Diverse QRS morphology reflecting variations in lead placement for left bundle branch area pacing

Abstract

Aims

Left bundle branch area pacing (LBBAP) is a potential alternative to His bundle pacing. This study aimed to investigate the impact of different septal locations of pacing leads on the diversity of QRS morphology during non-selective LBBAP.

Methods and results

Non-selective LBBAP and left ventricular septal pacing (LVSP) were achieved in 50 and 21 patients with atrioventricular block, respectively. The electrophysiological properties of LBBAP and their relationship with the lead location were investigated. QRS morphology and axis showed broad variations during LBBAP. Echocardiography demonstrated a widespread distribution of LBBAP leads in the septum. During non-selective LBBAP, the qR-wave in lead V1 indicated that the primary location for pacing lead was the inferior septum (93%). The non-selective LBBAP lead was deployed deeper than the LVSP lead in the inferior septum. The Qr-wave in lead V1 with the inferior axis in aVF suggested pacing lead placement in the anterior septum. The penetration depth of the non-selective LBBAP lead in the anterior septum was significantly shallower than that in the inferior septum (72 ± 11 and 87 ± 8%, respectively). In lead V6, the deep S-wave indicated the time lag between the R-wave peak and the latest ventricular activation in the coronary sinus trunk, with pacemaker leads deployed closer to the left ventricular apex.

Conclusion

Different QRS morphologies and axes were linked to the location of the non-selective LBBAP lead in the septum. Various lead deployments are feasible for LBBAP, allowing diversity in the conduction system capture in patients with atrioventricular block.

Graphical Abstract

Graphical Abstract.

What’s new?

• The study demonstrates various locations for septal deployment of pacing leads, on the echocardiogram, as reflected in diverse QRS morphology during left bundle branch area pacing (LBBAP).

• In patients with the qR-wave in lead V1, the primary deployment location of non-selective LBBAP leads was the inferior septum and deeper than the left ventricular septal pacing leads.

• In patients with the Qr-wave in lead V1 and the inferior axis in aVF, the non-selective LBBAP leads were located in the anterior septum but not in close proximity to the left ventricular endocardium.

• In lead V6, the deep S-wave indicated the time lag between the R-wave peak and the latest activation of the basal left ventricle. The pacemaker lead was deployed closer to the left ventricular apex.

Introduction

Recently, left bundle branch area pacing (LBBAP) has emerged as a potential alternative to His bundle pacing (HBP).^1–4 During LBBAP, different responses can be observed based on pacing output and lead location. Left bundle branch area pacing includes non-selective LBBAP, selective left bundle branch pacing (LBBP), or left ventricular septal pacing (LVSP).^1,2 Non-selective LBBAP is defined as the direct stimulation and recruitment of both local myocardium and left bundle branch (LBB) or fascicles, whereas LVSP is defined as the absence of direct recruitment of the LBB fibres during pacing.^1,2

The terminal R-wave in lead V1 is a characteristic during LBBAP. Various QRS complexes in lead V1 have been reported in previous studies.^5–14 They involved tall R-waves preceded by small Q-complexes (qR-waves) or deep Q-waves followed by small R-complexes (Qr-waves) in lead V1. The electrical QRS axes may vary with either the superior or inferior axis recorded in lead aVF. Additionally, an apparent S-wave could appear in lead V6. Different morphologies and axes of paced QRS complexes may represent variations in the electrical ventricular activation by LBBAP. In a European multi-centre study, LBBAP leads were implanted across a broad area on the interventricular septum; the study a wide range of conduction system capture, as evidenced by diverse QRS axes and a wide spectrum of conduction system potential to QRS intervals.⁴ This study aimed to confirm different septal locations of non-selective LBBAP leads on echocardiogram, as reflected in the diversity of paced QRS morphology in patients with atrioventricular (AV) block.

