Evaluation of electrophysiological characteristics and ventricular synchrony: An intrapatient‐controlled study during His‐Purkinje conduction system pacing versus right ventricular pacing

Abstract Objectives to Background To compare electromechanical ventricular synchrony when pacing from different sites, including right ventricular apex pacing (RVAP), right ventricular septum pacing (RVSP), His bundle pacing (HBP), left bundle branch pacing (LBBP), and RVSP during unipolar pacing from the ring electrode of LBBP lead (RVSPring) in each patient and evaluate the correlations between electrophysiological characteristics and ventricular synchrony. Methods Twenty patients with complete atrioventricular block indicated for dual‐chamber pacemaker implantation were included in the study. Unipolar pacing at different sites, including RVAP, RVSP, HBP, LBBP, and RVSPring, was successively performed in each patient. The pacing characteristics and echocardiogram parameters were collected and compared among intrinsic rhythm and pacing at different sites. Results Similar to HBP (114.84 ± 18.67 ms), narrower paced QRSd was found in LBBP (116.15 ± 11.60 ms) as compared to RVSPring (135.11 ± 13.68 ms), RVSP (141.65 ± 14.26 ms), and RVAP (160.15 ± 19.35 ms) (p < .001). LBBP showed comparable pacing parameters to RVAP or RVSP and was significantly better than HBP, with maintained cardiac function. TS‐12‐SD was significantly improved in LBBP (41.80 ± 20.97 ms) than RVAP (69.70 ± 32.42 ms, p = .003) and RVSP (63.30.56 ± 32.53 ms, p = .018) but similar to HBP (51.50 ± 25.67 ms, p = .283) or RVSPring (57.80 ± 25.65 ms, p = .198). Among these pacing strategies, negative values of interventricular mechanical delay (IVMD) were only identified in LBBP (−19.25 ± 18.43 ms), significantly different from RVAP (35.00 ± 30.72 ms), RVSP (22.85 ± 22.05 ms), HBP (5.20 ± 18.64 ms), and RVSPring (16.00 ± 26.76 ms (all p < .05). Using Pearson's analysis, Sti‐LVAT was positively correlated with QRS duration, IVMD, TS‐12‐SD, LVEDV, and LVESV, while a negative relationship could be observed for left ventricular ejection fraction. Conclusions His‐Purkinje conduction system pacing (HPCSP) achieved better electrical and mechanical synchrony than conventional RV pacing. For interventricular synchrony, only LBBP initiated earlier LV activation than RV, in accordance with the right bundle branch block (RBBB) pattern of paced QRS during LBBP. Sti‐LVAT might be a good parameter correlating with LV systolic function and mechanical synchrony.

His-Purkinje conduction system pacing (HPCSP), His bundle pacing (HBP), and left bundle branch pacing (LBBP), have emerged as two physiological conduction system pacing strategies and have been demonstrated to achieve narrower QRS duration and better mechanical synchrony than RVP. 4 Nowadays, HPCSP has been widely adopted from inside and outside the field. 5 Recent studies have also demonstrated that HPCSP significantly improved heart function in heart failure patients with left bundle branch block (LBBB), [6][7][8] and even better than BVP. 9,10 Theoretically, HBP is the most physiological pacing mode and it could achieve the same narrow QRS complex as the intrinsic and correct LBBB or right bundle branch block (RBBB) as well. However, pacing safety concerning high pacing threshold and low R wave amplitude of HBP limits its application in all pacing candidates, especially in infra-Hisian block cases. 11 While pacing captures the left conduction system more distal than His bundle, LBBP offers a low and stable threshold and high R wave amplitude comparable to RVP and its feasibility has been confirmed in large-scale studies with mid-long term follow-up. 12 Hence, the purpose of our present study is to draw meaningful comparisons between the electrophysiological and echocardiographic parameters during HPCSP and conventional RVP in AVB from an intrapatient analysis to further assess the relationship between electrical and mechanical synchrony.  (Figure 1), which is defined as the interval from the pacing stimulus to the peak of the R wave in lead V5, shortening abruptly by increasing output or remaining shortest and constant at the final depth. The lead depth inside the interventricular septum was measured via angiography through the sheath at LAO 35°( Figure 1P). During the procedure, the atrial lead placed at the right atrium appendage and ventricular lead at different pacing sites above were temporarily connected to the programmer (Medtronic 2290) with DDD mode, AV delay of 150 ms, and pacing output of 3.5 V/0.5 ms during unipolar configuration at different sites above in each patient.

| Statistical analysis
Continuous variables were expressed as mean ± SD and paired Student's t-test was used to compare the difference between baseline and 6-month follow-up in each group. Analysis of variance test was used to perform comparison among more than two groups and was followed by the least significant difference test for multiple compari-

| Correlation between electrophysiological characteristics and echocardiographic parameters
Correlations between electrophysiological characteristics and echocardiographic parameters are summarized in Table S3

| DISCUSSION
The present study directly compared electrophysiological characteristics and echocardiographic parameters at different pacing sites in each patient during the procedure and the main findings were as follows: (1)

| Electrical synchrony
Paced QRS duration is demonstrated to be narrower in LBBP 12

| Mechanical synchrony
Better LV mechanical synchrony has been previously confirmed during LBBP than RVSP using SPECT MPI and echocardiogram. 15,16 The latter study by Cai et al. 15

| Limitations
The study focused on the electrical and mechanical synchrony at different pacing sites during the procedure. These findings were acute hemodynamic results. Thus, the long-term hemodynamic effects of LBBP remain uncertain. In addition, it is a single-center, self-controlled observational study, with relatively small sample size. The main study population was AVB with narrow QRS or RBBB and normal LVEF. Consequently, the results of the study could not be generalized to patients with IVCD or heart dysfunction.

| CONCLUSIONS
HPCSP provided better electrical and mechanical left ventricular synchrony than conventional RVP. While interventricular synchrony during LBBP was significantly different as compared to HBP, RVAP, and RVSP, showing earlier LV activation than RV, which was consistent with the RBBB pattern of paced QRS during LBBP.
Sti-LVAT might be a good electrophysiological parameter correlated to LV systolic function and mechanical synchrony.

CONFLICTS OF INTEREST
The authors declare no conflicts of interest.

DATA AVAILABILITY STATEMENT
The datasets generated and/or analysed during the current study are not publicly available duebut are available from the corresponding author on reasonable request.