.. early device has only one wire and paces the ventricles at regular intervals. The pacing rate, usually around seventy beats a minute, is determined by a physician. The ECG in a patient with a VVI pacemaker shows a sharp spike of the pacemaker artifact before each paced beat, followed by a wide QRS wave. No pacemaker spike is present on sensed beats.
Retrograde conduction of the paced impulse from the ventricles to the atria, VA conduction, may not be present. If it is present, retrograde P waves follow the paced QRS complex. When VA conduction is absent, dissociated atrial activity is seen. Ventricular demand pacemakers are found in patients who: are physically inactive, regardless of age, and therefore do not require rate variability; have chronic atrial fibrillation or flutter, or giant, silent atria; or have mental incapacity or terminal illnesses that make dual-chambered pacing impractical. Another type of unit, atrioventricular sequential pacemakers (DVI), is capable of pacing in both the atrium and ventricle, senses only in the ventricle, and is inhibited by ventricular events.
Most AV sequential pacemakers are noncommitted. After a brief blanking period of 30 to 50 milliseconds following an atrial stimulus, sensing is continuous during the AV interval. Therefore, noncommitted DVI pacing systems may pace atrium and ventricle both, or atrium only, or be totally inhibited, depending on where the R wave is detected with respect to the pulse generator’s timing cycle. The ECG in a DVI pacemaker shows a sharp spike before each P wave on paced atrial beats and before each QRS on paced ventricular beats. The atrial and ventricular spikes are separated by a present or programmable AV interval.
Patients who have a sick sinus syndrome accompanied by AV nodal or His-Purkinje disease or an AV block with abnormal sinus node function and lack of ability to increase atrial rate with exercise typically benefit from these pacemakers. They are also useful in patients who have developed pacemaker syndrome with single-chambered ventricular demand units, since the normal atrioventricular relationship is then restored. A third, more commonly used type of pacemaker is the DDD pacemaker. A DDD pacemaker can sense intrinsic activity in the atrium and ventricle, pace either or both chambers when not inhibited by native activity, and thereby maintain atrioventricular synchrony over a wide range of heart rates. DDD units are noncommitted employing an atrial blanking period following atrial stimuli to avoid sensing of such events on the ventricular channel. All such pacemakers have upper rate characteristics and blocking modes to prevent 1:1 conduction during atrial arrhythmias such as flutter and fibrillation. Virtually all such devices are extensively programmable, and most have the ability to telemeter both programmed and real-time parameters.
One of the major initial problems encountered with DDD pacing is pacemaker-mediated tachycardia, which is where the pacemaker acts as one limb of a re-entrant circuit. However, this has been solved by the ability to program the interval at which atrial sensing resumes after a ventricular sensed or paced event. Normally, this device sequentially paces both the atrium and ventricle when atrial activity falls below the preset base rate and atrial pacing is not followed by a ventricular event. When the patient’s intrinsic atrial activity exceeds the base rate, and if a spontaneous QRS does not occur within the programmed AV interval, the pacemaker switches to an atrial sensing-ventricular pacing mode. In this case, the ECG shows a P wave that is followed by a sharp spike and a paced QRS.
Sensed ventricular events inhibit both atrial and ventricular output and reset the atrial escape interval. The DDD pacemakers are found in patients who possess: AV block with or without sinus node dysfunction; or moderate sick sinus syndrome and AV nodal or His-Purkinje disease, with at least some ability to increase atrial rate with exercise. Surgical implantation of cardiac pacemakers has dramatically improved over the years. During the late 1950’s and early 1960’s when artificial pacing was first being implemented, patients with severe Stokes-Adams attacks received some of the first battery operated pacemakers developed by William M. Chardack, chief of thoracic surgery at the Veterans Administration hospital and his colleague Wilson Greatbatch.
Physicians who implanted pacemakers in these patients reported numerous serious failures that required new operation: broken or dislodged leads, premature battery depletion, and leakage of body fluids into the pulse generator. Yet despite the problems, pacemakers proved effective at giving people months or years of life that they would not otherwise have enjoyed. The operative procedure during this particular era was carried out under general anesthesia with an endotracheal tube in place. Patients undergoing surgery were under the control of an external pacemaker with a cardiac electrode catheter passing through the right saphenous vein. Electrocardiographic leads were attached to the arms and legs, and a continuous ECG was displayed on an oscilloscope.
Two incisions were made: a six-inch incision near the umbilicus (naval) and a left sub mammary incision. A twin lead was passed up a subcutaneous tunnel, which connects the chest and abdominal incisions to the pericardium. The two electrodes were separated and implanted in the myocardium. The bared wire was passed back through to the entry point of the insulated portion of the electrode. The second electrode was implanted in the same fashion one centimeter from the first. The pacemaker was placed in the subcutaneous pocket and attached to the anterior rectus sheath. The external unit was taped to the abdomen and set between 80 and 90 pulses/min.
Today doctors who implant pacemakers almost never expose the patient’s heart. Instead, using local anesthesia, they make a two to three inch incision just below the left or right collarbone. Then, they cut into one of the prominent veins running across the upper chest toward the heart, either the cephalic or the subclavian vein. The pacing wire is contained within a venous catheter. While observing the process on a fluoroscope screen, the doctor advances and guides the catheter down the venous system, through the right atrium of the heart, and into the right ventricle.
Once the lead is positioned securely against the wall of the ventricle and tested for its electrical characteristics, the physician plugs it into the pulse generator and buries the generator beneath the chest muscle at the site of the incision. An experienced implanter can carry out this procedure in forty-five minutes or less, though complex cases take longer depending on the complexity. Tines at the tip of the lead hold it securely in position against the endocardium, the inner lining of the heart. Over a period of a week or two, fibrous tissue grows around the electrode and binds it tightly to the endocardium. About six weeks after the operation, the recipient goes back to the doctor’s office to have the pacemaker’s initial settings adjusted so that its batteries will last as long as possible. After that, a transmitter connected by telephone to a monitoring service can check the device.
This is done every two months for the first three years and then once a month until the battery runs out. Batteries need to be replaced about every seven to nine years for dual-chamber devices and every ten to twelve years for single-chamber units. Battery replacement surgery is an outpatient procedure. Artificial pacemakers have been around a long time and have improved dramatically with technology. Though there are several different types of pacemakers available on the market, they are all designed with the same intentions, to treat conditions such as bradycardia, sick-sinus syndrome, heart blockage, and various other irregular heartbeats by artificially controlling cardiac rhythm and output with electrical waves that propagate through the myocardium.
Cardiac pacing units have prolonged the lives of millions of Americans suffering from heart arrhythmias and other heart related diseases. Through technological advances in the health/sciences and engineering industries, patients are now able to resume their daily activities without having to worry about moderate physical exertion. BIBLIOGRAPGHY Glenn W. L., William. Cardiac Pacemakers.
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