Measuring access flow rate by use of blood treatment machine

A device for determining a fluid flow rate in a vascular access of a patient when connected to a blood treatment machine, said blood treatment machine comprising an extracorporeal blood flow circuit with first and second access devices for connection to the vascular access and having a blood pump operable to generate a flow of blood in the extracorporeal blood flow circuit from the first access device through a blood compartment of a dialyzer and to the second access device, and a treatment fluid flow circuit configured to generate a flow of treatment fluid through a treatment fluid compartment of the dialyzer, said treatment fluid compartment being separated from the blood compartment by a semi-permeable membrane, said device being configured to, during a measurement phase:

• cause the blood treatment machine to switch between a first operating state and a second operating state, wherein the second operating state differs from the first operating state at least by a change of flow direction of the blood or the treatment fluid through the dialyzer;
• acquire an output signal of at least one sensor in the blood treatment machine in the first and second operating states;
• compute, based on the output signal, a measurement value of a comparison parameter that compares treatment efficiency in the first operating state to treatment efficiency in the second operating state;
• obtain, for first and second sets of control values of operating parameters of the blood treatment machine in the first and second operating states, a current function that relates the comparison parameter to the fluid flow rate in the vascular access, the operating parameters including a cardiac output of the patient, a characteristic parameter of the dialyzer, a flow rate of the treatment fluid through the dialyzer, and a flow rate of the blood through the dialyzer;
• and
• determine, based on the measurement value, an estimated value of the fluid flow rate in the vascular access so that the current function yields the measurement value.