Macroreentrant atrial tachycardia

Macroreentry

The mechanism of MRAT is reentrant activation around a large central obstacle, generally several centimeters in diameter, at least in one of its dimensions. The central obstacle can consist of normal or abnormal structures and can be fixed, functional, or a combination of both (see Video 7.7 ). There is no single point of origin of activation, and atrial tissues outside the circuit are activated from various parts of the circuit. The CL of MRAT can vary widely and is not a reliable predictor of the mechanism. When the mechanism of MRAT can be further elucidated through the different mapping techniques, description of MRAT mechanisms must be made in relation to atrial anatomy, including a detailed description of the obstacles or boundaries of the circuit as well as the critical isthmuses.

Dual-loop reentry

Although MRAT can manifest as an isolated arrhythmia ( single-loop reentry ), it can also occur in conjunction with other reentry circuits (e.g., incisional MRAT and typical AFL). When two atrial macroreentrant circuits coexist and use neighboring anatomical structures, they create the so-called dual-loop reentry . The multiple loops can be engaged simultaneously or encountered sequentially. Not uncommonly, ablation of one tachycardia results in transition to the other; hence, ablation of both circuits is necessary for clinical success.

A figure-of-eight reentry is a specific type of dual-loop reentry involving two simultaneously coexisting loops with opposite directions of propagation and sharing a common segment of unidirectional activation.

Localized reentry

The term localized reentry has been used to refer to a reentrant circuit that is localized to a small area (covering a surface diameter of <2–3 cm) and does not have a central obstacle (see Video 14.1 ). Theses circuits typically form in close proximity to scars or prior ablation lines, where severe local conduction abnormalities allow for very slow conduction to support perpetuation reentry in such as small anatomical path. Typically, low-amplitude, long, fractionated electrograms occupying more than 70% of the tachycardia CL (TCL) can be recorded in a small region of the atrium, from where activation spreads out centrifugally to activate the rest of the atrium ( Fig. 14.2 ). Hence, these localized reentry circuits can mimic focal AT, especially if limited numbers of endocardial recordings are collected. Although localized reentry circuits are much smaller than macroreentry circuits, they are still significantly larger than focal “microreentrant” AT, whose site of origin cannot be mapped spatially beyond a single point or a few adjacent points with the resolution of a standard 4-mm-tip catheter.

Lower loop reentry

Lower loop reentry is a form of CTI-dependent MRAT with a reentrant circuit around the inferior vena cava (IVC); therefore, it is confined to the lower part of the RA ( Figs. 14.1 and 14.3 ) ( Video 14.2 ). The activation wavefront can rotate around the IVC in a counterclockwise (i.e., the impulse travels within the CTI in a medial-to-lateral direction) or clockwise fashion. A breakdown in the inferoposterior boundaries of the CTI produced by the Eustachian ridge and lower crista terminalis causes the circuit to revolve around the IVC (instead of around the tricuspid annulus), across the Eustachian ridge, and through the crista terminalis, with slow conduction because of transverse activation through that structure. Alternatively, the circuit can exit at the apex of the triangle of Koch and come behind the Eustachian ridge to break through across the crista terminalis behind the IVC and then return to the CTI.

Lower loop reentry often coexists with counterclockwise or clockwise typical AFL and is usually transient and terminates by itself or converts spontaneously into AFL or atrial fibrillation (AF).

Intra-isthmus reentry

Intra-isthmus reentry is a reentrant circuit usually occurring within the region bounded by the medial CTI and coronary sinus ostium (CS os). Circuits in the mid and lateral portion of the CTI can also occur but are less common.

This arrhythmia usually occurs in patients who have undergone prior, and often extensive, ablation at the CTI. Catheter ablation can result in islands of scarred areas and slowly conducting channels that can potentially provide the substrate for this arrhythmia. In one report, intra-isthmus reentry accounted for 21% of cases of “recurrent” AFL following prior successful CTI ablation. Of note, persistence of bidirectional CTI block following ablation of typical AFL does not exclude the possibility of intra-isthmus reentry, since the reentry circuit can exist medial to the line of CTI block. The reentry circuit can be quite small; fractionated potentials spanning one to two-thirds of the TCL are typically recorded within the CTI ( eFig. 14.1 ).

