Part II-Clinical presentation, electrophysiologic characteristics, and when and how to ablate atriofascicular pathways and long and short decrementally conducting accessory pathways.

Recognition of the presence, location, and properties of unusual accessory pathways for atrioventricular conduction is an exciting, frequently difficult, challenge for the clinical cardiac arrhythmologist. In this second part of our series of reviews relative to this topic, we discuss the steps required to achieve the correct diagnosis and appropriate management in patients with the so-called "Mahaim" variants of pre-excitation. We indicate that, nowadays, it is recognized that these abnormal rhythms are manifest because of the presence of atriofascicular pathways. These anatomical substrates, however, need to be distinguished from the other long and short accessory pathways which produce decremental atrioventricular conduction. The atriofascicular pathways, along with the long decrementally conducting pathways, have their atrial components located within the vestibule of the tricuspid valve. The short decremental pathways, in contrast, can originate in the vestibules of either the mitral or tricuspid valves. As a starting point, careful analysis of the 12-lead electrocardiogram, taken during both sinus rhythm and tachycardias, should precede any investigation in the catheterization room. When assessing the patient in the electrophysiological laboratory, the use of programmed electrical stimulation from different intracardiac locations, combined with entrainment technique and activation mapping, should permit the establishment of the properties of the accessory pathways, and localization of its proximal and distal ends. This should provide the answer to the question "is the pathway incorporated into the circuit underlying the clinical tachycardia". That information is essential for decision-making with regard to need, and localization of the proper site, for catheter ablation.

Unusual variants of pre-excitation: From anatomy to ablation: Part III-Clinical presentation, electrophysiologic characteristics, when and how to ablate nodoventricular, nodofascicular, fasciculoventricular pathways, along with considerations of permanent junctional reciprocating tachycardia.

The recognition of the presence, location, and properties of unusual accessory pathways for atrioventricular conduction is an exciting, but frequently a difficult, challenge for the clinical cardiac arrhythmologist. In this third part of our series of reviews, we discuss the different steps required to come to the correct diagnosis and management decision in patients with nodofascicular, nodoventricular, and fasciculo-ventricular pathways. We also discuss the concealed accessory atrioventricular pathways with the properties of decremental retrograde conduction that are associated with the so-called permanent form of junctional reciprocating tachycardia. Careful analysis of the 12-lead electrocardiogram during sinus rhythm and tachycardias should always precede the investigation in the catheterization room. When using programmed electrical stimulation of the heart from different intracardiac locations, combined with activation mapping, it should be possible to localize both the proximal and distal ends of the accessory connections. This, in turn, should then permit the determination of their electrophysiologic properties, providing the answer to the question "are they incorporated in a tachycardia circuit?". It is this information that is essential for decision-making with regard to the need for catheter ablation, and if necessary, its appropriate site.

Atypical bypass tracts: can they be recognized during sinus rhythm?

Atypical bypass tracts or variants of ventricular pre-excitation are rare anatomic structures often with rate-dependent slowing in conduction, called decremental conduction. During sinus rhythm, electrocardiographic recognition of those structures may be difficult because unlike in the Wolff-Parkinson-White syndrome where usually overt ventricular pre-excitation is present, the electrocardiogram (ECG) often shows a subtle pre-excitation pattern because of less contribution to ventricular activation over the slow and decrementally conducting bypass. Following the structure described by Ivan Mahaim and Benatt corresponding to a fasciculoventricular pathway, several other new variants of ventricular pre-excitation were reported. In this review, we aim to discuss the electrocardiographic pattern of the different subtypes of variants of ventricular pre-excitation, including the atriofascicular pathway, long and short decrementally conducting atrioventricular pathways, fasciculoventricular pathway, the atrio-Hisian bypass tract, and nodoventricular and nodofascicular fibres. Emphasis will be on the ECG findings during sinus rhythm.

Mahaim Tachycardia Induced Cardiomyopathy.

We present the case report of a 22-year man, with incessant palpitations, chest pain, shortness of breath, and pulsations in his neck for the past 7 months. He was referred to the cardiology unit for workup of wide complex tachycardia (WCT). His echocardiography, 6 months earlier, had demonstrated severe left ventricular (LV) systolic dysfunction, severe global hypokinesia, mild tricuspid regurgitation (TR), and mild mitral regurgitation (MR) which resolved with medical therapy including beta-blockers. He underwent electrophysiological study, which revealed a decremental right sided atriofascicular pathway causing a WCT with left bundle branch block (LBBB) morphology and left axis deviation (LAD, Mahaim tachycardia). This was successfully ablated by radiofrequency ablation (RF) with abolition of the tachycardia. This case report highlights Mahaim tachycardia induced cardiomyopathy, a rare but curable cause of cardiomyopathy.

