1. A family of anatomically distinct connections

The unusual variants of ventricular pre-excitation cannot be understood as a single form of accessory atrioventricular pathway. They include connections arising from atrial myocardium, the atrioventricular node or its extensions, and the insulated ventricular conduction axis. Their distal insertions may reach working ventricular myocardium, the right bundle branch, or one of the ventricular fascicles. The most informative name therefore identifies both ends of the connection. This anatomically explicit terminology prevents mechanisms with similar surface electrocardiograms from being grouped together and provides a safer basis for mapping and ablation [1–4].

The eponym “Mahaim” has acquired several meanings. Ivan Mahaim described pathways extending from the atrioventricular conduction axis toward the ventricular septum, including nodoventricular and fasciculoventricular connections. In contemporary electrophysiology, however, “Mahaim physiology” most often refers to an atriofascicular pathway with slow, decremental antegrade conduction. The eponym remains useful as clinical shorthand, but it should never replace a description of the demonstrated substrate. The terms atriofascicular, nodoventricular, nodofascicular, and fasciculoventricular are not interchangeable [1–4].

2. Mahaim physiology and atriofascicular pathways

The common substrate responsible for the modern Mahaim phenotype is an atriofascicular connection. Its proximal component is usually located in the right atrial free-wall vestibule, frequently along the inferior or anterolateral tricuspid junction. Histological observations support the presence of accessory nodal tissue at the atrial origin, followed by an insulated tract that courses within the right ventricular free wall and joins the distal right bundle branch or the moderator-band network. It is consequently an atriofascicular connection rather than a nodoventricular tract [1,2,5].

Conduction is usually antegrade, slow, and decremental, reflecting the nodal-like proximal component. Progressive atrial pacing shortens the effective His-to-ventricle contribution of the normal axis and increases activation delivered through the accessory tract. The resulting pre-excited QRS generally has a left bundle branch block configuration because ventricular activation begins through the distal right bundle branch and right ventricular Purkinje network. During antidromic atrioventricular re-entrant tachycardia, the atriofascicular pathway forms the antegrade limb and the normal His–Purkinje system and atrioventricular node provide the retrograde limb. Other circuits and bystander behavior are possible, so the diagnosis must be established electrophysiologically rather than inferred from morphology alone [2,5].

Mapping should recognize the long, oblique course of the pathway. A discrete high-frequency potential may identify activation of the tract along the tricuspid vestibule, but the signal must be distinguished from atrial, ventricular, and right bundle potentials. The atrial insertion is generally preferred for ablation because it is remote from the normal atrioventricular conduction axis. Mechanical interruption can occur during catheter contact and may temporarily remove the mapping target. Distal mapping near the right bundle branch or moderator band can demonstrate the anatomy, but ablation there carries a risk of right bundle injury and should not be treated as equivalent to targeting the atrial insertion [2,5].

Short decrementally conducting right-sided atrioventricular connections may resemble atriofascicular pathways yet insert directly into ventricular myocardium. They are better termed atrioventricular connections with decremental properties. Again, the physiological adjective “Mahaim” does not establish the anatomical endpoints.

3. Nodoventricular and nodofascicular connections

A nodoventricular connection arises from the atrioventricular node—potentially from its inferior extensions—and inserts into working ventricular myocardium. A nodofascicular connection has a similar proximal origin but inserts into a fascicle or another component of the specialized ventricular conduction system. Their atrial activation is therefore obliged to enter the atrioventricular node before reaching the accessory connection. Because both the normal axis and the accessory tract share part of the nodal route, these pathways differ fundamentally from atrioventricular connections that bypass the node [1,3,6].

Many nodoventricular and nodofascicular connections are concealed and do not generate overt pre-excitation during sinus rhythm. Depending on their direction of conduction and their relationship to the tachycardia circuit, they may act as bystanders, provide a concealed route linking the node to ventricle or fascicle, or participate in re-entry. A nodoventricular tract can connect nodal tissue to septal ventricular myocardium, whereas a nodofascicular tract can enter the right bundle or a left-sided fascicle. The observed QRS configuration consequently reflects the distal insertion and the balance between activation through the normal conduction axis and the accessory connection [3,6].

