PICM Definition and Left Ventricular Function
In the present study, we evaluated the impact of leadless pacing on left ventricular ejection fraction. Overall, there was no change in mean LVEF in our cohort of 48 subjects with LPs at 697 day follow up. We assessed the incidence of PICM in leadless pacemakers using both a liberal definition (Definition #1; drop in LVEF >10%) which has been reported to result in a PICM incidence of up to 39% in TVPs and a stringent definition of PICM (Definition #2; drop in LVEF >10% and mean LVEF <50%) which has been reported to result in a PICM incidence of approximately 13% [3]. The main finding of our study was an incidence of PICM in our study of LPs of 10.4% using Definition #1 and 7.0% using Definition #2.
Sanchez et al recently compared the incidence of PICM in 131 patients with TVPs and 67 patients with LPs who were all pacemaker-dependent. it was shown that LPs lead to significantly less PICM than TVPs, with a rate of 3% with LP and 13.7% with TVP [5]. That study used a definition for PICM which we have referred to as definition 2 in this study. The 13.7% incidence of PICM in the TVP group is similar to the typical published incidence as discussed above, which supports the validity of that study.
The etiology of PICM is commonly attributed to dyssynchronous right and left ventricular electromechanical activation which may also induce changes in coronary blood flow, hemodynamics, fibrosis, perfusion, and metabolism [10]. Some have suggested that this dyssynchrony could be reduced if the pacing lead is placed more basally on the septum, given studies showing higher rates of PICM with apical pacing [4]. The idea is that with a basally-placed LP, the capture of more proximal conduction tissue could result in a narrowed QRS complex and improve ventricular synchrony, as seen in conduction system pacing [11]. No other studies to our knowledge have assessed the impact of LP septal implantation location on LVEF. However, even in studies with TVP, the data is mixed, with some studies noting an increased risk of PICM in apical vs septal pacing [12] [13], while another study showed no association [14].
Our results are not consistent with the idea that a more basal location of the LP would result in less PICM as a result of the capture of the early conduction system, given that there was no statistically significant relationship identified between changes in LVEF and the QRS duration change or the LP implantation location. Although our study did not include a direct comparison group of patients with TVPs, the lower incidence of PICM in LPs could be explained by differences in myocardial activation at the level of the septum as well as the effects of tricuspid valve function as described below.