Left bundle branch area pacing: Electrocardiographic features

Abstract Background Left bundle branch (LBB) area pacing emerged as a promising alternative to His bundle (HB) pacing in difficult cases of physiological pacing and failed cases of cardiac resynchronization. So, it is important to understand ECG features of LBB area pacing in various subsets of patients. Objective We wanted to find out different morphological patterns and characteristic ECG features of LBB area pacing. Method Medtronic 3830 pacing lead was used to pierce the interventricular septum 1‐2 cm distal towards the RV cavity to a previously placed electrophysiology catheter at distal HB region to reach the LBB area in the right anterior oblique (RAO) 30 degree projection. We observed paced QRS morphology in lead V1 and paced QRS duration. Results We have analyzed ECG features of 60 patients who had undergone LBB area pacing and 60 patients with RV apical pacing. LBB area pacing resulted in narrower‐paced QRS complex than conventional RV apical pacing. In patients with baseline LBBB QRS shortening from LBB area pacing was more in comparison to patients with RBBB (34.45 ± 8.07 ms vs 19.78 ± 10.24 ms, P value .004). Paced QRS morphological pattern in lead V1 was most commonly qR pattern followed by Qr pattern. Conclusions LBB area pacing results in narrower‐paced QRS duration than RV apical pacing. The morphological pattern is most commonly a qR or Qr pattern in lead V1. Patients with baseline RBBB showed lesser paced QRS shortening in comparison to patients with baseline LBBB.

as a better alternative pacing site. Permanent His bundle (HB) pacing (selective or non-selective) was found to be the most physiological as it did not induce ventricular dyssynchrony (electrical or mechanical). 3,4 Left bundle branch (LBB) area emerged as an alternative to HB in difficult and failed cases of physiological pacing. 5 Moreover, LBB area pacing also revealed its potentials as an effective therapy for cardiac resynchronization. 6 So, it is important to understand ECG features of LBB area pacing in various subsets of patients.

| ME THODS
In our hospitals, from 1st January 2019 till 31st December 2019 we have analyzed ECG features of patients who had undergone successful LBB area pacing. We recruited all the patients who had All patients gave written consent agreeing to the implantation procedure, and this study was approved by the hospital's institutional review board.
Under local anesthesia with the coverage of intravenous antibiotics access to the left subclavian vein was done and the pacemaker pocket was prepared with blunt dissection in left infra-clavicular region in all patients. In LBB area pacing a select secure (Medtronic, model 3830) pacing lead was positioned 1-2 cm distal towards the RV cavity to the previously placed electrophysiology (EP) catheter (quadripolar catheter with 2-5-2 mm electrode spacing) at distal HB region (characterized by a typical triphasic signal with smaller "a," sharp H and bigger "V") catheter in the right anterior oblique (RAO) 30 degree projection ( Figure 1A). To avoid pinning of the septal leaflet of tricuspid valve the lead-sheath assembly was first advanced to the RV apex and then it was withdrawn to the desired location. Twelve lead ECG and the intra-cardiac electrograms were continuously recorded in an EP recording system (Labsystem Pro EP recording system, Boston Scientific). Baseline AH and HV interval were noted at the annulus in all cases of LBB area pacing by the same quadripolar EP catheter before it was positioned into the distal HB region. Unipolar pacing through the lead tip was performed in this basal part of the septum to identify the ideal site of penetration F I G U R E 1 (A) Fluoroscopic RAO 30 degree and LAO 40 degree projection showing relative position of the pacing lead (yellow arrow) and the His Bundle catheter, (B) From left to right showed baseline ECG of complete AV block with narrow QRS complex, unipolar pacing at basal septum before penetration and unipolar pacing at LBB area where the paced QRS complex morphology appeared as "W" pattern (notch in the nadir) and/or paced QRS duration of less than 145 ms in lead ( Figure 1B). Then the interventricular septum was penetrated with clockwise rotation on the lead body as described by Zhang et al to reach the left side of the septum where the paced QRS in lead V1 showed RBBB (rightward shift of the notch in lead V1) pattern ( Figure 1B). 7 Specific attention was given to identify any LBB potential recorded from the pacing lead tip. The definition of successful LBB area pacing was the presence of any two of the three criteria: (a) Narrow-paced QRS complex (≤130 ms), (b) RBBB morphological pattern of the paced QRS complex in lead V1, and (c) short peak LV activation time (pLVAT) (≤90 ms). In cases where we could not get success, we accepted the septal position of the lead for ventricular pacing and excluded them from the study.
In RV apical pacing ventricular pacing lead was positioned at the RV apex with the help of a simple curved stylet. All patients received a dual-chamber pacemaker with an atrial lead positioned at the right atrial appendage. After positioning, the leads were checked for parameters and stability. Once satisfied the leads were connected to the pulse generator and positioned in the preformed pocket. For patients with failed CRT, the LBB area pacing lead was connected to the LV port of the CRT device and RV lead output was programmed to sub-threshold level. The wound was closed after achieving proper hemostasis. ECG-gated AV optimization was done in all patients before discharge to achieve the narrowest paced QRS complex. We observed a change in paced QRS duration (from the intrinsicoid deflection in lead V1/V2 to the end of QRS complex) and QRS duration shortening (base-line QRS duration minus paced QRS duration) if any. Local ventricular myocardial depolarization time was measured as the interval from pacing stimulus to the peak of R wave in lead V4-6 in high (5 V) output and the highest value was termed as pLVAT.
All the measurements were done by 2 authors independently (AD and SC). In case of discrepancy of 2 observers, the highest value was taken. We looked for paced QRS morphology in lead V1 specifically for qR, Qr, rSR' or rSr', M-shaped, or monophasic R wave.

