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ransient atrioventricular conduction block withryoablation following normal cryomapping

eter S. Fischbach, MD,a Elizabeth V. Saarel, MD,b MacDonald Dick, II, MDa

From the University of Michigan, Ann Arbor, Michigan and

Cleveland Clinic Foundation, Department of Pediatrics, Cleveland, Ohio.

OBJECTIVES The purpose of this study was to demonstrate that adverse but reversible effects on AVconduction may be observed during cryoablation despite no evidence of deleterious effects evidentduring cryomapping.BACKGROUND Transcatheter cryoablation has recently been introduced for treatment of supraventric-ular tachycardia. Potential advantages compared with radiofrequency ablation include decreased risk ofthromboembolism and perforation, less patient discomfort, more sharply demarcated lesions, anddecreased risk of inadvertent damage to adjacent structures due to the ability to observe the electro-physiologic effects of lesion generation prior to permanent tissue damage. Occasionally, however,adverse electrophysiologic effects may be observed during cryoablation when prior cryomappingsuggests a safe location.METHODS We reviewed the experience with our first 32 consecutive patients who underwent cryoa-blation for supraventricular tachycardia. Patients who experienced transient heart block during cryoa-blation following uneventful cryomapping were identified.RESULTS Five patients in whom cryomapping was performed with no evidence for deleterious effectson AV conduction were identified. However, application of cryoablation at these mapped sites resultedin transient complete heart block. This experience demonstrates that the cryolesion created duringcryoablation may expand relative to that suggested by cryomapping.CONCLUSIONS It is important to maintain vigilance in monitoring for AV nodal conduction abnor-malities during cryoablation, even when cryomapping suggests a safe location for cryoablation.

KEYWORDS AV conduction; Cryoablation; Supraventricular tachycardia

(Heart Rhythm 2004;1:554–557) © 2004 Heart Rhythm Society. All rights reserved.

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ranscatheter cryoablation has recently been introduceds an alternative to radiofrequency ablation for treatingupraventricular arrhythmias.1–5 Previously, cryoablationad been used both epicardially and endocardially duringardiac surgery for treatment of both ventricular andupraventricular arrhythmias. Cryoablation offers severaldvantages compared with radiofrequency ablation. Ben-ficial features include decreased risk of thromboembo-ism due to little or no endocardial disruption6; moreharply circumscribed lesions potentially limiting inad-ertent damage to adjacent structures7; a catheter elec-

Address reprint requests and correspondence: Dr. Peter S. Fischbach,.S. Mott Children’s Hospital, University of Michigan Medical Center, 1500. Medical Center Drive, L1242 Women’s/Box 0204, Ann Arbor, Michigan8109-0204.

E-mail address: petersf@umich.edu.

e(Received April 27, 2004; accepted June 28, 2004.)

547-5271/$ -see front matter © 2004 Heart Rhythm Society. All rights reserved

rode tip on which an ice ball forms and adheres to thendocardial surface, generating a more specific lesion8;ess patient discomfort during energy application9; and anpportunity to observe the electrophysiologic effects ofhe lesion prior to inducing cell death.10 This last at-ribute allows detection of a transient, reversible loss oflectrical conduction in the tissue of interest prior toermanent tissue damage.

The ability to cryomap prior to tissue damage providesn added measure of safety and is advanced as the principalactor distinguishing this procedure from the establishedadiofrequency ablation technique. In short, it has beenuggested that if no adverse effects are observed during thepplication of the cryomap (tip temperature �30°C), thenryoablation (tip temperature �70°C) can be performedafely. We herein report on 5 of the first 32 patients whonderwent cryoablation in our laboratory in whom cryo-apping was performed with no evidence of deleterious

ffects. However, when the tip temperature was lowered to

. doi:10.1016/j.hrthm.2004.06.018

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70°C for cryoablation, prolongation or block of conduc-ion through the AV node was observed.

