A 70-year-old woman with symptomatic bradycardia caused by persistent atrial fibrillation and atrioventricular block was referred to our institution for pacemaker implantation. After we failed to obtain adequate His bundle capture thresholds (>2.5 V at 1.0 ms) at three pacing sites, left bundle branch pacing was attempted as an alternative technique. The tip of the 3830 lead was screwed towards the left side of the interventricular septum. Contrast medium was injected through the C315 sheath, which was placed close to the right side of the interventricular septum to determine the exact depth of the 3830 lead inside the septum. Unexpectedly, the vessels in the interventricular septum were revealed by the contrast, which showed that the lead had penetrated one of the septal vessels. To the best of our knowledge, this is the first reported case of a patient in whom injection of a contrast agent through a delivery sheath showed damage to the interventricular septal vessels. Findings from this case suggest that injection of contrast medium through a C315 sheath that is placed close to the interventricular septum is a potential method for excluding damage to interventricular septal vessels.