Measurements:
Following active fixation of His bundle pacing lead a baseline H-V interval plus ventricular activation time (VAT) in Lead 1 was measured and designated as baseline HVAT interval (Fig.1)
In addition, the following intervals were measured during paced beats at each
1-V decrement.
  1. The interval from His bundle pacing artifact (H) to the peak deflection of the QRS in Lead 1, i.e., His (H) to peak ventricular activation time (VAT), designated as HVAT of paced beat) (Fig. 1).
  2. Stimulus to QRS (Stim-q) interval in S-HBP (Fig. 1).
  3. Lead I voltage (Fig. 1).
  4. QRS axis in frontal plane was measured using an online ECG analysis tool.
  5. V lead transition in chest leads.
We used a Cardiolab (GE Healthcare) signal acquisition and recording system. The 12 lead ECG filter setting was 0-100 Hz. The His bundle electrogram
Study data including QRS morphology was analyzed offline on a review monitor. Activation times were measured using an on-screen multi-leg caliper.
Statistics: A paired t test was applied to the data, using an online statistics calculator (GraphPad). Mean difference at a p value <0.05 was considered significant.
Results
Threshold testing resulted in non-selective (NS) and selective (S) His bundle pacing (HBP as follows.
  1. NS-S HBP group: Both NS and S HBP was observed in 17 patients (11 chronic and 6 acute RBBB).
  2. NS-HBP group): Only non-selective HBP was observed in 18 patients (14 chronic, 4 acute)
  3. S-HBP group: Only selective HBP was see in 4 patients (2 chronic, 2 acute).
The changes in baseline characteristics are shown in Table 1. The His Purkinje system-peak ventricular activation time (HVAT), showed no change with S-HBP however slight but significant decrease was seen with NS-HBP. Whereas the
S-HBP complex was nearly identical to baseline, the NS-HBP complex showed characteristic changes which included a significant increase in L1 voltage (all patients) as well as a significant