Selective transepicardial ablation in the immature canine myocardium

J

THORAC CARDIOVASC SURG

1989;97:893-9

Selective transepicardial ablation in the immature canine myocardium A more precise method The numerous surgical and transcatheter ablation methods widely applicable to the adult heart have had limited use and efficacy in the immature myocardium. This study introduces a more precise modification of the transepicardial approach to ablation, applicable to the immature heart, by combining selective mapping and simultaneous ablation techniques. To determine the specificity of this method, we attempted His bundle and sinus node ablations in 14 beagle puppies (aged 2 to 4 months). After a thoracotomy, a custom bipolar needle mapping/ablation probe, attached to a standard electrophysiologic recorder, was applied along the aortoatrial and atrial-superior vena caval junctions to record maximal epicardial His bundle and sinus node impulses, respectively. Complete ablation was achieved by the single injection of 10% formalin specificaUy into the desired target region of the conduction system after the probe was advanced into the myocardium proper. Ventricular pacing was then instituted, the thoracotomy closed, and the animals aUowed to recover. After 4 months' observation, bidirectional complete atrioventricular block persisted. High right atrial electrical activity was absent in the animals that underwent sinus node ablation. Programmed stimulation failed to induce any arrhythmias. Histologic study demonstrated selective His bundle and sinus node destruction with minimal involvementof surrounding tissue. This study demonstrates a reproducible transepicardial approach to precise ablation in the immature heart.

Peter P. Karpawich, MD, Saroja Bharati, MD, Jacqueline O. Roskamp, MD, and Maurice Lev, MD, Detroit, Mich., and Browns Mills, N.J.

Ablation of atrioventricular (A V) junction and ectopic foci, either in the animal laboratory or for clinical management of medically refractory tachyarrhythmias, has been performed with numerous surgical and transvenous catheter techniques with variable success and morbidity.!" However, most ablation methods have been reported only in the adult heart. Experience From the Section of Pediatric Cardiology, Department of Pediatrics, Children's Hospital of Michigan, Wayne State University School of Medicine, Detroit, Mich., and the Congenital Heart and Conduction System Laboratory, Department of Pathology, Deborah Heart and Lung Institute, Browns Mills, N.J. Supported in part by Grants 34-86 from the American Heart Association of Michigan, HL 30558-05 from the National Institutes of Health, National Heart, Lung, and Blood Institute, Bethesda, Md., and a grant from Children's Hospital of Michigan. Received for publication Feb. 8, 1988. Accepted for publication Nov. I, 1988. Address for reprints: Peter P. Karpawich, MD, Children's Hospital of Michigan, 3901 Beaubien, Detroit, MI 48201.

Fig. 1. Appearance of the custom mapping/ablation probe and attachment to permit electrophysiologic readings.

with the immature myocardium has been limited and, even with the newer transvenous catheter techniques, associated with less effective and potentially more arrhythmia-inducing results.':'? 893

894

The Journal of Thoracic and Cardiovascular Surgery

Karpawich et al.

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rHl l l l l l lTfl lTFnT Fig. 2. Surface lead II: Intracardiac atrial and epicardial His bundle tracings during epicardial mapping and after needle penetration into the His bundle region. As the probe is advanced, thre is an observed prolongation of initial low atrial-His bundle (arrows) interval of 50 (left panel) to 100 (right panel) msec, which progresses to complete AV block below the His bundle after formalin is injected. LRA, Low right atrial; HBE, His bundle electrogram. Paper speed 100 mm/sec,

Successful transepicardial chemical ablation of the AV junction was reported by Steiner and Kovalik" by infiltration of high-concentration formalin into the aortic root of adult dogs. This approach was later modified by cryothermal ablation applied to the same area." However, such indirect approaches are imprecise. Likewise, any ablation method that interposes myocardial tissue between the ablation source and target area or requires tissue resection may cause unnecessary damage, especially in the small target area and more friable tissue of the immature myocardium. The need for a more precise ablation technique is therefore required for any long-term study of the immature heart. The purpose of this study was to attempt a more precise modification of this older injection technique by using a combination of direct transepicardial electrophysiologic mapping and simultaneous selective chemical ablation of the bundle of His and, as a substitute for an ectopic focus, the sinus node, in the immature canine myocardium. Methods Experimental preparation. Fourteen beagle puppies (2 to 4 months of age, mean weight 5.3 kg) were used in this study. All animal care and procedures were in accord with Wayne State University's Animal Investigation Committee guidelines. All puppies were first anesthetized with intravenous sodium pentobarbital (25 mg/kg), After baseline transvenous hemodynamic and intracardiac electrophysiologic measurements were made, all were intubated and the lungs ventilated with room air. A thoracotomy was performed and the pericardium

