Methods, systems and devices for effectively and safely accessing and verifying a target site within a vessel or body cavity undergoing a catheter-based procedure or treatment. The present invention further includes the accurate delivery an instrument, an implantable device and/or materials to the target site. The present invention is particularly useful for transvenously assessing the suitability of a target site within the coronary vasculature for placement of a pacing electrode and transvenously placing the pacing electrode at the target site.
