Implant procedure:
HBP was performed using the 3830 SelectSecure lead (Medtronic, Minneapolis, MN) delivered through either the fixed curve (C315-HIS) or deflectable curve (C304) sheath. Standard techniques for HBP were used, including mapping for the distal His recording prior to fixation or pace-mapping when the His recording was not located (infrequently). The procedure was deemed successful if selective or nonselective His-bundle capture was demonstrable. RV apical pacing leads were also implanted in 12% (6/50) of patients receiving HBP with the apical lead programmed to pace 80 ms after His-pacing. At implant, pacing outputs were generally programmed at 3.5 V at 1 ms and then adjusted based on the HBP capture threshold (confirmed by 12 lead EKG morphology) during follow-up, such that outputs were programmed to ~twice the capture threshold for His-capture.
LBBAP was also performed using the above lead and sheaths. Standard techniques were used, as previously described.15Briefly, the lead was advanced 2 cm distal to the His electrogram toward the RV apex or on the basis of unipolar paced morphology.20 With the sheath held flush against the septum, the lead was rotated and advanced until the lead perforated the RV septal myocardium. The paced QRS morphology and impedance was continuously monitored and the lead rotated until the paced morphology approximate a RBBB morphology. Contrast injection was performed in select cases. At implant, outputs were generally programmed at 2-2.5 V at 0.5 ms, and then adjusted only if needed during follow-up based on threshold (confirmed by 12 lead EKG morphology) such that outputs were programmed to ~ twice that of threshold.
Data Collection and Follow-Up :
Baseline demographic, clinical, electrocardiographic, and procedural data were collected by chart review. Follow-up data from the first post-procedure follow-up and from the most recent follow-up were collected for all patients. Pacing threshold was defined as the lowest voltage required to capture the conduction system with either selective or nonselective morphology. Pacing parameters were recorded in detail with 12-lead ECG performed during threshold testing. Pacing thresholds, R-wave sensing and impedances were all carefully documented at implant and in follow-up. Stimulus to peak R wave in lead V6 (RWPT) was defined as the time in milliseconds between the pacing stimulus and the peak of the R-wave in V6 on the surface electrocardiogram. Pacemaker stimulation energy was analyzed as a function of voltage and pulse width (E = V2t/R where E = energy, V = voltage, t = pulse width and R = impedance).14 Though pulse widths of 0.4, 0.5 and 1 ms were used variably across the population, we were able to use this method to adjust thresholds to reflect an amplitude measured at a pulse width of 0.5 ms for consistency across all measurements (adjusted pulse amplitude = sqrt[(recorded pulse amplitude)2(recorded pulse width)/(0.5)]. For the purpose of analysis, an ‘acceptable pacing endpoint’ (APE) included the following parameters: sensing R-wave amplitude >5 mV, pacing threshold <2.5 V @ 0.5 ms and impedance between 400 and 1200 Ohms. APE captures normal lead behavior as seen with legacy pacing. Total fluoroscopy duration and procedure duration was obtained. Follow-up for each patient stopped with any lead revision or generator replacement.
Statistical Analysis :
Pre-procedural, procedural and follow-up data for patients who underwent HBP were compared with patients who underwent LBBP. Continuous variables were reported as mean +/- SD and compared with two-sample t-tests. Categorical variables were reported as percentages and compared using chi squared or two-tailed Fisher exact tests as appropriate. The Kaplan-Meier method was used to generate failure curves for descriptive purposes with censoring performed at either the date of loss of APE, date of last follow-up, or date of death. All analyses were performed with the use of Stata software version 16.1. Statistical significance was defined by p-values <0.05.