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Limits of mitral valve apparatus distensibility: Observations from balloon mitral valvotomy in a canine model
Volume Number
114 6
Brief
REFERENCES
1. Kloster 2.
3.
4. 5.
6.
7.
F. Diagnosis and management of complications of prosthetic heart valves. Am J Cardiol 1975;35:872. Kotler MN, Mintz GS, Panidis I, Morganroth J, et al. Noninvasive evaluation of normal and abnormal prosthetic valve function. J Am Coll Cardiol 1983;2:151. Wilkins GT, Gillam LD, Kritzer GL, Levine RA, et al. Validation of continuous-wave Doppler echocardiographic measurements of mitral and tricuspid prosthetic valve gradients: a simultaneous Doppler-catheter study. Circulation 1986;74:786. Hatle L, Angelsen B. Doppler ultrasound in cardiology. Philadelnhia: Lea & Febieer. 1985:97. Weinstein IR, Marbarge; JP, Perez JE. Ultrasonic assessment of the St. Jude’s prosthetic valve: M mode, twodimensional, and Doppler echocardiography. Circulation 1983;68:897. Holen J, Simonsen S, Froysaker T. An ultrasound Doppler technique for the noninvasive determination of the pressure gradient in the Bjork-Shiley mitral valve. Circulation 1979;59:436. Kurzrok S, Singh AK, Most AS, Williams DO. Thrombolytic therapy for prosthetic cardiac valve thrombosis. J Am Co11 Cardiol 1987;9:592.
Limits of mitral valve apparatus distensibility: observations from balloon mitral valvdtomy in a canln6 model Donald E. Rediker, M.D., J. Luis Guerrero, David S. Block, James F. Southern, M.D., John T. Fallon, M.D., Ph.D., and Peter C. Block, M.D. Boston, Mass.
Percutaneous balloon mitral valvotomy (PMV) is being used increasingly as palliative treatment for rheumatic mitral stenosis (MS).le6 Recent reports5B6 suggest t,hat a larger mitral valve area is achieved with a double balloon technique. Prior to clinical trials that used the double balloon technique, we investigated the limits of mitral valve apparatus distensibility in a dog model. Nine mongrel dogs weighing 17 to 27 kg were anesthetized with chloralose (60 mg/kg, intravenously) and urethane (600 mg/kg, intravenously). Tracheal intubation was performed and mechanical ventilation was instituted. A left thoracotomy was performed, and the heart was suspended in a pericardial cradle. The first four dogs were placed on standard cardiopulmonary bypass. Pressure measurements were determined with standard fluid-filled catheters inserted into the left atrium (LA) through a pulmonary vein, and into the left ventricle through the apex. An incision was made in the LA appendage, and in the first four dogs a breakable purse-string suture was From the Department of Medicine Hospital, Harvard Medical School. Reprint General
requests: Hospital,
Peter Boston,
(Cardiac
C. Block, M.D., MA 02114.
Unit), Cardiac
Massachusetts Unit,
General
Massachusetts
Communications
I5I3
placed through the mitral valve leaflets, creating a gradient across the mitral valve. Balloon valvotomy was then performed through the LA appendage, at an inflation pressure of 4 atm, measured with a standard balloon pressure gauge (USCI, Billerica, Mass.). The initial two mitral valves were dilated with 15 mm and 25 mm diameter balloons positioned side by side, while the other seven mitral valves were dilated with two adjacent 25 mm diameter balloons. Pathologic observations were made by observers blinded both to the procedure performed and to the hemodynamic results (Table I). Five dogs had a mitral annular tear (Fig. l), which correlated with a smaller annular circumference (6.5 + 0.7 vs 7.8 t- 0.7 cm, p = 0.02) than those with an intact anulus. With the application of standard geometric and trigonometric formulas, the effective circumference of adjacent 15 and 25 mm balloons is 10.4 cm, and that of two 25 mm balloons is 12.9 cm. Therefore the total balloon circumference exceeded that of the mitral anulus by 53 % to 122 % . This ratio of balloon-to-annular circumference correlated with annular damage (1.91 rt_ 0.26 damaged vs 1.57 +- 0.04 intact, p = 0.04). The only dog (dog No. 1) without damage to the mitral apparatus was the only dog with a ratio < 1.55. There was a trend toward lower weight dogs having more annular damage, but this was not significant (21.0 i 3.4 vs 24.8 + 1.7 kg, p = 0.07). Three dogs had torn mitral leaflets (Fig. l), but valve leaflet damage did not correlate with balloon size or the balloon-to-annular circumference ratio. Five dogs had ruptured chordae, and two had ruptured papillary muscles. Subvalvular (chordal or papillary muscle) injury did correlate with balloon circumference (12.9 f 0.0 damaged vs 11.2 + 1.4 cm intact, p = 0.02). When chordal rupture occurred, it was always on the posterior aspect of the anterior mitral leaflet. Of the five dogs with no mitral regurgitation (MR), two had annular tears, one had a torn leaflet, three had ruptured bhordae, and one had a ruptured papillary muscle. In the four dogs with surgically produced MS, the mean diastolic gradient across the mitral valve decreased from 15 ? 