A flexible tube designed to extend the length of a vascular guide-catheter during percutaneous transluminal passage and placement of a diagnostic/interventional device (e.g., pacing lead, balloon dilatation catheter) within either the coronary or peripheral vasculature. It is intended to be inserted through the lumen of the guide-catheter, and includes a proximal handle and delivery shaft connected to a distal catheter-extending tube. This is a single-use device.