Surface electrocardiographic clues suggesting presence of a nodofascicular Mahaim fiber

Arrhythmias Involving Variants of Accessory Pathways

In some cases, atrioventricular reentrant arrhythmias are sustained by accessory pathways with peculiar electrophysiologic properties related to their specific anatomy. Most of these fibers, which may be responsible for variants of ventricular preexcitation, show decremental conduction properties due to a nodelike aspect or a peculiar tortuous anatomic route across the atrioventricular groove. Moreover, some fibers do not actively sustain any reentrant circuit and can be only involved as bystander in other arrhythmias. Although rare, these accessory pathway variants should be properly diagnosed using noninvasive and invasive methods to guide catheter ablation procedures when needed.

Preexcitation Syndromes

The classic electrocardiogram in Wolff-Parkinson-White (WPW) syndrome is characterized by a short PR interval and prolonged QRS duration in the presence of sinus rhythm with initial slurring. The clinical syndrome associated with above electrocardiogram finding and the history of paroxysmal supraventricular tachycardia is referred to as Wolff-Parkinson-White syndrome. Various eponyms describing accessory or anomalous conduction pathways in addition to the normal pathway are collectively referred to as preexcitation syndromes. The latter form and associated eponyms are frequently used in literature despite controversy and disagreements over their actual anatomical existence and electrophysiological significance. This communication highlights inherent deficiencies in the knowledge that has existed since the use of such eponyms began. With the advent of curative ablation, initially surgical, and then catheter based, the knowledge gaps have been mostly filled with better delineation of the anatomic and electrophysiological properties of anomalous atrioventricular pathways. It seems reasonable, therefore, to revisit the clinical and electrophysiologic role of preexcitation syndromes in current practice.

Is the 12-lead electrocardiogram during antidromic circus movement tachycardia helpful in predicting the ablation site in atriofascicular pathways?

AIMS

Unlike in the Wolff-Parkinson-White syndrome, there has been no systematic study on the role of the pre-excitation pattern in predicting the ablation site in patients with atriofascicular (AF) pathways. We assessed in a large cohort the value of the 12-lead electrocardiogram (ECG) during antidromic tachycardia (ADT) to predict the site of ablation.

METHODS AND RESULTS

Forty-five patients were studied, 23 males (51%), mean age of 27 ± 12 years with 46 AF pathways and 48 ADT using the AF pathway for A-V conduction. Inclusion required induction of a sustained ADT and successful ablation. Ablation site was assessed during LAO 45° projection and clockwise classified as hours in posteroseptal, posterolateral, lateral, anterolateral, and anteroseptal tricuspid annulus as follows: 05:00-07:00, >07:00-08:00, >08:00-09:00, >09:00-11:00, and >11:00-13:00 o'clock. The QRS axis was assessed during ADT and classified as normal (>+15°), horizontal (+15° to -30°), and superior (<-30°). During ADT axis was superior (-57° ± 10°) in 15 (31%), horizontal (-11° ± 14°) in 22 (46%), and normal (+45° ± 16°) in 11 (23%) patients. The correct ablation site did not differ between the different groups of QRS axis. QRS width during ADT was narrower in patients with a normal when compared with a horizontal and leftward axis (127 ± 14 vs. 145 ± 12 ms, P < 0.0001), and the V-H interval was shorter (4 ± 3 ms vs. 19 ± 22 ms, P = 0.03).

CONCLUSIONS

There was no correlation between the AF pathway ablation site and the QRS axis during ADT. The 12-lead ECG during maximal pre-excitation does not predict the proper site of tricuspid annulus ablation in patients with A-V conduction over an AF pathway.

