Atypical Atrial Flutter A.S. M

A.S. M

Background

With the use of endocardial activation mapping and stimulation studies, sev- eral investigators have shown that different types of macroreentrant atrial tachycardias are possible. The circuit is usually located in the right atrium and a critical component of slow conduction is frequently present, often located at the isthmus of atrial tissue which is between the tricuspid annu- lus, the inferior vena cava, and the coronary sinus with eustachian valve and ridge. Of the different types of atrial flutter, the typical common or type 1 atrial flutter is the most frequent. In this type of flutter right atrial activation rotates in a counterclockwise direction. Less common are reverse (clockwise) typical flutter. More recently, other types of isthmus-dependent and non- isthmus-dependent flutter patterns have been described, even including left atrial flutter [1–3].

Lower Loop Atrial Flutter

Lower loop reentry is defined as macroreentrant tachycardia maintained by circus movement of the activation wave front around the inferior vena cava instead of around the tricuspid annulus as in typical right atrial flutter. An earlier study from Cheng et al. [1] showed that early breakthrough over the tricuspid annulus occurred over the low lateral right atrium. Later on the same group extended these observations showing that one annular break or more could occur at the lateral or anterolateral regions of the annulus [2].

Lower loop reentry uses the same isthmus between the tricuspid annulus and inferior vena cava as in common atrial flutter and therefore is similarly

Department of Cardiology and Arrhythmia Centre, Policlinico MultiMedica, Sesto S.

Giovanni (Milan), Italy

amendable by ablation of the isthmus. Recently Zhang et al. [4] have investi- gated the exact reentry circuit in patients with atrial flutter that was associ- ated with surface ECG flutter wave morphology and endocardial recordings that are characteristic of clockwise atrial flutter. The major finding of this study is that most of the isthmus between the tricuspid annulus and the infe- rior vena cava dependent-flutter with positive flutter wave in the inferior ECG leads involves a reentrant circuit around the inferior vena cava. It pro- vided the first evidence that figure-of-eight double loop reentry may occur around the inferior vena cava and tricuspid annulus and mimic typical clockwise atrial flutter in patients without prior atriotomy. Because the tri- cuspid isthmus constitutes the common pathway between the two reentrant loops, this double loop reentry is also amendable by ablation that results in bidirectional conduction block.

SI Short Circuit

Earlier studies by Olgin et al. [5] and Nakagawa et al. [6] showed that break- through over the eustachian ridge posterior to the coronary sinus (CS) ostium may be observed in 25–50% of patients with typical forms of flutter.

In these reports, there was almost simultaneous activation of the septum by wave fronts advancing both anterior and posterior to the CS. Later Yang et al.

[2] showed another novel finding: demonstration of a circuit with early acti- vation of the CS region. In this circuit, a typical counterclockwise wave front negotiated the lateral portion of the isthmus and skirted posterior to the CS ostium and the septum. One possible explanation is the presence of a pecti- nate muscle band from the crista effectively separating the isthmus into anterior and posterior compartments. The authors defined this type of flut- ter as ‘partial isthmus-dependent’ atrial flutter. Bidirectional isthmus block induced by a radiofrequency lesion would terminate the tachycardia.

Upper Loop Reentry Tachycardia

Upper loop reentry (ULR) is interpreted as the ‘converse’ of lower loop reen-

try (LLR), with a clockwise circuit and break over the lateral or anterolateral

annulus with impulse collision in the isthmus. It should be emphasised that

electroanatomic mapping studies were not available during ULR; hence, the

precise confines of the circuit are not clear, although detailed entrainment

mapping in some of these patients showed concealed entrainment at the pos-

terior right atrial septal region between the fossa ovalis and either the supe-

rior or the inferior vena cava. In addition, work by Yang et al. [2] supports

the finding of spontaneous conversion of either typical clockwise flutter or

LLR to ULR. In one patient with LLR and multiple breakthroughs, conduc- tion block over the isthmus was associated with the start of an ULR loop.

Previous reports [7] have described an atypical flutter circuit similar to the ULR described by Yang et al. [2]. In addition, a very complete report by Shah et al. [8], who used electroanatomic mapping, revealed a variable pat- tern of activation of the superior right atrium in patients with typical coun- terclockwise flutter. They showed an apparent isthmus between the superior vena cava and the superior portion of the tricuspid annulus. They hypothe- sise that ULR might use the channel between these structures. The descrip- tions provided by Yang et al. [2] of hypothetical circuits were derived largely from deductive reasoning based on typical flutter circuits. They appreciated that precise delineation of the tachycardia circuit(s) was not possible with- out advanced imaging techniques.