Electroanatomic mapping guided cryoablation of Mahaim pathways in children with limited fluoroscopy exposure.

BACKGROUND: The presence of Mahaim accessory pathways (MAP) with anterograde decremental conduction is a disorder that leads to antidromic atrioventricular reentrant tachycardia. There are rare reports of cryoablation use in MAP. This study aims at sharing our experience with using cryoablation to treat MAP in children. METHODS: Electrophysiology study and catheter ablation were performed in 14 patients diagnosed with Mahaim tachycardia between January 2010 and December 2013. Cryoablation was used in nine of the cases. A three-dimensional navigation system with surface electrode patches (EnSite System, St. Jude Medical Inc., St. Paul, MN, USA) was used for all procedures. RESULTS: The patients (two girls and seven boys) had a median age of 11.5 years (8-18 years) and a median weight of 67 kg (31-80 kg). Mahaim conduction was localized in the right posterolateral (n = 4), right lateral region (n = 2), right posteroseptal (n = 1), right anterolateral (n = 1), and right anterior (n = 1). A pathway potential was noted in six of nine cases at the tricuspid annulus. Catheter choices and acute success rates were as follows: cryoablation in four (three of four successful), radiofrequency catheter ablation (RFA) and cryoablation in five (successful in four of five). No fluoroscopy was used in six of nine patients. The mean procedure duration was 249 ± 90 minutes. No major complications were observed. The final long-term success rate for cryoablation was seven of nine (78%). CONCLUSIONS: Cryoablation can be used as a reliable and effective alternative to RFA in the treatment of Mahaim accessory conduction pathways in children. Prospective comparative studies are necessary in order to further evaluate the long-term efficacy of this method.

Successful cryoablation of Mahaim tachycardia in a child with Ebstein's anomaly.

Mahaim fibers with decremental atrioventricular (AV) node-like conduction properties comprise less than 3 % of accessory pathways. Radiofrequency ablation of right atriofascicular pathways guided by a distinct Mahaim potential detected at the anterolateral to posterolateral tricuspid annulus or in the right ventricular free wall is a safe and highly effective treatment method. The case report presents a 16-year-old boy with Ebstein's anomaly and symptomatic wide complex tachyarrhythmia. The electrophysiologic study and the entire ablation procedure were performed using a three-dimensional mapping system (EnSite Velocity; St. Jude Medical Inc., St. Paul, MN, USA). No fluoroscopy was used during the procedure. Electrophysiologic evaluation demonstrated typical atrioventricular nodal reentrant tachycardia and Mahaim tachycardia with a wide QRS and a left bundle branch block pattern. After Mahaim potential was located at the lateral tricuspid annulus, successful cryoablation was performed with an 8-mm-tip catheter followed by slow pathway ablation to eliminate typical atrioventricular nodal reentrant tachycardia. Cryoablation with an 8-mm-tip catheter can be an alternative treatment option for children with Mahaim tachycardia.

Postablation-acquired short atrioventricular Mahaim-type fibers: observations on their clinical, electrocardiographic, and electrophysiologic profile.

BACKGROUND: The electrophysiologic characteristics of decrementally conducting accessory pathways (APs) are well described; however, little is known about decrementally conducting APs caused by the radiofrequency ablation of a rapidly conducting AP. OBJECTIVE: To report the clinical, electrocardiographic, and electrophysiologic characteristics of 6 patients who developed a decremental AP after an attempt at ablation. METHODS: We compared the clinical and electrophysiologic characteristics of 295 consecutive patients with the Wolff-Parkinson-White syndrome who underwent radiofrequency ablation of 311 manifest APs (group A) with those of 6 patients with the Wolff-Parkinson-White syndrome in whom a decrementally conducting AP was detected after an attempt at ablation. RESULTS: The AP ablation site in group B patients was at the coronary sinus ostium region in 3 patients, middle cardiac vein in 2 patients, and left posteroseptal region in 1 patient. Sixty-two bypass tracts in group A patients and all 6 in group B patients were ablated at these locations, while 249 bypass tracts in group A patients and none in group B patients were ablated elsewhere (P = .0001). Five of the 6 patients (83%) with acquired Mahaim physiology had an AP located in the venous system. The odds for developing an acquired decremental antegrade atrioventricular AP when it was located inside the venous system were 1 in 6. All group B decremental APs were sensitive to adenosine, but none in 85 group A patients (P <.0001). CONCLUSIONS: The risk for developing decremental conduction after the ablation of a rapidly conducting AP is greater for APs inside the coronary venous system. Acquired decremental antegrade atrioventricular APs are electrophysiologically similar to de novo ones. They are capable of being part of an arrhythmia circuit and, therefore, should be targeted for ablation.

