To introduce a sufficient quantity of fluid into a vascular resion to be handled or treated in diagnostic or therapeutic micro-invasive surgical procedures, while the outer diameter of a guide catheter is kept relatively small.

In a guide catheter for a device for use in micro-invasive surgical procedures, a bypass section 5 having an enlarged hydraulic cross section is provided, into which, an instrument 10, for example a balloon, on an instrument catheter 11 can be retracted after treatment has been administered, thus, allowing a sufficient quantity of fluid to flow pass the instrument 1 and out of the guide catheter 4. By providing the bypass section 5 with an enlarged cross section, it becomes possible to use instruments 10 with very close-fitting guide catheters 4.