The atrial activation sequence around the tricuspid annulus can exhibit spontaneous or pacing-induced shifts from counterclockwise to clockwise or fusion patterns. Additionally, entrainment mapping from the lateral CTI demonstrates a postpacing interval (PPI) longer than the TCL, a finding indicating that the lateral CTI is not part of the reentrant circuit. On the other hand, pacing from the region of the medial CTI or CS os demonstrates concealed entrainment with PPI equal to the TCL. Linear ablation across the medial CTI, usually at the site of a very prolonged electrogram, can eliminate the tachycardia.

Incisional right atrial macroreentry

Incisional or scar-mediated RA macroreentry circuits are the most common form of non-CTI-dependent RA MRATs. The incision on the RA free wall is a common access site for corrective surgery for congenital or acquired heart disease. The atriotomy scar, suture lines, and cannulation sites form fixed obstacles that can potentially promote reentry by providing multiple protected isthmuses along with natural conduction barriers (such as valve annuli and caval ostia) and regions of atrial scarring caused by the underlying heart disease. In its simplest form, incisional RA MRAT utilizes a circuit rotating around the atriotomy scar, with the lower turning point located between the end of the scar and the IVC, and the upper turning point located between the upper end of the scar or the superior vena cava (SVC) ( Video 14.3 ). Other obstacles can also be included in the reentry circuit, such as anatomical structures located in the vicinity of the scar (e.g., SVC or IVC) or functional obstacles (anisotropic conduction delay or block).

These patients often exhibit multiple clinical or inducible tachycardias, particularly typical AFL, indicating the complexity of the atrial substrate (see Fig. 15.1 ). Importantly, the atriotomy scar in the lateral or posterolateral RA forms a fixed posterior barrier to conduction in the superoinferior direction between the venae cavae, and a lateral boundary necessary for the development of a peritricuspid macroreentry (typical AFL) by preventing short-circuiting of the tricuspid annulus via the posterior atrium. This explains why typical AFL is the most common AT in this patient population, accounting for more than 70% of all MRATs. Other RA MRATs involving free wall atriotomy become progressively more common with more extensive atrial incisions.

RA free wall atriotomy is also widely used to access the left atrial (LA) for surgical replacement of the mitral valve. In fact, RA MRATs are more common than LA macroreentry in this group of patients. Other approaches for mitral valve surgeries involve incisions through the interatrial (Waterston) groove, transseptal incisions, and the superior transseptal approach. The latter surgical approach utilizes an extensive series of incisions in the lateral and superior RA, the interatrial septum, and the LA roof. Consequently, the superior transseptal approach has been associated with higher risk of RA MRATs.

Patients with congenital heart disease have a high prevalence of MRAT, particularly after they have undergone reparative or palliative surgical procedures. For MRATs in adults with repaired congenital heart disease, three RA circuits are generally identified: (1) lateral wall circuits with reentry around or related to the lateral atriotomy scar; (2) septal circuits with reentry around an atrial septal prosthetic patch (when present); and (3) typical AFL circuits using the CTI. These arrhythmias are discussed separately in Chapter 15 .

Right atrial macroreentry in the absence of previous atrial surgery

Rarely, RA MRATs have been observed in patients without prior atrial surgery (i.e., no prior atriotomy) in whom the central obstacle is a large area of abnormal atrial myocardium and electrically silent zones, indicating scarring ( Video 14.4 ). The mechanism of atrial scarring remains uncertain, but it has been suggested that these low-voltage areas may reflect areas of fibrosis potentially related to inflammatory, infiltrative, or ischemic processes. These patients frequently exhibit multiple clinical or inducible tachycardias. The most common circuit involves macroreentry around the RA free wall, but the vast majority of these patients also have clinical or inducible typical AFL, and many had prior CTI ablation. The electrically silent area forms a fixed posterior conduction barrier, which likely facilitates the development of peritricuspid reentry.

