A method of deriving central aortic systolic pressure comprises (a) creating a set having a predetermined number of blood pressure measurements, the set representative of an arterial waveform (b) determining an integer interval value (c) averaging a series of consecutive blood pressure measurement readings in the set equal to the integer interval value commencing from the fth blood pressure measurement in the set (d) storing the averaged value in a central aortic systolic pressure set and (e) setting the central aortic systolic pressure as the highest value in the central aortic pressure set. Steps (c) and (d) are repeated with the value of f being incremented by 1 each time until the value of f plus the integer interval value equals the predetermined number of blood pressure measurements in the set.