opened to expose the base of the aorta. Temporary epicardial right atrial and ventricular pacing wires (Model AS 633, Cooner Wire Co., Chatsworth, Calif.) were attached and the right atrial appendage retracted to expose the aortoatrial groove. To ensure precise localization of the conduction area during ablation, we used a custom-made bipolar needle electrode capable of both simultaneous epicardial His bundle mapping and eventual precise ablation. This electrode was fashioned from two partially Teflon-coated 26-gauge by 2'/2-inch stainless steel short beveled needles (Microcath, Burron Medical, Bethlehem, Pa.), attached 3 mm apart with siliconeconnectors and electrically isolated along their lengths. The distal silicone connectors also served to limit the depth of needle penetration. The needle tips were exposed for 6 mm at the distal end with an additional I mm exposure proximally (Fig. I). The proximal ends were attached to a standard Electronics for Medicine (AR-6) recorder (PPG Biomedical Systems, Pleasantville, N.Y.) set at a frequency range of 30 to 250 Hz with standard limb lead electrodes at direct current to 100 Hz. Ablation technique. Without prior dissection of the aortic fat pad, the bipolar "mapping" electrode was applied along the aortoatrial groove opposite the right and noncoronary cuspsas described for epicardial His bundle localization." At the position of greatest His bundle potential, the tip of the electrode was advanced into the myocardium proper with continuous recording for guidance specifically toward the bundle and aspiration to ensure avoidance of any cardiac chambers. Exact needle positioning could be confirmed by an observed transient increase in low atrial-His bundle intervalor transient AV block (Fig. 2) as the bundle of His was approximated. This could be achieved either by positioning of the mapping probe itself or after preliminary injection of 0.1 ml of 0.9% sodium chloride to confirm needle approximation. In each instance, permanent ablation was not attempted until

Volum e 97 Number 6

Transepicardial ablation in immature myocardium

June 1989

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Fig. 3. Sinus node ablation after His bundle ablat ion with ventricular pac ing. Intr acardi ac high right atrial (H RA ) impulse (small arrows) at a cycle length of 390 msec (left panel). Injection of formalin (large arrow) results in immediate change in sinus cycle length to 360 msec with dispersion of the intracardiac signal. T his is followed by progressive atten uatio n of high right atrial activity. precise needle positioning was confirmed. Permanent complete AV block was then ac hieved by the single injection of 10% formalin in volumes ranging from 0.3 to 0.1 ml per an imal. Immediate ventricular (VVI) pacing was instituted to prevent any morbidity associated with the sudden decrease in ventricular rate. Th is a blation techniqu e was additionally applied to the sinus node in six puppies by mapping the right atrialsuperior vena cava junction compared with tem porar y reference electrodes for a rea of earliest atrial ac tivation. Insertion of the probe altered sinus node cycle length. Th is was followed by 0.1 ml formalin infiltrat ion into the sinus node region for ablation (Fig . 3). Perm anent VVI pacing at 129 beats/min was continued with left ventricular epicardial electrodes (MOOtronic model 69 17A-35T, Med tronic Inc., Minnea polis, Minn.) attach ed to implanted pulse genera tors (Medtronic Model 8423). Th e thoracotomy was closed and the animals allowed to recover. Follow-up studies. All puppies were permitted routine activity and monitored with serial electrocardiograms for a total of 4 months. They were then anesthetized as described, and tran sthoracic two-dimensional pulsed-wave Doppler and transvenous hemodynam ic studies were performed to evaluate valve competency. Electrophysiologic reevaluation for persistence of AV a nd ventriculoatrial (VA) block and inducibility of a rrhythmias was performe d with progra mmed stimulation by means of single, double, a nd burst premature right atrial and ventricular impulses at 2 paced cycle lengths. The a nimals were then killed and the excised heart s of six puppies exami ned for histologic appearance. Histologic ex amination. Serial sections through the AV node, His bund le, and bundle branches were obtained from three puppies, each receiving 0.3 or 0.2 ml form alin. All sections were stained with hematoxylin-eosin. Extensive and detailed studies were perfor med in three other puppies that received 0.1 ml formalin including one that underwent sinus node ablati on. In these, the sinus a nd AV nodes and their approaches, the His bundle, and the bundle branches up to the level of the moderator band were serially sectioned a nd every twentieth section retained. In additio n, multiple sections were taken from the free walls of both at ria and ventricles and the rema inder of the ventricula r septum. Sections were sta ined

with both hemat oxylin-eosin a nd Weige rt-van Gieson stai ns. In this mann er, a total of 1036, 1416, and 1036 sections were examined in each of these three pupp ies, respectively.

