A system for use in prevention of stroke, comprising: a compression device for applying a compressive force to a carotid artery, wherein the compression device is adjustable so as to apply the compressive force and so as to not apply the compressive force; an actuation device for causing the compression device to apply the compressive force; and an emboli monitoring device; wherein the compressive force is applied when the emboli monitoring device detects the presence of emboli (18, 20) in a circulatory system to which the carotid artery is a component thereof and is of such a degree that the carotid artery is compressed to such an extent that blood flow through the carotid artery is limited or completely blocked, and wherein the compression device comprises an expandable member (27, 32, 46) which applies the compressive force, characterised in that the actuation device is operable to expand the expandable member (27, 32, 46) such that the pressure in the expandable member (27, 32, 46) exceeds the patient's systemic arterial blood pressure by at least 10 mm Hg (133.32 N/m 2), in that the compression device comprises as least two expandable members (27, 46) which are inflatable to compress both the right and left carotid arteries at the same time. and in that the expandable members (27, 46) upon inflation expand in a radial direction towards a central axis of the compression device and do not expand in the radial direction away from the central axis, by making the portions of the expandable members (27, 46) facing away from the central axis non-expandable in the radial direction while making the portions of the expandable members (27, 46) facing towards the central axis expandable. a compression device for applying a compressive force to a carotid artery, wherein the compression device is adjustable so as to apply the compressive force and so as to not apply the compressive force; an actuation device for causing the compression device to apply the compressive force; and an emboli monitoring device; wherein the compressive force is applied when the emboli monitoring device detects the presence of emboli (18, 20) in a circulatory system to which the carotid artery is a component thereof and is of such a degree that the carotid artery is compressed to such an extent that blood flow through the carotid artery is limited or completely blocked, and wherein the compression device comprises an expandable member (27, 32, 46) which applies the compressive force, characterised in that the actuation device is operable to expand the expandable member (27, 32, 46) such that the pressure in the expandable member (27, 32, 46) exceeds the patient's systemic arterial blood pressure by at least 10 mm Hg (133.32 N/m 2), in that the compression device comprises as least two expandable members (27, 46) which are inflatable to compress both the right and left carotid arteries at the same time. and in that the expandable members (27, 46) upon inflation expand in a radial direction towards a central axis of the compression device and do not expand in the radial direction away from the central axis, by making the portions of the expandable members (27, 46) facing away from the central axis non-expandable in the radial direction while making the portions of the expandable members (27, 46) facing towards the central axis expandable.