Device for extracorporeal blood treatment comprising a blood treatment unit comprising at least one compartment, a first blood line which is connected to a connection of the compartment and comprises a first patient connection, a second blood line which is connected to a connection of the compartment and comprises a second patient connection, a blood pump for conveying blood, an apparatus (14A, 14B) for measuring a physical or chemical characteristic variable, a control and evaluation unit which is connected to the blood pump and to the apparatus for measuring a physical or chemical characteristic variable and is configured such that, for one measurement cycle of two measurement cycles, the blood pump is operated in an operating mode involving a reversed blood flow, such that blood flows from the second patient connection to the blood treatment unit and from the blood treatment unit to the first patient connection, and, for the other measurement cycle of the two measurement cycles, the blood pump is operated in an operating mode involving a normal blood flow, such that blood flows from the first patient connection to the blood treatment unit and from the blood treatment unit to the second patient connection, a haemodynamic parameter being determined from the measured physical or chemical characteristic variable in the measurement cycle when the blood flow is normal and when the blood flow is reversed, or, for one measurement cycle, the blood pump is operated in an operating mode involving a reversed blood flow, such that blood flows from the second patient connection to the blood treatment unit and from the blood treatment unit to the first patient connection, a haemodynamic parameter being determined from the measured physical or chemical characteristic variable in the measurement cycle when the blood flow is reversed, the device for extracorporeal blood treatment comprising an input unit for inputting a time interval, characterised in that the control and evaluation unit is configured such that the operation of the blood pump in the operating mode involving a reversed blood flow is only enabled during the time interval input by means of the input unit. a blood treatment unit comprising at least one compartment, a first blood line which is connected to a connection of the compartment and comprises a first patient connection, a second blood line which is connected to a connection of the compartment and comprises a second patient connection, a blood pump for conveying blood, an apparatus (14A, 14B) for measuring a physical or chemical characteristic variable, a control and evaluation unit which is connected to the blood pump and to the apparatus for measuring a physical or chemical characteristic variable and is configured such that, for one measurement cycle of two measurement cycles, the blood pump is operated in an operating mode involving a reversed blood flow, such that blood flows from the second patient connection to the blood treatment unit and from the blood treatment unit to the first patient connection, and, for the other measurement cycle of the two measurement cycles, the blood pump is operated in an operating mode involving a normal blood flow, such that blood flows from the first patient connection to the blood treatment unit and from the blood treatment unit to the second patient connection, a haemodynamic parameter being determined from the measured physical or chemical characteristic variable in the measurement cycle when the blood flow is normal and when the blood flow is reversed, or, for one measurement cycle, the blood pump is operated in an operating mode involving a reversed blood flow, such that blood flows from the second patient connection to the blood treatment unit and from the blood treatment unit to the first patient connection, a haemodynamic parameter being determined from the measured physical or chemical characteristic variable in the measurement cycle when the blood flow is reversed, the device for extracorporeal blood treatment comprising an input unit for inputting a time interval, characterised in that the control and evaluation unit is configured such that the operation of the blood pump in the operating mode involving a reversed blood flow is only enabled during the time interval input by means of the input unit.