Left bundle branch pacing: the new kid on the block

Right ventricular pacing is has deletorius effects due to left ventricular dysynchrony and remodelling and may result in heart failure. Over the last decade, His bundle pacing has emerged as the most physiologic form pacing. However, it has limitations, such as higher capture thresholds, lower R wave amplitudes, atrial oversensing, and increased risk for lead revisions from late threshold increase with subsequent premature battery depletion, which has prevented a wider adoption of this technique in routine clinical practice. Left bundle branch pacing has been developed as an alternative physiologic pacing strategy that overcomes most of His bundle pacing limitations. This article summarizes the current status of left bundle branch pacing. Keywords: His bundle pacing, left bundle branch pacing, cardiac resynchronization, therapy heart failure.


INTRODUCTION
Left bundle branch block (LBBB) has been shown to increase morbidity and mortality in heart failure patients due to ventricular dyssynchrony. The marker of ventricular dyssynchrony is the wide QRS duration (QRSd) that sugest nonsimultaneous activation of the ventricular walls. As a result of prolonged ventricular conduction time and dyssynchronous activation of the ventricles, systolic dysfunction may occur in patients with LBBB 1 .
Using (HBP) the native conduction system, His bundle pacing has been proved to be the most physiologic form of ventricular pacing, than can correct wide QRSd, ensure the ventricular electrical activation synchrony and improves the clinical outcomes in heart failure patients [1][2][3] . However the technique is limited by diffi cult implantation, long fl uoroscopic exposure time, high capture threshold and fails to provide adequate pacing in patients with infrahisian block or proximal left bundle branch block 4 .
By providing a stable and low capture threshold, left bundle branch pacing has emerged to be a potential altenative to His bundle pacing in heart failure patients with left bundle branch block and furthermore an attractive option for pacing in patients with infranodal conduction disease [5][6][7] .
Abstract: Right ventricular pacing is has deletorius effects due to left ventricular dysynchrony and remodelling and may result in heart failure. Over the last decade, His bundle pacing has emerged as the most physiologic form pacing. However, it has limitations, such as higher capture thresholds, lower R wave amplitudes, atrial oversensing, and increased risk for lead revisions from late threshold increase with subsequent premature battery depletion, which has prevented a wider adoption of this technique in routine clinical practice. Left bundle branch pacing has been developed as an alternative physiologic pacing strategy that overcomes most of His bundle pacing limitations. This article summarizes the current status of left bundle branch pacing. Keywords: His bundle pacing, left bundle branch pacing, cardiac resynchronization, therapy heart failure. lead and proximity to the LBBB include: unipolar ring pacing capture the septum, fulcrum sign, estimation of depth by injecting contrast through the sheath and delineating the septum, and recording of LBB potential when present. [8][9][10] Lead depth can also be assessed by echocardiogram.
The following criteria for LBB capture have been proposed: paced RBB pattern; recording of LBB potential; evidence of direct LBB capture; short (≤60-90 msec) and constant at high and low output left ventricular activation time (LVAT) defi ned as interval between pacing stimulus to peak of R wave in lead V5/ V6; determination of selective vs non-selective LBB pacing 8 .
With selective LBB capture, the paced QRS morphology usually has a rSR pattern in V1 and there is an isolectric interval between the pacing stimulus and the onset of QRS complex 11 . Nonselective LBBP is defi ned by a paced QRS with a QR pattern in V1 and no isoelectric interval between the pacing spike and the QRS complex 11 .
Another method to differentiate LBB capture vs LV septal myocardial capture relies on the differen-LBBP is typically performed using a Select Secure 3830 pacing lead (Medtronic Inc, Minneapolis, MN) and a catether-delivering sheath (C315His or Select site C304His) ( Figure 1) via left subclavian or left axillary vein access 8 . Intracardiac and 12-lead electrocardiograms are continuously recorded and displayed on an electrophysiological recording system. The catheter and pacing lead are placed into the His bundle area in 30° right anterior oblique fl uoroscopic view and the His bundle potential is recorded 8,9 . After noting the His bundle location, the sheath/pacing lead is advanced 1.5-2 cm apically 8,9 . Pacing showing a "W" with the notch closer to the nadir in lead V1 may indicate an ideal postion 8 . The sheath is manipulated, usually by rotating it counterclockwise, until the pacing asssumes a position perpendicular to the interventricular septum ( Figure 2B, C). The lead is the gradually screwed in with a clockwise rotation. During lead advancement, paced QRS morphology and pacing impedance are monitored [8][9][10] .
As the lead is advanced, the paced QRS morphology changes from a LBB pattern to a right bundle branch block (RBBB) pattern in lead V1 (qR or rsR´) 11 . Initially the impedance increases gradually before it drops by about 100 Ω when the lead tip reaches the LBB. Other means to confi rm suffi cient depth of the Although HBP represent the most physiologic form of pacing, it is technically challenging due to anatomic location, long fl uoroscopic exposure times, high capture thresholds, sensing issues and often fails to provide adequate pacing in patients with infrahisian block or proximal left bundle branch block. According to Upadhyay et al. 13 the site of block usually is located within the His or proximal left bundle. Left bundle branch pacing overcomes the limitations of HBP by implanting the lead distal to the level of block 5 .
Studies of LBBP have focused on several issues: methods to improve technique feasability, safety, hemodynamic advantages, and pacing in common scenarios: bradycardia, complete or advanced degree AV block, atrial fi brillation with AV node ablation, and as an alternative to biventricular pacing for CRT in patients with heart failure. tial effective refractory periods of the two structures 12 . Premature beats are delivered either during sinus rhythm or after an 8-beat drive train at 600 ms with a 450 ms coupling interval decremented by 10 ms. Non-selective LBB pacing was characterized by paced QRS morphology changes to a myocardial-only capture (broader QRS, with slur/notch/plateau and/ or with change in amplitude/polarity in several leads). Selective LBBP was characterized by changed in QRS morphology to a typical RBB morphology preceded by a latency.
Once LBBP capture is recorded (narrow QRS with RBB morphology, higher R-wave amplitudes, short left ventricular activation time (LVAT) ≤60-90 msec, low capture threshold) no further lead advancement should be performed. A low capture threshold and high pacing impedance suggest that the pacing lead is located inside the septum. Sudden decrease of lead impedance, sensed R-wave amplitude, and/or loss of capture indicate that the helix of the lead has entered the chamber of the left ventricle, for example, LV perforation [8][9][10] . Other potential complications that may occur during procedure are RBB injury, septal hematoma, coronary arterial injury, tricuspid regurgitation, risk of lead extraction.  LV mechanical synchrony evaluated by a 3-months follow-up echocardiography, was preserved or improved by LBBP. Guo et al. reported a 89% success rate in a series of 164 patients and no complications 23 . LBBP was shown to be safe in a small cohort of patients with heart failure, atrial fi brillation, and AV node ablation. Improvement in LVEF and reduction in LV end-systolic volume were signifi cantly higher than in a control group that received standard ICD implant and had AV node ablation and resulted in a decreased incidence of inappropriate shocks 24 .
LBBP appears to be a good alternative for CRT. Zhang et al. 18 evaluated the value of LBBP in eleven patients with heart failure, low LVEF and LBBB. They showed that LBBP has the potential to be an effective alternative to cardiac resynchronization therapy, by correcting the LBBB, providing ventricular electric and mechanical synchrony with LV reverse remodelling and improving the NYHA functional class. In propensity matched small cohort of patients with heart failure, Li et al., found that resulted in greater reduction in the QRSd (58.0 vs. 12.5 ms, P < 0.001), a greater increase in LVEF (15.6% vs. 7.0%, P < 0.001), and greater echocardiographic (88.9% vs. 66.7%, P = 0.035) and super response (44.4% vs. 16.7%, P = 0.007) to biventricular pacing 25 .

