Abstract: A method of detecting changes in blood flow in a head of a subject includes measuring a value of a parameter of a cardiac bioelectrical signal at a scalp area of the subject relative to a reference cardiac bioelectrical signal. The method also includes comparing the value of the measured parameter with a predetermined value of the parameter to determine any change in blood flow in the head of the subject. The determined change can be used to detect changes in perfusion in the brain of a subject for example, as a result of anti-coagulation medication used to dissolve a clot in a blood vessel of the brain of a subject who has experienced ischaemic stroke.