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Exploring Atrial Macroreentrant CircuitsETIENNE DELACRETAZ, M.D.

From the Swiss Cardiovascular Center Berne, University Hospital, Berne, Switzerland

Editorial Comment

Late after surgical correction of congenital heart dis-ease (CHD), patients frequently develop atrial tachyarrhyth-mias that lead to significant morbidity. While antiarrhythmicagents have limited efficacy in the treatment of these arrhyth-mias, recent studies have demonstrated the feasibility of usingradiofrequency (RF) ablation to treat them.1-4 However, RFablation may be challenging because of complex arrhythmiamechanisms.

In operated CHD patients, the right atrium can supporta variety of arrhythmia circuits referred to as macroreen-trant tachycardias (MRT) or intraatrial reentrant tachycardia(IART).3-5 Isthmus-dependent atrial flutter may frequentlyoccur.6,7 Other arrhythmia mechanisms have been described,including reentry related to the lateral right atriotomy scar (orincisional atrial tachycardia),2,8 and less commonly, lower-loop or higher-loop atrial flutter (around the inferior or su-perior vena cava), reentry around a septal patch or linkedto the coronary sinus, and left atrial flutter.4 Tachycardiaswith an apparently focal origin have also been described.The availability of three-dimensional mapping techniqueshas greatly improved our understanding of these arrhythmiamechanisms.9,10

In this issue of the Journal, the study by Magnin-Poullet al.11 confirms that isthmus-dependent flutter is the mostcommon ensuing reentry mechanism in patients late after sur-gical repair of an atrium septal defect.6 Using extensive three-dimensional electroanatomic mapping data acquired duringsinus rhythm and tachycardia, the authors relate lines of dou-ble potentials to sites of conduction block and depict patternsof wavefront propagation within the right atrium. The posi-tions of these lines of block match the location of the surgicalincisions made parallel to the intercaval line. In a majorityof patients, there is a relatively narrow isthmus between theatriotomy scar and the inferior vena cava, which is a suitabletarget for RF ablation if this low pivot point is critical for themaintenance of a periatriotomy reentry. In a few patients, theatriotomy scar had two segments separated by an isthmus ofconducting myocardium, while in some other patients, therewere two parallel intercaval lines of block, outlining an ad-ditional potential corridor for reentry. These findings reflectthe diversity of the arrhythmia substrates in CHD patients.

Magnin-Poull et al.11 claim that 70% of patients withIART late after surgical closure of an atrial septum defect have

J Cardiovasc Electrophysiol, Vol. 16, pp. 688-689, July 2005.

Etienne Delacretaz is supported by a grant from the Swiss National Foun-dation for Scientific Research.

Address for correspondence: Etienne Delacretaz, M.D., F.E.S.C., SwissCardiovascular Center Berne, University Hospital, CH-3010 Berne,Switzerland. Fax: +41 31 632 14 14; E-mail: etienne.delacretaz@insel.ch

doi: 10.1111/j.1540-8167.2005.50121.x

double-loop reentrant circuits, mainly based on the visualiza-tion of lines of conduction block and high-density activationmaps. When confronted with figure-eight reentrant circuits,it is essential to determine whether the “second loop” is acodominant or a nondominant loop, which will keep rotatingonce the first loop is abolished by RF ablation, or whether it isa dead-end pathway, an “innocent” bystander that will not re-quire additional treatment. This issue cannot be addressed byactivation mapping alone. The problem is relevant, since thecommon isthmus of double-loop atrial circuits (contrastingwith that of ventricular circuits) is usually not a vulnerablesite and cannot easily be transected with a short line of RFablation (see below).

Figure-eight reentry was first described by El-Sherif inepicardial mapping studies of postinfarction dog models.12

Figure-eight circuits were then demonstrated to be frequentin scar-related ventricular tachycardia in humans, and func-tional classification of the circuit sites using entrainment map-ping was shown to be essential to facilitate RF ablation.13 Inthe right atrium, most of the reentry circuits have a single-loop pattern.4 CHD patients often have more than one circuit,but the circuits do not rotate simultaneously.4,7,14,15 How-ever, more complex reentry patterns have been inferred by anumber of electrophysiologists who had to grapple over thetransformation of tachycardia during RF ablation of IART.Shah et al.16 were the first investigators to delineate an entirecircuit of dual-loop atrial reentry in 5 patients late after surgi-cal closure of an atrial septum defect. Although double-loopactivation circuits may have been overlooked in some series,they appear to be uncommon. In a series of 36 CHD patientswith IART, dual-loop atrial reentry was present in only 3 pa-tients (8%).17 This is in contradiction with Magnin-Poull’sassertion.

