An apparatus comprising: a first displacement current sensor comprising a first sensing electrode and a first guard electrode, wherein the first displacement current sensor is configured to measure a first sensed signal dependent upon electrical activity of a subjects heart； a second displacement current sensor comprising a second sensing electrode and a second guard electrode, wherein the second displacement current sensor is configured to measure a second sensed signal dependent upon electrical activity of a subjects heart； and circuitry configured to process at least the first sensed signal to compensate for artefacts arising from motion of the subject.