Upper loop reentry

This type of MRAT involves the upper portion of the RA, with transverse conduction across the crista terminalis and wavefront collision occurring at a lower part of the RA or within the CTI. The central obstacle of the circuit is composed of the crista terminalis (functional) and the SVC (fixed) ( Video 14.5 ). The impulse rotating in the circuit can be in a counterclockwise or clockwise direction (see Fig. 15.10 ). The CTI is not an intrinsic part of the reentrant circuit. The lower turnaround point for this circuit is by conduction through a gap in functional block in the crista terminalis. Ablation of this gap can abolish the arrhythmia. Upper loop reentry can exist as a single loop or as a part of a dual-loop (figure-of-eight) reentry with lower-loop or with free-wall circuits.

Perimitral left atrial macroreentry

Perimitral macroreentry circuit involves reentry around the mitral annulus in a counterclockwise or clockwise fashion ( Fig. 14.4 ) ( Video 14.6 ). The mitral annulus forms the anterior boundary of the reentry circuit, while the posterior boundary is formed by low-voltage zone or scar in the posterior wall of the LA.

Perimitral MRAT is more common in patients with structural heart disease; however, it has been described in patients without obvious structural heart disease. In the latter group, voltage mapping often shows scar or low-voltage areas on the posterior wall of the LA forming the posterior boundary of the reentry circuit. As noted, perimitral macroreentry is the most common MRAT in patients with prior LA ablation procedures for AF, particularly when incomplete linear ablation across the lateral mitral isthmus has been performed.

Incisional left atrial macroreentry

A variety of incisional LA macroreentry circuits have been described in patients with prior cardiac surgery involving the LA or atrial septum. The atriotomy scar forms a fixed obstacle that, in combination with the often intrinsically diseased atrial myocardium (providing functional obstacles) and normal structural obstacles in the vicinity of atrial incisions, can provide the substrate for macroreentry. Following mitral valve surgery, the risk of AT does depend on the surgical approach, with AT being more likely to occur among patients who had a superior transseptal approach for mitral valve surgery compared with the left atrial approach.

Left atrial macroreentry post ablation of atrial fibrillation

ATs occur frequently after catheter ablation of AF, and LA macroreentry accounts for the majority of organized ATs. The incidence and nature of ATs developing post AF ablation are, in large part, determined by the ablation strategy and energy sources employed in the index procedure as well as the underlying arrhythmogenic substrate (see Chapter 18 ). The incidence seems to be lower following segmental ostial pulmonary vein (PV) isolation than circumferential PV isolation and higher following circumferential or linear LA ablation (up to 30%). The incidence of MRAT is much higher (up to 50%) for approaches of catheter ablation incorporating extensive lesions in the LA to terminate persistent AF. Linear ablation lesions, together with anatomical structures and intrinsically abnormal adjacent atrial myocardium, provide an ideal substrate for reentry. Gaps in the ablation lines further increase the chance for reentry by creating a region of slow conduction ( Fig. 14.2 ).

The most common form of macroreentry occurring after AF ablation is perimitral MRAT (accounting for >40% of all MRATs) ( eFig. 14.2 ). Macroreentrant circuits traversing the LA roof account for approximately 20%. Less common sites of macroreentry involve the right or left PVs, LA septum, and the base of the LA appendage ( Fig. 14.5 , eFig. 14.3 ). Not infrequently, multiple macroreentrant circuits and multiple-loop circuits are encountered ( eFig. 14.2 ). Additionally, localized reentrant circuits comprise a sizable minority of the cases, and they usually arise from the vicinity of the isolated PVs or linear lesions ( Fig. 14.2 ) ( Video 14.7 ). ^,

LA MRATs comprise the majority of arrhythmias developing following surgical ablation of AF. Perimitral macroreentry is the most common, but macroreentry circuits incorporating the LA roof, septum, and posterior wall, and circuits around the PVs also have been observed. Localized reentrant ATs comprise a sizable minority of the cases.

