A device for treating heart failure in a patient. The device comprising a body, at least one passage through the body, at least one one way valve in the passage and a mounting means adapted for mounting the body in an opening provided in the patient's atrial septum. In use, the device is oriented such that, when the patient's left atrial pressure exceeds the patient's right atrial pressure by a predetermined amount, the one way valve(s) opens to allow blood flow through the passage(s) from the left atrium to the right atrium to thereby reduce the left atrial pressure.