ethods

ryoablation was performed on 32 consecutive patientsresenting to the electrophysiology laboratory between Au-ust 2003 and March 2004. The patients had AV nodaleentrant tachycardia (AVNRT; n � 22), AV reentrantachycardia (AVRT) using either an anterior or midseptalccessory pathway (n � 6), ectopic atrial tachycardia (n �), or congenital junctional ectopic tachycardia (n � 1).tandard electrophysiologic testing was performed using

hree intracardiac catheters in all patients. The electrophysi-logic procedure included a quadripolar catheter in the highight atrium, a multipurpose catheter used to record both theis and right ventricular apical signals,11 and a coronary

inus catheter. The LocaLisa catheter navigation systemMedtronic Inc., Minneapolis, MN, USA) was used to lo-alize the ablation catheter tip, the coronary sinus os, andhe His bundle in all patients. After completion of thelectrophysiologic study, if AVNRT or AVRT using annterior or midseptal accessory pathway was detected, a 7r cryocatheter was inserted for mapping and ablation. Foratients with AVNRT, mapping was initiated in the regionnterior to the roof of the coronary sinus os. In general,ased on our prior experience, the mapping catheter wasdvanced slightly more anteriorly toward the apex of theriangle of Koch at the beginning of the procedure than wassed with conventional radiofrequency ablation. Mappingas performed similar to mapping in radiofrequency abla-

ion looking for an atrial to ventricular electrogram ratioize of approximately 1:3 with a complex atrial component.ryomapping was performed at sites that fulfilled the afore-entioned criteria. After achieving a temperature of30°C, atrial extrastimulus testing was performed in an

ttempt to document loss of slow pathway conduction in thease of AVNRT or accessory pathway conduction in thease of AVRT. Concurrently, the PR interval was measuredo ensure no change. If no indication for compromisedonduction through the AV node was observed, then theemperature was lowered to �70°C for cryoablation prior toewarming, ensuring that the site of cryomapping was iden-

able 1 Patient characteristics for the five patients described

Pt. no.Age(y)

Weight(kg) Arrhythmia

Time to Ablock (s)

13 60 AVNRT 1214 68 AVNRT 1217 70 AVNRT 1417 85 AVRT 914 40 AVNRT 9

The presence of a His signal on the distal ablation tip prior to or follAVNRT � atnoventricular nodal reentrant; tachycardia; AVRT � atriov

ical to the site of cryoablation. For patients with AVNRT, f

f slow pathway conduction persisted during cryomapping,he catheter tip was allowed to rewarm, and the catheter wasoved anteriorly toward the apex of the triangle of Koch.

atients

atient 1A 13-year-old, 60-kg boy with AVNRT underwent slow

athway cryoablation (Table 1). During cryomapping in theidseptal region, no alterations in AV conduction were

etected with elimination of slow pathway conduction. Af-er the temperature was lowered to �70°C, the patienteveloped a low atrial rhythm with PR prolongation andhen third-degree AV block after 12 seconds of cryoabla-ion. The cryoapplication was terminated, and the AV nodeonduction recovered rapidly with return of normal sinushythm. The catheter then was moved slightly more poste-iorly (away from the compact AV node). Successful abla-ion of the slow pathway was performed in the more pos-erior location. The patient’s PR interval was normal at theonclusion of the procedure and during 24-hour Holteronitoring following the procedure.

atient 2A 14-year-old, 68-kg girl with AVNRT underwent cryo-

apping at �30°C for 30 seconds in the midseptal regionithout alterations in AV conduction prior to engaging

ryoablation at �70°C. After 12 seconds at �70°C, the PRnterval prolonged from 150 to 220 ms. Cryoenergy waserminated, and the PR interval rapidly returned to normal.he catheter was moved more posteriorly, and successfullow pathway ablation was performed. The PR interval wasormal at the end of the study, and no abnormalities in AVonduction were detected during postprocedural 24-hourolter monitoring.

atient 3A 17-year-old, 70-kg girl with AVNRT underwent cryo-

apping at �30°C for 60 seconds prior to cryoablation,ith no change in the PR interval. After 14 seconds at70°C, 2:1 AV block was observed. Cryoenergy was

urned off, and normal AV conduction returned within 5econds. The catheter was moved posteriorly and a success-

CryomappingHis signalpresent

Follow-up(mo) Recurrence

Y No 4 NoY No 7 NoY No 4 NoY No 2 YesY No 3 No

ryoablation is noted.lar reentrant tachycardia.

