The heart has its own natural “pacemaker”- a system inside the heart that functions in much the same way as an electrical circuit in a house or automobile. Impulses develop periodically and are carried throughout the heart by “wires” (nerves) which end in the small heart muscle fibers. These impulses signal the muscle to contract. The origin of the normal impulse is the sino-atrial or S-A node in the right atrium. This pacemaking structure can be likened to the distributor in an auto engine, which sends signals to the spark plugs and causes them to fire.There are only two main “wires” in the heart which link the natural pacemaker to the ventricles, which pump the blood. Just as in a two-cylinder engine, if one of the spark plug wires were cut, only a single spark plug would fire. If both spark plug wires were cut, even though the distributor still functioned, the engine would be dead.The two wires (nerves) in the heart are called the right and left bundle branches. When one of the wires is not functioning, the problem is identified by the term right or left bundle branch block. A typical electrocardiographic picture is produced by these malfunctions which, of course, aids in the diagnosis. If both of the wires are nonfunctional, the patient is said to have a complete heart block. In this circumstance the natural pacemaker will still be functioning, but the message will not be transmitted to the ventricles. The ventricles will either not beat at all, or a secondary natural pacemaker in the ventricle itself may take over and initiate a periodic heart beat. This standby secondary pacemaker is always very slow, producing only 30 to 40 beats per minute, which may or may not be adequate to provide blood flow in sufficient quantity to allow the body to function. In some instances, the primary natural pacemaker fails – and no impulse is then generated to flow through the wires, a circumstance called sinus arrest.Interruptions in function may occur suddenly and be permanent, or may occur intermittently, with normal heart function in the interim. The usual symptom is faintness or fainting due to inadequate blood flow to the brain. The patient is usually not aware of a change in his heart beat before the faint. During the period of unconsciousness, the patient may have a convulsion, again due to inadequate blood flow to the brain.These problems with the heart’s pacemaker or with the conducting wires usually occur in people who have atherosclerotic heart disease. Rarer instances concern people who have had heart disease due to diphtheria or other infections of the heart (myocarditis), rheumatic heart disease, or some forms of congenital heart disease (heart defects that children are born with). A complete heart block can occur as a complication of an acute heart attack can develop several years later.The treatment of this condition is based primarily upon its recognition. Of course, every person who faints does not have a heart block. Some people faint at the sight of blood or with pain or sudden fright. This involves another mechanism and is harmless unless the person hurts himself when he falls. But if a person who is over 35 or 40, and who is feeling fine, suddenly faints, this is probably a serious symptom.Once the condition is recognized, the treatment is to replace the heart’s electrical circuits with a mechanical device as soon as possible. Some degree of haste is in order because there is no way to predict when the next heart stoppage will occur, and the next episode may be the last.The device used is called a pacemaker. Many misconceptions exist concerning them. The most common one is that the pacemaker is an artificial heart. It is not. It does not pump blood, nor does it prevent a person from having another heart attack or from dying of heart disease in another form. A pacemaker is an electronic unit with its own power supply (battery) connected to a wire (or wires) that are attached to the inside or the outside of the heart. The pacemaker regularly supplies an impulse that is carried by the wires to the heart muscle, where the impulse stimulates the heart to beat. If the heart is too weak from disease to contract, the pacemaker will not cause a heartbeat to be produced.Some forms of pacemakers beat constantly and other forms operate only on demand. The latter form samples each of the heart’s own beats and if one or more are missed, the pacemaker functions to supply the missing impulses as needed.The pacemaker unit itself is usually placed within a pocket just beneath the surface of the skin of the chest or abdomen. This location is chosen to facilitate replacement of the electronic unit or batteries. A wire runs from the pacemaker to the heart. The original method for establishing this connection was to perform chest surgery, opening the chest cage and sewing the end of the wire to the surface of the heart. A newer technique has been developed to avoid a major operation. The wire is threaded through a vein and directed to the interior of the heart. The wire is usually started in a vein in the neck or the shoulder. The progress of the wire is followed by a fluoroscope. When the tip of the wire reaches the interior of the heart, it is lodged in a corner of the right ventricle. The other end of the wire is connected to the pacemaker. The exposed piece of wire between the pacemaker and the neck vein is covered with skin, so that the entire system is beneath the body surface.Batteries must be replaced every one to three years, and in some instances the pacemaker unit itself must be replaced if it malfunctions. Physicians are usually able to test the function of the units by means of an electrocardiogram and external magnets, which temporarily alter pacemaker function in a predictable manner. This permits batteries and units to be replaced before serious failures occur.*21/309/5*