5 to 1 + 1 mm Hg (p = 0.03) (Table II). Clinical reports2*5~6 of PMV have not described more adverse effects with two balloons than with the use of a single balloon. However, the numbers have been small, the maximal balloon size has been a 25 mm diameter balloon with a 15 mm diameter balloon5 and pathologic examination has not been reported. In our experimental model, we found more pathologic changes than we anticipated. Annular injury occurred in valves with smaller annular circumference and higher balloon-to-annular circumference ratio. Subvalvular damage was seen more often with larger balloon size. The absence of hemodynamic MR after balloon valvotomy did not preclude injury to the mitral valve apparatus. Chordae on the posterior aspect of the anterior mitral rupture. Finally,
leaflet balloon
nated MS in a surgically
were particularly susceptible to valvotomy successfully elimi-
created
canine model.
December
1514
Brief
Communications
American
Fig. 1. Dog No. 4 shows a lacerated leaflet and a vertical anulus in dog No. 5 is completely disrupted.
Table
I. Hemodynamic
Dog no.
and pathologic
v waue
tear in the mitral
anulus. In contrast,
Ruptured chordae
Torn leaflet
Ruptured papillary muscle
Annular tear
MR
1
9
0
0
0
0
0
2 3 4 5 6 7 8 9
16 25 50 5 43 2 5 3
+ ++ +++ 0 +++ 0 0 0
0 + 0 0 + + + +
0 0 + 0 + + 0 0
0 0 0 + 0 0 0 +
+ + + + 0 0 0 +
MR = mitral
regurgitation;
1987 Journal
the mitral
findings
f&J
(mm
llearl
Annular circ (cm) 6.8 6.5 6.1 5.8 1.5 8.3 8.2 8.0 6.8
Balloon/
an&us 1.53 1.60 2.11 2.22 1.72 1.55 1.57 1.61 1.90
circ = circumference.
This study is limited by the inability to extrapolate the results in a canine model to humans. There is no animal model of rheumatic MS, and the validity of using a surgical model is unknown. In addition, the effects of
balloon valvotomy through the LA appendage may be different from those when the transseptal approach is used. There is a large interspecies difference in size, but given a normal human mitral valve anulus circumference
Volume Number
114 6
Brief
II. Surgical model of mitral stenosis
Table
Dog no. 1 2 3 4 Mean
f SEM
Gradient pre valvotomy (mm Hd 9 29 7 15 15 k 5
Gradient
post valvotomy (mm f&J 0 2 3 0 1 2 1
(p = 0.03)
of approximately 11 cm, use of balloon circumferences proportionate to those in this study, i.e., double 30 mm balloons, would be expected to yield similar results. Nonetheless,thesefindings suggestthat caution should be exercisedin the clinical useof an oversized double balloon technique, in which the effective balloon circumference exceedsthe mitral annular circumference by >55%, for PMV. The circumference of the double balloonsin current clinical usedoesnot exceed that of a normal mitral anulus, and therefore this should not causethe damageobserved in the canine model. REFERENCES 1. Inoue K, Owaki T, Nakamura T, Kitamura F, Miyamoto N. Clinical application of transvenous mitral commiasurotomy by a new balloon catheter. J Thorac Cardiovasc Surg 1984; 87:394. 2. Palacios IF, Block PC, Brandi SC, Blanc0 P, Casal H, Pulido JI, Munoz S, D’Empaire G, Ortega MA, Jacobs ML, Vlahakes G. Percutaneous balloon valvotomy for mitral stenosis [Abstract]. Circulation 1986;74(suppl II):II-208. 3. Lock JE, Khalilullah M, Shrivastava S, Bahl V, Keane JF. Percutaneous catheter commissurotomy in rheumatic mitral stenosis. N Engl J Med 1985;313:1515. 4. McKay RG, Lock JE, Safian RD, Mandell VS, Bairn DS, Diver DJ, Royal HR, Come PC, Grossman W. Percutaneous balloon valvuloplasty in adult patients with critical mitral stenosis [Abstract]. Circulation 1986;74(suppl II):II-209. 5. Palacios I. Block PC. Brandi S. Blanc0 P. Casal H. Pulido JI. Munoz S,‘D’Empairk G, Ortega MA, Jacobs M, Vlahakes G: Percutaneous balloon valvotomy (PMV) for patients with severe mitral stenosis. Circulation 1987;75:778. 6. McKay CR, Ruiz C, Kawanishi D, Rahimtoola SH. Catheter balloon valvuloplasty treatment of mitral stenosis in adult patients: Initial experience with double balloon technique [Abstract]. Circulation 1986;74(suppl II):II-208.