Differential diagnosis of tachycardia with a typical left bundle branch block morphology

The evaluation of wide QRS complex tachycardias (WCT) remains a common dilemma for clinicians. Numerous algorithms exist to aid in arriving at the correct diagnosis. Unfortunately, these algorithms are difficult to remember, and overreliance on them may prevent cardiologists from understanding the mechanisms underlying these arrhythmias. One distinct subcategory of WCTs are those that present with a "typical" or "classic" left bundle branch block pattern. These tachycardias may be supraventricular or ventricular in origin and arise from functional or fixed aberrancy, bystander or participating atriofascicular pre-excitation, and bundle branch reentry. This review will describe these arrhythmias, illustrate their mechanisms, and discuss their clinical features and treatment strategies.

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.

Electrocardiogram during tachycardia in patients with anterograde conduction over a Mahaim fiber: old criteria revisited

OBJECTIVES

The aim of this study was to prospectively evaluate the sensitivity, specificity, and positive and negative predictive values of previously described ECG criteria to identify preexcited tachycardia due to decrementally conducting accessory pathways (QRS axis between 0 and -75 degrees , QRS width < or = 0.15 seconds, an R wave in lead I, an rS pattern in lead V(1), RS > 1 QRS transition > V(4), and cycle length between 220 and 450 ms).

BACKGROUND

Preexcited tachycardia associated with decrementally conducting right-sided accessory pathways usually shows a rather "narrow" QRS complex and can be difficult to differentiate from supraventricular tachycardia (SVT) with left bundle branch block (LBBB) aberrant conduction.

METHODS

We analyzed three groups of patients: 32 patients with an atriofascicular pathway (group I); 8 patients with long (n = 3) or short (n = 5) decrementally conducting right-sided AV pathway (group II); and a control group that consisted of 35 patients with SVT and LBBB (group III).

RESULTS

Presence of all six criteria had 87.5% sensitivity in group I and a 0% sensitivity in group II. There were four false negatives in group I. The negative predictive value was 82.5%, with six false positives in group III (five patients with an aberrant LBBB-shaped tachycardia with ventriculoatrial conduction over an accessory AV pathway). The criterion cycle length was not helpful.

CONCLUSIONS

Criteria for identifying a tachycardia with anterograde conduction over a Mahaim fiber are helpful only in atriofascicular pathways, with a sensitivity of 87.5% and a negative predictive value of 82.5%. The major cause of false positives was a tachycardia with aberrant LBBB conduction and ventriculoatrial conduction over an accessory AV pathway.

The electrocardiogram during sinus rhythm and tachycardia in patients with Mahaim fibers: the importance of an "rS" pattern in lead III

OBJECTIVES

The purpose of the study was to identify the electrocardiographic (ECG) characteristics of the Mahaim fiber.

BACKGROUND

Mahaim fibers are slowly conducting accessory pathways reaching into the right ventricle. They often play a role in tachycardias.

METHODS

We retrospectively analyzed 40 patients with Mahaim fibers. Five patients had associated Wolff-Parkinson-White syndrome and were excluded from the study. Two patients had a short atrioventricular decremental accessory pathway and were also excluded. The remaining 33 patients had a tachycardia with anterograde conduction over a Mahaim fiber. Twenty were female. Their mean age was 24 +/- 10 years.

RESULTS

The most common pattern of minimal preexcitation during sinus rhythm was an rS pattern in lead III. This was found in 20 patients. There was a match between the presence of rS in lead III during sinus rhythm and left axis deviation during tachycardia with anterograde conduction over the Mahaim fiber. After ablation, a different QRS pattern emerged in lead III, indicating the absence of conduction over the Mahaim fiber. To obtain information on the prevalence of an rS pattern in lead III in age-matched controls with palpitations and without structural heart disease, the 12-lead ECG of 200 young individuals were examined. An rS pattern in lead III was found in 6%.

CONCLUSIONS

A narrow QRS with an rS pattern in lead III during sinus rhythm in a patient with a history of palpitations should alert the physician to the possibility of a Mahaim fiber. During tachycardia, these patients typically show a left bundle branch block-like QRS complex with left axis deviation.

The right ventricular tachycardias

A variety of tachycardias originate from the right ventricle or use right ventricular structures as part of their circuit. They are characterized by a left bundle branch block pattern. Many of these tachycardias are relatively easy targets for radiofrequency catheter ablation. Ventricular tachycardia (VT) is the most common manifestation of arrhythmogenic right ventricular dysplasia, an often familial disease that can cause sudden death. Catheter ablation, antiarrhythmic drugs, or an implantable cardioverter-defibrillator may be used as therapy. Idiopathic right ventricular tachycardia has a benign course. It most often arises from the septal region of the right ventricular outflow tract. It commonly presents as nonsustained, repetitive monomorphic VT. The success rate of catheter ablation is greater than 90%. Bundle branch reentry occurs in patients with cardiomyopathy and His-Purkinje disease. It uses the right bundle branch anterogradely and the left bundle branch retrogradely. The QRS is very similar during VT and sinus rhythm. It can be cured by catheter ablation of the right bundle branch. VT seldom originates from the right ventricle in patients with coronary artery disease, idiopathic cardiomyopathy, or myocarditis. Atriofascicular (so-called Mahaim) fibers can sustain antidromic AV reentrant tachycardia. They represent an accessory AV node and His-Purkinje-like conduction system with atrial insertion in the right free wall near the tricuspid annulus and distal insertion directly into the right bundle branch. The accessory connection is ablated at the level of the tricuspid ring.

Ventricular tachycardias with left bundle branch block morphology

The differential diagnosis of VTs with LBBB morphology includes several well-defined syndromes. Although the majority of cases are attributable to acquired structural heart disease, including ischemia, prior infarction, or dilated cardiomyopathy, consideration of specific right ventricular processes is essential to proper evaluation and treatment. The approach to older patients or those with evidence for heart disease should begin with an evaluation for coronary artery disease and an assessment of biventricular function. Careful evaluation for bundle branch reentry should be performed during electrophysiological study, especially when there is underlying conduction system disease. Younger patients, those without overt heart disease, or those with isolated right ventricular disease, should receive a complete noninvasive evaluation of right and left ventricular size and function. An abnormal SAECG or identification of intracardiac late potentials suggest right ventricular dysplasia or cardiomyopathy, whereas responsiveness to adenosine and absence of detectable heart disease support the diagnosis of idiopathic right VT. Newer techniques, including MRI, show promise in identifying subtle right ventricular disease not otherwise detectable even in the setting of presumed idiopathic right VT. Following surgical repair of selected congenital heart defects, particularly tetralogy of Fallot, symptoms of recurrent palpitations, near syncope, syncope, or aborted sudden death may be attributable to recurrent VT, and diagnostic electrophysiological study should be considered for these patients. Finally, SVTs with LBBB morphology, particularly cases associated with right-sided or septal accessory pathways, should always be considered in this differential diagnosis.

Therapeutic and diagnostic efficacy of adenosine in wide-complex tachycardia

Wide-complex tachycardia is one of the most challenging problems faced by emergency physicians. Adenosine is an endogenous nucleoside that has gained recent popularity as an anti-arrhythmic drug in the setting of supraventricular tachycardia. We present the cases of two patients with stable wide-complex tachycardia in which adenosine was administered for therapeutic and diagnostic effects. Both patients quickly converted into sinus rhythm with subsequent diagnoses of atrioventricular reciprocating tachycardia secondary to Wolff-Parkinson-White syndrome. The role of adenosine in the acute management of wide-complex tachycardia is discussed.

Characteristic findings on the standard 12-lead ECG in patients with the fasciculoventricular Mahaim fiber

Standard 12-lead electrocardiograms with a Q wave in lead V1 were obtained from 32 subjects without organic cardiac disease and analyzed for features that might characterize an abnormal atrioventricular conduction through the fasciculoventricular Mahaim fiber. Following an infusion of ajmaline, the Q wave in V1 vanished abruptly and changed to an rS pattern in the 12 ajmaline responders. Discriminant analysis was performed to distinguish the ajmaline responders from the others. The explanatory variables were number of precordial leads with the abnormal Q wave, existence of the septal q waves, existence of the slurring of the Q wave in V1, existence of clockwise rotation, and existence of high voltage (RV5 + SV1 > 3.5 mV). Three variables, the absence of the septal q waves, the presence of the slurring, and the absence of clockwise rotation, were found to predict a positive response to ajmaline (discriminant probability = 77%). These findings associated with the Q wave in V1 suggest that the fasciculoventricular fiber may be present.

Late development of conduction block over the Mahaim fibers after electrical atrioventricular junction ablation for Mahaim fiber tachycardia

A patient suffering from so called "Mahaim fiber" tachycardia is presented who developed complete heart block over Mahaim fibers after 18 months of AV junction ablation. Antegrade conduction along the Mahaim fiber was intact immediately after the procedure. This finding has not been previously described and suggests that permanent pacemaker implantation is mandatory in such patients.

Hypothesis testing as an approach to the analysis of complex tachycardias--an illustrative case of a preexcitation variant

The correct elucidation of the electrophysiological substrate and mechanism(s) responsible for a complex arrhythmia requires a systematic approach to the analysis of the electrophysiological data. One approach calls for the formulation of a set of hypotheses that could explain the data obtained during the study. The hypotheses are then tested for compatibility with phenomena observed and the one that agrees with the majority of the findings would represent the most tenable explanation. We present the case of a young girl with a wide QRS complex tachycardia and a history of ventricular preexcitation that illustrates this approach. The complexities were resolved only after intraoperative analysis and surgical ablation of a right-sided accessory pathway with decremental properties, and provides further insight into our understanding of the nodoventricular Mahaim fiber.

Catheter ablation of Mahaim fibers with preservation of atrioventricular nodal conduction

Three patients with refractory preexcited tachycardia implicating Mahaim fibers underwent attempted catheter ablation of the accessory pathway. In the absence of demonstrable retrograde conduction in Mahaim fibers, we located the accessory pathway ventricular insertion site using the criteria of concordance between paced and spontaneous QRS morphologies during pace-mapping and earliest onset of local electrogram relative to surface preexcited QRS. At this site, a QS-like pattern of unfiltered unipolar electrograms with steep downstroke was recorded. The optimal site appeared radiologically at the right ventricular anterior wall or the adjacent septum, 2-4 cm from the tricuspid anulus. Three to six 160-J shocks were delivered at this site using an anterior chest wall plate as anode. After fulguration, conduction through the Mahaim tract was absent. A right bundle branch block persisted in two patients. All patients remained free of preexcited tachycardia during 12-16 months of follow-up. Postablation electrophysiological assessment showed no preexcitation in any patient. No reciprocating tachycardia was inducible, even during isoproterenol infusion. Atrioventricular nodal conduction parameters were unchanged from baseline study. Catheter ablation of Mahaim fibers is an effective alternative method for the treatment of tachycardias that include the accessory pathway in the circuit.

Pharmacological characterization of conduction over a Mahaim fiber: evidence for adenosine sensitive conduction

The presence of a nodoventricular pathway (Mahaim fiber) has been invoked to explain certain distinctive electrocardiographic and electrophysiological observations. The presence of an atrioventricular or atriofascicular fiber with decremental conduction properties has been documented in many of these patients. We report the case of a patient with a Mahaim fiber and the response to conduction over this pathway after adenosine, procainamide, encainide, verapamil, edrophonium, phenylephrine and isoproterenol. Conduction over the Mahaim fiber was blocked by adenosine, but not verapamil. The time course of adenosine induced block over the Mahaim fiber differed from adenosine induced AV nodal block. Mahaim fibers are decrementally conducting pathways that are adenosine sensitive. These findings support the concept that conduction in accessory pathways manifesting decremental properties is not mediated by the calcium channel.

The scintigraphic characteristics of ventricular pre-excitation through Mahaim fibers with the use of phase analysis

The phase image pattern of blood pool scintigrams was blindly assessed in 11 patients exhibiting conduction through Mahaim pathways, including 6 nodoventricular and 5 fasciculoventricular. These patterns were compared with the phase image findings in normal subjects, patients with left and right bundle branch block in the absence of pre-excitation and patients with pre-excitation through atrioventricular (AV) connections. In all patients with a Mahaim pathway, the site of earliest phase angle was septal or paraseptal. Phase progression was asymmetric and the pre-excited ventricle demonstrated the earliest mean ventricular phase angle in 10 of 11 patients. This pattern, and the associated ventricular phase difference, appeared to vary from that in normal subjects and in those with a septal AV connection, in whom phase progression is generally symmetric. Scintigraphic phase analysis provided localizing information and presented patterns consistent with Mahaim pathways. Although not able to differentiate among Mahaim pathway subtypes, these phase patterns differed from those in normal subjects, those with right and left lateral free wall pathways and most patients with a septal AV pathway. However, the phase pattern of patients with a Mahaim pathway may not differ from that of patients with a septal AV connection displaying an asymmetric pattern of phase progression, or those with left and right bundle branch block in the absence of pre-excitation. Objective, yet imperfect phase measurements supported these differences. Such image findings may complement the often complex electrophysiologic evaluation of patients presenting with pre-excitation.

Coexistent Mahaim and Kent accessory connections: diagnostic and therapeutic implications

Six patients with coexistent Mahaim and Kent accessory connections are described. Two had left nodoventricular Mahaim connections, the first reported cases demonstrating these findings. In neither were the left-sided Mahaim connections components of a tachycardia and their presence was incidental. In two of four with nodoventricular connections, associated atrioventricular (AV) node conduction and coexistent posteroseptal accessory pathways were found. One of these had the unusual finding of a right-sided Mahaim connection arising from a "fast" AV node pathway. In only one patient did the tachycardia incorporate the Mahaim connection. In this patient, anterograde conduction during tachycardia occurred over a right nodoventricular connection whereas retrograde conduction occurred through a concealed right free wall Kent connection. Two patients had fasciculoventricular connections that were associated with either septal (one patient) or left free wall (one patient) Kent connections. The latter also had evidence of enhanced AV node conduction. This report is unique in that it describes in detail two patients with left nodoventricular connections (Mahaim) inserting in or near the left posterior fascicle. Combined Kent and Mahaim connections, present in the six patients, appear to occur in approximately 5% of patients with the Wolff-Parkinson-White syndrome. Precise identification of bypass connections critical for reentrant circuits is essential for intelligent application of treatment options.

When is the vectorcardiogram superior to the scalar electrocardiogram?

The clinical usefulness of the vectorcardiogram is well documented by the numerous reports published in the last 3 decades. It has been found more reliable than the electrocardiogram for the diagnosis of atrial enlargement and right ventricular hypertrophy. It is more sensitive than the electrocardiogram in the recognition of myocardial infarction, especially if the infarction is inferior or if it occurs in the presence of left bundle branch block or left anterior hemiblock. It is helpful in the diagnosis of ventricular pre-excitation and in the localization of the bypass tract. Some repolarization abnormalities are more clearly demonstrated by the vector display. However, some information, such as that on cardiac chamber size and myocardial damage, can also be obtained by other noninvasive tests that are often performed on the same patients. With the increasing awareness of cost-effectiveness of various laboratory procedures in medicine, the vectorcardiogram should no longer be considered a routine cardiac test and should be requested only for a specific clinical purpose. When properly utilized, vectorcardiography should remain a valuable diagnostic as well as teaching tool.