Diagnosis relies on physiological perturbation of the circuit. Changes produced by His-refractory ventricular premature beats, responses to ventricular overdrive pacing, dissociation of atrial activation from tachycardia, and the sequence of His and ventricular activation help determine whether a connection is present and whether it is required for the arrhythmia. No single maneuver should be interpreted without the anatomy of the proposed endpoints. Apparent nodoventricular behavior can be reproduced by an atrioventricular pathway with nodal-like conduction or by other forms of pre-excitation [3,6].

When ablation is required, the intended target depends on the demonstrated mechanism. Ablation within or immediately adjacent to the compact atrioventricular node would expose normal conduction to unacceptable risk. A safer strategy may target a more remote distal insertion or, when the pathway participates indirectly, another indispensable and anatomically safer component of the circuit. The decision must distinguish elimination of a clinical tachycardia from attempted eradication of an incidental connection.

4. Fasciculoventricular connections

A fasciculoventricular connection originates distal to the atrioventricular node, from the non-branching or branching His bundle, a bundle branch, or a fascicle, and inserts into ventricular septal myocardium. It is therefore not an accessory atrioventricular pathway. Atrial impulses still traverse the atrioventricular node and the proximal conduction axis before entering the connection. This arrangement produces a limited ventricular pre-excitation that begins only after conduction has reached the fascicular origin [1,4,7,8].

In a manifest fasciculoventricular connection, changes in atrioventricular nodal conduction alter the PR interval while the degree of pre-excitation and the interval from His activation to the onset of ventricular activation remain relatively fixed. Atrial pacing or adenosine may delay or block conduction in the atrioventricular node without showing the progressive pathway recruitment typical of an atrioventricular bypass tract. The surface pattern can resemble a superior or mid-paraseptal accessory pathway, making intracardiac confirmation essential before energy is delivered near the His bundle [7,9].

Most fasciculoventricular connections behave as innocent bystanders and do not provide an atrioventricular limb capable of sustaining conventional reciprocating tachycardia. The principal clinical task is therefore recognition and avoidance of unnecessary ablation. Delivery of energy at the apparent site of earliest activation risks damaging the His bundle or proximal bundle branches while leaving the underlying diagnosis misunderstood. Rare reports describe participation in more complex circuits, and fasciculoventricular pre-excitation can occur in association with PRKAG2-related glycogen-storage cardiomyopathy; these situations require individualized electrophysiological and clinical assessment [4,7,9].

Recent anatomical investigation has added an important perspective. Serial histological studies identified connections from the branching atrioventricular axis to the crest of the muscular ventricular septum in a high proportion of examined normal hearts. Complementary clinical work has proposed that some such connections remain electrically silent during ordinary conduction but can be revealed by selective His-bundle pacing. These findings support a spectrum extending from anatomically present but concealed connections to clinically manifest pre-excitation. They should be interpreted as evidence for a frequent anatomical substrate and a testable physiological hypothesis, not as proof that every heart has a functionally conducting fasciculoventricular pathway [8,10].

5. Electrophysiological differentiation and clinical implications

The diagnostic sequence should begin with the proximal origin. A right atrial free-wall origin with decremental antegrade conduction supports an atriofascicular pathway. Dependence on conduction through the atrioventricular node before entry into an accessory route raises a nodoventricular or nodofascicular connection. Fixed, limited pre-excitation arising distal to the His recording supports a fasciculoventricular connection. The distal insertion then explains the ventricular activation pattern and identifies the structures at risk [1–4].

This distinction changes treatment. An atriofascicular pathway participating in tachycardia is commonly approached at its atrial insertion. A nodoventricular or nodofascicular pathway requires proof of participation and selection of the safest accessible component. A fasciculoventricular connection usually requires no ablation. Thus, anatomical naming is not semantic refinement: it separates a treatable free-wall accessory tract from connections whose ablation could injure the normal conduction axis without clinical benefit.