Successful catheter ablation of a noninducible ventricular tachycardia Paul G. Colavita, M.D., W. Kenneth Haisty, Jr., M.D., and John S. Kelley, M.D. Winston-Salem, N.C. From the Department of Medicine, Bowman Gray School of Medicine. Reprint requests: Paul G. Colavita, M.D., Bowman Gray School Medicine, 300 S. Hawthorne Rd., Winston-Salem, NC 27103.
of
Communications
15 15
Ablation of ventricular tachycardia guided by electrophysiologic mapping’ or by visual inspection* has been accomplishedintraoperatively with much success.Catheter ablation of ventricular tachycardia has also been accomplished, but the experience is limited.3a4Prior to catheter ablation, endocardial activation mapping is required during ventricular tachycardia to isolate the area showing earliest endocardial activity relative to multiple reference leads. The purpose of this article is to report successfulcatheter ablation of a ventricular tachycardia focus guided by endocardial pace-mapping5without the aid of endocardial activation mapping. The patient suffered inferior and lateral myocardial infarctions in 1972 and 1980, respectively. Ventricular tachycardia with a right bundle branch morphology (RBBB) occurred in 1980 and was initially treated with quinidine and beta blockers. Quinidine was ineffective, and disopyramide was begun in 1981 with no recurrence until 1984. Ventricular tachycardia with a left bundle branch morphology (LBBB) wasalsonoted in September, 1984. An electrophysiologic study was performed in December, 1984, and the LBBB ventricular tachycardia was easily induced with premature atria1 stimulation. Amiodarone therapy was initiated, but combination therapy with tocainide, procainamide, or flecainide was required becauseof recurrent RBBB ventricular tachycardia. Flecainide and procainamide were discontinued due to exacerbation of congestive heart failure, and tocainide wasdiscontinued becauseof nauseaand vomiting. During a repeat electrophysiology study in April, 1986, RBBB ventricular tachycardia waseasily induced with pacing at 400 msecfrom the right ventricular apex, despite therapy with amiodarone and mexiletine. The patient was not considered a suitable candidate for ablative surgery and catheter ablation of the RBBB ventricular tachycardia focus was performed after we obtained informed consent. The patient wasbrought to the cardiac catheterization laboratory in the fasting, nonsedated state, after all antiarrhythmic drugs had been discontinued for at least 5 half-lives (except amiodarone). With the extrastimulus technique, RBBB ventricular tachycardia was initially induced from the right ventricular outflow tract after a prolonged effort. Following introduction of a No. 7F tripolar catheter (USCI, Billerica Mass.) into the left ventricle, ventricular tachycardia could not be reinitiated despite burst pacing, single,double, and triple extrastimuli at two cycle lengths from the right ventricular apex, outflow tract, and from multiple sites in the left ventricle. With the use of a No. 6F steerable tripolar catheter (Electra-Catheter Corp., Rahway, N. J.), pace-mapping was performed from multiple sites in the left ventricle.5 Pace-mappingfrom a localized area on the posteromedial left ventricular wall resulted in a QRS morphology almost identical to the clinical RBBB ventricular tachycardia (Fig. 1). Slight shifts in catheter position would produce dramatic changesin QRS morphology, particularly in lead V,. Slow ventricular conduction wasconfirmed in this area by latency from stimulus to onset of ventricular activation
114 6
Brief
REFERENCES
1. Kloster 2.
3.
