An alarm apparatus for estimating a patient's propensity to suffer from symptomatic hypotension during extracorporeal blood treatment, the apparatus comprising: an output interface adapted to generate an electromagnetic test signal (s) adapted to be fed to at least one transmitter electrode via which the electromagnetic test signal (s) is applied over a thoracic region of the patient; an input interface adapted to receive a result signal (r) via at least one receiver electrode on the patient's body, the result signal (r) being produced in response to the electromagnetic test signal (s); and an analysis unit adapted to determine a bio-impedance parameter based on the result signal (r), characterized in that the analysis unit comprises: a processing module adapted to derive a test parameter based on the result signal (r), the test parameter expressing a fluid status of the thoracic region of the patient, a comparison module adapted to test whether or not the test parameter fulfills an alarm criterion, and an alarm triggering module adapted to cause an alarm signal (α) to be generated if the alarm criterion is fulfilled, the electromagnetic test signal (s) comprising at least two signal components (S LF, S HF) with mutually different spectral properties, the input interface is adapted to receive a set of result signal components via the at least one receiver electrode, the set of result signal components being produced in response to the electromagnetic test signal (s), and the processing module is adapted to derive the test parameter based on the set of result signal components, or the electromagnetic test signal (s) comprising a source signal component of a well-defined frequency, and the processing module is adapted to derive the test parameter based on a phase shift (Δφ) of the result signal (r) relative to the source signal component and an attenuation (ΔA) of the result signal (r) relative to the source signal component. an output interface adapted to generate an electromagnetic test signal (s) adapted to be fed to at least one transmitter electrode via which the electromagnetic test signal (s) is applied over a thoracic region of the patient; an input interface adapted to receive a result signal (r) via at least one receiver electrode on the patient's body, the result signal (r) being produced in response to the electromagnetic test signal (s); and an analysis unit adapted to determine a bio-impedance parameter based on the result signal (r), a processing module adapted to derive a test parameter based on the result signal (r), the test parameter expressing a fluid status of the thoracic region of the patient, a comparison module adapted to test whether or not the test parameter fulfills an alarm criterion, and an alarm triggering module adapted to cause an alarm signal (α) to be generated if the alarm criterion is fulfilled, the electromagnetic test signal (s) comprising at least two signal components (S LF, S HF) with mutually different spectral properties, the input interface is adapted to receive a set of result signal components via the at least one receiver electrode, the set of result signal components being produced in response to the electromagnetic test signal (s), and the processing module is adapted to derive the test parameter based on the set of result signal components, or