Results Electrophysiology. Complete antegrade and retrograde AV block was created in all 14 puppies with the single formalin injection. In each instance, onset of block was immediate after formalin infiltration. Although His bundle electrograms became markedly attenuated and difficult to measure, no definite block proximal to the His bundle was observed. However, after initial needle insertion or saline injection, trans ient bundle branch block was observed with inaccurate probe positioning. Immediate ventricular pacing was required in each instance to prevent morbidity associated with the prolonged ventricular recovery rhythm after AV block formation. The ablation technique was not associated with initiation of sustained atri al or ventricular arrhythmias. There were no intraoperative deaths. Chronic ventricular pacing was required in all puppies. Unfortunately, the youngest puppies had bradycardia- induced congestive failure at the maximum programmable rate of 129 beats/min of the implanted pulse generators. During the postoperat ive phase, all four smaller animals (aged 2 to 3 months, mean weight 3.4 kg) had dyspnea, lethargy, and diminished perfusion within 48 hours of instrumentation and were put to death . In one instance, the pulse generator was found to be malfunctioning. No histologic studies were performed. All 10 larger puppies (aged 3 to 4 months, mean weight 6.1 kg) were followed up for the 4-month postoperative period without symptoms attributable to cardiac dysfunction. After the study period, repeat intracardiac electro-

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The Journal of Thoracic and Cardiovascular Surgery

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Table I. Microscopic findings in the conduction system Case

A-AVN

SAN

A-SAN

Chronic epicarditis, fibrosis, mono. cells

Chronic epicarditis, fat, focal fibrosis, young connective tissue

2'

Not recognized

3

Normal

Chronic inflammation, fibrosis, hemorrhage, necrosis, cartilage in endocardium Chronic epicarditis +4 Fat

PB

AVN

+3 Fatty infiltration, mono. cells, increased connective tissue Cartilage in endocardium, fibrosis, +4 fat in approach

BrB

BiB

+ 2 Fibrosis, spaces, lobulation of node

Increased spaces, fibrosis, and finally disa ppeared

Fibrosis, cartilage replace parenchyma

Fibrosis, cartilage replace parenchyma

+2 Fibrosis

+2 Fibrosis

+ 3 Fibrosis

+4 Fibrosis

Marked chronic inflammation, + I fibrosis

+4 Fibrosis: basophilic degeneration of fat around cartilage

_+2/+3 Fibrosis, spaces

+4 Fibrosis

A, Approachesto; AVN, atrioventricular node; BiB,bifurcating bundle; BrB, branching bundle; mono.. mononuclear; LBB. left bundle branch; PB. penetrating bundle: RBB, right bundle branch; SAN, sinus node; + I, mild: +2, mild-moderate; +3, moderate-severe; +4, marked. 'Sinus node ablated.

Table II. Microscopic findings of the myocardium and valves Case

CFB

AV

2

+ 3 Fat, basophilic degeneration, cartilage +1/+2 Cartilage

Increased spongosia, basophilic degeneration, + 3 fat, cartilage in part Proliferation of spongiosa

3

Cartilage with fibrosis, ulceration

VS

TV Increased spongiosa, basophilic degeneration

Beginning of cartilage formation at base, increased spongiosa with degeneration Increased spongiosa, Increased spongiosa, cartilage at commissures basophilic degeneration, +3 fat

MV

LV

Focal fibrosis, fat

Increased spongiosa

Foci of calcification

Focal degeneration

Degeneration and hemorrhage

Prominent Purkinje cells

Focal fibrosis below YS, early focal necrosis, cartilage, mono. cells

Increased basal spongiosa

Prominent Purkinje cells

AV, Aortic valve;CFB, central fibrousbody; Mono.. mononuclear; MV, mitral valve; LV,left ventricle; PS apex. posteriorseptum apex; RV, right ventricle; TV. tricuspid valve; VS. ventricular septum; + 1. mild; +2, mild-moderate; +3, moderate-severe; +4, severe.

