Pacemaker implantation
The left subclavian approach was preferred per study protocol. After the venous system was reached, the atrial pacing lead was temporarily placed in the apex of the right ventricle. Then the His bundle region was mapped using a SelectSecure™ lead (model 3830, 69 cm, Medtronic Inc., Minneapolis, MN), which was delivered through a fixed-curve sheath (C315 HIS, Medtronic, Minneapolis, MN), as previously described [8]. Once the His bundle signal was identified, to ensure the para-hisian ventricular position of the lead tip, it was moved to the right ventricle, behind the hinge of the septal leaflet of the tricuspid valve. An X-ray and an electrogram (EGM) of the atrial, His bundle, and ventricular signals were used to navigate the lead tip to the area of the basal interventricular septum. At this location, atrial signals significantly decreased or disappeared, but His bundle signals were present before lead fixation - Figure 1. In case of uncertainty, an injection of contrast agent through a C315 HIS sheath was performed to verify ventricular lead tip placement. Once the proper ventricular septal position of the lead tip was achieved, it was fixed using 3–5 clockwise rotations to anchor it into the septal ‘para-hisian’ position, which was followed by pacing with outputs between 0–5V at 0.5 ms. cSp was confirmed when a wide, notched, or slurred QRS complexes during low output pacing, which narrowed and had a reduced R wave peak time (RWPT) at V6 (LVAT) during high output pacing were observed. Thereafter the atrial lead with a pre-shaped stylet was moved from the apical position and temporarily placed in pre-specified locations in the right ventricle (RV septum, anterior wall, and lateral wall). For appropriate lead placement in these locations, the SelectSecure™ lead and C315His sheath, already positioned in the basal septal region, were used as landmarks – Supplementary Figure 1. For all pacing locations, X-ray cine loops in two projections (RAO 20–30° and LAO 20–30°) were generated and stored for further analysis. In patients with an indication for a dual-chamber pacemaker, the atrial lead was removed from the ventricular position and was fixed in the right atrium at the end of the procedure. Stored ventricular pacing positions of the atrial lead were later retrospectively reviewed by an experienced physician blinded to the intended position. The exact location of the lead tip was determined to be one of the following: RV myocardial septal – mSp, apical, anterior wall or lateral wall. Moreover, mSp positions were further classified as (a) the septal right ventricular inflow tract (RVIT) or (b) the septal right ventricular outflow tract (RVOT) in the RAO projection – Figure 2.
The distances between cardiac silhouette lines (a+b) and the lead with cSp capture to the lower cardiac silhouette (a) were measured in mm – Figure 2. The location of the para-hisian region with cSp capture, with respect to the diameter of the heart silhouette, was calculated as (a/a+b) x 100 and reported as percentages. All ECG recordings during pacings of the right ventricular septum were reviewed retrospectively using EP Labsystem software (Boston Scientific, MA, USA) at 25, 100, and 200 mm/s, as needed. HV intervals (calculated as the mean value between 2 consecutive beats), LVAT, the types of ventricular capture, and the ratio between the atrial and ventricular signal amplitudes in mV (A/V amplitude ratio) were measured after lead fixation in the para-hisian area during spontaneous rhythm and pacing at 100 mm/s.