A catheter includes a hub and a stylet extending through the hub to a distal end of the catheter. A balloon fixed at its proximal end to the hub and at its distal end to the stylet is axially extendible by operation of the stylet. Inserting the catheter, in its extended state, into an arterial conduit enables the balloon to be inflated and to exert only radial forces against the interior of the vessel. This apparatus and procedure overcomes spasms normally associated with preparation of an arterial conduit, such as the internal mammary artery, for bypass surgery. Extension of the balloon to as much as six times its initial length can be accomplished by positioning a separation spring between the stylet and the balloon.