physiologic tracings demonstrated persistence of complete A V and VA block at rest and with both atrial and ventricular pacing, The escape ventricular rhythm (1200 msec) demonstrated a wide QRS complex (>0,10 second) with a discrete bundle branch impulse occurring 10 to 15 msec before the onset of ventricular activation. Specific His bundle potentials could not be recorded. This correlated with histologic observations of the most extensive involvement occurring in the main bundle itself. Neither sustained atrial nor ventricular arrhythmias could be induced with programmed stimulation. In the puppies that additionally underwent sinus node ablation, high right atrial activity was absent. A noncon-

ducted left atrial rhythm was recorded. Doppler and hemodynamic studies confirmed absence of tricuspid or aortic valve insufficiency. Gross and histologic findings. In the puppies that received 0.3 and 0.2 ml formalin, transmural fibrosis, inflammation, and localized areas of calcification were present from the AV node to bifurcating portions of the bundle of His. The extent of this involvement precluded any histologic examination of specific areas of the specialized AV conduction system. In the three puppies that received 0.1 m1 formalin, involvement was specific to the His bundle. Gross inspection revealed only thickening of the medial leaflet of the tricuspid valve

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Transepicardial ablation in immature myocardium

June 1989

RBB

LBB Replaced by connective tissue in beginning

Disappears in beginning, later fibrosis

Intact

Degeneration of endocardium near RBB

+I

Fibrosis, spaces

Spaces

RV

PS apex

Epicardium thick, focal fibrosis, prominent Purkinje cells Focal mono. cells, fibrosis, hemorrhage

+4 Epicardial fibrosis

Prominent Purkinje cells

No change

No change

and the atrium immediately adjacent without evidence of sinus of Valsalva aneurysm or septal defects. Microscopic examination of specimens from the three puppies is discussed in Tables I and II. In the conduction system, the approaches to the AV node revealed marked fatty metamorphosis. The AV node itself showed mild to moderate fibrosis. The penetrating and the branching bundle showed moderate to marked fibrosis in all puppies (Fig. 4, A) and increase in space formation (Fig. 4, B) with complete disappearance of the branching bundle in one (Fig. 5). The bundle branches showed moderate fibrosis. In the puppy that underwent sinus node ablation, the node itself and supplying artery were replaced by fibrosis and could not be distinguished.

897

Prominent Purkinje cells were seen in the subendocardium of the right ventricular side at a distance from the ablated area. The tricuspid and aortic valves showed an increase in spongiosa, with basophilic degeneration, fatty infiltration, and cartilage formation at the base. The mitral valve revealed hemorrhage and an increase in spongiosa. The summit of the ventricular septum showed focal fibrosis. Discussion Application of ablation methods to the immature myocardium. Although transcatheter fulguration, laser, and epicardial cryoablation have been effective in the adult, application of any of these techniques to the immature myocardium is largely unknown. Despite the advantage of a closed chest approach, transcatheter fulguration requires critical positioning of a relatively unstable catheter.r" Recently, Moak, Friedman, and Garson'" reported anatomic and electrophysiologic consequences of electrical ablation in the immature canine atrium. They and others concluded that the inaccuracies of electrode catheter mapping and creation of an electrophysiologic substrate for development of spontaneous arrhythmias may limit the usefulness of transcatheter fulguration in the immature myocardium." Similar problems may be associated with laser ablation." The newer radiofrequency ablation technique, although having advantages over other catheter methods in the adult heart, has not yet been applied to the immature myocardiuin.t'
8 9 8 Karpawich et al.

The Journal of Thoracic and Cardiovascular Surgery

Fig. 4. A, Histologic appearance of A V bundle showing fibrosis and fatty infiltration. Weigert-van Gieson stain. (Original magnification X45.) CFB. Central fibrous body; V. ventricular septum; B. A V bundle; D. basophilic degenerat ion with vacuolization in central fibrous body beneath aorta; A . atrial septal musculature. B, Histologic appearance further down the A V bundle from A at beginning of branching bundle (ar rows). Bundle shows moderate fibrosis with space formation, and endocardium (E ) is markedly thickened with basophilic and vacuolar degeneration. (Original magnification X45.) a pplica ble to use of lower formalin concentrations or other agents capable of producing local tissue fibroSis.IB. 19 These other options may cau se fewer side effects, which may result only in mod ification of A V conduction or ectopy and may be more ad vantageous for use in children. In this regard, this method may potentially be a pplied to the newer radiofrequency a blation technique, with energy applied directly through the mapping needles to the desired a blation site . T hese concepts are under investigation in our laboratory at the present time.

Pathologic findings in low-dose formalin infiltration. The pathologic finding s of th e conduction system

Fig. 5. Histologic appearance at base of aortic valve (AV) and tricuspid valve (TV) showing basophilic and vacuolar degeneration. Bundle is replaced by cartilage tissue. Central fibrous body (CF B) has marked cartilage (Ca) formation throughout. Arrows point to remnant of bundle itself. V, Ventricular septum. (Original magnificat ion x30.)

in these growing puppies are significant. After ablation, the sinus nod e and arter y wer e not recogni zed. Th ese were replaced by chronic inflam mation, fibrosis, necrosis, and cartilage in the end ocardium. Likewise, the His bundle region of all puppies revealed varying amounts of fibrosis, some fat , and some focal calcification or cartilage formation . In one animal, there was complete replacement of the His bundle and the beginning of the bundle branches by fibrosi s and cartilage formation.

