The main causes of atrioventricular reentrant tachycardia include atrioventricular bypass (Kent bundle), atrioventricular His bundle bypass (James bundle), junctional-ventricular bypass (Mahaim bundle), and bundle-ventricular bypass (Mahaim bundle). Atrioventricular bypass (Kent bundle) presents as WPW syndrome, short P-R interval, and δ wave; while atrioventricular His bundle bypass (James bundle) shows L-G-L syndrome on ECG, short P-R interval, and no δ wave; Mahaim bundle presents with atypical pre-excitation syndrome, with normal P-R interval, δ wave, and widened QRS complex on surface ECG.
▲ Professor Ma Jian
Clinically, the difficulty of ablating the right-sided bypass is often significant, making it challenging to determine whether the target has A or V waves. Unipolar mapping can quickly clarify whether A or V waves are present, thus confirming the success of the target. The unipolar electrogram of a successful target shows P-V fusion, with an “rS” pattern, and a sharply rising V wave. It is important to note that unipolar mapping is not suitable for right posterior and posterior septal bypasses. Chinese scholar Professor Jiang Hong has pointed out that when ablating the right-sided bypass, positioning the ablation target requires observing unipolar electrograms, while assessing catheter contact quality involves looking at bipolar potentials, thereby determining the bypass location and observing the ablation effect. Among these, the most complex and challenging is the mapping and ablation of the anterior decremental atrioventricular bypass—Mahaim bundle. In Professor Ma Jian’s statistics from 1997 to date, there have been 49 patients with Mahaim syndrome undergoing radiofrequency ablation, with 20 cases of atrial-bundle type and 29 cases of atrial-ventricular type. There were 2 cases combined with Ebstein’s anomaly, 4 cases with atrioventricular nodal reentrant tachycardia (AVNRT), 3 other atrioventricular bypass cases, 1 case of atrial fibrillation, and 2 cases of short runs of atrial tachycardia. All patients were successfully ablated, and there were no recurrences during postoperative follow-up. Among these cases, 24 had previous failed ablations outside the hospital, with failure primarily due to unclear or incorrect diagnoses. Most were misdiagnosed as ventricular tachycardia (11 cases), with the rest misdiagnosed as AVNRT and right ventricular septal AVRT. Therefore, Director Ma Jian emphasizes that preoperative analysis of ECG characteristics is crucial. Often, when patients with Mahaim syndrome experience tachycardia, their ECG shows LBBB pattern, left axis deviation, and no visible atrioventricular dissociation. The ECG may show tall R waves in lead I, Rs wave in V1, with R wave transition occurring after V4. Additionally, clues supporting the Mahaim bundle need to be sought, which requires observing the degree of ventricular pre-excitation under sinus rhythm; most patients have no or mild ventricular pre-excitation, while a few exhibit significant ventricular pre-excitation. The ECG of patients often presents as “delayed excitation” or terminal notching of the QRS wave. The most common type is atrial-ventricular type without significant pre-excitation. Since Mahaim fibers exist around the tricuspid valve annulus, they may exhibit autonomous cardiac rhythm, with an incidence of about “8.6%-12.5%”, often misdiagnosed as “premature ventricular contraction” or “accelerated idioventricular rhythm.” Therefore, when interpreting surface ECGs, it is important to note that Mahaim syndrome often presents as left bundle branch block-type tachycardia, with sinus rhythm showing no or mild pre-excitation, terminal notching in lead I or V6 QRS, and rS in lead III, sometimes exhibiting autonomous rhythm.
During mapping, multiple electrode catheters should be placed whenever possible to clearly record His bundle potentials. Electrophysiological examination of ventricular stimulation confirms that this bypass has no retrograde conduction function, or confirms decremental conduction through the atrioventricular node, or retrograde conduction through another bypass. Atrial stimulation confirms the decremental antegrade conduction of this bypass, with intra-cardiac electrograms showing rapid pacing of the atrium, increasing degrees of ventricular pre-excitation, gradually shortening H-V intervals, and gradually lengthening S-δ wave intervals. Finally, differential diagnosis with other tachycardias is also necessary. Rapid atrial pacing captures the ventricle, and the QRS morphology is identical, with a 1:1 atrioventricular conduction ratio, ruling out the possibility of ventricular tachycardia; when tachycardia is induced, the H-V interval shortens, ruling out supraventricular tachycardia with differential conduction.
Additionally, since Mahaim bundles are classified into long and short Mahaim bundles, attention should be paid to different targets during mapping. For long Mahaim bundles, the earliest V wave is located at the right ventricular free wall apex, with a significant delay of the V wave at the tricuspid valve annulus free wall. The successful target is located at the TA free wall, presenting an isolated “M” potential between A and V. For short Mahaim bundles, the earliest V wave is located at the TA, with a significant delay of the V wave at the right ventricular apex. The successful target is also located at the TA, with the V wave starting to merge into an “M” type potential.
The effectiveness of ablation is demonstrated by the blockade of antegrade conduction through the bypass after discharge, but the difficulty lies in the catheter’s tendency to displace. Atrial pacing under discharge helps observe the ablation effect. Among these, 26% of patients may exhibit autonomous rhythms accompanying the bypass during ablation, similar to the junctional rhythm seen during AVNRT slow pathway ablation.