A user interface (21) for an extracorporeal blood treatment machine is provided, as well as a system (200) for feedback control of a user. The system (200) comprises a controller (22), a sensor (108) and a device (107) for administration of a substance. The controller (22) is enabled to receive input from the user interface and send instructions to the user interface and the controller is arranged to control the operation of the device (107) for administration of a substance. The device (107) for administration of a substance is monitored by the sensor (108) and the controller (22) is enabled to receive input from the sensor (108) such that the controller is enabled to provide feedback to the user interface by said instructions. A method (40) for performing a critical action in a system (200) is also provided.
