An apparatus and method for determining a calibration parameter for a blood pressure measurement device. According to an aspect, there is provided an apparatus for determining a calibration parameter for a first blood pressure, BP, measurement device, the apparatus comprising a control unit that is to be coupled to a first BP measurement device that is for obtaining physiological characteristic measurements of a physiological characteristic of a subject and for determining a blood pressure measurement of the subject from the physiological characteristic measurements, a second BP measurement device that is for obtaining measurements of the blood pressure of the subject and a ventilator that is for providing a controlled flow of gas to the subject.