The invention relates to a device A for monitoring a vessel opening for an extracorporeal blood treatment device B with an extracorporeal blood circulation (I), which has an arterial blood line (20) with an arterial cannula (19) and a venous blood line (21) with a venous cannula (22). Moreover, the invention relates to an arrangement with a device A for monitoring a vessel opening and an extracorporeal blood treatment device B, and a method for monitoring a vessel opening during an extracorporeal blood treatment. The monitoring device A comprises a sensor (1) for detecting at least one physical variable, which is characteristic for the state of the vessel opening, and an evaluation apparatus (7), connected to the sensor (1) by way of a connection cable (6), for producing signals characteristic for the state of the vessel opening. The evaluation apparatus (7) has a data transmission unit (38) for establishing a wireless connection between the evaluation apparatus (7) and the blood treatment device B. The evaluation apparatus (7) is distinguished by a control unit (39) configured in such a way that an interruption in the wireless connection between the evaluation apparatus (7) and the blood treatment device B is identified, for example if the patient were to lie on the evaluation apparatus (7). Moreover, the monitoring device A has a signal unit (40), which has a vibration transducer (40A) which causes the housing (8) of the evaluation apparatus (7) to vibrate when the wireless connection is interrupted.