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556 Heart Rhythm, Vol 1, No 5, November 2004

atient 4A 17-year-old, 85-kg boy had AVRT using a concealed

ight-sided midseptal accessory pathway supporting su-raventricular tachycardia. Radiofrequency ablation per-ormed 2 years previously was unsuccessful due to theroximity of the pathway to the AV node. The site chosenor ablation based on the shortest VA times during ventric-lar pacing lay immediately posterior to a region in whichis signals were recorded. Following cryomapping for 45

econds with no alteration in AV conduction, 9 seconds ofryoablation resulted in PR prolongation (172 to 190 ms)ith development of AV block during atrial pacing at arive cycle length of 400 ms (sinus cycle length � 570 ms).ryoenergy was terminated, and AV conduction normalized

apidly. The catheter was repositioned in the same antero-osterior plane, moving toward the atrial side of the AVing. The accessory pathway was eliminated with nohanges observed in the PR interval. The PR interval wasormal at the end of the procedure, and no PR prolongationas detected on 24-Holter monitoring on the day following

he procedure.

atient 5A 12-year-old, 40-kg girl with AVNRT underwent cryo-

apping at �30°C for 60 seconds prior to cryoablation,ith no change in the PR interval. After 9 seconds of

ryoablation at �70°C, the PR interval prolonged from 165o 190 ms, and 2:1 AV block was observed (Figure 1).ryoablation was immediately turned off, and normal con-uction was rapidly restored. The catheter was positionedlightly more posteriorly, and successful cryoablation waschieved.

iscussion

hese five patients demonstrate that adverse but reversibleffects on AV conduction may be observed during cryoab-ation, despite no evidence of deleterious effects duringryomapping. In these five cases, the delay or block in

igure 1 Patient 5: 2:1 AV block occurs 9 seconds into cryoa-lation. Cryothermal energy was discontinued immediately afterV block was observed. Normal AV conduction returned shortly

hereafter.

onduction through the AV node was detected within 15 A

econds of the change from cryomapping to cryoablationnd were rapidly reversed with discontinuation of tissueooling.

During the cryoablation procedure, test lesions are cre-ted by lowering the catheter tip temperature to �30°C for0 seconds, a process known as “cryomapping” or “freezeapping.” When the catheter tip temperature is lowered to30°C, the tip adheres to the endocardium, forming an ice

all. The local cooling leads to prolongation of the tissue’sefractory period and eventually conduction block.12 If theemperature is left at �30°C for an extended period of timeseveral minutes), permanent tissue damage ensues.

hereas permanent tissue damage results from sustainedooling, no permanent tissue injury results if the tissue isllowed to return to physiologic temperature following shortpplications of cryotherapy. When the desired electricalffects are observed during cryomapping, the catheter tipemperature is lowered to �70°C for 4 minutes, creating aermanent lesion in the myocardium with well-circum-cribed borders.

The mechanism of lesion creation by cryoablation isomplex,2 and much of the data on tissue injury have beenbtained in noncardiac muscle. Tissue injury occurs both byirect thermal injury induced by ice formation in the extrand intracellular space and by more poorly understoodechanisms induced by the freeze/thaw cycle. Additional

ell death occurs in the hours following the procedure viaecrosis and apoptosis.13 An advantage to the cryoablationrocedure is that the extracellular matrix and endocardiumre not disrupted by cryoablation, maintaining the tensiletrength of the myocardium and lowering the risk of perfo-ation and thromboembolism.6 Lesion size is dependent onhe temperature at the catheter tip, size of the catheter tip,nd the duration and number of freeze/thaw cycles. Thisariation may explain why electrophysiologic effects areifferent between cryomapping and cryoablation. Followinghe decrease in temperature at the catheter tip, the zone ofypothermia expands, impinging on the electrophysiologicroperties of a larger zone of myocardium. This possibilityas briefly noted by Gaita et al14 when they observed

ransient AH prolongation during the third minute of cryoa-lation during ablation of a septal accessory pathway.

With prolonged application of cryotherapy, the lesionxpands slightly, with extent determined by regional bloodow. Because the adverse effect on AV conduction oc-urred very early in our patients (�14 seconds) duringelivery of cryoablation (�70°C), one can infer that theatheter tip was close to the AV node. If the AV conductionoes not change until later in the application as the lesion isxpanding, it follows that the catheter tip is more distantrom the AV node. Thus, an early change in AV conductionuring cryoablation calls for immediate termination of cryo-hermal energy because it threatens the adjacent AV node. Aater change in AV conduction may or may not be betterolerated, as it suggests greater separation of catheter and

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557Fischbach et al Transient AV Conduction Block with Cryoablation

This experience underscores the fact that AV conductionay remain at risk following uneventful cryomapping that

emonstrated no effect on AV conduction, suggesting noisk to AV conduction. Fortunately, the observed AV blockas transient in all cases, with restoration of normal AV

onduction following tissue rewarming. This report servess a cautionary note that the operator needs to maintainigilance in monitoring AV nodal conduction during cryoa-lation, even when cryomapping suggests a safe location forV slow pathway ablation.

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