Apparatus and methods are described including a tube configured to traverse an aortic valve of a subject. A frame is disposed within at least a portion of the tube. An impeller rotates such as to pump blood from the subject's left ventricle to the subject's aorta. The impeller is disposed inside the tube such that, when the impeller and the tube are deployed inside the subject, and prior to rotation of the impeller, a gap between an outer edge of the impeller and an inner surface of the tube is less than 1 mm. A rigid stabilizing element stabilizes the impeller with respect to the tube, such that during rotation of the impeller, a gap between the outer edge of the impeller and the inner surface of the tube is maintained. Other applications are also described.