Accessory Atrioventricular Connections

The anatomic substrate of accessory AV connections is the myocardium bridging the AV annuli, which, in normal individuals, are fibrous and electrically insulating (Box 93-1).¹ The sequence of normal initial ventricular septal depolarization is altered by conduction through these connections inserting into the ordinary myocardium and bypassing the normal insulated and septally conducting His-Purkinje system. The relatively slow spread of activation through the ordinary myocardium contrasts with the coordinated septal endocardial breakthrough of Purkinje ramifications and results in the δ-wave in the surface electrocardiogram (ECG). In addition to providing an additional route for impulse conduction between the atria and the ventricles, nearly all accessory connections exhibit conduction properties different from the AV node. Decremental conduction is not ordinarily seen; that is, with increasing frequency or shortening coupling intervals, the conduction time across the pathway does not significantly increase.

Ventricular Pre-excitation and Its Mechanisms

Pre-excitation is defined as ventricular myocardial activation by a pathway other than the His-Purkinje system during sinus rhythm or atrial pacing. The normal H-V interval includes the time required for activation to proceed from the bundle of His recording site down the bundle branches to the distal ramifications of the Purkinje fibers before exiting to depolarize the working myocardium. Therefore pre-excitation is inferred if the H-V interval is abnormally short during sinus rhythm or atrial pacing. The H-V interval may be normal if too little myocardium is pre-excited (consequently generating feeble voltage) to be evident on the surface ECG. With an increased frequency of supraventricular impulses, more of the ventricular myocardium is pre-excited through the accessory connection (with a progressive widening of the QRS and shortening of the H-V interval) because of decremental conduction through the AV node and nondecremental conduction through accessory AV connections. Incremental atrial pacing is an integral part of the evaluation of accessory AV connections and increases pre-excitation, thus allowing the optimal surface ECG localization of pathway insertion. Pre-excitation may be difficult to discern on the surface ECG in sinus rhythm in case of accessory connections with long anterograde conduction times or short conduction times through the AV node. Pre-excitation should, however, be detectable by rapid pacing or slowing conduction through the AV node.

An accessory pathway with a long antegrade conduction time can manifest with an isoelectric interval separating the end of the P wave from the onset of ventricular activation, which may persist even during atrial pacing or AT. An electrical connection between the AV node and the ventricular myocardium bypassing the His-Purkinje system has also been postulated to be responsible for such an ECG but has not been conclusively demonstrated. The so-called fasciculoventricular connections can also produce similar ECG manifestations with a high septal breakthrough from the normally insulated bundle of His or bundle branch being the anatomic correlate. Interestingly, no clinical arrhythmia correlate has been described for what may be no more than an anatomic variant.

Electrophysiological Characteristics of Accessory Pathways

Normal retrograde (ventriculoatrial [VA]) activation over the AV conduction system depolarizes the atria from the septal region and decrements by at least 20 ms with faster stimulation. Nondecremental free wall activation (the so-called eccentric activation) suggests VA conduction over an accessory AV connection. Dynamic maneuvers are required to distinguish between septally situated accessory pathways and normal routes of VA conduction.

During sinus rhythm, ventricular extrastimuli resulting in atrial activation preceding retrograde bundle of His activation indicate an accessory connection. If moving the ventricular pacing site from the apex toward the septum decreases the stimulus to atrial activation time instead of increasing it, an accessory VA connection should be considered. Moving away from the apex increases the conduction time to the normal AV conduction system through the distal Purkinje myocardial interface, whereas it decreases the conduction time to the annular insertion of an accessory pathway.² Similarly, high output–dependent capture of the insulated right bundle or the bundle of His contrasted with lower output ventricular myocardial capture at the same site can show changes in atrial activation sequence, retrograde His to atrial activation time, and stimulus to atrial activation timing, which suggest the presence of more than one retrograde pathway of VA conduction.³ Unchanged atrial activation sequence coupled with a constant H-A interval and prolongation of the stimulus-A interval resulting from loss of His–right bundle capture indicate the presence of the normal VA conduction alone. Conversely, the absence of change in any of the intervals and sequences indicates the sole presence of accessory pathway retrograde conduction. If the accessory pathway is remote from the pacing site or is captured only with a long conduction time or if conduction through the AV node is very rapid, conduction through the accessory pathway may be completely masked. In practice, left free wall pathways remote from a right ventricular pacing site may fulfill these conditions and are therefore likely to be masked.

