DISCUSSION
The main findings of this study were: 1) LBBAP may improve FTR in patients with persistent AF and bradycardia; 2) A high ventricular pacing percentage of LBBAP maybe needed to improve FTR.
Long-term hemodynamic disturbance resulting from an irregular and slow heart rate may be the main mechanism of FTR in patients with persistent AF and bradycardia. Compared to regular rhythm, irregular rhythm leads to hemodynamic effects including cardiac output decreases, capillary wedge pressure increases and right atrial pressure increases in AF patients[10, 22, 23]. And the slow heart rate causes a prolonged diastole phase, resulting in ventricular filling pressure increasing, which is correlated with a higher PASP and right atrial pressure. Those hemodynamic disturbances may contribute to RAV enlargement, PASP increase, and FTR development subsequently. Preclinical and exploratory clinical studies suggested that pacing at moderately accelerated heart rates was able to decrease ventricular filling pressures and improve hemodynamic effects[24-26]. Besides, the clinical trial also demonstrated the benefits of moderately accelerated heart rate compared to usual care[27]. In our study, LBBAP elevated and regularized the heart rate, and the RAV and PASP reduction were observed, which may contribute to the FTR improvement.
The physiological pacing effect of LBBAP was essential for FTR improvement. Lead interference and pacing-induced ventricular dyssynchrony were considered to be the main mechanisms of TR deterioration following right ventricular pacing[11]. With a thinner pacing lead and physiological pacing mode, LBBAP has the potential to preserve tricuspid valve function during long-term follow-up[28-30]. Lead interference was avoidable, as relevant studies[29, 30] showed that farther distance from the electrode fixation site to the tricuspid annulus (Lead-TA-dist) was correlated with a lower incidence of TR worsening after LBBAP. With the Lead-TA-dist of 19.7 ± 5.0 mm in our study, the interference of 3830 lead on tricuspid valve (TV) was relatively small. And pacing effect of LBBAP could preserve tricuspid valve function in the acute phase also had been proved[31]. Taken together, the effect of a high percentage of LBBAP on FTR in persistent AF patients with bradycardia challenged the old belief that pacemaker implantation was correlated with TR deterioration only. PVC burden can decrease VP percentage and impact the improvement of FTR. Drugs such as beta blocker or catheter ablation could be considered to eliminate PVC burden.