Methods

Study design

This was a single-centre prospective observational study. Left bundle branch area pacing was attempted during pacemaker implantation in patients with a third- or second-degree AV block between February 2019 and October 2022. Consecutive patients who underwent successful non-selective LBBAP were enrolled. We also included patients with LVSP. Patients with indications for cardiac resynchronization therapy with a defibrillator were excluded. The Institutional Review Board of Kyorin University approved the study protocol, and all patients provided written informed consent. This study complies with the principles of the Declaration of Helsinki.

Implantation procedures

Left bundle branch area pacing and HBP were attempted using SelectSecure 3830 leads through C315HIS catheters (Medtronic Inc., Minneapolis, MN, USA), described as the dual-lead technique.^5,6 Intracardiac electrograms were recorded using an electrophysiology recording system (Prucka CardioLab; GE Healthcare, Waukesha, WI, USA). Contrast material was injected from the catheter to visualize the anterior interventricular septum. An HBP lead was fixed at the distal His bundle via the right ventricle.¹⁵

In the right anterior oblique view, another sheath was advanced 1.5–2 cm towards the right ventricular apex from the tip of the HBP lead.^5–9 The pacemaker lead was gradually deployed with a few rapid rotations. After ∼10 clockwise rotations, contrast material was injected via the catheter. If the entire helix and weld core had not been penetrated in the septum, we extracted and redeployed the pacemaker lead posteriorly or anteriorly. Pacemapping was continued during lead deployment. When the notch on the QRS nadir in lead V1 gradually ascended to form the R-wave, the deployment was discontinued.

Measurements at implantation

Paced QRS duration was measured from the pacing spike to the end of the QRS complex. We determined the stimulus to R-wave peak time (Stim–RWPT) in lead V6,^10–12 which was described as the stimulus to left ventricular activation time in pioneering studies.^6–9 This reflects the depolarization time of the lateral precordial myocardium.

Following deployment of LBBAP leads, a quadripolar mapping catheter was placed into the coronary sinus (CS) to record the left ventricular electrogram (LV EGM) at the anterolateral, lateral, and posterolateral CS trunks (Figure 1).¹⁶ The left AV groove was presumed to be a circle linked to the tip of the HBP lead, ring of the LBBAP lead, and along the lateral cardiac silhouette. The apex was assumed to be located at two-thirds of the radius from the centre. In this study, the lateral CS from the base of the circle to 90° anteriorly was divided into three regions. During LBBAP, we identified the stimulus to the latest positive peak of the left ventricular electrogram (Stim–latest LV EGM) interval in the whole CS trunk and each CS region. From the analysis, we excluded two cases in which the mapping catheter did not reach the anterolateral CS. The electrical axis of the QRS complex was determined during non-selective LBBAP or LVSP. In lead aVF, the superior axis was determined when the paced Q-wave had greater amplitude than the paced R-wave.

Figure 1.

Cases with qR- and Qr-wave in lead V1. (A) The qR-wave was recorded in lead V1 during non-selective LBBAP. The most delayed left ventricular electrogram (*) was identified in the CS trunk during non-selective LBBAP in the left anterior oblique view. (B) The Qr-wave in lead V1 and the inferior axis in aVF were recorded during non-selective LBBAP. The cross indicates the left conduction system potential. Single, grey, or black double-headed arrows represent the direction of activation sequence in the CS trunk, stimulus to R-wave peak time in lead V6, and stimulus to the latest left ventricular electrogram in the CS trunk, respectively. A ratio of the R-wave to Q-wave amplitude from the baseline (vertical double-headed arrows) was measured in lead V1. CS, coronary sinus; HBP, His bundle pacing; LBBAP, left bundle branch area pacing.