CONCLUSIONS
LBBP represents an attractive option for physiologic pacing in patients with conduction disease by providing a stable and low capture threshold. As it bypasses the level of block, LBBP can potentially be a fi rst line pacing method in patients with iatrogenic AV block due AV node ablation. LBBP appears to provide signifi cant advantages in heart failure patients by achieving electrical and mechanical synchrony of the left ventricle. However, randomized prospective clinical trials are needed to establish the patients that would most likely benefi t from this pacing strategy. Furthermore, studies are needed to asses the long-term effects of septal contractile stress with LBBP.

Confl ict of interest: none declared.
Abbreviations list AV = atrioventricular BIV = biventricular pacing CRT = cardiac resynchronization therapy ECG = electrocardiographic HBP = His bundle pacing HF= heart failure Left bundle branch pacing is performed mostly using the 3830 lead and C315 sheath. 5 Mafi -Rad et al. 14 developed another technique using a custom-made lead implanted in mid-distal septum. They observed that left ventricular septal pacing resulted in a narrower QRS with a RBB morphology and compared with RV pacing provided better hemodynamic benefi ts. 3D mapping guidence can be used to guide lead position and depth but it adds costs to the procedure 15 . The accurate location for LBBP can be targeted during implantation using characteristic changes in paced QRS morphology during lead deployment 16 . Similarly, Chen et al. 10 in a prospective study explored the feasibility of LBBP using a transventricular septal approach and demonstrated that the transition of ECG morphology during lead implantation can guide the appropriate lead placement in the basal septum. At 3-months follow-up the pacing thresholds were comparable and remained low and stable. LBBP generated a narrower QRS duration (QRSd) due to fast left atrial activation and produced a better synchronization of the left ventricle than conventional RV pacing.
LBBP preserves satisfactory LV synchrony and result in favorable hemodynamic effects in patients with narrow QRS when evaluated by echocardiography 17 . LBBP has benefi cial effects in re-establishing ventricular electric and mechanical synchrony and LV reverse remodelling in patients with heart failure and LBBB 18 . In a single centre prospective study, using single-photon emission CT (SPECT) myocardial perfusion imaging, Hou et al. 19 reported that mechanical synchrony parameters after LBBP were similar to HBP and better than RVP.
LBBP appears to be safe and feasible. In a two center study involving 341 patients, there were only 3 complications reported: all lead dislodgements 20 . LBBP was achieved in 89% of patients. In a long-term followup prospective study Vijayaraman et al. 21,22 reported that His bundle capture threshold and the generator replacement rate in HBP was higher than that in RVP (9% vs. 1%). They also described a high success rate (88%) in the correction of LBBB by LBBP, and found that LBBP it is feasible in a high percentage of patients with low thresholds during acute follow-up and may signifi cantly increase the overall success of physiologic pacing.
Li and colleagues 16 tested the feasibility and safety of LBBP in patients with AV nodal disease. LBBP was successful in 90% of patients, with a stable low pacing output and narrow paced QRS duration. The