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etermining inferior vena cava–tricuspid isthmus blockfter typical atrial flutter ablation

rancisco G. Cosío, MD, Paula Awamleh, MD, Agustín Pastor, MD,mbrosio Núñez, MD

rom the Cardiology Service, Hospital Universitario de Getafe, Madrid, Spain.

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avotricuspid isthmus role in typical flutter

n typical atrial flutter, circular activation around the tricus-id ring is possible because the terminal crest preventshort-circuiting on the posterior wall, and the myocardiumetween the inferior vena cava (IVC) and the lower rim ofhe tricuspid ring is the obligatory pathway to close theircuit in the low right atrium (RA) (Figure 1). This IVC–ricuspid ring isthmus (cavotricuspid isthmus) has becomehe preferred target for ablation because it is the narrowestoint of the circuit, it is easily accessible, and it is locatedar from the AV junction. Variations of the circuit all sharehe cavotricuspid isthmus as the obligatory path in the lowA (Figure 1).

Bidirectional block at the cavotricuspid isthmus has becomehe accepted endpoint of ablation during sinus rhythm.1,2

hecking conduction/block at the isthmus

he cavotricuspid isthmus extends from the low anteriorA to the low septal RA, located posteromedially. During

ypical atrial flutter, activation through the cavotricuspidsthmus can be traced from its anterior to its posterior endor vice versa in reverse typical atrial flutter) by sequentialecordings with a roving catheter or by placing a multipolaratheter along the course of the cavotricuspid isthmus (Fig-re 2). Once typical atrial flutter is interrupted, conductions checked during pacing at both ends of the cavotricuspidsthmus, the anterior isthmus (or the low anterior RA) andhe posterior isthmus (or the low septal RA) (Figure 1). Bylacing an additional catheter in the coronary sinus (CS) ory advancing a multipolar reference catheter from the an-erior RA to the CS, CS os pacing often is substituted foracing the low septal RA because of catheter stability (Fig-re 2). The possibility of block at the eustachian ridge3

hould be considered when pacing at the CS os or the

Address reprint requests and correspondence: Francisco G. Cosío,hief Cardiology Service, Hospital Universitario de Getafe, Carretera deoledo, km 12.5, 28905 Getafe, Madrid.

cE-mail address: fcosio@vitanet.nu.

547-5271/$ -see front matter © 2005 Heart Rhythm Society. All rights reserved

osteroinferior septal RA, because pacing beyond this lineelays arrival of activation to the cavotricuspid isthmus andould mimic block (see later and Figure 1).

A activation sequence outside theavotricuspid isthmus

ith intact cavotricuspid isthmus conduction, ascendingctivation from pacing the low septal RA collides withescending activation on the anterior wall. With pacingrom the low anterior RA, the opposite occurs, with colli-ion of ascending and descending activation fronts on theeptal RA. Cavotricuspid isthmus block after ablation pre-ents ascending activation on the RA wall opposite theacing point. However, ablation can cause long local con-uction delays, indistinguishable from complete block (Fig-res 1 and 3B). Nevertheless, recording RA activation se-uence is helpful because the appearance of changes inctivation marks at least some degree of effectiveness andndicates when to check for complete block by recordingslong the ablation line.

CG changes with cavotricuspid isthmus block

-wave configuration when pacing the low RA can reflectavotricuspid isthmus block when loss of ascending activa-ion on the RA wall opposite the pacing site changes theerminal P wave from negative to positive in leads II, III,nd aVF (Figure 3). P-wave duration also can increase,specially with pacing the anterior isthmus4,5 (Figure 3).ecause P-wave change reflects RA activation and notavotricuspid isthmus activation, it will not allow differen-iation between complete block and very slow conduction.owever, P-wave change is a useful indicator of some

ffect taking place during RF application and helps to mon-tor recovery of isthmus conduction during a postablation

onfirmation period.6

. doi:10.1016/j.hrthm.2004.12.008

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329Cosío et al IVC–Tricuspid Isthmus Block After Typical Atrial Flutter Ablation

irect recording of isthmus conduction

ollision of two activation fronts

The demonstration of extremely slow conduction throughhe cavotricuspid isthmus after ablation shows the need forirect recordings from the cavotricuspid isthmus to confirmlock. A multipolar catheter, introduced from the IVC andurved in the RA to rest on the cavotricuspid isthmus, isommonly used. If the catheter tip is advanced to the CS os,t also can be used to pace both sides of the ablation lineFigure 2). Alternatively, a multipolar catheter can recordnd pace multiple sites of the anterior and septal RA, fromigh to low, using the ablation catheter to record the abla-ion line (Figure 2).

