A device and a method which monitors the pulse transit time of a heart beat is disclosed, i.e. the time taken for a heart impulse to travel from the heart to a limb of a subject. The time taken to travel is calibrated to blood pressure of the subject by a pre-determined formula. However, the calibration is augmented by empirical observations, obtaining several optional calibrations. The most suitable of the calibrations is selected to interpret each pulse transit time reading based on proximity of the pulse transit time. Hence, it is possible that every pulse transit time reading may use a different calibration. The method diminishes the effect of physiological change in the subject which causes drift in a solely formula based calibration.