Left bundle branch area pacing

Unipolar pacing from the LBBAP lead with selective capture of the left conduction system was defined as selective LBBP.^2,9 The EHRA clinical consensus statement defines LBBP as the capture of the pre-divisional LBB with simultaneous activation of all of its fascicles.¹ Non-selective LBBAP includes LBBP and left fascicular pacing.^1,4 Currently, there are no distinct criteria differentiating between them, except LBB–Purkinje potential to QRS onset interval.⁴ Jastrzębski et al.⁴ defined LBBP as when this interval was in the range of 25–35 ms with positive QRS complex in leads II and III or with positive QRS in lead II and negative in lead III. Left bundle branch pacing or left fascicular pacing was differentiated from LVSP by identifying the LBB–Purkinje capture. It was proved by either (i) the LBB–Purkinje potential to R-wave peak interval in lead V6 being equal to the Stim–RWPT (±10 ms),^4,10 (ii) transition from non-selective to selective LBBP during threshold testing (or programmed stimulation),⁹ (iii) His bundle retrograde activation, (iv) abrupt shortening (≥10 ms) and then constant Stim–RWPT in lead V6, (v) V6–V1 interpeak interval >40 ms,^11,12 or (vi) Stim–RWPT ≤74 ms.^9,10

Left ventricular septal pacing was defined as (i) the terminal R-wave in lead V1, (ii) Stim–RWPT <90 ms,¹⁰ (iii) lead placement in the left ventricular septum on the echocardiogram, and (iv) no evidence of the LBB–Purkinje capture as described above.

Patients with non-selective LBBAP were divided based on QRS morphology. In lead V1, a ratio of the R-wave to Q-wave amplitude from the baseline was measured during non-selective LBBAP, with a pacing output of 2 V (Figure 1). The qR-wave (including the rsR-wave) in lead V1 was defined as the ratio of ≥0.8, whereas the Qr-wave as <0.8, as described in the Results section. In lead V6, the deep S-wave was defined as the amplitude below baseline of >20% of the total amplitude of the QRS complex.

Echocardiogram

A follow-up echocardiogram was performed to determine the location of the pacing lead tip in the septum. In the short-axis view, the position of the pacemaker lead was classified into anterior or inferior ventricular septum. The penetration depth of the lead was quantified using the ratio of the lead length in the septum to the wall thickness (Figure 2). In the four-chamber view, the distances from the lead tip to the mitral valve and to the left ventricular apex were measured (Figure 3).

Figure 2.

Various lead locations linked to QRS morphologies. Short-axis and four-chamber views of the echocardiogram were obtained. The non-selective LBBAP lead was located in (A) the inferior septum beneath the left ventricular endocardium with the qR-wave in lead V1 and (B) the anterior septum but not close to the endocardium with the Qr-wave in lead V1 and the inferior axis in aVF. The pacing leads were deployed more posteriorly and deeply in the qR-wave group. Left bundle branch, left fascicle, and left ventricular septum pacing were identified in 10, 40, and 21 patients, respectively. In seven patients with positive QRS in lead II and negative in lead III, the LBBP lead was deployed at the border between the anterior and inferior septum. The penetration depth of the lead was quantified as the ratio of the lead length in the septum (broken arrow) to the wall thickness (arrow). Asterisks indicate the pacing lead tips.

Figure 3.

The deep S-wave in lead V6. (A) The deep S-wave in lead V6 was recorded during LBBAP, accompanied by the superior axis in aVF and the qR-wave in lead V1. There was a long interval between the R-wave peak in lead V6 and the latest LV EGM in the CS trunk (the double-headed arrows). (B) This interval was longer in the deep S-wave group than in the non-S-wave group, while the Stim–RWPT was shorter in the deep S-wave group. (C and D) The proportion of the distance from the lead tip (*) to the mitral valve (grey line) in that from the left ventricular apex to the mitral valve (white dotted line) was greater in the deep S-wave group than in the non-S-wave group. CS, coronary sinus; LBBAP, left bundle branch area pacing; LV EGM, left ventricular electrogram; Stim–RWPT, stimulus to R-wave peak time.

Statistical analysis

Data are presented as mean ± standard deviation. Student’s t-test and the Mann–Whitney U test were performed to compare continuous variables, whereas the χ² or Fisher exact test was performed to compare categorical variables. A receiver-operator characteristic curve analysis was performed to determine an optimal value for identifying the non-selective LBBAP in the inferior septum. All analyses were performed using SPSS statistical software (version 26, SPSS, Chicago, IL, USA).

