Prosthetic heart valve and pacemaker. Coloured X- ray of a side-view of a 72-year-old woman's chest. She has a prosthetic heart valve (yellow ring) and a pacemaker (white oblong, upper right). Stitches (rings, right) used to close the chest are also seen. Heart valves control the flow of blood between chambers of the heart. Valve failure can lead to malfunctioning of the heart and death. The pacemaker supplies electrical impulses, along a lead, to the heart to keep it beating regularly. Pacemakers are fitted in patients with a defective sinoatrial node, the part of the heart that initiates the heartbeat, or in patients with a heart block, which impairs the nerve impulse generated by the node.