A complete activation sequence of the low RA, includinghe cavotricuspid isthmus, displays a double activation frontn the presence of conduction block: one early, enteringrom the pacing site, the other very late, entering the oppo-

igure 1 Schematic representation of the right atrium in the leftnterior oblique view. The tricuspid orifice is artificially increasedo show the endocardium with the openings of the superior venaava, inferior vena cava, and coronary sinus. A: Arrows showirection of activation during typical flutter (TFL) and lower loopLL) macroreentry closing the circuit through the terminal crest. B:ctivation in reverse typical flutter (RTFL). C: Activation se-uence pacing the anterior end of the cavotricuspid isthmus beforeblation showing collision of ascending and descending fronts onhe septal wall. D: Activation sequence pacing the septal end of theavotricuspid isthmus before ablation showing collision of ascend-ng and descending fronts on the anterior wall. E: Activationequence pacing the anterior end of the cavotricuspid isthmus aftersthmus block showing fully descending activation on the septalall. F: Activation sequence pacing the septal end of the cavotri-

uspid isthmus after isthmus block showing fully descending ac-ivation on the anterior wall. G: Activation sequence pacing theeptal end of the cavotricuspid isthmus with partial block (slowonduction) showing fully descending activation on the anteriorall. H: Block at the eustachian ridge causing delay in arrival of

ctivation to the cavotricuspid isthmus that can mimic cavotricus-id isthmus block.

ite end of the cavotricuspid isthmus. At the block site, the c

lectrogram shows two deflections separated 60 to 150 msy an isoelectric segment. However, the presence of theseouble electrograms should be checked along the entireblation line, from the tricuspid ring to the IVC7 because theidth of the cavotricuspid isthmus is enough to show locallock at one point and persistent conduction with continu-us electrogram recordings at another. In the presence oflow conduction, even if activation of the RA opposite theacing site is reversed, cavotricuspid isthmus recordingshow electrograms, often prolonged and fragmented, bridg-ng the interval between the stimulus and the opposite sidef the cavotricuspid isthmus.

Activation time across the cavotricuspid isthmus, ofteneasured as the separation between the components of the

ouble electrogram, has been used to indicate completelock. A positive predictive value of 100% has been pro-osed for a separation of 110 ms and a specificity of 80%or a 50% increase of transisthmus conduction time fromaseline. These values are not absolute, and the gold stan-ard remains the demonstration of opposite activation frontsolliding at the ablation line.8,9

hanges in local electrogram configuration

Cavotricuspid isthmus block shifts direction of activationn the side of the ablation line opposite from the pacing site,hich changes electrogram configuration. Unfiltered unipo-

ar electrograms show an RS configuration when activationoves through the area, changing to a single positive de-ection when block occurs and the recording site becomes

igure 2 Coronal (top left) and left anterior oblique (top right)agnetic resonance imaging cuts of the heart showing the position

f the cavotricuspid isthmus (CTI). Bottom: Anterior obliqueuoroscopic views of (A) multipolar catheter covering the anterioright atrium, CTI, and proximal coronary sinus and (B) multipolaratheter covering the septal and anterior right atrium and ablation

atheter on the CTI.

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dead end.10,11 The main problem with unipolar unfilteredlectrograms is that they are technically demanding because

igure 3 Top panel. A: Recordings from the right atrium (RA)nd cavotricuspid isthmus with the setup shown in Figure 2B.Roof � anterior roof; A1–A4 � anterior wall from top (1) toottom (4) close to anterior isthmus; CTI � cavotricuspid isthmus;1–S4� septal wall from bottom (4) to top (1); pRoof � posterioroof. A: Before CTI block, pacing the low septum, close to theTI, collision of activation occurs on the anterior wall. The CTIlectrogram is recorded between the stimulus and A4. B: With CTIlock, anterior wall activation is fully descending. The CTI showsdouble electrogram, with the second component recorded after4. C: Before CTI block, pacing the low anterior RA, close to theTI, shows descending activation of the septal RA, with almost

imultaneous S4 and S3 electrogram. The CTI electrogram isecorded between the stimulus and S4. D: After CTI block, pacinghe low anterior RA, close to the CTI, now shows a fully descend-ng activation of the septal wall with further delay and change inonfiguration of S3 and S4 electrograms. The CTI shows a veryate electrogram inscribed after S4. Note marked prolongation ofhe PR interval as a result of increased P wave duration. B: Falseppearance of CTI block in typical atrial flutter recurring afterblation. Bottom panel. A: Descending activation sequence of thenterior RA wall from high (A1) to low (A3) pacing close to theeptal CTI. B: Descending activation sequence of the septal RAall from high (S1) to low (S3) pacing close to the anterior CTI.: Typical atrial flutter with descending activation of the anteriorall and ascending of the septal wall, with a very slow cycle length

s a result of very slow conduction through the CTI. Note the veryide, continuous, fragmented CTI electrogram.

