In this study, we investigated the short-term effects of LBBP on mechanical synchrony and myocardial work in patients with normal heart function. The results showed that LBBP induced more synchronized contraction when compared to RVP. LBBP induced a significant effective myocardial work which was equal to that in native conduction and was much better than RVP. Thereafter, LBBP reduced LV global wasted work, increased effective work, and even improved LV global systolic function.
Permanent pacemaker implantation is the only effective therapy for non-reversible bradycardia. Whereas the goal of pacemaker implantation is not only to increase ventricular rate but also to achieve physical conduction and subsequently mechanical synchrony. The pacing site is one of the major factors for this. RVP has been used for more than half a century. However, the research showed that it may induce ventricular arrhythmia, heart failure, and even increase hospitalization rate and mortality[2]. HBP is a more physiological pacing technique but is challenging in native electrical conduction damage and lead-placing technology. LBBP is a more advanced innovation to pace on the left bundle branch area. In a prospective study including 100 patients, Vijayraman P et al reported that LBBP was associated with lower and more stable capture thresholds[11]. In the patients with HF and LBBB, Wang et al found that LBBP improved clinical symptoms and LVEF after 6 months[12]. Wu et al compared LBBP, HBP, and traditional biventricular pacing in CRT indicators[13]. The results showed that LBBP induced better electrical synchrony and LVEF than biventricular pacing. The clinical improvement in LBBP was nearly equal to that in the HBP, but the pacing parameters were better—the pacing threshold was lower and R-wave amplitude was higher.
Mechanical dyssynchrony is an independent predictor of adverse cardiovascular outcomes[14]. However, it is not associated with electrical conduction pattern (QRS duration) straightforwardly. Nearly 29.5% of HF patients with normal QRS duration have mechanical dyssynchrony[15]. PSD from strain analysis is a more specified method to quantify mechanical synchrony accurately and was widely used in CRT, His bundle pacing, and RVP patients[16]. In the present study, LBBP induced nearly normal QRS duration. More importantly, PSD and IVMD in LBBP were much shorter than that in RVP, and even the same as that under the native heart rhythm. It demonstrated that LBBP provides normal mechanical synchronization both inter-and intra-ventricle. Such synchronization could have significant effects on the myocardium at molecular, cellular, and structure levels, such as modulation of calcium handling, ß-adrenergic signaling, ion channel expression and function, cell survival signaling, and mitochondrial energetics, and eventually prevented myocardial remodeling[17].
Myocardial function was the most important predictor of all-cause death and HF hospitalization. The pressure-volume analysis is the gold standard for evaluating myocardial contractility, but with limited clinical application because of complexity and invasiveness. As a novel technique, the pressure-strain loop has been invented to replace pressure-volume analysis in a simplified and non-invasive way. Based on 2D speckle-tracking echocardiography, it takes into consideration the loading conditions by combining blood pressure measurements. The validity of this approach is supported by 18fluorodeoxyglucose (FDG) PET analysis, which showed an excellent correlation between segmental work and regional myocardial glucose usage[18]. The Pressure-strain loop evaluates segmental/ global myocardial contractility from work index, constructive work, waste work, and work efficiency. GCW is the effective heart work for blood ejection, and GWW represents the energy loss for blood ejection. GWI represents the power generated by LV during a cardiac cycle. GWE, which means the weight of effective work in a whole heart cycle, is an estimate of LV mechanical performance and energy consumption that considers loading conditions[19].
In the present study, LBBP implantation did not change GLS. Meanwhile, when compared with RVP, LBBP increased GCW and decreased GWW, hence both GWI and GWE increased. It demonstrates that LBBP induces more effective contraction in the systole, and avoids non-effective contraction in the diastole. RVP induced LBBB-like contraction pattern[2], then septal segments shortened from early systole and followed by lengthening in the middle and late systole, while LV lateral walls shorten from middle-late systole to the early diastole. The net result is that GCW is markedly reduced and GWW significantly increases, contributing to global ventricular dysfunction progression. Conversely, LBBP synchronized contraction on the septum and lateral wall from early systole when LV pressure is low, followed by relaxation in the diastole. Subsequently, GCW significantly increased because of homogeneous ventricular work distribution. Meanwhile, work efficiency and heart function improved significantly and is linked to a better long-term prognosis[19].
Study limitation
There are several methodological limitations in this study. First, this was a small sample size, single-center, nonrandomized, and non-blinded observational research. Secondly, mechanical synchrony was dependent on the echocardiography image quality. Because of age, obesity, or pulmonary disease, some images were suboptimal and could not be analyzed. Thirdly, this study only focused on instant myocardial work, but without long-term complications and prognosis. It needs a randomized, multi-center, large sample size, and long-term follow-up study to further prove our findings.