Circuits involving the pulmonary veins

Various reentrant circuits involve the PVs, especially in patients with AF or mitral valve disease and those with prior catheter ablation of AF (particularly with linear LA ablation lesions), including circuits around one or more PVs ( Fig. 14.5 ) and posterior scar or low-voltage areas. PV-LA conduction recovery is frequently a critical element in these arrhythmias.

Left septal circuits

Left septal circuits occur primarily in patients with previous heart surgery, particularly after mitral valve surgery requiring septal incision, and those with prior AF ablation. Rarely, MRATs involving the LA septum have been reported in the absence of prior cardiac surgery. These circuits involve the septum primum, which acts as a central obstacle for the reentrant circuit. The right PV ostia serve as its posterior boundary, whereas the mitral annulus serves as the anterior boundary. Atrial dilation and concomitant antiarrhythmic drug therapy also appear to play a role via the prolongation of left intraatrial conduction, which then allows stable macroreentry circuits to persist. In patients with a history of surgery for atrial septal defects, scars or the patch on the septum can serve as the anatomical substrate of left septal circuits.

Left atrial macroreentry in the absence of previous surgery or ablation

LA circuits can be found in patients without a history of atriotomy or prior ablation. Electroanatomical maps in these patients often show electrically silent (low-voltage or scar) areas in the LA, which act as a central obstacle or barrier in the circuit. These low-voltage areas appear to be more frequently located in the anterior LA, where they form a corridor of slow conduction between the scar and the mitral annulus. The pathogenesis of these areas with no electrical signals is not well established. Potential causes include volume and pressure overload (mitral valve disease, hypertension, heart failure), ischemia (atrial branch occlusion), postinflammation scarring (after myocarditis), atrial amyloidosis, atrial dysplasia, and tachycardia-related structural remodeling. A novel type of cardiomyopathy (“isolated fibrotic atrial cardiomyopathy”) was recently described and appears to underly some cases of the unexplained scar-related MRATs in younger patients. These conditions are frequently associated with diffuse or patchy areas of scarring, which are typically located at the posterior wall (45%), roof (28%), or anteroseptal region (27%) of the LA. The macroreentrant circuits in these patients show considerable anatomical variability and frequently involve multiple simultaneous loops (dual- or triple-loop reentry, or figure-of-eight reentry).

Incisional right atrial macroreentry

The surface ECG morphology of free wall MRAT in a patient with a previous atriotomy is highly variable, depending on factors including the location of the scars and low-voltage areas, the direction of rotation, the presence of coexisting conduction block in the atrium, and the presence of a simultaneous typical AFL. The morphology of the atrial complex on the surface ECG can range from that similar to typical AFL to one characteristic of focal AT (see Fig. 15.2 ). Often, inverted P waves can be observed in lead V ₁ . Depending on the predominant direction of septal activation, RA free wall MRAT can mimic either clockwise or counterclockwise typical AFL.

Upper loop reentry

The surface ECG of upper loop reentry closely mimics that of clockwise typical AFL (negative P waves in lead V ₁ and positive P waves in the inferior leads) because, in most cases, both arrhythmias share a similar activation sequence of the LA, the septum, and caudocranial activation of the lateral RA. However, negative or isoelectric or flat P waves in lead I favor upper loop reentry over clockwise typical AFL. Conversely, positive P waves in lead I with an amplitude of more than 0.07 mV favor clockwise typical AFL. Additionally, the CL of upper loop reentry is usually shorter in comparison with typical AFL.

Lower loop reentry

The surface morphology of lower loop reentry is highly variable and can be similar to that of counterclockwise or clockwise typical AFL, but lower loop reentry associated with somewhat higher crista terminalis breaks can produce unusual ECG patterns. Sometimes, the changes are manifested by decreased amplitude of the late positive waves in the inferior leads, probably as a result of wavefront collision over the lateral RA wall.

