A system (10) is for performing ECG measurements and comprises a probe (12) with an integrated one or more ECG electrodes (22) and an ultrasound sensing means (18), such as a transducer arrangement. The probe thus provides a mobile ECG electrode which can be sequentially moved between a set of different locations on the body for acquiring ECG measurements from different angles relative to the heart (i.e. different ‘leads’). Positioning of the probe in each required location is guided by a position guidance function which uses ultrasound data acquired by the ultrasound sensing means to locate the probe (with reference to an ultrasound body atlas (28) or map), and uses a stored set of reference body locations to guide a user with guidance information as to how to move the probe to arrive at a next target electrode location. In examples, the user may be guided through a sequence of electrode locations, with ECG data acquired at each one, thereby sequentially building up a set of standard ECG lead measurements.