During a tachycardia, evidence of conduction through an accessory AV connection can be obtained by delivering late ventricular extrastimuli coincident with or 10 ms before activation of the bundle of His, thus ensuring the encountering of complete refractoriness within the bundle of His. If the extrastimulus is earlier than the His electrogram, the lack of anticipation of the ventricular electrogram, the bundle of His electrogram, or both confirms His-Purkinje refractoriness. The presence of conduction through an accessory connection is indicated if the ventricular extrastimulus advances or delays atrial activation or terminates the tachycardia without conduction to the atria.⁴ Tachycardia termination by a His-synchronous ventricular extrastimulus without conduction to the atria or with anticipation of the succeeding ventricular or bundle of His electrogram indicates participation of the accessory pathway in the tachycardia.

In addition to establishing the presence of an accessory connection, the electrophysiological study (EPS) allows assessment of the arrhythmogenic potential of the accessory connection. The indications for curative ablation of accessory pathways chiefly depends on their proven threat—pre-excited AF degenerating to VF—or their potential threat, indicated by R-R intervals shorter than 200 to 250 ms during AF or the presence of clinical tachycardias using the accessory pathway.⁵

Successful ablation of an accessory AV connection requires precise localization, and the surface ECG is a vital starting point. Although many algorithms have been described, those using the δ-wave vector are more difficult to use compared with the mean QRS vector during full or maximal pre-excitation. ECG pattern recognition allows the planning of a strategy specific to the presumed location.

Left Lateral Atrioventricular Accessory Connections

Left free wall pathways are found in the arc extending from approximately the 12 o’clock position to the 7 o’clock position on the mitral annulus, as viewed in the left anterior oblique view. The anteroseptal aspect of the mitral annulus is the region of aortomitral fibrous continuity established in early embryonic life, which means that accessory pathways do not occur in this area. Left free wall pathways have been considered to be the most straightforward locations for catheter ablations because they are far away from the AV conduction axis and are easily pinpointed by a multi-electrode coronary sinus catheter. In contrast, the proximity of the mitral valve and the circumflex coronary artery and the possibility of a true mid-myocardial pathway can represent significant technical challenges.

Retrograde Transaortic Approach

In our laboratory, a retrograde arterial approach is preferred, whereas the trans-septal approach is used secondarily. Trans-septal access is the first-line approach in case of aortic or arterial abnormalities, such as the presence of prosthetic valves; aortic stenosis; or severe aortic, femoral, or iliac atherosclerosis. In pediatric patients, the trans-septal approach may be preferred to avoid injury to the aortic valve.

Entering the left ventricle in a retrograde fashion across the aortic valve is an important part of the retrograde arterial approach. When the catheter is brought down to the root of the aortic valve, it meets the resistance of the aortic valve, and catheter flexion combined with continued gentle pressure facilitates the formation of a loop. The loop generally crosses the aortic valve into the left ventricular cavity before a 180-degree flexion. This may be facilitated by gentle torquing. It is imperative to avoid the catheter tip entering a coronary artery, and the catheter should be promptly withdrawn in case of any doubt. Rare instances of complications resulting from an unrecognized position within the left coronary system have been reported. The catheter can also easily enter the right coronary artery ostium, particularly if it has a downward takeoff. Entry into the left ventricle occasionally produces mechanical trauma and block within the normal AV conduction axis, which does not become apparent until the accessory pathway conducting in an anterograde fashion has been ablated. Fortunately, spontaneous recovery of normal conduction is the most common outcome. After crossing the aortic valve, the catheter should be straightened before it is gently advanced toward the posterolateral left ventricular free wall. Progressive flexion of the catheter tip as it touches the free wall brings the catheter tip near or at the level of the mitral annulus and under the mitral valve leaflet, as indicated by the recording of a significant atrial electrogram. The posterior and lateral mitral annulus should be mapped at this level.

Although catheter stability is the strong suit of the retrograde left-sided approach, this same characteristic renders mapping the mitral annulus difficult. Moving from one position to another requires catheter withdrawal from under the leaflet and repositioning it anew. Clockwise rotation positions the catheter tip more laterally and anteriorly, whereas counterclockwise rotation brings the tip around more medially. The size of the catheter curve is important; a large curve does not allow the catheter tip to reach the annulus level, and a small curve means that the catheter tip “floats” or bounces without stable contact. A more atrial position (where the catheter tip makes contact with the atrial side of the mitral leaflet) can be achieved by torquing the catheter counterclockwise so that it slips medially onto the atrial side through the posterior commissure. The further anterior the accessory pathway, the more difficult it is to reach the atrioventricular annulus with the catheter tip from the retrograde approach. This situation may call for a larger curve or a trans-septal approach. The catheter tip can be much more freely moved to map the annulus on the atrial side of the mitral leaflet but, typically, is less stable than when positioned under the leaflet. Ectopy, not uncommon during RF delivery in this position, can easily dislodge the catheter.

