Sensor system and sensing method for use in assessment of circulatory volume

In a sensor system (12), output signals of at least one PPG sensor (14) are processed to derive a modified pulse amplitude variation (PAV) value, being modified to take account of a baseline variation of the PPG sensor output signal.
In particular, the modified PAV is derived through performing a modification step (42) in which either: a baseline variation of the PPG sensor output is derived and combined with a previously derived PAV, or, a PPG sensor output is first processed to perform baseline variation compensation, in advance of then deriving a PAV from the compensated signal.