The invention relates to a device and to a method wherein access to a patient, in particular access to vessels, can be monitored during extracorporeal blood treatment during which a loop (27) is formed in the arterial and/or venous line (6, 7). Inventive means (26) for fixing a section of the line in the form of a loop are provided. When the line is stressed under tension, the loop contracts until the line finally kinks. The modification of the diameter of the line and finally the formation of a kink in the line leads to a rise of pressure in the line, which is monitored by a pressure monitoring device. If the pressure exceeds a predetermined threshold value, the aspirating cannula is in danger of slipping out of the vessel access, has partially or has completely slipped out thus allowing the threshold value to be determined for prealarms or alarms.
