A device in an apparatus for extracorporeal blood treatment is configured to monitor a fluid flow rate (Q) of a cardiovascular system of a subject. The apparatus comprises an extracorporeal blood circuit and a connection (C) for connecting the extracorporeal blood circuit to the cardiovascular system. The device comprises an input for obtaining a time-dependent measurement signal (d(n)) from a pressure sensor in the extracorporeal blood circuit. The pressure sensor is arranged to detect a subject pulse originating from a subject pulse generator in the cardiovascular system of the subject, wherein the system further comprises a signal processor connected to the input. The signal processor is configured to process the measurement signal to obtain a pulse profile (e(n)) which is a temporal signal profile of the subject pulse, and to calculate a fluid flow rate (Q) based at least partly on the temporal signal profile.
