1 Introduction

Around 1940, Mahaim firstly reported the existence of Mahaim fibers which connected His bundle or the atrioventricular node to the ventricle anatomically [1,2,3]. Although some reports were published about these fibers after his report, no one knew the electrophysiological characteristics of them at that time. In 1975, Lev et al. described that Mahaim fibers histologically passed from the atrioventricular node to the left and to the right side of the posterior ventricular septum [4], and these pathways might play a role in the genesis of the preexcitation [5, 6]. In same year, Anderson proposed that Mahaim fibers might be classified into two anatomical types [7]. One was nodoventricular (NV) fibers connecting between the atrioventricular node and the ventricle, and the other was fasciculoventricular (FV) fibers connecting between the His bundle or the bundle branches and ventricle. However, there were rare cases having NV fibers or FV fibers, and the demonstration of reentrant circuit during reciprocating tachycardias might be difficult [6, 8]. In 1988, Tchou proposed that an accessory pathway which behaved a typical NV fiber actually arose directly from the right atrium and inserted into the right bundle branch, named as atriofascicular accessory pathway [9]. Many data have supported this proposal [10,11,12], and most accessory pathways with anterograde decremental conduction properties referred to as Mahaim fibers have been recognized as originating from the right lateral atrium in these days.

2 Classification of Mahaim Fibers in Cardiac Arrhythmias

Uncommon accessory pathways presenting decremental properties and inserting into the right ventricle have been frequently described as Mahaim pathways, and these fibers are distinct form of preexcitation. Recently, these anomalous pathways are subdivided into three types (Fig. 29.1), and it is said that most of those pathways in fact originate from the right atrial free wall near the tricuspid annulus and terminate in the distal right bundle branch, which is called as atriofascicular pathways. The most important feature in Fig. 29.1 is that the circuit of atriofascicular pathway includes right atrial muscle, in contradistinction to other circuit of Mahaim pathways. By using this feature, atriofascicular pathways can be ruled out of other Mahaim pathways. The differential diagnosis between atriofascicular pathway and other Mahaim pathways are noted in the following section. This Fig. 29.1 also indicates that Mahaim fibers including atriofascicular pathways can play a role of only anterograde pathway when macro reentrant reciprocating tachycardia occurs. These pathways typically conduct only in the anterograde direction.

Fig. 29.1
figure 1

Three types of schematic representations of antidromic reciprocating tachycardia containing Mahaim fibers. (a) Nodofascicular fibers connecting between the atrioventricular node and the right bundle. (b) Nodoventricular fibers connecting between the atrioventricular node and the right ventricle. (c) Atriofascicular fibers connecting between the right atrium and the right bundle. AV node; atrioventricular node, LB left bundle, RB right bundle

Full size image

3 Electrocardiographic Characteristics of Atriofascicular Pathway

The electrocardiogram during preexcited tachycardia using atriofascicular pathway is usually recorded as left bundle branch block pattern and left axis deviation, since the distal part of atriofascicular pathway connects to right ventricle or right bundle branch. On the other hand, although the detailed electrocardiogram depends on the transmission velocity or insertion site of these fibers, typical electrocardiogram of the patients with atriofascicular pathway shows little or no preexcitation during sinus rhythm. Häissaguerre also reported that ten patients with atriofascicular pathways had narrower QRS complexes than seven patients with atrioventricular accessory pathways (133 ± 10 versus 165 ± 26 milliseconds, p = 0.02) [13], and narrower initial r wave in leads V2 through V4 during maximal preexcitation. As those pathways have the characteristics of decremental property, PR duration is normal in most cases.

