Cardiac Pacemakers




- ECEN 4011/5011 -
-- MAY 7, 1998 --
The heart is bestowed with a specialized system that automatically generates rhythmic control via the sinus node, located in the superior lateral wall of the right atrium near the opening of the superior vena cava. The specialized pacemaker cells dictate control of the rest of the heart through regular electrical impulses that propagate from the right atria to the lower ventricles. The rapid conduction of these impulses cause the muscle cells of the atria to contract and squeeze blood into the ventricles, which contract and force blood into the aorta and pulmonary arteries. Abnormalities of the heart rhythm, called arrhythmias, can disrupt this normal cardiac control making it necessary to use some artificial means to regulate the rhythm of the heart. Today, some half a million men and women, most of them over the age of sixty, carry implanted cardiac pacemakers that take over the duties of the natural conduction system. Tens of thousands of these devices are implanted each year in this country alone. Over the past thirty years cardiac pacemakers have evolved from simple devices only capable of fixed-rate stimulation of a single chamber of the heart to more sophisticated "implanted computers" that medical personnel can interrogate and reprogram from outside the patient\'s body. These refinements have allowed for more physiologic pacing with maintenance of atrioventricular synchrony and cardiac output. There are various types of cardiac pacemakers available today that can be surgically implanted to treat specific arryhythmic disorders in the heart.
Abnormal rhythms in the heart are one of the most frequent causes of heart malfunction, and in most cases necessitate some type of cardiac pacing unit. Cardiac arrhythmias are common in the elderly, in whom age-related physiologic changes often alter the conduction system of the heart. Such changes may remain asymptomatic, or they may progress to syncope, or possibly sudden death. In the event of acute myocardial infarction, arrhythmias are no more frequent in the elderly than in younger subjects; in fact, ventricular premature beats are seen less commonly in patients aged seventy years and older. Age is also not a factor in determining the success of resuscitation from cardiac arrest, although it may be a predictor of six-month survival. In general, there is nothing unique about arrhythmias in the elderly. All of the commonly encountered arrhythmias may be seen in older patients. Arrhythmias may occur in otherwise normal hearts, but with increasing age, associated cardiac disease becomes more likely. A possible exception is atrial flutter; in younger patients, its presence almost always indicates a serious cardiac disorder. There are two indications for antiarrhythmic therapy: relief of symptoms and prevention of more malignant arrhythmias. In elderly patients, pacemakers are the preferred treatment for bradyarrhythmias.
Most arrhythmias occur in response to the aging heart. In the sinoatrial node, the number of pacemaker cells decreases, until often less than 10% of the normal complement remain after age 75. Beginning at age 60, there is a detectable loss of fiber from the fascicles of the left bundle branch. Commonly, less than one-half the original number remain, the others having been replaced by fibrous tissue. Microcalcification is often found in this region, and can be related to both age-associated change and pathologic processes. There is also some fibrous tissue replacement of conduction fibers in the distal conduction system, as well as occurrences of fibrosis and hyalinization in the media of the blood vessels supplying the conduction tissue. Any of these age related processes can lead to a disrupted rhythmic and conduction system of the heart.
One type of arrhythmia, bradycardia, normally necessitates the surgical implantation of a pacemaker device. Bradycardia is a circulatory condition in which the myocardium contracts steadily but at a rate of less than sixty contractions a minute. This condition may be normal in some physically fit people, where their pulse may be quite slow. This is because an athlete\'s heart is considerably stronger and is capable of pumping a larger volume of blood per heart beat than someone who is less physically active. However, in other people, cardiac output is decreased which can cause faintness, dizziness, chest pain, and eventually syncope and circulatory collapse. The cause of bradycardia can be an increase in the