract (Tro) (open circ/ej. The exit to pacing site interval was shorter at the apex than that at the oufflow tract. The solid stcrr marks the entrance to the area of slow conduction; the open star marks the exit. Dotted lines represent various conduction pathways. LV = left ventricle.

Heart .I 1984;109:1034-1037. 4. Ohe T, Shimomura K, Aihara N, Kamakura S, Matsuhisa M, Sate I, Nakagawa H, Shimizu A. Idiopathic left ventricular tachycardia: clinical and elecuophysio- logic characterization. Circulation 1988;77:56&568. 5. Aizawa Y, Naitoh N, Kitazawa H, Shibata A. Frequency of presumed reentry witb an excitable gap in sustained ventricular tachycardia unassociated with coro- nary artery disease. Am J Cardiol 1993;72:9&921. 6. Aizawa Y, Niwano S, Cbinushi M, Kusano Y, Miyajima T, Shibata A. Incidence and mechanism of interruption of reentrant ventricular tachycardia with rapid ven- hicular pacing. Circulation 1992;85:589-595. 7. Stevenson WG, Weiss JW, Wiener I, Nadamanee K, Gelemter D, Y&man L, Klitzner T. Resetting of ventricular tachycxdia: implications for localizing the area of slow conduction. JAm Coil Cardiol 1988;11:522-529. 8. Fontaine G, Frank G, Tonet J. Identification of a zone of slow conduction for VT ablation: theoretical and practical consideration. PACE 1989; 12:262-267. 9. Washizuka T, Aizawa Y, Chinushi M, Naitoh N, Miyajima T, Uchiyama H, Takxhashi K, Shibata A. Alteration of QRS morphology and effects of radiofre- quency ablation in idiopathic ventricular tachycardia. PACE 1994:18:18-27. IO. Stevenson WG, Nademanee K, Weiss JN, Wiener I, Baron K, Yeatman LA, Sherman CT. Programmed electrical stimulation at potential ventricular reentry cir- cuit sites. Comparison of observations in humans with predictions from computer simulations. Circulation 1989;80:793-806.

BRIEF REPORTS 319