4 Electrophysiological Characteristics of Atriofascicular Pathway

4.1 Differentiation Between Atriofascicular Pathway and Atrioventricular Pathway

Atriofascicular pathways electrophysiologically differ from typical atrioventricular pathways in some respects. First, these atriofascicular pathways are found to conduct only in the anterograde direction with a long conduction time, and these pathways can only form antidromic tachycardia or as bystanders during atrioventricular nodal reentrant tachycardia or atrial fibrillation. Therefore, the antidromic reentrant tachycardia using atriofascicular pathway has characteristic features which demonstrate a relatively short ventriculoatrial interval and a long atrioventricular interval. Second, these pathways characterize decremental conduction property as like atrioventricular node. Needless to say, typical atrioventricular accessory pathways have no decremental conduction property. Third, these pathways are vulnerable to atrioventricular block by administering adenosine, which also seem to be acted as like atrioventricular node. Moreover, the timing of the right ventricular apical electrocardiogram may be helpful in distinguishing an atriofascicular pathway from a slowly conducting, right free-wall atrioventricular pathway [14]. The right ventricular apical electrocardiogram is relatively early in the former and late in the latter, compared to the delta wave on the surface electrocardiogram.

4.2 Differentiation Between Atriofascicular Pathway and Other Mahaim Pathways

There are also several differences of electrophysiological characteristics between atriofascicular and other Mahaim pathways. However, it is very difficult to distinguish and identify those three types of Mahaim fibers by checking the earliest atrial activation site during ventricular pacing, or by using para-Hisian pacing method, as these pathways conduct only in the anterograde direction. To diagnose and elucidate the circuit with atriofascicular pathways, recordings of both right bundle branch and His electrocardiogram are firstly needed.

Especially, it is important to confirm the relationship between the activation sequence of His and right bundle potential. During sinus rhythm, surface electrocardiogram shows little or no preexcitation, and activation of His potential precedes right bundle potential since that the stimulus from sinus node goes through atrioventricular node pathway. However, His bundle to right bundle activation sequence reverses during right atrial rapid pacing at cycle lengths of associated with maximal preexcitation (Fig. 29.2). This reversed sequence demonstrated that the right bundle branch and His bundle were activated in a retrograde direction when electrocardiogram showed maximal preexcitation of the QRS configuration.

Fig. 29.2
figure 2

Intra-electrocardiagraphic findings of the relationship between His bundle activation and right bundle activation. (a) His bundle activation (solid arrow) precedes right bundle branch activation (dot arrow) during sinus rhythm. (b) Right bundle branch activation precedes His bundle activation during right atrial pacing with preexcited QRS morphology. ECG electrocardiogram, HB His bundle, RA right atrium, RB right bundle

Full size image

Seconds, insertion of a single late atrial pacing can make reset the tachycardia in the cases with atriofascicular pathways. An atriofascicular pathway mostly plays a role of antidromic reciprocating tachycardia, in which the anterograde circuit is the accessory pathway and the retrograde route is atrioventricular node. During tachycardia, ventricular activation was advanced by the atrial pacing delivered at the time of refractoriness of the atrial septum, while the morphology of the QRS configuration was exactly same as that during tachycardia [9]. It suggested that the origination of the accessory pathway directly from the right atrium rather than from the atrioventricular node. However, it should be taken care that the advanced ventricular electrocardiogram could not be obtained by atrial pacing at random location. This lack of advancement was due to the distance from the pacing site to the reentrant circuit of tachycardia. As might be expected, the advanced ventricular electrocardiogram resets the tachycardia by atrial pacing located near the pathway of accessary fibers.

Third, the intra-electrocardiograms of the patients who have nodoventricular fibers occasionally represent ventriculoatrial dissociation during tachycardias. The tachycardia shows wide QRS configuration and each ventricular response is preceded by a His deflection. It clarifies that an atrial tissue is not involved in the circuit of reciprocating tachycardia. Gallagher reported that ventriculoatrial dissociation occurred during tachycardia in three of the six patients with having nodoventricular fibers, and the mechanism of this tachycardia circuit is a macroreentry using the nodoventricular fiber for the anterograde limb and the His-Purkinje system with a portion of the atrioventricular node for the retrograde limb [6].

With regard to fasciculoventricular pathway, no reciprocating tachycardia using this pathway could be observed [6].