4. 5.
6.
7.
F. Diagnosis and management of complications of prosthetic heart valves. Am J Cardiol 1975;35:872. Kotler MN, Mintz GS, Panidis I, Morganroth J, et al. Noninvasive evaluation of normal and abnormal prosthetic valve function. J Am Coll Cardiol 1983;2:151. Wilkins GT, Gillam LD, Kritzer GL, Levine RA, et al. Validation of continuous-wave Doppler echocardiographic measurements of mitral and tricuspid prosthetic valve gradients: a simultaneous Doppler-catheter study. Circulation 1986;74:786. Hatle L, Angelsen B. Doppler ultrasound in cardiology. Philadelnhia: Lea & Febieer. 1985:97. Weinstein IR, Marbarge; JP, Perez JE. Ultrasonic assessment of the St. Jude’s prosthetic valve: M mode, twodimensional, and Doppler echocardiography. Circulation 1983;68:897. Holen J, Simonsen S, Froysaker T. An ultrasound Doppler technique for the noninvasive determination of the pressure gradient in the Bjork-Shiley mitral valve. Circulation 1979;59:436. Kurzrok S, Singh AK, Most AS, Williams DO. Thrombolytic therapy for prosthetic cardiac valve thrombosis. J Am Co11 Cardiol 1987;9:592.
Limits of mitral valve apparatus distensibility: observations from balloon mitral valvdtomy in a canln6 model Donald E. Rediker, M.D., J. Luis Guerrero, David S. Block, James F. Southern, M.D., John T. Fallon, M.D., Ph.D., and Peter C. Block, M.D. Boston, Mass.
Percutaneous balloon mitral valvotomy (PMV) is being used increasingly as palliative treatment for rheumatic mitral stenosis (MS).le6 Recent reports5B6 suggest t,hat a larger mitral valve area is achieved with a double balloon technique. Prior to clinical trials that used the double balloon technique, we investigated the limits of mitral valve apparatus distensibility in a dog model. Nine mongrel dogs weighing 17 to 27 kg were anesthetized with chloralose (60 mg/kg, intravenously) and urethane (600 mg/kg, intravenously). Tracheal intubation was performed and mechanical ventilation was instituted. A left thoracotomy was performed, and the heart was suspended in a pericardial cradle. The first four dogs were placed on standard cardiopulmonary bypass. Pressure measurements were determined with standard fluid-filled catheters inserted into the left atrium (LA) through a pulmonary vein, and into the left ventricle through the apex. An incision was made in the LA appendage, and in the first four dogs a breakable purse-string suture was From the Department of Medicine Hospital, Harvard Medical School. Reprint General
requests: Hospital,
Peter Boston,
(Cardiac
C. Block, M.D., MA 02114.
Unit), Cardiac
Massachusetts Unit,
General
Massachusetts
Communications
I5I3
placed through the mitral valve leaflets, creating a gradient across the mitral valve. Balloon valvotomy was then performed through the LA appendage, at an inflation pressure of 4 atm, measured with a standard balloon pressure gauge (USCI, Billerica, Mass.). The initial two mitral valves were dilated with 15 mm and 25 mm diameter balloons positioned side by side, while the other seven mitral valves were dilated with two adjacent 25 mm diameter balloons. Pathologic observations were made by observers blinded both to the procedure performed and to the hemodynamic results (Table I). Five dogs had a mitral annular tear (Fig. l), which correlated with a smaller annular circumference (6.5 + 0.7 vs 7.8 t- 0.7 cm, p = 0.02) than those with an intact anulus. With the application of standard geometric and trigonometric formulas, the effective circumference of adjacent 15 and 25 mm balloons is 10.4 cm, and that of two 25 mm balloons is 12.9 cm. Therefore the total balloon circumference exceeded that of the mitral anulus by 53 % to 122 % . This ratio of balloon-to-annular circumference correlated with annular damage (1.91 rt_ 0.26 damaged vs 1.57 +- 0.04 intact, p = 0.04). The only dog (dog No. 1) without damage to the mitral apparatus was the only dog with a ratio < 1.55. There was a trend toward lower weight dogs having more annular damage, but this was not significant (21.0 i 3.4 vs 24.8 + 1.7 kg, p = 0.07). Three dogs had torn mitral leaflets (Fig. l), but valve leaflet damage did not correlate with balloon size or the balloon-to-annular circumference ratio. Five dogs had ruptured chordae, and two had ruptured papillary muscles. Subvalvular (chordal or papillary muscle) injury did correlate with balloon circumference (12.9 f 0.0 damaged vs 11.2 + 1.4 cm intact, p = 0.02). When chordal rupture occurred, it was always on the posterior aspect of the anterior mitral leaflet. Of the five dogs with no mitral regurgitation (MR), two had annular tears, one had a torn leaflet, three had ruptured bhordae, and one had a ruptured papillary muscle. In the four dogs with surgically produced MS, the mean diastolic gradient across the mitral valve decreased from 15 ? 