Pacemakers

A pacemaker is a device that is used to speed up the heart rate when the rate is too slow. The pacemaker senses the electrical signals from the heart and sends out electrical impulses to cause the heart to beat if the heart itself fails to generate an electrical impulse after a selected period of time. A pacemaker system has two parts: a pulse generator that contains the battery and electronic circuitry; and a pacing wire or lead that connects the pulse generator to the heart and carries electrical impulses back and forth. The pacemaker generator is usually implanted, just below the collarbone, beneath the skin and fat and above the muscle and rib cage. It senses a person's heartbeat and responds accordingly. The pacemaker (or pulse generator) sends tiny electrical impulses to start a heartbeat. These pulses travel through the insulated wires of a pacing lead until they reach the metal electrode at the tip of the lead. The electrode, which is in direct contact with your heart, is designed to relay information (sense) about your heart's own electrical activity to the pacemaker and to deliver electrical impulses (paces) only when the heart needs them. If the pacemaker detects the need for pacing, then it allows the electrical impulses to flow through the lead to stimulate your heartbeat. If the pacemaker detects that your heart does not need pacing, it withholds the electrical impulse. In rate-responsive pacemakers, a special sensor works to detect changes in your body such as motion of your limbs or frequency of respiration. The pacemaker's circuitry interprets these changes and increases or decreases the pacing rate to meet your body's requirements for oxygen.

The lead is the wire that connects the pacemaker generator to the heart. Early pacemaker systems used leads that were attached to the outside of the heart, but the majority of current pacemaker systems employ endocardial ("inside the heart") leads. The cardiac electrophysiologist inserts the endocardial pacing lead or leads into a vein in the upper chest area. The lead is threaded through the vein until it reaches into the heart chamber. The number of leads used depends upon which chambers require connection to the pacemaker generator. A single-chamber pacemaker, which senses and delivers impulses to one chamber, requires only one lead to be threaded through a vein. A dual-chamber pacemaker typically uses two leads with one lead in the right ventricle and then another lead in the right atrium.
Leads consist of wires that carry the electrical signals and insulation. Unipolar leads carry only one wire, while bipolar leads carry two separate wires. The tip of each lead is has a metal electrode which makes contact with the myocardium. The lead tip is anchored to the myocardium using either a small screw which actually screws into the heart muscle or silicone tines or anchors that entangle the irregular surface of the heart wall.