An unusual preinduction arrhythmia resulting from the presence of a Mahaim fiber.

A potentially life-threatening arrhythmia appeared on the preinduction electrocardiogram of an asymptomatic young woman prior to spine surgery. The patient was evaluated by electrophysiology and had a rare accessory pathway, a Mahaim Fiber.

Nodoventricular pathway associated with twin AV nodes: complexity of ablation in single ventricle physiology.

We report the case of a patient with heterotaxy syndrome including complex single ventricular morphology and interrupted IVC in association with twin conduction systems and a nodoventricular accessory pathway connection. The presence of 3 distinct QRS morphologies was inadvertently discovered during a hemodynamic catheterization study and prompted formal EP testing prior to hepatic venous inclusion into the Fontan circuit and loss of access to the atrial chamber for testing and therapy. This patient underscores the importance of close surveillance and high index of suspicion of arrhythmia mechanisms in patients with heterotaxy syndrome in conjunction with single ventricle morphology.

Electrophysiological study and catheter ablation of a Mahaim fibre located at the mitral annulus-aorta junction.

Accessory pathways with slow and anterograde decremental conduction (Mahaim fibres) are responsible for a minority of atrioventricular reentrant tachycardias. While usually located along the tricuspid annulus, left-sided Mahaim fibres have been occasionally reported. We here report on a unique case of radiofrequency catheter ablation of a Mahaim pathway located at the supero-septal aspect of the mitral annulus, in a region known as mitral annulus-aorta junction, between the right and left fibrous trigons. Electrophysiological properties and embryological implications of this unusual accessory pathway are discussed.

Nodoventricular accessory pathways in PRKAG2-dependent familial preexcitation syndrome reveal a disorder in cardiac development.

BACKGROUND: Familial preexcitation syndrome is linked to mutations in PRKAG2. Previous studies on the R302Q mutation have provided evidence for a remarkably high proportion of otherwise rare accessory pathways with atrioventricular (AV) node-like conduction properties (Mahaim fibers). Yet, histopathologic proof is still lacking. We aimed to provide such proof. METHODS AND RESULTS: We retrospectively studied the medical records of 17 members of a 5-generation family. Five subjects died prematurely. The R302Q mutation was found in 8 living subjects and 2 deceased subjects (obligate carriers). Cardiac hypertrophy was found in 7 mutation carriers. ECGs compatible with preexcitation were found in 13 subjects and AV block at varying degrees in 5 subjects. All mutation carriers had electrocardiographic evidence of preexcitation, AV block, or both. Three individuals had high-grade AV block with preexcited conducted beats. Electrophysiological studies in 3 individuals revealed bypasses with AV node-like properties. Histopathologic studies of 1 suddenly deceased mutation carrier revealed concentric hypertrophy of the left ventricle with extensive myocardial disarray associated with slight interstitial fibrosis but no lysosomal-bound glycogen. Moreover, there were 3 small nodoventricular tracts (Mahaim fibers) passing through the central fibrous body and connecting the AV node with the working myocardium of the interventricular septum. CONCLUSIONS: Preexcitation associated with the R302Q mutation in PRKAG2 is associated with Mahaim fibers. These findings support the novel insight that PRKAG2 may be involved in the development of the cardiac conduction system.

Spontaneous automaticity arising from a successfully ablated Mahaim fiber.

The authors describe a 22-year-old woman with regular and irregular arrhythmias exhibiting left bundle branch block (LBBB) morphology at various heart rates. An atriofascicular fiber was diagnosed as the underlying mechanism for the antidromic reciprocating tachycardia. In addition, spontaneous automaticity of the Mahaim fiber was present during electrophysiologic study. The accessory pathway was ablated successfully, targeting a Mahaim potential at the supero-anterior tricuspid valve annulus. Relatively slow automatic rhythms with identical LBBB morphology were recorded immediately after ablation, as well as during long-term follow-up in a more sporadic and subclinical form. Abnormal automaticity arising from the distal portions of the remnant pathway was considered to be the origin of the slow ventricular rhythms in this peculiar case.

Radiofrequency ablation of a "concealed" Mahaim-type accessory pathway.

We report a case of supraventricular tachycardia associated with left bundle branch block due to a slowly conducting right sided accessory pathway. Preexcitation of the ventricles could not be demonstrated during sinus rhythm or incremental atrial pacing but its presence was confirmed during antedromic tachycardia with critically timed atrial extrastimuli. The pathway was mapped during tachycardia and successfully ablated.

The 12-lead ECG in patients with Mahaim fibers.

The aim of this review article is to discuss the electrocardiographic presentation of the so called variants of pre-excitation ("Mahaim fibers") during sinus rhythm and tachycardia.