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 , wherein the control unit is configured to: obtain a first physiological characteristic measurement of the subject using the first BP measurement device ; control the second BP measurement device to obtain a first BP measurement of the subject ; control the ventilator to change one or more properties of the flow of gas to the subject , and thereby change the BP of the subject ; analyze a signal from the first BP measurement device to determine when the BP is stable following the change of the one or more properties of the flow of gas; once the BP is determined to be stable following the change of the one or more properties of the flow of gas, obtain a second physiological characteristic measurement of the subject using the first BP measurement device ; once the BP is determined to be stable following the change of the one or more properties of the flow of gas, control the second BP measurement device to obtain a second BP measurement of the subject ; and determine a calibration parameter for determining BP measurements from physiological characteristic measurements obtained by the first blood pressure measurement device from an analysis of the first physiological characteristic measurement, the second physiological characteristic measurement, the first BP measurement and the second BP measurement. obtain a first physiological characteristic measurement of the subject using the first BP measurement device ; control the second BP measurement device to obtain a first BP measurement of the subject ; control the ventilator to change one or more properties of the flow of gas to the subject , and thereby change the BP of the subject ; analyze a signal from the first BP measurement device to determine when the BP is stable following the change of the one or more properties of the flow of gas; once the BP is determined to be stable following the change of the one or more properties of the flow of gas, obtain a second physiological characteristic measurement of the subject using the first BP measurement device ; once the BP is determined to be stable following the change of the one or more properties of the flow of gas, control the second BP measurement device to obtain a second BP measurement of the subject ; and determine a calibration parameter for determining BP measurements from physiological characteristic measurements obtained by the first blood pressure measurement device from an analysis of the first physiological characteristic measurement, the second physiological characteristic measurement, the first BP measurement and the second BP measurement. <EOS>