The term “double-loop” or “dual-loop” reentry should bereserved to those circuits in which there is a functional sec-ond loop that is either a codominant or nondominant loop,according to the terminology defined for ventricular tachy-cardia circuits. The second loop will continue to rotate afterdisconnection of the first tachycardia limb and needs to beaddressed with ablation. The term should not be used forcircuits with an “innocent” dead-end pathway. The absolutedemonstration of a double-loop reentry requires transforma-tion of the tachycardia into a different tachycardia, without apause in between, with some change in F-wave morphologyor intracardiac activation pattern (as in the case illustrated inFigure 5 of Magnin-Poull’s article). Tachycardia changes aremost often associated with a sudden change of tachycardiacycle length, although this is not essential for the diagnosis ofdouble-loop reentry. Sudden slowing of tachycardia duringRF current application may also be due to the creation of slowconduction within the reentry isthmus being neutralized.

What is the best approach if a double-loop reentry issuspected?

Entrainment mapping is an important complement tothree-dimensional activation mapping in the exploration of

Delacretaz Editorial Comment 689

arrhythmias in CHD patients.7,18 It has also proved useful toinvestigate double-loop circuits, but entrainment data need tobe carefully interpreted.19 For example, if the revolution timeof both circuits is different (>20-second difference), entrain-ment mapping at sites within the nondominant (slower) loopshows long postpacing intervals and classifies the sites as“out of the circuit.” On the other hand, entrainment mappingis useful to examine the effect of RF ablation: comparisonof entrainment mapping before and after elimination of afirst conduction isthmus allows one to demonstrate whethera limb of the circuit has been successfully disconnected fromthe remaining circuit.19 In most cases of double-loop reentrycircuits described to date, the common isthmus is situatedbetween the lateral aspect of the tricuspid annulus and a rightatrial lateral longitudinal scar. Stabilization of the ablationcatheter may be difficult in this location. Moreover, the corri-dor between the atriotomy scar and the tricuspid annulus maybe broad, as observed in the study of Magnin-Poull11 (on av-erage 41 mm). Thus, ablation of two separate isthmuses isusually preferred to an attempt to transect the common isth-mus. During RF ablation, detecting an abrupt change of theactivation pattern or cycle length is crucial, and remappingafter this change may be necessary.

Reoccurring arrhythmias requiring repeat ablation proce-dures were frequent (54%) in Dr. Magnin-Poull’s series, eventhough conduction block was documented following abla-tion. A possible interpretation is that conventional cathetersmay not allow the creation of adequate RF lesions. Maximalpower delivered during RF current application is sometimeslimited due to insufficient electrode cooling in some atrialrecesses with low blood flow, and the depth of the lesion maynot be sufficient to create a transmural scar after the heal-ing process. Several studies suggest that using irrigated-tipcatheters may be helpful in patients with CHD.17,20 Anothernoteworthy point is that CHD patients who underwent suc-cessful ablation of one circuit may develop new arrhythmiacircuits over a longer follow-up period. Thus, RF ablation inthese patients should not be considered to be curative beforelong-term follow-up is available.

In conclusion, the treatment of arrhythmias by catheterablation in the CHD population offers unique challengesthat inspires further investigation. The interesting article byMagnin-Poull brings to our attention the variability and com-plexity of arrhythmia substrates in patients late after sur-gical repair of CHD. Double-loop reentry circuits are apotential cause for ablation failure and should be routinelyinvestigated. Although there are several approaches to studyarrhythmia circuits, entrainment mapping at a few key lo-cations rapidly delivers important information. Entrainmentmapping should be viewed as an invaluable opportunity to ex-plore the fascinating physiology of complex reentry circuitsbefore “burning.” But ultimately, we should not be embar-rassed by “learning by burning,” at least as far as the ablationof complex reentry circuits is concerned.

References

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2. Kalman JM, VanHare GF, Olgin JE, Saxon LA, Stark SI, Lesh MD: Ab-lation of ‘incisional’ reentrant atrial tachycardia complicating surgery

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4. Cosio FG, Martin-Penato A, Pastor A, Nunez A, Goicolea A: Atypicalflutter: A review. Pacing Clin Electrophysiol 2003;26:2157-2169.

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