5 Catheter Ablation

About thirty years ago, surgical operation of Mahaim fibers has been reported by several institutions for drug-refractory antidromic reciprocating tachycardia [11, 12]. In that era, some therapeutic approaches to patients with Mahaim fibers were also carried out including pharmacological therapy [15] and His bundle ablation [16]. Next, Häissaguerre described that three patients whose Mahaim fibers were successfully ablated by endocardial DC shocks applied at the ventricular insertion of the pathway in 1990 [17]. All three patients were free from tachycardia during 12–16 months of follow-up. Since then, a lot of cases of successful catheter ablation for Mahaim fibers were reported [13, 18,19,20,21,22,23,24], and the utility of radiofrequency catheter ablation for those fibers has been established. Especially, catheter ablation for atriofascicular pathway targeting Mahaim potential at tricuspid annulus is useful and widely recognized. In this chapter, several methods of catheter ablation for atriofascicular pathway are noted.

5.1 Catheter Ablation of Ventricular Insertion of Atriofascicular Pathway (Fig. 29.3)

As already described, Häissaguerre reported successful catheter ablation of Mahaim fibers in three patients [17]. Their ablation target sites of those fibers were ventricular insertion of the pathway found by using the criteria of concordance between paced and spontaneous QRS morphologies during pace mapping and the earliest onset of local electrocardiogram relative to surface preexcited QRS morphology. The complication related to the procedure in this paper was the creation of a permanent right bundle branch block in two of the three patients, but anomalous conduction in right bundle branch was present before the procedure in one of these two patients. Five years later, they also described the characteristics of the ventricular insertion sites of Mahaim fibers in ten patients with atriofascicular pathway and in eleven with atrioventricular accessory pathway [13]. In the first eleven patients, radiofrequency energy was delivered to the distal ventricular insertion sites of accessory pathway with a mean of 8 ± 5 applications. Although electrocardiographic preexcitation patterns have been changed either progressively in three patients or suddenly in one patient and right bundle branch block occurred in one patient by catheter ablation, successful catheter ablation at this site was obtained in only two patients. The procedure outcome in those cases might indicate that those pathways have a broad distal ventricular insertion supporting by the fact that spike potentials fusing with the earliest bipolar ventricular potentials could be recognized over the wide areas. Ventricular insertion of atriofascicular fibers usually occur near the right ventricular apex. It might be difficult that the distal insertion of those pathways can be separated from the right bundle branch by a few applications of catheter ablation.

Fig. 29.3
figure 3

The electrocardiogram recorded by ablation catheter located at the ventricular insertion site of atriofascicular pathway during antidromic reciprocating tachycardia (yellow arrow). ABLd, ABLm, ABLp; distal, mid, and proximal electrodes of ablation catheter, P (red arrow); Purkinje potential, M (blue arrow); Mahaim potential, LAO; left anterior oblique, RAO; right anterior oblique

Full size image

5.2 Catheter Ablation of Atrial Insertion of Atriofascicular Pathway

Due to the distal arborization of atriofascicular pathway fibers, radiofrequency catheter ablation of the atrial insertion of those fibers was performed [18, 25]. The atrial insertion site of this pathway is identified by introducing premature atrial stimulations at the tricuspid annulus during tachycardia to determine the site from which the latest atrial pacing beat preexcited right ventricle without advancing the atrium. This “reset phenomenon” clarified that the pacing site might be located on the circuit of antidromic reciprocating tachycardia, that is, on the atrial insertion site of atriofascicular pathway. The other approach is stimulus-to-delta wave mapping [18, 25]. Pace mapping in the right atrium during sinus rhythm can lead an atrial origin of accessory pathway, and the atrial insertion site was predicted by seeking the shortest stimulus-to-delta wave interval around the tricuspid annulus. However, those two techniques have a big same problem which each atrial pacing site at the tricuspid annulus should be severely limited. Moreover, the catheter is likely to be moved during tachycardia, and reliable stimulus-to-delta wave measurements might be difficult to obtain during the mapping procedure by changing heart rate or autonomic tone.

