Bachmann Bundle Block Occurring During Radiofrequency Ablation at the Inter-Atrial Septum

Case presentation

A 69-year-old woman was admitted for an elective ra- diofrequency ablation (RFA) of symptomatic paroxys- mal atrial fibrillation, without concomitant structural heart disease except for mild left atrial dilatation. The baseline electrocardiogram (ECG) is shown in Figure 1.

The catheter ablation was performed under gen- eral anaesthesia, with a right femoral endovascular ac- cess. After the transeptal puncture, complete isolation of the four pulmonary veins was performed including ablation of an atrial tachycardia/flutter originating from the left common ostium. During burst stimulation another atypical flutter was provoked with a critical isthmus localised on the left side of the high inter- atrial septum by entrainment mapping. Two radiofre- quency ablation lesions were performed at this point during sinus rhythm. The continuous electrocardio- gram eight seconds after the start of the first ablation lesion is shown in Figure 2.

Questions

• What is the diagnosis?
• What is the mechanism of the modification?
• What are the implications of the ECG changes?

Commentary

Radiofrequency ablation (RFA) of atrial fibrillation (AF) implies a transeptal left atrial access with crea- tion of thermal lesions around the four pulmonary veins in the left atrium to interrupt the mechanism of transmission of the arrhythmia.

If the tachyarrhythmia circuit or the critical isth- mus of a re-entry circuit is localised to the left side of the inter-atrial septum, as was the case with this patient, it may be necessary to deliver RF lesions in this particular anatomical zone. Lesions in the inter-atrial septum may, as in this case, create damage to the major impulse-conducting pathways of inter-atrial conduction.

Although there is no histopathologic evidence of specialised conduction tracts, three zones of preferen- tially rapid conduction across the human inter-atrial septum have been described [1]. The upper zone corre- sponds to the location of the Bachmann’s bundle (BB), the middle is localised to the fossa ovalis and the infe- rior zone corresponds to the ostium of the coronary si- nus and connections of the proximal coronary sinus.

The BB, first described by Bachmann in 1916 [2], is thought to be responsible for the rapid conduction of impulses from the right atrium (RA) to the LA during sinus rhythm, synchronising atrial activation. It emerges from the anterior-superior-septal aspect of the right atrium to form a trapezoid-shaped bundle of parallel fibres, courses along the superior quadrant of the inter-atrial sulcus from the RA to the LA, traverses the curvature of the atrial wall across the inter-atrial septal roof and then courses leftward and bifurcates to encircle the neck of the left atrial appendage. Most frequently, the fibres of this inter-atrial muscular bridge are separate in orientation and perpendicular to the transverse atrial myocardial fibres.

Typically, the median bundle measurements are 4 mm in thickness and 9 mm in height with upper and lower bundle lengths of 10 mm and 3 mm, respectively.

In our patient, the baseline prolonged P wave du- ration (160 ms) with notching (see lead I) as well as a small terminal negative deflection in lead III reflects prolongation of inter-atrial conduction time with minor changes in the terminal P wave vector. This correlates well with major change in the second half of the P wave produced by a localised RFA lesion in the region of the BB’s insertion. This lesion probably completely blocked or severely prolonged conduction through the superior inter-atrial septum (predominantly through BB) forc- ing inter-atrial conduction to proceed during sinus rhythm via the lower inter-atrial connection and pro- ducing an upward vector best seen in the inferior leads as a negative deflection. This upward vector indicates depolarisation of the left atrium from below in an upwards direction during sinus rhythm (Figure 2).

Normal duration of the P wave is <110 ms and P waves in BB conduction block are often ≥110 ms, and in our case 160 ms at baseline which prolonged to 180 ms after BB block. Figure 2 was recorded during RFA on the left inter-atrial septum and show the progres- sive generation of biphasic, prolonged P waves in II, III and aVF leads and in V3 to V6 leads starting from the fourth complex, a BB block that will likely be perma- nent in our patient.

The IAB is now believed to be precursor for atrial tachyarrhythmias and LA electromechanical dysfunc- tion, and may be an important mechanism predispos- ing to, and even maintaining, AF [3].