Apparatus and methods are provided, including a sensor, configured to sense a cardiac cycle of a patient and to generate a signal in response thereto. A pressure mechanism is coupled to a site of an artery of the patient, and, in response to the signal, enhances blood flow of the patient in a downstream direction by compressing the artery. A valve is implanted upstream of the site, and inhibits blood flow in an upstream direction due to operation of the pressure mechanism. Other applications are also described.