An apparatus for analysing a pulse wave signal (PWS) obtained from a subject to determine an indication of a blood pressure or a change in blood pressure of the subject, wherein the PWS comprises pulse wave measurements for a plurality of cardiac cycles of the subject during a first time period, the apparatus comprising an input interface configured to receive the PWS, the apparatus configured to:
- (i) analyse the PWS to determine a first average cardiac cycle waveform for a first time point in the first time period and a second average cardiac cycle waveform for a second time point in the first time period;
- (ii) determine a difference signal representing a change in morphology from the first average cardiac cycle waveform to the second average cardiac cycle waveform;
- (iii) determine an absolute value of the change in morphology from the difference signal;
- (iv) determine a direction of the change in morphology;
- and (v) determine a relative blood pressure change by combining the absolute value and the direction of the change in morphology.