Results

Patients

Non-selective LBBAP or LVSP was achieved in 71 of the 82 patients (87%) with AV block (Table 1). The mean QRS duration was 117 ± 27 ms. Third-degree and infranodal AV blocks were determined in 58 and 49% of the patients, respectively. Left and right bundle branch block were identified in 6 and 42% of the patients, respectively. Infranodal conduction disturbance was observed in 70% of the patients.

Table 1.

Clinical characteristics of the patients

	All	Non-selective LBBAP	LVSP	Non-selective LBBAP
				qR-wave	Qr-wave
	(n = 71)	(n = 50)	(n = 21)	(n = 27)	(n = 23)
Age, years	79 ± 7	78 ± 7	82 ± 6	79 ± 6	78 ± 9
Male sex	42 (59)	32 (64)	10 (48)	18 (67)	14 (61)
Weight, kg	57 ± 11	57 ± 11	58 ± 11	57 ± 13	57 ± 9
Body mass index, kg/m2	22 ± 4	22 ± 4	23 ± 3	22 ± 4	22 ± 4
Hypertension	48 (68)	33 (66)	15 (71)	17 (63)	16 (70)
Diabetes mellitus	22 (31)	14 (28)	8 (38)	9 (33)	5 (22)
LVEF, %	64 ± 7	63 ± 12	64 ± 7	62 ± 9	63 ± 15
Cardiomyopathy	8 (11)	6 (12)	2 (10)	5 (19)	1 (4)
Atrioventricular block
Third-degree	41 (58)	28 (56)	13 (62)	13 (48)	15 (65)
Infranodal	35 (49)	23 (46)	12 (57)	14 (52)	9 (39)
Persistent atrial fibrillation	6 (9)	4 (8)	2 (10)	3 (11)	1 (4)
QRS duration, ms	117 ± 27	115 ± 25	122 ± 31	121 ± 26	109 ± 22
Right bundle branch block	30 (42)	21 (42)	9 (43)	12 (44)	9 (39)
Left bundle branch block	4 (6)	3 (6)	1 (5)	3 (11)	0 (0)
Procedure time (min)	107 ± 29	105 ± 27	111 ± 33	104 ± 30	107 ± 23
Fluoroscopy time (min)	30 ± 10	29 ± 9	33 ± 11	30 ± 11	28 ± 8

Values are presented as mean ± standard deviation or n (%). LBBAP, left bundle branch area pacing; LVEF, left ventricular ejection fraction; LVSP, left ventricular septal pacing. No parameters were significantly different between non-selective LBBAP and LVSP or between non-selective LBBAP sub-groups.

Pacing features at implantation

Non-selective LBBAP was identified in 50 out of 71 patients (70%). The LBB–Purkinje potential criterion and the Stim–RWPT criterion were satisfied in 39 of the 50 patients (78%) and 35 patients (70%), respectively. Selective LBB–Purkinje capture during threshold testing was observed in 10 patients with non-selective LBBAP (20%). The Stim–RWPT in lead V6 of the non-selective LBBAP was shorter than that of the LVSP (Table 2). In the CS trunk, the Stim–latest LV EGM interval of the non-selective LBBAP was shorter than that of the LVSP.

Table 2.