5 to 1 + 1 mm Hg (p = 0.03) (Table II). Clinical reports2*5~6 of PMV have not described more adverse effects with two balloons than with the use of a single balloon. However, the numbers have been small, the maximal balloon size has been a 25 mm diameter balloon with a 15 mm diameter balloon5 and pathologic examination has not been reported. In our experimental model, we found more pathologic changes than we anticipated. Annular injury occurred in valves with smaller annular circumference and higher balloon-to-annular circumference ratio. Subvalvular damage was seen more often with larger balloon size. The absence of hemodynamic MR after balloon valvotomy did not preclude injury to the mitral valve apparatus. Chordae on the posterior aspect of the anterior mitral rupture. Finally,
leaflet balloon
nated MS in a surgically
were particularly susceptible to valvotomy successfully elimi-
created
canine model.
December
1514
Brief
Communications
American
Fig. 1. Dog No. 4 shows a lacerated leaflet and a vertical anulus in dog No. 5 is completely disrupted.
Table
I. Hemodynamic
Dog no.
and pathologic
v waue
tear in the mitral
anulus. In contrast,
Ruptured chordae
Torn leaflet
Ruptured papillary muscle
Annular tear
MR
1
9
0
0
0
0
0
2 3 4 5 6 7 8 9
16 25 50 5 43 2 5 3
+ ++ +++ 0 +++ 0 0 0
0 + 0 0 + + + +
0 0 + 0 + + 0 0
0 0 0 + 0 0 0 +
+ + + + 0 0 0 +
MR = mitral
regurgitation;
1987 Journal
the mitral
findings
f&J
(mm
llearl
Annular circ (cm) 6.8 6.5 6.1 5.8 1.5 8.3 8.2 8.0 6.8
Balloon/
an&us 1.53 1.60 2.11 2.22 1.72 1.55 1.57 1.61 1.90
circ = circumference.
This study is limited by the inability to extrapolate the results in a canine model to humans. There is no animal model of rheumatic MS, and the validity of using a surgical model is unknown. In addition, the effects of
balloon valvotomy through the LA appendage may be different from those when the transseptal approach is used. There is a large interspecies difference in size, but given a normal human mitral valve anulus circumference
Volume Number
114 6
Brief
II. Surgical model of mitral stenosis
Table
Dog no. 1 2 3 4 Mean
f SEM
Gradient pre valvotomy (mm Hd 9 29 7 15 15 k 5
Gradient
post valvotomy (mm f&J 0 2 3 0 1 2 1
(p = 0.03)
of approximately 11 cm, use of balloon circumferences proportionate to those in this study, i.e., double 30 mm balloons, would be expected to yield similar results. Nonetheless,thesefindings suggestthat caution should be exercisedin the clinical useof an oversized double balloon technique, in which the effective balloon circumference exceedsthe mitral annular circumference by >55%, for PMV. The circumference of the double balloonsin current clinical usedoesnot exceed that of a normal mitral anulus, and therefore this should not causethe damageobserved in the canine model. REFERENCES 1. Inoue K, Owaki T, Nakamura T, Kitamura F, Miyamoto N. Clinical application of transvenous mitral commiasurotomy by a new balloon catheter. J Thorac Cardiovasc Surg 1984; 87:394. 2. Palacios IF, Block PC, Brandi SC, Blanc0 P, Casal H, Pulido JI, Munoz S, D’Empaire G, Ortega MA, Jacobs ML, Vlahakes G. Percutaneous balloon valvotomy for mitral stenosis [Abstract]. Circulation 1986;74(suppl II):II-208. 3. Lock JE, Khalilullah M, Shrivastava S, Bahl V, Keane JF. Percutaneous catheter commissurotomy in rheumatic mitral stenosis. N Engl J Med 1985;313:1515. 4. McKay RG, Lock JE, Safian RD, Mandell VS, Bairn DS, Diver DJ, Royal HR, Come PC, Grossman W. Percutaneous balloon valvuloplasty in adult patients with critical mitral stenosis [Abstract]. Circulation 1986;74(suppl II):II-209. 5. Palacios I. Block PC. Brandi S. Blanc0 P. Casal H. Pulido JI. Munoz S,‘D’Empairk G, Ortega MA, Jacobs M, Vlahakes G: Percutaneous balloon valvotomy (PMV) for patients with severe mitral stenosis. Circulation 1987;75:778. 6. McKay CR, Ruiz C, Kawanishi D, Rahimtoola SH. Catheter balloon valvuloplasty treatment of mitral stenosis in adult patients: Initial experience with double balloon technique [Abstract]. Circulation 1986;74(suppl II):II-208.
