Mahaim fiber tachycardia

A-20- years female came with a history of recurrent palpitation for one year. The last episode remains for one hour for which she was hospitalized and tachycardia was terminated by intravenous medication. The ECHO showed a structurally normal heart.  

ECG during tachycardia (Kindly click on the ECG to enlarge it)

Source of the Image: Rajiv Gandhi superspecialtiy hospital, Dilshad Garden, New Delhi

ECG is showing broad complex regular tachycardia @ 250 beats per minute, QRS duration of 130 msec, QRS of LBBB morphology,  left axis deviation with axis is less than -30 degrees (lead I and avF negative, lead II is negative so the axis is less than -30 degrees),  negative concordance in seen in lead V1 to V6, initial activation of QRS in lead V1 is sharp (kindly see initial R wave in lead V1), no capture beat, no fusion beat, or no AV dissociation seen. 

ECG diagnosis Broad complex regular tachycardia. 

Differential diagnosis: Ventricular tachycardia (Bundle branch reentrant tachycardia), or SVT with underlying aberrancy or  SVT with rate related aberrancy or AVRT with accessory pathway conduction(Mahaim fiber tachycardia in view of LBBB morphology) or antidromic AVRT over the right accessory pathway.

However, in ECG there is no evidence of Brugada or avR criteria for VT seen. Also, morphological criteria for VT in the tachycardia ECG is not seen. The initial activation of the ventricle is rapid as evidenced by sharp activation in lead V1. All these favors SVT rather than a VT.  The age of the patient along with structurally normal heart favors SVT then a VT. 

Sinus ECG

Sinus ECG: Sinus rhythm @100 bpm, normal axis, PR160 msec, QT interval 280 msec, Loss of Q waves in lead V5, and lead V6 along with QRS transition in lead V4 suggestive of subtle pre-excitation.
There is no evidence of any short PR interval or any delta wave. Also, the baseline QRS is also normal. 

All these go against Antidromic AVRT, SVT with underlying aberrancy, and bundle branch reentrant VT.  The possibility of Mahim fiber tachycardia was kept and the patient was referred to AIIMS New Delhi for electrophysiological study. 

 Mahaim fiber tachycardias

INTRODUCTION — The term cardiac preexcitation was originally used to describe premature activation of the ventricles in patients with the Wolff-Parkinson-White (WPW) pattern. This term has been broadened to include all conditions in which antegrade ventricular activation or retrograde atrial activation occurs partially or totally via an anomalous pathway distinct from the normal cardiac conduction system.

The classic form of cardiac preexcitation is the WPW pattern, which is characterized by a short PR interval and a broad QRS complex with a delta wave. The anatomic substrate for this is a band of myocytes that bridges the fibrous atrioventricular junction, also known as the bundle of Kent. The electrocardiographic features are a result of premature ventricular activation due to conduction over the accessory pathway. 

Several other pathways, such as Mahaim fibers, have been postulated to result in cardiac preexcitation. However, most lack the histopathologic correlation that has been demonstrated for the WPW pattern. This topic will discuss the Mahaim fiber tachycardias. WPW and other non-WPW forms of preexcitation are discussed separately. 

ANATOMIC AND FUNCTIONAL FEATURES — In 1937, during pathologic examination of the heart, Mahaim and Benatt identified islands of conducting tissue extending from the His bundle tissue into the ventricular myocardium. These fibers were termed Mahaim or fasciculoventricular fibers . This description was subsequently expanded to include connections between the atrioventricular (AV) node and the ventricular myocardium (nodoventricular fibers). These findings have been confirmed by other investigators, but true continuity of these pathways is less common than was initially suspected.

Mahaim fibers were originally classified into two main groups depending upon their site of origin :

• Nodoventricular fibers
• Fasciculoventricular fibers

This classification had a functional as well as an anatomic significance, since the two groups were thought to be associated with different clinical features . The nodoventricular connections were presumed responsible for the generation of an AV reentrant tachycardia (AVRT) with a left bundle branch block morphology that had unique electrophysiologic features . This arrhythmia was not seen in patients with fasciculoventricular fibers.

This classification for Mahaim fibers persisted until evidence suggested that the anatomic cause for the tachycardia with characteristics previously attributed to nodoventricular fibers is a slowly conducting AV accessory pathway with decremental conduction (ie, conduction slows at faster heart rates). This accessory pathway, which only conducts in an antegrade fashion, most often arises in the anterior wall of the right atrium and inserts into the right ventricle, close to the right bundle branch.

These unusual accessory pathways are responsible for the constellation of electrophysiologic features that define Mahaim tachycardias. Histologic and functional examination of tissue from patients treated surgically has demonstrated an accessory pathway with features similar to normal atrioventricular nodal tissue. The presence of nodal tissue in the accessory pathway would account for the decremental properties seen in Mahaim fibers, which is not characteristic of other accessory pathways. 

Two types of decremental right-sided accessory pathways, both arising from the tricuspid annulus but with different ventricular insertions, may be responsible for Mahaim tachycardia :

• Atriofascicular connections, which account for approximately 80 percent Mahaim fibers, have a long intracardiac course and insert into the distal right bundle or right ventricle near its apex, often with arborization. A possible morphologic and functional explanation for these connections, involving the moderator band, has been proposed.
• Atrioventricular pathways, accounting for approximately 20 percent of Mahaim fibers, insert proximally into the right ventricle near the atrioventricular annulus close to the conducting system.

The term Mahaim tachycardia describes the typical constellation of electrophysiologic features that characterize this unusual form of reentrant tachycardia, without implication as to the underlying anatomic cause.

ELECTROCARDIOGRAPHIC FEATURES — The resting electrocardiogram (ECG) in patients with Mahaim fibers is usually normal 

In contrast to the WPW pattern, there is no delta wave with Mahaim fiber conduction. As noted above, the Mahaim pathways terminate in the ventricles into or near the conducting system; in contrast, insertion occurs into the ventricular myocardium, with slow muscle fiber-to-muscle fiber conduction in patients with the WPW pattern. It is this combination of preexcited and slowed conduction that is responsible for the delta wave. 

There are several ECG features that suggest Mahaim fibers as the cause of a tachycardia with a left bundle branch block pattern  These include:

• QRS axis between 0 and minus 75º
• QRS duration of 0.15 seconds or less
• R-wave in lead 1
• rS complex in lead V1
• Precordial transition in lead V4 or later
• Cycle length between 220 and 450 milliseconds (heart rates of 130 to 270)

Although these criteria are useful, they are not diagnostic of a Mahaim tachycardia.