Volume 97 Number 6 June 1989

The replacement of connective tissue by large spaces within the conductive cells is a previously unreported consequence after ablation. This, however, has been seen in degenerative disease of the conduction system. Likewise, the development of prominent Purkinje cells is a unique finding not previously reported in growing puppies. These cells may result from chronic loss of AV synchrony and hypertrophy and further growth of the normal distal myocardium. Further study will be required in this regard. Cartilage formation may be a consequence of His bundle ablation in dogs. We wish to thank Cheryl Justice and Diane Cavitt, CCVT, for technical assistance and Janice McLarty and Darlene Delmonte for manuscript preparation. REFERENCES 1. Starzl TE, Gartner RA, Baker RR. Acute complete heart block in dogs. Circulation 1955;55:82-9. 2. Harrison L, Gallagher JJ, Kasell J, et al. Cryosurgical ablation of the A-V node-His bundle: a new method for producing A-V block. Circulation 1977;55:463-70. 3. Gonzalez R, Scheinmann M, Margaretten W, Rubinstein M. Closed-chest electrode-catheter technique for His bundle ablation in dogs. Am J Physio1 1981;241: H283-7. 4. Giannelli S, Ayers SM, Gromprecht RF, Conklin EF, Kennedy RJ. Therapeutic surgical division of the human conduction system. JAMA 1967;199:155-60. 5. Saksena S, Hussain S, Gielchinsky I. Surgical ablation of tachyarrhythmias: reflection for the third decade. PACE 1988;11:103-8. 6. Ward DE, Camm AJ. The current status of ablation of cardiac conduction tissue and ectopic myocardial foci by transvenous electrical discharges. Clin Cardiol 1986; 9:237-44. 7. Moak JP, Friedman RA, Garson A. Electrical ablation of atrial muscle: early and late anatomic observations in canine atria. Am Heart J 1987;113:1397-1404. 8. Moak JP, Friedman RA; Garson A. Electrical ablation of

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atrial muscle. Early and late electrophysiologic observations in canine atria. Am Heart J 1987;113:1404-14. 9. Gillette PC, Garson A, Porter CJ, et al. Junctional automatic ectopic tachycardia: new proposed treatment by transcatheter His bundle ablation. Am Heart J 1983;106:619-23. 10. Morady F. A perspective on the role of catheter ablation in the management of tachyarrhythmias. PACE 1988; 11:98-102. 11. Steiner C, Kovalik ATW. A simple technique for production of chronic complete heart block in dogs. J Appl Physiol 1968;25:631-2. 12. Painvin GA, Gillette PC, Zinner A, et al. Epicardial cryoablation of the bundle of His. J THORAC CARDIOVASC SURG 1984;88:273-6. 13. Karpawich PP, Gillette PC, Lewis RM, Zinner A, McNamara DG. Chronic epicardial His bundle recording in awake nonsedated dogs: a new method. Am Heart J 1983;105:16-21. 14. Bardy GH, Ideker RE, Kasell J, et al. Transvenous ablation of the atrioventricular conduction system in dogs: electrophysiologicand histologic observations. Am 1 Cardiol 1983;51:1775-81. 15. Narula OS, Bharati S, Chan MC, Embi AA, Lev M. Microtransection of the His bundle with laser radiation through a pervenous catheter: correlation of histologic and electrophysiologic data. Am 1 Cardiol 1984;54:186-92. 16. Huang SK, Bharati S, Lev M, Marcus FI. Electrophysiologic and histologic observations of chronic atrioventricular block induced by closed-chest catheter desiccation with radiofrequency energy. PACE 1987;10:85-16. 17. Rudolph AH, Heymann' MA, Fishman N, Lakier lB. Formalin infiltration of the ductus arteriosus. N Engl J Med 1975;292:1263-8. 18. Scherlag BJ, Sweidan R, Sakurai M, Lazzara R. Basis of unexpected V-A conduction after complete A-V block. Circulation 1986;74(Pt 2):II258. 19. Chilson DA, Peigh PS, Mahomend Y, Waller BF, Zipes DP. Chemical ablation of ventricular tachycardia in the dog. Am Heart J 1986;111:1113-7.