Trans-septal and Other Approaches

The trans-septal route probably offers greater freedom in mapping the left AV annulus and allows the use of bipolar electrogram polarity reversal to bracket the atrial insertion of accessory pathways. Long sheaths improve catheter stability and facilitate precise mapping. Anterolaterally located pathways can be directly accessed, whereas septal pathways are difficult to access trans-septally. A left free wall pathway ordinarily calls for both femoral venous and arterial access. An anteroseptal pathway pattern should suggest the requirement for subclavian or jugular vein access, a right free wall pattern, the need for a long sheath, and posteroseptal pathways suggest the possible need for coronary sinus angiography.

Electrophysiological Localization

A multi-catheter approach can cover both AV annuli and provide corroboration of localization rapidly. Successful and equally rapid ablation can, however, be achieved with fewer catheters—typically two or three. In the case of evident pre-excitation, a single ablation catheter may be successfully used, which may be followed by an adenosine test; but the assessment of retrograde VA conduction usually requires an additional intracardiac catheter.

When even the best unipolar and bipolar endocardial electrograms are not good enough, an epicardial or intramyocardial pathway insertion may need to be evaluated or considered. Ventricular electrograms close to or at the site of insertion can be late, not only because the insertion may be far from the endocardium but also because of the endocardial insertion of an oblique pathway. Changing the pacing site (e.g., from the right ventricular apex to the lateral left ventricular or the right ventricular infundibulum) can help distinguish apparently early atrial electrograms (during ventricular pacing) because of an oblique pathway course. Simultaneous comparison of endocardial and epicardial recordings obtained from within the coronary sinus is useful; bracketing, as well as electrogram timing and dv/dt (rate of ventricular electrogram depolarization) comparison, can provide valuable clues. Ablation within the coronary sinus may be necessary (Figure 93-1), although conventional RF delivery achieves only low power and is frequently ineffective. Ablation in the coronary sinus and veins with a catheter with an open irrigated tip can achieve good results; however, stepwise increments in RF power (a cautious maximum of 25 W) are prudent. Pops in the thin-walled coronary venous structure can be devastating; damage to adjacent coronary arteries has also been reported.

FIGURE 93-1 An epicardial concealed left-sided accessory atrioventricular connection successfully ablated within the distal coronary sinus at about the 2 o’clock position in the left anterior oblique (LAO) view; coronary sinus angiogram with catheter position (arrow) is shown below. Top, The recording at the ablation site within the coronary sinus (RF dist). The star marks the accessory pathway potential, which was found to be dissociated from atrial and ventricular activity in sinus rhythm after successful ablation.

Localizing a pathway conducting in an antegrade fashion involves sampling the annulus of interest for the shortest local AV intervals and the earliest V (local ventricular electrogram)-δ intervals. Some posteroseptal pathways exhibit long AV times at successful sites, which suggests slow conduction through the accessory pathway. The correct assessment of the timing of ablation catheter electrograms requires comparison with the surface ECG lead showing the maximum pre-excitation.

Local Electrogram Characteristics

Bipolar and unipolar electrograms should both be used for mapping (Figure 93-2)—the former because of their higher signal/noise ratio and the latter because of their simple morphologic pattern recognition–based analysis.⁶ Localization based on bipolar electrograms requires distinction of atrial electrograms from ventricular electrograms by using late-coupled ventricular and atrial extrastimuli. However, these maneuvers can be difficult to perform or analyze and may even induce arrhythmias. The contribution of the proximal ring electrode to bipolar electrograms from the distal bipole can be misleading. Atrial electrograms can be distinguished from ventricular electrograms by using unipolar electrograms from the distal electrode.

FIGURE 93-2 Left, Site of successful ablation for a pre-excited left lateral accessory atrioventricular connection. Right, Prompt elimination of pre-excitation during radiofrequency (RF) delivery. Vertical dashed line marks intrinsic deflection of ventricular electrogram preceding δ-wave onset by less than 10 ms. Note QS morphology in the unipolar electrograms. The arrow indicates presumptive accessory pathway potential interposed between atrial and ventricular deflections identified, respectively, by comparison after ablation (atrial electrogram) and correlation with unipolar (uni) electrogram (ventricular electrogram).