Left bundle branch area pacing features

	Non-selective LBBAP	LVSP	P-value	Non-selective LBBAP		P-value
				qR-wave	Qr-wave
	(n = 50)	(n = 21)		(n = 27)	(n = 23)
Paced QRS duration, ms	132 ± 9	133 ± 9	0.76	135 ± 8	129 ± 8	0.02
QRS axis, °	−17 ± 51	−15 ± 43	0.78	−38 ± 39	8 ± 52	<0.01
Superior axis in lead aVF	35 (70)	15 (71)	0.96	24 (89)	11 (48)	<0.01
Stim–RWPT, ms	71 ± 6	80 ± 4	<0.01	70 ± 7	72 ± 5	0.27
Stimulus to latest LV EGM interval, ms	110 ± 11	120 ± 11	<0.01	111 ± 13	109 ± 10	0.62
CS area with the latest LV EGM: anterolateral, lateral, posterolateral, %	34, 50, 16	37, 58, 5	0.49	52, 48, 0	13, 52, 35	<0.01
Capture thresholds, V at 0.5 ms	0.4 ± 0.1	0.4 ± 0.2	0.42	0.4 ± 0.1	0.4 ± 0.2	0.87
Bipolar amplitude, mV	14 ± 5	15 ± 6	0.57	14 ± 4	14 ± 5	0.62
Impedance, Ω	577 ± 130	587 ± 141	0.77	546 ± 118	613 ± 137	0.07
Lead placement
Inferior/anterior septum, %	72/28	67/33	0.65	93/7	48/52	<0.01
Penetration depth (all), %	83 ± 11	79 ± 11	0.15	87 ± 7	77 ± 13	<0.01
(Inferior septum), %	87 ± 8	80 ± 11	0.01	—	—	—

Values are presented as mean ± SD or n (%).

CS, coronary sinus; LBBAP, left bundle branch area pacing; LVSP, left ventricular septal pacing; latest LV EGM, latest left ventricular electrogram; Stim–RWPT, stimulus to R-wave peak time in lead V6.

Paced QRS morphology and location of pacing lead

During non-selective LBBAP, qR- or Qr-wave was recorded in lead V1 (Figure 1). Superior or inferior axis was identified in 70 and 30%, respectively. Paced QRS axes were dispersed with a large standard deviation (−17° ± 51°). Follow-up echocardiography demonstrated that LBBAP lead placement varied substantially in the septum (Figure 2). The distribution of non-selective LBBAP lead was linked to the different QRS morphologies.

A higher ratio of the R- to Q-wave amplitude was observed during non-selective LBBAP in the inferior septum than in the anterior septum (1.6 ± 1.8 and 0.5 ± 0.3, respectively, P = 0.027). A ratio of 0.8 was determined to be the optimal value for identifying non-selective LBBAP in the inferior septum by receiver-operator characteristic curve analysis, with a specificity of 0.86, sensitivity of 0.69, and area under the curve of 0.79. In the qR-wave group, with a ratio of ≥0.8, the non-selective LBBAP leads were located mostly in the inferior septum (93%) (Table 2). The non-selective LBBAP lead was deployed deeper than the LVSP lead in the inferior septum (87 ± 8% and 80 ± 11%, respectively, P = 0.01).

The penetration depth of the non-selective LBBAP lead in the anterior septum was significantly less than that in the inferior septum (72 ± 11 and 87 ± 8%, respectively, P < 0.01). In all 12 patients with the Qr-wave in lead V1 and the inferior axis in aVF, the non-selective LBBAP lead was placed in the anterior septum and the left ventricular activation sequence in the CS trunk was directionally concordant between non-selective LBBAP and HBP (see Supplementary material online, Figure S1).

Based on the criteria of the MELOS study,⁴ LBBP was identified in 10 out of 71 patients (14%). In seven patients with positive QRS in lead II and negative in lead III, the LBBP lead was deployed at the border between the anterior and inferior septum. The left fascicular pacing was identified in 40 patients (56%), in whom pacing lead was deployed in the inferior septum (n = 31) or anterior septum (n = 9).

Deep S-wave in lead V6

In lead V6, the deep S-wave was recorded in 15 of 71 patients (21%), of whom 13 belonged to the non-selective LBBAP group (Figure 3). The deep S-wave was accompanied by the superior axis in all patients and the qR-wave in 73% of patients. The Stim–RWPT was shorter in the deep S-wave group than in the non-S-wave group. In the CS trunk, Stim–latest LV EGM interval was longer in the deep S-wave group than in the non-S-wave group (121 ± 9 and 110 ± 11 ms, respectively, P < 0.01). Thus, the time lag between the R-wave peak in lead V6 and the latest LV EGM in the CS trunk was greater in the deep S-wave group than in the non-S-wave group. At baseline, the left anterior fascicular block tended to be more frequently present in the deep S-wave group (33%) than in the non-S-wave group (18%) (P = 0.28).