f sensitivity to artifact, baseline motion, and far-field p

otentials. The bipolar electrogram shows only relativehanges in configuration. Nevertheless, a change of polar-ty of the initial deflection of the electrogram closest to theblation line opposite the pacing site is a reliable sign oflock12 (Figure 3A, top panel).

ynamic changes: Differential pacing

he double electrograms recorded from the ablation lineay have variable separation (60–120 ms) often too narrow

o reach the absolute numbers indicative of complete block.n this situation, changes induced by moving the pacing siteway from the ablation line help demonstrate block. If thewo components of the electrogram represent slow conduc-ion from some unidentified gap in the ablation line, thenacing from higher on the septal or anterior walls increaseshe interval between stimulus and electrogram for bothomponents in parallel. On the other hand, if the two com-onents represent two opposing activations fronts, the de-ection corresponding to the same side of the ablation line

s delayed but that due to activation on the other side of theine is advanced, or not delayed, and the separation of theocal electrograms decreases13 (Figure 4).

igure 4 Differential pacing to diagnose cavotricuspid isthmusCTI) block. Recordings are labeled as in previous figures. Atrialchemas are constructed as in Figure 1. Numbers on top of the CTIlectrogram show the interval from stimulus to last component ofTI electrogram. Numbers below show separation of the twoomponents of the CTI electrogram. A: With pacing the lownterior right atrium, close to the CTI, septal wall activation isully descending. The CTI electrogram has two components, theast later than S4. B: Pacing 1 cm higher on the anterior wall. Notehe second component of the CTI electrogram is closer to thetimulus and electrogram separation decreases. C: Descendingnterior wall activation pacing the low septum, close to the CTI,nd a double electrogram in the CTI. D: Pacing 1 cm higher on theeptal wall. The interval from the stimulus to the second CTIlectrogram component decreases, as does the separation betweenhe electrogram components. The terminal portion of the P wave is

ositive in lead II.

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erminal crest permeability and clockwiselock

t is generally assumed that the terminal crest blocks con-uction during pacing at the usual cycle lengths of 500 to00 ms, but this is not true in a large proportion of pa-ients.14–16 It is especially during low septal or CS ostialacing that conduction across the terminal crest can producecollision pattern on the anterior RA wall, suggesting

onduction across the cavotricuspid isthmus (Figure 4).onduction through the terminal crest can be confirmed by

ecording a similar collision pattern pacing the low posteriorA with a shorter interval between the stimulus and the lownterior RA electrogram, which would be unexpected if itere due to conduction through the cavotricuspid isthmus

Figure 5).

nidirectional cavotricuspid isthmus block

nidirectional block has been observed in some cases afterblation, with a higher incidence of counterclockwise (an-eroposterior) unidirectional block6,17 possibly as a result ofocal anisotropy. Because diagnosis of block can be techni-ally difficult in a given direction, it appears important toonfirm block in both directions in every case so that bidi-ectional block is the endpoint in all cases.

ersistent versus reversible block

avotricuspid isthmus block can be transient, lasting from a

igure 5 Conduction through the terminal crest mimickingavotricuspid isthmus conduction. A: Collision of ascending (A4o A3) and descending (aRoof to A3) activation on the anterioright atrium (RA) pacing close to the septal end of the CTI. B:acing the low posterior RA with the same collision pattern on thenterior wall, but a shorter stimulus A4 interval. Atrial schemas areonstructed as in Figure 1.

ew seconds to minutes or more than 1 hour while observed

ontinuously in the laboratory. In fact, recovery of conduc-ion can occur probably hours or days after ablation, be-ause at least 5% of patients with bidirectional cavotricus-id isthmus block at the end of the procedure have recurrentutter at follow-up of 6 to 12 months.4 Conduction recoveryccurs in most of recurrences within 20 to 30 minutes ofbservation.7,18 This should be a minimum observation pe-iod after block is produced. Isoproterenol infusion has beenroposed to disclose reversible block.19 However, no evi-ence indicates this is better than an observation period. Inact, the incidence of conduction recovery after isoprotere-ol is very similar to that reported during a waiting period.

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