Unipolar electrograms with a steep QS morphology are particularly useful for localizing the site of ventricular insertion on the basis of a steep QS morphology (the absence of an initial R wave) during sinus rhythm, pacing, or even ongoing AF and also in patients with Ebstein’s anomaly who exhibit low-amplitude, fractionated bipolar electrograms on the tricuspid annulus. Unipolar electrograms should be recorded with wide band filters—0.05 to 500 Hz—because the low-frequency content makes important contributions to the generation of RS or QS patterns. Instead of Wilson’s central terminal, a remote cutaneous or inferior vena cava (IVC) electrode may be useful as a ground, allowing common mode rejection of contaminating 50- or 60-Hz line noise. Notch filters should also be used with caution, if at all. Once the atrial and ventricular electrograms have been recognized, the intervening deflections represent presumptive accessory pathway potentials (see Figure 93-1).⁷ Certainly, the best validation is the prompt abolition of accessory pathway conduction by RF ablation at this site (assuming appropriate power delivery and contact). In practice, accessory pathway potential validation often is a retrospective exercise.

For patients without pre-excitation, the target of choice is the shortest VA interval during orthodromic AVNRT because this effectively rules out the fusion of activation through the normal AV axis with activation through the accessory connection. Ablation during ongoing AVNRT can, however, lead to dislodgment of the ablation catheter at tachycardia termination. Prompt initiation of ventricular pacing after tachycardia termination has been advocated to minimize or prevent instability. If no tachycardia is inducible and retrograde conduction through the normal AV axis can be excluded or distinguished, earliest atrial activation during ventricular pacing is a reasonable target. In the presence of an obliquely coursing accessory pathway, changing the ventricular pacing site is useful in evaluating electrogram timing as pointed out above.

Individual Pathway Locations

Specific Situations

During an ablation procedure, sustained AF is not infrequent, which renders mapping difficult. No alternative to electrical cardioversion may be available because type I or III antiarrhythmic drugs may alter the accessory pathway properties and may even eliminate pre-excitation. Mapping the earliest ventricular activation during the widest QRS (indicating maximum pre-excitation) is feasible, as is ablation, particularly when guided by unipolar electrograms. Verification of bi-directional conduction block (assessment of VA conduction) is not possible during AF.

Multiple pathways are not common but may be encountered, particularly in association with Ebstein’s anomaly. Changing patterns of pre-excitation and VA intervals and sequences are important clues. However, the same principles of mapping and ablation described above are usually effective.

The substrates of the “Mahaim” pathways (decremental atriofascicular or atrioventricular pathways) and permanent junctional reciprocating tachycardias are both thought to be accessory pathways with long conduction times—antegrade in case of the atriofascicular or AV Mahaim pathways and retrograde in case of persistent junctional reciprocating tachycardia (PJRT). In addition, these two accessory pathway variants share another characteristic—that of one-way conduction only. The few available histologic studies suggest that the anatomic substrate of PJRT is a long and tortuous muscular fascicle, whereas in the case of the Mahaim fiber, an accessory node–like structure is thought to exist at its atrial origin.¹⁰ Atriofascicular and AV Mahaim fibers are most effectively ablated by targeting the pathway potentials at the level of the annulus; they resemble the bundle of His potentials but continue to precede ventricular activation even during pre-excitation. PJRT is ablated by targeting the earliest atrial activation during the tachycardia, and, as with every ablation within that posteroseptal area, care must be taken to ensure a reasonable distance from the normal AV conduction axis.

An accessory pathway with a large insertion or an insertion with multiple branches may occasionally be encountered.¹¹ Multiple coalescent lesions, each of which modifies local electrogram parameters, have been used.

Another uncommon variant is an appendage to the ventricular connection characterized by an insertion bridging the appendage tip to the ventricle away from the annulus.¹² Careful mapping, aided by three-dimensional mapping as needed, can clarify the exact location of the insertion. Similarly, the unusual variant of surgically acquired pre-excitation is encountered rarely after right atrial appendage anastomosis to the right ventricular outflow tract (RVOT) (historically performed as a palliative procedure for tricuspid atresia). In the appropriate surgical context and with the pre-excited QRS resembling an RVOT tachycardia, careful mapping has allowed successful ablation.

An additional arrhythmia substrate such as AVNRT or AT may coexist. The electrophysiological maneuvers described above can assist in deciding whether the accessory pathway participates in the tachycardia.¹³ However, in practice, elimination of the accessory pathway substrate typically unmasks the AVNRT or AT, which can then be ablated in the standard fashion.

Indications for catheter ablation include the following: