Bachmann’s Bundle’s Unique Physiology

Abstract

A 69-year-old man with a history of previous ablation and cardiac surgery was found on cardiac electrophysiology study to have a macro–re-entrant left atrial flutter initially misdiagnosed as a micro–re-entrant right atrial tachycardia resulting from the unique conduction properties of Bachmann’s bundle. (Level of Difficulty: Advanced.)

Central Illustration

Case Report

A 69-year-old man presented to our hospital (University of Maryland School of Medicine, Baltimore, Maryland) as a referral for palpitations. The patient had a history of atrial fibrillation with a previous radiofrequency pulmonary vein isolation 10 years earlier but continued to experience symptoms of refractory atrial fibrillation. He had a diagnosis of severe mitral regurgitation secondary to mitral valve prolapse and subsequently underwent a mitral valve repair with placement of a mitral valve ring 9 months before presentation, during which a left atrial (LA) posterior wall cryomaze procedure with additional lesions to the mitral isthmus and external coronary sinus (CS) was performed. His recovery was complicated by ongoing symptoms of palpitations resistant to beta blockade and amiodarone therapy. He had undergone several earlier attempts at cardioversion, with prompt return to his tachyarrhythmia.

Learning Objectives

• •Review the function and physiology of Bachmann’s bundle.
• •Understand the location and course of Bachmann’s bundle.
• •Summarize how Bachmann’s bundle may affect basic diagnostic maneuvers to differentiate SVT during an EP study.

At presentation, the patient’s vital signs were within normal limits, and his examination demonstrated an otherwise healthy 69-year-old patient without focal pathologic findings. The patient’s electrocardiogram (ECG) demonstrated a regular narrow complex rhythm with regular P waves suggestive of an atrial tachycardia or slow atrial flutter with 2:1 conduction (Figure 1). Localizing the P-wave origin by ECG was deemed unreliable because of his previous atrial ablations, although suspicion of an atypical LA flutter was high, also because of his previous ablation procedures. We reviewed treatment options and ultimately decided to pursue an electrophysiology (EP) study with a plan for ablation.

Figure 1.

Electrocardiogram Demonstrating 2:1 Conduction of an Atrial Tachycardia or Slow Atypical Atrial Flutter

The patient was in the arrhythmia at the time of the EP study. Standard quadripolar catheters were placed in the high right atrium, the His position, and the right ventricular apex. A decapolar catheter was placed in the CS. On placement of the CS catheter, we noted a line of block across the CS, consistent with his history of external CS ablation during his previous cardiac surgery (Figure 2).

Figure 2.

Coronary Sinus Catheter Placement

Coronary sinus catheter placement confirmed supraventricular tachycardia with 2:1 conduction and an atrial cycle length of 352 milliseconds. Coronary sinus catheter placement also demonstrated a line of block along the coronary sinus catheter poles 7 to 8, consistent with previous external ablation performed during cardiac surgery.

We pursued entrainment to elucidate the tachycardia further. The tachycardia was easily entrained, and entrainment behavior was consistent with varying pacing durations, most consistent with a re-entry circuit. Initial entrainment suggested the distal poles of the CS catheter were closest to the tachycardia circuit (Figure 3). We then created an electroanatomic map of the right atrium using Carto software and the Carto Pentaray mapping catheter (Biosense Webster). Activation mapping included 100% of the tachycardia cycle length (TCL) and was suggestive of a point source, with a focal site of interest at the anterior superior vena cava (SVC), just superior to the cavoatrial junction (Figures 4A and 4B), with a unipolar signal at the location demonstrating a QS signal. Entrainment from the site of interest confirmed that the site was within the circuit (difference between the postpacing interval and the TCL [PPI-TCL] = 6 milliseconds) (Figure 5), with a narrower PPI-TCL compared with the distal CS. Of note, entrainment was performed at low output (2 mV at 2 milliseconds) to prevent capture of adjacent structures. We consequently suspected a micro–re-entry focus originating from the anterior SVC just superior to the cavoatrial junction behaving as a point source. Although we remained cognizant that the CS activation suggested that the lower left atrium activated from left to right, our entrainment results, with considerably better entrainment from the right atrium compared with the left, was highly compelling that the origin of the tachycardia was from the right atrium. Given the strength of the entrainment results, we thought it prudent to ablate within the right atrium first, before subjecting the patient to the additional clinical risks of a LA procedure. However, focal ablation to the site of interest failed to terminate or affect the tachycardia (Figure 6).

Figure 3.

Entrainment from the Distal Coronary Sinus

Entrainment from the distal coronary sinus (coronary sinus 3-4), with a pacing cycle length of 340 milliseconds (tachycardia cycle length, 354 milliseconds) demonstrated the difference between the postpacing interval and the tachycardia cycle length to be 26 milliseconds, suggesting that the distal coronary sinus was within the circuit. Of note, pacing from this site captured both atrium and ventricle.

Figure 4.

Activation Mapping Identifying a Site of Interest Within the RA Localized to the Anterior Superior Vena Cava Just Superior to the Cavoatrial Junction

(A) Coherence mapping demonstrated typical distributive activation of a focal atrial tachycardia or micro–re-entrant flutter, originating from the site of interest (red) at the anterior superior vena cava and distributing throughout the right atrium (RA) and around the superior vena cava. (B) Entrainment suggested the site of interest to be within the tachycardia circuit. Of note, the green dots are within the circuit (difference between the postpacing interval and the tachycardia cycle length <30 milliseconds), the yellow areas are near the circuit (difference between the postpacing interval and the tachycardia cycle length, 30-50 milliseconds), and the brown dots are outside the circuit (difference between the postpacing interval and the tachycardia cycle length >50 milliseconds). CL = cycle length.

Figure 5.

Entrainment From Right Atrial Site of Interest

Entrainment with a pacing cycle length of 330 milliseconds (tachycardia cycle length, 352 milliseconds) from the right atrial site of interest (ablation distal) yielded a difference between the postpacing interval and the tachycardia cycle length of 6 milliseconds, a finding suggesting the site of interest to be within the tachycardia circuit.

Figure 6.

Coherence Map of the RA Following Ablation

Ablation lesions are demarcated by pink or red dots. Ablation performed within the right atrial site of interest did not affect the tachycardia. RA = right atrium; CL = cycle length.

Electroanatomic and activation mapping of the left atrium through a standard transseptal access was then performed. Activation mapping demonstrated a macro–re-entrant tachycardia (Figures 7A and 7B) of the left atrium. Entrainment confirmed a circuit circulating up the LA posterior wall to the LA roof, down the right anterior wall, around the right inferior pulmonary vein, before returning up posterior wall. PPI-TCL within the circuit was <5 milliseconds. The arrhythmia terminated on completion of an LA roof line ablation (Figure 8). Pacing on either side of the lesion set confirmed block across the roof of the left atrium. The right pulmonary veins were not fully isolated on voltage mapping of the left atrium, thus necessitating additional lesions along the antrum of the right pulmonary veins. The arrhythmia did not recur following ablation or after a 30-minute waiting period.

Figure 7.

Coherence Maps of the LA

The maps show the (A) anterior and (B) posterior left atrium (LA) with associated entrainment sites, demonstrating a macro–re-entrant circuit involving the atrial roof, the location of the overlying Bachmann’s bundle. The green and blue dots are within the circuit (difference between the postpacing interval and the tachycardia cycle length <30 milliseconds), the yellow areas are near the circuit (difference between the postpacing interval and the tachycardia cycle length, 30-50 milliseconds), and the brown dots are outside the circuit (difference between the postpacing interval and the tachycardia cycle length >50 milliseconds). A postmaze residual leak along the right pulmonary vein carina was noted. CL = cycle length.

Figure 8.

Coherence Map of the Posterior Wall

The map shows an overlying line of ablation (red and pink dots) performed along the left atrial roof, which terminated the tachycardia. Additional lesions were performed along the right superior pulmonary vein antra to isolate the right-sided veins fully. LA = left atrium; CL = cycle length.

Discussion

Bachmann’s bundle, often referred to as the interatrial bundle, is a muscular bundle composed of parallel muscle strands connecting the left and right atria and serving as the primary electrical connection between the right and left atria.¹ It is an anterior conduction pathway inserting within the right atrium superiorly along the cavoatrial junction near the sinus node and inferiorly from the subepicardium of the right atrial (RA) vestibule before traversing the interatrial grove with insertions along the anterior LA wall before termination at the base of the LA appendage.^2,3 It is distinguished within the atrium by its rapid conduction properties, with conduction velocities of 1.7 m/s compared with 0.4 m/s of adjacent myocardium, and its action potential characteristics are similar to those of the Purkinje system.^4,5

Here, we present a case wherein standard SVT diagnostic techniques of activation and entrainment mapping presented a macro–re-entrant LA flutter as a micro–re-entrant RA flutter as a result of conduction across Bachmann’s bundle. Activation mapping of the right atrium led to identification of a focal SVC tachycardia with a PPI-TCL of 6 milliseconds suggesting a micro–re-entrant mechanism localized to the site. After ablation failed to affect the tachycardia, an LA circuit breaking out into the right atrium through Bachmann’s bundle was suspected. Activation and entrainment mapping of the left atrium confirmed the circuit to include the anterior left atrium, which is the typical exit site of Bachmann’s bundle. Overlying electroanatomic maps of the right and left atria further demonstrated the site of interest to be at the RA insertion site of Bachmann’s bundle (Figure 9). Pacing from the RA site of interest was performed at low output, thus making capture of adjacent atrial tissue unlikely. Moreover, the closest left-sided structure was the superior aspect of the right superior pulmonary vein, which was confirmed to be isolated after voltage mapping of the left atrium. We consequently concluded that the very early activation and the results of entrainment were caused by the rapid conduction properties of Bachmann’s bundle.

Figure 9.

Activation Map of the Right and Left Atria

The map shows ablation lesions (red dots) demonstrating the close proximity of the right atrial site of interest to the left atrium and highlighting the site to be along the course of Bachman’s bundle. RA = right atrium; LAT = local activation time.

Conclusions

To our knowledge, this is the first reported LA macro re-entrant flutter masquerading as focal RA micro–re-entrant tachycardia through the use of Bachmann’s bundle. It demonstrates that arrhythmias can manifest atypically when they involve the entry or exit sites of Bachmann’s bundle because of the bundle’s rapid conduction properties. Consideration for macro–re-entrant circuits within the contralateral atria should be made whenever a tachycardia localizes to the regions of Bachmann’s bundle.

Funding Support and Author Disclosures

The authors have reported that they have no relationships relevant to the contents of this paper to disclose.