The echocardiogram showed that the pacemaker leads were placed mostly in the inferior septum (93%) in the deep S-wave group. The lead tip was deployed further away from the mitral valve in the deep S-wave group than in the non-S-wave group (31 ± 5 and 28 ± 4 mm, respectively, P < 0.01) and closer to the left ventricular apex (35 ± 5 and 43 ± 6 mm, respectively, P < 0.01).

A ventricular septal defect was not detected on follow-up echocardiography. One patient underwent lead revision due to lead dislodgement 1 day after implantation. The pacemaker lead was extracted and placed in the right ventricular septum.

Discussion

The main findings of this study are as follows: first, the QRS morphology and axis of non-selective LBBAP showed great variations. Echocardiography revealed that LBBAP lead placements were broadly distributed in the septum. Secondly, the qR- or Qr-wave in lead V1 was recorded during non-selective LBBAP in 54 and 46% of the patients, respectively. In the qR-wave group, the pacing leads were deployed mostly in the inferior septum (93%). The non-selective LBBAP lead was deployed deeper than the LVSP lead in the inferior septum. In patients with the Qr-wave in lead V1 and the inferior axis in aVF, non-elective LBBAP leads were located in the anterior septum but not in close proximity to the left ventricular endocardium. Thirdly, in lead V6, the deep S-wave indicated the time lag between the R-wave peak and the latest ventricular activation in the CS trunk. The pacemaker lead was deployed closer to the left ventricular apex. Diverse QRS morphologies and axes are linked to the location of the non-selective LBBAP lead in the septum.

Potential alternative to His bundle pacing

In recent years, physiologic pacing has implied His–Purkinje conduction system pacing. The feasibility of permanent HBP has already been demonstrated in patients with bradycardia, although limited efficacy for infranodal conduction diseases and a high capture threshold remain critical issues.^15,17 Since Huang et al.⁵ presented an initial case, LBBP has been established as a promising alternative to HBP. A lower capture threshold and higher ventricular amplitude were obtained in patients with LBBP.^5–8 The LBBP lead was deployed at 1.5–2.0 cm from the distal His bundle in the apical direction in the initial studies,^5–7 while the LBBAP lead can be deployed more liberally over a wide area on the septum.^4,8

Various lead locations in the septum

QRS morphologies and axes had wide variability during non-selective LBBAP. The lead locations were broadly distributed from the inferior to the anterior septum on the echocardiogram. The initial site for the LBBP lead deployment has been described in previous studies.^5–9 The final fixation sites have been discussed mainly based on the penetration depth of the lead.^5–10 Our study provides the detailed locations of the LBBAP lead in the septum. The widespread distribution of the leads was linked to various QRS morphologies of LBBAP.

In the qR-wave group, the non-selective LBBAP leads were placed more inferiorly than in the Qr-wave group. The qR-wave in lead V1 may reflect electrical propagation from the inferior septum towards the electrode as well as delayed right ventricular activation. In 12 patients with the Qr-wave in lead V1 and the inferior axis in aVF, the non-selective LBBAP lead was placed in the anterior septum. This Qr-wave suggests propagation from the anterior septum away from the electrode. Moreover, the ventricular activation sequence in the CS trunk of non-selective LBBAP was directionally concordant with that of HBP in these patients. Thus, the Qr-wave in lead V1 and the inferior axis may indicate left anterior fascicle capture.⁴

In the European multi-centre study, the left anterior, septal, or posterior fascicular pacing was the most predominantly observed, occurring in 70% of patients.⁴ In our study, the left fascicular pacing was mainly identified, occurring in 56% of patients. It was differentiated from the LBBP by the LBB–Purkinje potential to QRS onset interval. The cut-off value of 25 ms was determined based on the length of LBB (10–15 mm) and conduction velocity in His–Purkinje system (1.5–1.8 m/s). The LBBP lead was placed in the border between the anterior and inferior septum of seven patients and in the anterior septum of three patients. These probably indicated the location of proximal LBB in the septum.^4–6

The Stim–RWPT of non-selective LBBAP was shorter than that of LVSP.^9,10 Non-selective LBBAP enabled early activation of the left ventricular free wall. Left ventricular septal pacing was not approved as successful LBBAP in initial studies. However, LVSP was included in LBBAP in recent studies.^4,13 We added the criterion of Stim–RWPT <90 ms to define strictly the successful LVSP and regarded it as successful LBBAP.