| RE SULTS
In our hospitals, in a period from 1 January 2019 till 31 July 2020 we have analyzed ECG features of 60 patients with successful LBB area pacing (case group) (in 15 cases we failed to achieve successful LBB area pacing; success rate 80%) and 75 patients with RV apical pacing (control group). Table 1

| D ISCUSS I ON
RV apical pacing results in abnormal late activation of the lateral wall of the LV because of a differential muscle strain and fiber shortening resulting from which in turn leads to increased myocardial work and oxygen consumption. These changes in cardiac hemodynamics cause LV cellular abnormalities (both at a gross and ultrastructural level) may lead to ventricular remodeling which is associated with a higher risk of development of LV systolic dysfunction, heart failure, and atrial fibrillation. 2  with significantly shorter QRS duration with shorter R' wave duration.
They also commented that the integrity of the bundle branch conduction, rather than the presence of an LBB potential, had the most significant impact on the characteristics of the ECG during LBB area pacing.
In our study, the majority of the patients with LBB area pacing showed qR and Qr patterns (40% and 36.67%, respectively).

| Ventricular activation
LBB area pacing resulted in narrow (<130 ms) paced QRS complex with an RBBB pattern in lead V1 and short isoelectric stimulus-QRS intervals at low outputs. 10

| Paced QRS morphology
The RBBB morphology resulted from LBB area pacing was distinctly different from typical RBBB. The rapid conduction through the RBB could lessen the degree of RV delay and contribute to the narrowing of the terminal R' wave in lead V1. The ventricular activation during LBB area pacing in patients with intact bundle branch conduction was biphasic with the initial vector directed posteriorly and towards left, appearing as a Q or q wave in lead V1 F I G U R E 2 Ventricular activation during LBB area pacing: LV depolarization is rapid as a result of rapid downstream stimulation via native conduction system from the pacing site. RV gets depolarized by 2 wave-fronts: (a) slow conduction wave from the LV through septal myocardium, and (b) from the pacing site conduction wave goes retrogradely through the native conduction system to reach the distal HB where it is recorded as "retro" His in a prepositioned EP catheter. From there the impulse reaches the RBB through slower conducting transverse connections (may be slower conducting sometimes). Although relative contribution of each one is unknown. Now the impulse reaches the RV myocardium by rapid conduction through the native RBB. Solid red arrow indicates faster conduction and curved red arrow indicates slower conduction. HB, His bundle; LBB, left bundle branch; LV, left ventricle; RBB, right bundle branch; RV, right ventricle on the surface ECG, followed by a synchronized rightward vector formed the narrow terminal R' wave in lead V1 (Figure 3). The LBB area pacing ECG typically displayed a biphasic qR or Qr morphology without an initial r wave in lead V1, in contrast to the rsR' pattern typically during RBBB.

| AV optimization
Adjusting

| Intervals
In cases of LBB area pacing stimulus to earliest surface QRS should be ideally equivalent (<10 ms variability) to baseline P-potential (LBB potential recorded from the pacing lead) to the earliest surface QRS when the recording was available. Normally the P-potential to earliest surface QRS interval is a short (≤30 ms) isoelectric segment. Usually, in patients with LBBB, this potential is not recordable. However, alternatively LBB captures during LBB area pacing can be ascertained in cases where it is not recordable. The sum of stimulus to retrograde His interval and stimulus to earliest surface QRS interval should be lesser than or equal to baseline His to earliest surface QRS interval ( Figure 6). Peak LV activation time (pLVAT) is a measurement of local ventricular myocardial depolarization time and is measured as the interval from pacing stimulus to the peak of R wave in lead V4-6. In patients with LBB area pacing remains very short (60-90 ms) (Figure 7).

| Selective vs non-selective
There are no unified criteria to differentiate between selective and non-selective LBB area pacing. If paced QRS duration and pLVAT in different pacing outputs remained unaltered, the pacing was considered selective. If pLVAT was prolonged during low output (1.0 V) compared to high output (5.0 V), it suggested the lead was closer to the LBB but not directly on it, hence a nonselective one. However, to avoid this we measured it at the high output in all patients.

| Limitations
The major limitation of the present study was the relatively small sample size. So, the results may not be generalized. Secondly, hemodynamic response and clinical outcomes evidence were not measured in our study. The definition of successful LBB area pacing in our study was not on the EP features (ie, recording of LBB potential, getting different VA intervals during unipolar pacing from cathode and anode with 1:1 VA conduction, or finding different refractory periods with pacing by extra-stimulus technique from cathode and anode).
Retrograde His potential would be recorded better in narrowly spaced multipolar catheter with a smaller electrode. We have checked the ECG features in high output which may be altered in lower pacing output. However, standard setting of pacing output (safety margin of 3 times of threshold) was usually close to high output.

| CON CLUS ION
LBB area pacing results in narrower-paced QRS complex in comparison to conventional RV apical pacing. The most common pattern of paced QRS complex in lead V1 was qR or Qr pattern and less commonly rSr' or QS pattern. Patients with baseline RBBB showed lesser paced QRS shortening in comparison to patients with baseline LBBB.

CO N FLI C T O F I NTE R E S T
Authors declare no conflict of interests for this article.