5.3 Catheter Ablation Targeting to the Tricuspid Annulus

Although atriofascicular pathways do not insert at the tricuspid annulus, the fact that they always pass through the annulus gave an important clue to separate those pathways [18, 19, 26, 27]. When the atriofascicular pathway mapping is performed during sinus rhythm, it is useful to be targeted to look for a single, discrete, high-frequency accessory pathway potential recorded between atrial and ventricular electrocardiogram. The catheter position and the timing of the accessory pathway potential serve to distinguish it from His potential. McClelland reported that those potential was recorded at the lateral, anterolateral, or posterolateral tricuspid annulus in 22 of the 23 patients 63 ± 12 ms after the local atrial potential and 83 ± 23 ms before the local ventricular potential during sinus rhythm [26]. Although the pathway potential can be identified from the mapping catheter only at a limited space at the tricuspid annulus, accessory pathway potentials were recognized at multiple sites along the right ventricular free wall, between the tricuspid annulus and the distal insertion near the right ventricular apex in many cases. As approaching right ventricular free wall close to the apex, this potential was recorded later. This suggests that an atriofascicular fiber runs through between the tricuspid annulus and the distal right bundle branch. From the previous paper, the right atriofascicular pathway was successfully ablated in a single session in all 23 patients [26]. The successful application of radiofrequency catheter ablation was performed close to the tricuspid annulus in 20 patients and at the right ventricular free wall in three patients [26].

Catheter ablation should be performed during right atrial pacing or antidromic reciprocating tachycardia to confirm the separation of accessory pathway in real-time. During radiofrequency energy, accessory pathway automaticity sometimes occurs, and presence of Mahaim automatic rhythm and its abolition during ablation is associated with long-term success of the procedure [28]. Besides, catheter trauma occasionally eliminates accessory pathway conduction, which clarifies that the atriofascicular pathways run through superficial right ventricular endocardium. Transient block of anterograde conduction induced by catheter manipulation at the subannular level proved to be reliable for precise mapping of atrial insertion of the accessory pathway [19].

Electroanatomical mapping is also useful for successful ablation of atriofascicular accessory pathway, as well as other various arrhythmias. Conventional mapping has a risk of prolonged traumatic loss of accessory pathway conduction. On the other hand, electroanatomical mapping can decrease such risk and make easy to perform the earliest ventricular activation mapping during preexcitation [21].

5.4 Catheter Ablation in the Patients Who Have No Mahaim Potential

Now that, a large, high-frequency Mahaim accessory pathway potential is helpful to identify an alternative site for catheter ablation. However, this potential could not be found along the tricuspid annulus in 14 of 29 study patients (48%) who had an atriofascicular pathway [23]. Catheter ablation targeting at the ventricular insertion is an option for such cases [24]. And, Mahaim potential mapping using electroanatomical mapping system during tachycardia seems to be another therapeutic option. We experienced a patient who had an antidromic reciprocating tachycardia related to an atriofascicular pathway. In this case, although a discrete accessory pathway potential could be recognized at free wall of right ventricle during tachycardia, we failed to identify this potential around the tricuspid annulus. Since right bundle branch block occurred by the catheter ablation targeting at ventricular insertion (Fig. 29.4), we speculate the atrial insertion site of the accessory pathway at the tricuspid annulus by using Mahaim potential mapping using electroanatomical mapping system, which could clearly visualize the course of this pathway (Fig. 29.5). When the ablation catheter was shifted to the presumed atrial insertion site during right atrial pacing, the delta wave was suddenly disappeared, which suggested atriofascicular pathway was transiently eliminated by catheter trauma. Radiofrequency energy was delivered at this site in order to achieve complete ablation lesion, which led to favorable long-term outcome.

Fig. 29.4
figure 4

Radiofrequency catheter ablation during antidromic reciprocating tachycardia. Tachycardia was terminated and transient right bundle branch block occurred (red arrow) by catheter ablation. Ablation catheter was placed at the ventricular insertion site of atriofascicular pathway

Full size image
Fig. 29.5
figure 5

Mahaim potential mapping during antidromic reciprocating tachycardia. Purple arrow shows Mahaim potential. Electroanatomical mapping system (CARTO; Biosense Webster, Inc., Diamond Bar, California, USA) can clearly visualize the course of the atriofascicular pathway during tachycardia

Full size image

5.5 The Outcome of Catheter Ablation

The acute success rate of catheter ablation of atriofascicular accessory pathway is more than 95%, and the recurrence rate is not so high [13, 18, 23, 26].

Conflicts of Interests

None declared.