Cover Story

Do Mahaim Arrhythmias Cause Mayhem in the Lab?

Diane Augustine, RN, CCRN1, Lynn Butner, RN, CCRN1,
William DeForge, PhD2, Michael Pittaro, MSC, MD3

1Staff nurse at Norwalk Hospital, Cardiac Vascular Lab;
2St. Jude Medical, Field Clinical Engineer; 3Director of Cardiac EP, Cardiology Physicians of Fairfield Co. at Norwalk Hospital 

Norwalk, Connecticut

Diane Augustine, RN, CCRN1, Lynn Butner, RN, CCRN1,
William DeForge, PhD2, Michael Pittaro, MSC, MD3

1Staff nurse at Norwalk Hospital, Cardiac Vascular Lab;
2St. Jude Medical, Field Clinical Engineer; 3Director of Cardiac EP, Cardiology Physicians of Fairfield Co. at Norwalk Hospital 

Norwalk, Connecticut


Mahaim is used to describe a group of arrhythmias originating from the atrioventricular node (AV) to the ventricular myocardium. It is noted to be a decremental anterograde conduction with increasing preexcitation. One may see a left bundle branch block (LBBB) on an ECG whenever the left ventricle is stimulated after the right ventricle. Since the initial discovery and identification of Mahaim fiber arrhythmias, we have learned much more through EP studies, radiofrequency ablations, and surgical and histopathological labs.

Case Report

A 51-year-old male presented to the emergency department with chest pain radiating down the left arm, anxiety, tachycardia, and a concerning ECG for anterior ischemia. Past medical history was not significant except for a history of complaints of palpitations with presyncope. The patient had measured his own heart rate at 220 beats per minute with a monitor he uses during exercise, with sudden jumps from his baseline of 160 to 220 bpm on exertion. He underwent a negative EP study two years prior for the same symptoms. A cardiac catheterization was performed, which showed normal coronary arteries with right dominant circulation. A transesophageal echocardiogram was also performed, which showed the left ventricular size was normal, with mild generalized hypokinesis, and an estimated ejection fraction of 45%. The patient was discharged on day two without further complications.

The patient returned to the emergency department 24 hours later. ECG revealed a wide complex tachycardia with a left bundle branch morphology, consistent with a right ventricular (RV) outflow tract ventricular tachycardia. It was felt this patient may have VT, or less likely, supraventricular tachycardia (SVT) with aberrancy. The patient was medicated for his anxiety, which also helped with rate control, and was brought to the EP lab where a comprehensive electrophysiology study was performed, including coronary sinus recording and pacing. 

Catheters were placed in the right femoral vein using St. Jude Medical Fast-Cath introducers, with a St. Jude Medical Josephson quadripolar catheter inserted to the RV apex, a St. Jude Medical CSL catheter, St. Jude Medical coronary sinus catheter, and a St. Jude Medical Safire 4 mm Large Curl ablation catheter. A baseline atrial-His high atrium interval of 110 ms and His-ventricular interval of 40 ms were noted, with AV nodal Wenckebach at less than 400 ms.

The coronary sinus and the right atrium were mapped with RV pacing, and there was no apparent retrograde bypass tract conduction, with retrograde conduction being midline, concentric, and decremental. There appeared to be no significant anterior classic bypass tract conduction. The patient did develop a left bundle with atrial pacing. Isoproterenol was administered, starting at 1mcg/min. Single and double ventricular extrastimuli were applied. No significant ventricular dysrhythmias were initially noted; however, after isoproterenol infusion was increased, there were nonsustained runs of the wide complex tachycardia up to 200 bpm. The same wide complex morphology was produced with atrial pacing, so it was felt the arrhythmia was an SVT, and that it might be a Mahaim bypass tract.

The area of earliest activation was noted to be in the right ventricle near the outflow tract. Pace mapping was performed, which confirmed the morphology. Radiofrequency (RF) energy was applied to this area with production of the wide complex tachycardia, which slowly terminated. With atrial pacing there was still evidence of Mahaim pathway, although the SVT was not as readily supported with burst pacing and single and double extrastimuli. The earliest site of ventricular activation was mapped to an area just above the His bundle. RF energy at this site terminated the Mahaim preexcitation, but also transiently caused AV conduction disturbance, and RF energy was immediately discontinued with resumption of conduction. AV nodal Wenckebach was again less than 400 ms. The SVT was no longer inducible, and the Mahaim pathway no longer conducted at this time. At this point, the procedure was terminated. The overall impression was this patient had SVT likely due to the bypass tract from a Mahaim fiber. He was observed overnight and then discharged the following day. Two weeks after discharge, he was further evaluated with exercise stress testing, which failed to reproduce the wide complex tachycardia.


The Mahaim arrhythmia, first described by Mahaim and Benatt in 1938,7 is used to describe a rhythm disorder with fibers connecting the AV to the ventricular myocardium. These can be divided into two main groups, nodoventricular (NV) fibers, which arise from the AV node, and fasciculoventricular (FV) fibers, which arise from the His bundle and bundle branches.1,4 

Mahaim fibers can present with several variants, including: minimal or no preexcitation in normal sinus rhythm; reversal of normal sequence of His followed by RBB activation during maximal preexcitation and during atriofascicular reentrant tachycardia; absence of retrograde conduction over pathway; participation of the right atrium in the antidromic AV reentrant tachycardia, confirmed by preexcitation of the QRS by a late atrial extrastimulus at a time when septal atrium is refractory, and resetting of the tachycardia with an atrial extrastimulus.3 Other variants include ventricular activation preceded by activation of the distal right bundle branch during maximal preexcitation and during AV reentrant tachycardia, suggesting a fascicular insertion to the distal right bundle branch.3  

The treatment options for patients with a Mahaim arrhythmia are either pharmacology or ablation. Prior to ablation, empiric drug therapy was the only option, with no clear comparative trials as to the different classes of drugs. Patients who cannot tolerate or have not had positive results with pharmacological treatment and who are symptomatic may consider curative ablation.

Today there is a choice of surgical ablation, catheter ablation, and a hybrid of both techniques. RF catheter ablation has been highly successful in recent years,8 and offers the greatest chance at definitive therapy for patients with decremental accessory pathways.8


Mahaim fibers can lead to wide complex tachycardias with a LBBB pattern that are amenable to RF ablation. This type of arrhythmia should be suspected when the presenting ECG reflects wide complex tachycardia and a left bundle branch pattern. Long-term success of catheter ablation has approached 87% in one study.9 Although this particular patient has had several hospital admissions for non-cardiac reasons in the past eight months, there has been no more evidence of his presenting arrhythmia. ■ 



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  7. Mahaim I, Benatt A. Nouvelles recherches sur les connexions superieures de la branche gauche du faisceau de His-Tawara avec cloison interventriculaire. Cardiologia. 1938;1:61-73.
  8. Podrid P. Mahaim Fiber Tachycardias. Wolters Kluwer Health, UpToDate, Inc. 2014. Available online at Accessed December 3, 2014.
  9. Monnig G, Wasmer K, Milberg P, et al. Predictors of long-term success after catheter ablation of atriofascicular accessory pathways. Heart Rhythm. 2012:9(5):704-708.

Disclosures: The authors have no conflicts of interest to report regarding the content herein.   

Editor’s Note: This article underwent peer review by one or more members of EP Lab Digest®’s editorial board.