Successful catheter ablation of a noninducible ventricular tachycardia Paul G. Colavita, M.D., W. Kenneth Haisty, Jr., M.D., and John S. Kelley, M.D. Winston-Salem, N.C. From the Department of Medicine, Bowman Gray School of Medicine. Reprint requests: Paul G. Colavita, M.D., Bowman Gray School Medicine, 300 S. Hawthorne Rd., Winston-Salem, NC 27103.
of
Communications
15 15
Ablation of ventricular tachycardia guided by electrophysiologic mapping’ or by visual inspection* has been accomplishedintraoperatively with much success.Catheter ablation of ventricular tachycardia has also been accomplished, but the experience is limited.3a4Prior to catheter ablation, endocardial activation mapping is required during ventricular tachycardia to isolate the area showing earliest endocardial activity relative to multiple reference leads. The purpose of this article is to report successfulcatheter ablation of a ventricular tachycardia focus guided by endocardial pace-mapping5without the aid of endocardial activation mapping. The patient suffered inferior and lateral myocardial infarctions in 1972 and 1980, respectively. Ventricular tachycardia with a right bundle branch morphology (RBBB) occurred in 1980 and was initially treated with quinidine and beta blockers. Quinidine was ineffective, and disopyramide was begun in 1981 with no recurrence until 1984. Ventricular tachycardia with a left bundle branch morphology (LBBB) wasalsonoted in September, 1984. An electrophysiologic study was performed in December, 1984, and the LBBB ventricular tachycardia was easily induced with premature atria1 stimulation. Amiodarone therapy was initiated, but combination therapy with tocainide, procainamide, or flecainide was required becauseof recurrent RBBB ventricular tachycardia. Flecainide and procainamide were discontinued due to exacerbation of congestive heart failure, and tocainide wasdiscontinued becauseof nauseaand vomiting. During a repeat electrophysiology study in April, 1986, RBBB ventricular tachycardia waseasily induced with pacing at 400 msecfrom the right ventricular apex, despite therapy with amiodarone and mexiletine. The patient was not considered a suitable candidate for ablative surgery and catheter ablation of the RBBB ventricular tachycardia focus was performed after we obtained informed consent. The patient wasbrought to the cardiac catheterization laboratory in the fasting, nonsedated state, after all antiarrhythmic drugs had been discontinued for at least 5 half-lives (except amiodarone). With the extrastimulus technique, RBBB ventricular tachycardia was initially induced from the right ventricular outflow tract after a prolonged effort. Following introduction of a No. 7F tripolar catheter (USCI, Billerica Mass.) into the left ventricle, ventricular tachycardia could not be reinitiated despite burst pacing, single,double, and triple extrastimuli at two cycle lengths from the right ventricular apex, outflow tract, and from multiple sites in the left ventricle. With the use of a No. 6F steerable tripolar catheter (Electra-Catheter Corp., Rahway, N. J.), pace-mapping was performed from multiple sites in the left ventricle.5 Pace-mappingfrom a localized area on the posteromedial left ventricular wall resulted in a QRS morphology almost identical to the clinical RBBB ventricular tachycardia (Fig. 1). Slight shifts in catheter position would produce dramatic changesin QRS morphology, particularly in lead V,. Slow ventricular conduction wasconfirmed in this area by latency from stimulus to onset of ventricular activation