in fig2 a 2e in use the vein 12 of a patient is punctured by a puncture cannula 14 such as a seldinger puncture cannula close to a thrombus 13 a guide wire 16 is then introduced through the puncture cannula 14 close to the thrombus 13 in the next stage the puncture cannula 14 is drawn out via the guide wire 16 which stays behind the catheter set 2 with in particular the sheath 3 and preferably with the catheter 8a in its larger lumen 4 and the safety change wire 11 located in the smaller cross section and the second lumen 6 are introduced by the guide wire 16 into the vein 12 and also close to the thrombus 13 optionally the safety change wire 11 can be inserted through the second lumen 6 after the introduction of sheath 3 the catheter 8a is preferably a so called van andel catheter which has a pointed distal end 17 and a greater length than the sheath 3 so that its distal end 17 projects over the distal end 18 of the sheath 3 as a result of the construction of the van andel catheter 8a the insertion of the catheter set is facilitated guide wire 16 is then removed from the lumen 9 of catheter 8a at their proximal ends sheath 3 and or catheter 8a have in an area.