BACKGROUND: Multiple screw-in attempts under fluoroscopy are often needed to place the pacing lead tip near or at the left bundle branch (LBB). This study was conducted to evaluate the feasibility of implanting an LBB pacing lead in the proximal LBB (PLBB) guided by intracardiac echocardiography (ICE). METHODS: The distribution of the LBB was initially determined by ICE anatomic imaging and 3-dimensional electrical mapping of His and LBB potentials in 20 patients in the first parts of the study. In the second part, 101 consecutive pacemakerindicated patients were randomized into the ICE-guided and non-ICE groups for LBB pacing implantation. The procedural details and electrophysiological characteristics of the 2 groups were compared. RESULTS: In the first part of the study, PLBB was identified at 10 to 20 mm from the tricuspid annulus toward the apex with an area of 4.5 +/- 1.1 cm(2). In the second part, the number of lead screw-in attempts in the septum was fewer in the ICE group than in the non-ICE group (1.43 +/- 0.62 versus 1.98 +/- 0.75, P=0.0002). The duration of the procedure (26 +/- 8 versus 43 +/- 9 minutes, P<0.001) and fluoroscopy for LBB pacing implantation (7.4 +/- 1.8 versus 10.7 +/- 2.4 minutes, P<0.001) in the ICE group was significantly shorter than those in the non-ICE group. LBB pacing in the ICE group generated a lesser QRS duration with more cases of LBB trunk pacing (46.8% versus 25%, P=0.031) and PLBB (91.5% versus 72.7%, P=0.0267) pacing compared with that in the non-ICE group. CONCLUSIONS: The basal left ventricular septum can be better visualized using ICE. ICE-guided PLBB pacing is feasible and safe, with a shorter duration required for the procedure and fluoroscopy, and generates greater LBB trunk pacing and PLBB pacing.