Implantation procedure
A single cardiologist was responsible for implantation, while other operators participated at various stages of the procedure (venous cannulation, atrial lead placement).
Fluoroscopy-guided cannulation of the axillary vein was the venous access of choice, using a lumen-less 4-Fr lead (Select-Secure model 3830 69 cm, Medtronic Inc., Minneapolis, MN) through a fixed preformed sheath (C315 HIS, Medtronic Inc.). A deflectable sheath (C304, Medtronic Inc.) was used when it proved impossible to record the His electrogram. The lead was connected to a digital recording system (Electrophysiology Lab System, Boston Scientific, MA) in a unipolar configuration to measure the intracavitary signal.
HBP was performed as described in the literature.10 In brief, rotation and advance-retreat maneuvers were used to find the His bundle electrogram with lowest atrial signal amplitude, evaluating the selective and non-selective HBP with the lowest possible threshold. The bundle block correction threshold was also considered in patients with a wide baseline QRS.
After localization of the His electrogram in the LBBAP procedure, the sheath was apically advanced 1–2 cm; next, if a paced QRS with “W” morphology was obtained in lead V1, the electrode was inserted into the septum with 5–6 clockwise turns until the notch in the paced QRS complex migrated to the end of the QRS wave (or disappeared) and the QRS width narrowed (Figure 1). The depth at which the lead was affixed in the interventricular septum was guided by the impedance values and target QRS obtained. Recording of the left bundle branch potential was not an objective. The fluoroscopy time included the time for the axillary puncture and for placing all of the leads.
The QRS complex was recorded before and after implantation by another operator using a digital recording system at a speed of 100 mm/s. The QRS duration was measured from QRS onset in the 12 leads when an isoelectric interval was observed between pacing stimulus and QRS onset or from stimulus to the end of the QRS when it was not. Pacing parameters (threshold, sensed R-wave amplitude, impedance) were compared between implantation and 3 months post-implantation.
The procedure was considered successful when a baseline QRS of <120 ms was achieved in HPB, as recommended by the Multicenter HBP Collaborative Working Group1, and when a QRS width ≤ 130 ms was obtained in LBBAP.12 In patients with bundle branch block (BBB), HPB and LBBAP procedures were considered successful when the paced QRS narrowed by ≥20% or to a QRS width of <130ms 13. Accepted pacing parameters were a pacing threshold of ≤ 3.5 V, an R-wave amplitude ≥ 0.8 mV, and a pulse width of 1 ms with HBP and 0.5 ms with LBBAP.