A fixator assembly comprising a fixator for fixing inside a blood vessel, a guide tube attached to the fixator and a pump for supplying a liquid to the blood vessel or from the blood vessel via the guide tube. Blood flow past the fixator is possible during pumping so that critical situations are avoided. A method of providing a liquid to a blood vessel using the assembly is also claimed. Sampling or liquid dosing thus may be performed during extended periods of time while performing this at a well-defined position in the blood vessel and potentially close to or at the desired position in the person.