Scar-Related Atrial Tachycardia

Patients with who have previously undergone atriotomy may suffer from a scar-related tachycardia that mimics a counterclockwise atrial flutter. This tachycardia arises from a large low-voltage area of the posterolateral right atrium and travels around the scar with more than one critical isthmus. The identification and ablation of these isthmi may represent a challenge.

Left Atrial Flutter

Few data are available about left atrial flutters. In a dog model, Schuessler et al. [9] demonstrated atrial flutters, usually rotating around anatomical and functional zones of block. However, in dogs with enlarged and/or hypertro- phied left atria, most of the circuits were located in the right atrium. Pure left atrial circuits were rarely found, usually rotating around the pulmonary vein (four dogs in Schuessler’s experience). Therefore, both human and ani- mal data suggest that the right atrium is more frequently involved in flutters than the left. However, the exact incidence of left atrial flutters in humans is currently unknown.

The diagnosis of left atrial flutter can be established by comprehensive mapping, including a 3D electroanatomic system, and confirmed by the results of catheter ablation achieving sinus rhythm. Various circuits can be demonstrated. In most cases, the arrhythmia may rotate around the mitral annulus, a zone of block including the pulmonary veins, or a silent area.

Lines of block and silent areas also act as lateral barriers, probably allowing

stabilisation of the circuit and preventing short circuiting. In a few patients,

the circuit is more complex, with two or three loops rotating concomitantly.

There is no marked area of slow conduction in these macroreentrant cir- cuits, in contrast to the cases of small reentrant circuits, in which a zone of very slow conduction has been demonstrated, accounting for more than two- thirds of the cycle length. Slow-conduction areas have been frequently reported in animal models, usually being the centre of the circuit, either alone or in association with anatomical obstacles. In contrast, a silent area has not previously been reported clinically. It seems to be a distinctive and relatively common feature of human left atrial flutter, present in 50% of the patients in the series of Jaïs et al. [10]. This is probably related to severe atri- al fibrosis (and atrial myocardial cell modification/disappearance), a com- mon phenomenon in patients with structural heart disease. It may also be possible that in patients who have suffered from atrial arrhythmias for many years, histological changes have occurred as a result of atrial arrhythmia.

Marrouche et al. [11] recently reported on patients with a novel macroreen- trant left atrial arrhythmia: the so-called left septal atrial flutter. Most of the patients in this study developed left atrial flutter after initiation of anti- arrhythmic therapy for atrial fibrillation. Slowing of atrial conduction by anti-arrhythmic drugs may be a factor that allows left septal flutter. The zone of double potentials anterior to the right pulmonary veins, corresponding to the limbus of the fossa ovalis for the right atrium, and the mitral annulus represent functional and anatomical barriers for left septal atrial flutter, respectively. In addition, the results from the Marrouche series appeared to suggest that the anterior ablation approach is more likely to provide a long- term cure. Concealed entrainment has been used to identify a protected isth- mus between barriers, which are critical for the maintenance of arrhytmia.

Concealed entrainment documented by pacing between the zone of double

potentials between the right pulmonary veins and the septum primum and

between the septum primum and the mitral annulus. Thus, the left septal

atrial flutter circuit appeared to revolve around the septum primum with the

use of two protected isthmi. The zone of double potential seems to play a

role similar to that of the crista terminalis during typical atrial flutter. The

zone of double potentials recorded in patients corresponds to the limbus of

the fossa ovalis on the right septum, which seems to complete the embry-

ological separation of the two atria. The region between the septum primum

and the mitral annulus defines another critical isthmus for the left septal

atrial flutter circuit. A similar situation has been described by Nagakawa et

al. [6] in the right atrium, where the tricuspid annulus forms a continuous

anterior barrier in patients with typical flutter. Marrouche has also demon-

strated a similar role of the septal part of the mitral annulus in forming an

anterior anatomical barrier for the left septal flutter circuit. The left atrial

septum possesses a unique muscular architecture that would allow the main-

tenance of left septal atrial flutter. Moreover, Jaïs et al. [10] also recognised

the mitral annulus as a critical anatomical barrier in 36% of patients with left atrial flutter. Linear radiofrequency lesions between the membranous septum and the mitral annulus seem to be an effective and safe therapy for this arrhythmia.

Conclusions

The identification of the macroreentrant nature of atrial flutter and the abili- ty to localise the circuit by endocardial activation mapping and pacing resulted in attempts to interrupt the circuit by ablative interventions.

Nowadays, catheter ablation of atrial flutter has become a safe, curative, and highly successful procedure, particularly when the right atrial isthmus is incorporated in the flutter circuit. Demonstration of bidirectional isthmus block after ablation predicts a high long-term success rate. Scar-related and left atrial flutters present more complex patterns of activation, making the ablation more difficult and the 3D system often mandatory.

References

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