Intra-isthmus reentry

The surface ECG in intra-isthmus reentry commonly exhibits a pattern similar to counterclockwise typical AFL. The same ECG pattern can exist whether the intracardiac atrial activation sequence around the tricuspid annulus exhibits a counterclockwise, clockwise, or fusion pattern. The proximity of the exit site of reentrant circuit to the CS os promotes more rapid CS and septal activation of the LA, which largely determine inscription of the P wave, regardless of the peritricuspid activation pattern.

Perimitral left atrial macroreentry

Most of these tachycardias show prominent forces in leads V ₁ and V ₂ , with diminished amplitude in the inferior leads ( Fig. 14.4 ). It has been suggested that a posterior LA scar allows for domination by anterior LA forces. This constellation of findings can mimic counterclockwise or clockwise typical AFL, but the decreased amplitude of frontal plane forces suggests an LA circuit. In patients with prior PV isolation procedures, the surface ECG morphology of counterclockwise perimitral MRAT can be different from that in patients without prior ablation, possibly related to varying degrees of prior LA ablation or scar. In these patients, counterclockwise perimitral MRAT demonstrates positive P waves in the inferior and precordial leads and a significant negative component in leads I and aVL. Furthermore, counterclockwise perimitral MRAT in these patients can have a morphology similar to that of left PV ATs. However, counterclockwise perimitral MRAT is suggested in the presence of a more negative component in lead I, an initial negative component in lead V ₂ , and a lack of any isoelectric interval between P waves. Clockwise perimitral MRAT has a limb lead morphology that is the converse of that of counterclockwise perimitral MRAT and an initial negative component in the lateral precordial leads. A positive P wave in leads I and aVL differentiates clockwise perimitral MRAT from counterclockwise CTI-dependent AFL and left PV AT.

Pulmonary vein circuits

Because these circuits are related to low-voltage or scar areas, the surface ECG usually shows low-amplitude or flat P waves. These tachycardias have the most variable surface ECG patterns.

Left septal circuits

Because the reentry circuit is on the septum, the surface ECG shows prominent, usually positive, P waves only in lead V ₁ or V ₂ and almost flat waves in most other leads. This pattern can be caused by a septal circuit with anteroposterior forces projecting in lead V _{1 1} and the cancellation of caudocranial forces. This pattern was 100% sensitive for an LA septal circuit, but the specificity of this pattern for any type of LA MRAT was only 64%.

Atrial extrastimulation during tachycardia

An AES commonly results in resetting of the MRAT circuit. The closer the site of atrial stimulation to tissue in the circuit, the easier the resetting of the MRAT circuit at longer coupling intervals. In response to AES, MRATs typically demonstrate an increasing or mixed (flat, then increasing) resetting response. AES usually fails to terminate the AT. The ability to capture the atrium without affecting (resetting) the MRAT circuit timing indicates that the pacing site is outside the MRAT circuit. Focal ATs and small reentry circuits are characterized by the ability of AES to dissociate large portions of the entire atrial mass from the tachycardia.

Importantly, AES at short coupling intervals can result in transformation of the tachycardia to a different AT or to AF. To minimize the risk of transformation or interruption of the tachycardia, which would hinder tachycardia mapping, atrial stimulation maneuvers during the tachycardia should be used sparingly and only when needed to confirm the diagnosis or more precisely localize the critical portion of the AT circuit as guided by the initial activation mapping.

Atrial pacing during tachycardia

Burst pacing from the CS or along the Halo catheter is started at a CL 10 to 20 milliseconds shorter than the atrial CL, and then the PCL is progressively shortened by 10 to 20 milliseconds. Consistent capture by atrial stimuli and acceleration of all recorded atrial electrograms to the paced rate should be verified before analyzing the tachycardia response to overdrive pacing. The response of AT to overdrive pacing is evaluated for entrainment, overdrive suppression, transformation into distinct AT morphologies or AF, and ability and pattern of termination.