Deep S-wave in lead V6

In lead V6, the S-wave can be accompanied by the right bundle branch block, right ventricular hypertrophy, and left anterior fascicular block. Deep S-wave may also be apparent during left posterior fascicular ventricular tachycardia.¹⁸ In the deep S-wave group, 13 patients (87%) belonged to the non-selective LBBAP group. While the superior axis in aVF was present in all patients, the pacemaker leads were mostly placed in the inferior septum (93%). It was noted that the lead was deployed further away from the mitral valve and closer to the left ventricular apex in the deep S-wave group. Both a longer Stim–latest LV EGM interval in the CS trunk and the presence of the S-wave in lead V6 suggested electrical propagation from the apex to the base of the left ventricle. The presence of the S-wave was closely related to the earlier peak of the R-wave within the QRS complex. Consequently, the time lag between the R-wave peak and the latest LV EGM in the CS trunk was longer in patients with the S-wave in lead V6. During LBBAP with the deep S-wave in lead V6, Stim–RWPT may less accurately reflect the timing of the local depolarization wavefront.¹⁰

Clinical translation

Previous studies have suggested that LBB–Purkinje capture was achieved by deeper deployment of a lead with a higher R-wave in lead V1.^6–10 The qR-wave was obtained mostly at the inferior septum (93%). The non-selective LBBAP lead was deployed deeper than the LVSP lead in the inferior septum. When the low R-wave in lead V1 and the superior axis are obtained during the deployment, the pacing lead may be advanced more until the high R-wave appears to capture the LBB fibres. Finally, unipolar tip pacing might have been discontinued. The pacemaker lead should be rotated more slowly while monitoring the intracardiac electrogram and impedance to prevent septal perforation.¹⁴

Notably, non-selective LBBAP leads were located in the anterior septum but not in close proximity to the left ventricular endocardium. The risk of septal perforation is probably lower during the anterior septal deployment. The qR-wave was rarely obtained during non-selective LBBAP from the anterior septum. When the inferior axis is recorded during the deployment, additional attempts to obtain the tall R-wave may result in the excessive anterior migration of the pacemaker lead, after which the capture threshold may be increased. Thus, during lead deployment, the operator can anticipate the lead-tip location on the echocardiogram by evaluating the paced QRS morphologies.

Study limitations

In this study, a small number of patients were enrolled from a single centre. Multi-centre studies with a larger number of patients based on various imaging modalities are needed to confirm our findings.

Recordings of LV EGM were limited to the CS trunk. Three-dimensional multi-site mapping and recording of Purkinje potential in the left ventricle can reveal the detailed electrical propagation and capture of the left anterior or posterior fascicle.¹⁹ Left septal fascicle may exist,⁴ although the left septal fascicular pacing has not been evaluated in detail. Right ventricular activation should be further investigated. Interventricular conduction delay may differ between groups and improve during bipolar LBBAP.

Infra-Hisian and intraventricular conduction disturbances certainly affect the QRS morphology. Patients with normal QRS duration may have been selected for this study.

Conclusions

In patients with AV block, diverse QRS morphologies and axes were linked to the location of the non-selective LBBAP lead in the septum. Various lead deployments are feasible for LBBAP, allowing diversity in the conduction system capture. Left bundle branch area pacing is a promising alternative to HBP in patients dependent on ventricular pacing.

Supplementary material

Supplementary material is available at Europace online.

Funding

None declared.

Data availability

The data underlying this article will be shared on reasonable request to the corresponding author.