Pacemaker description and guide
Modern pacemakerts have revolutionized the care of patients with slow heart rates. Until the introduction of the first implantable permanent pacemaker fifty years ago, there was no effective treatment for patients with symptoms due to slow heart rates. Modern pacemakers have revolutionized the care of patients with slow heart rates and can restore normal heart rates both at rest and during exercise. Furthermore, modern dual chamber pacemakers can restore the normal sequence of atrial and ventricular contraction; thus, optimizing the cardiac output. No oral medications are available which will increase the heart rate or correct atrial ventricular conduction abnormalities.
Atrial Paced Rhythm
In a patient with severe symptomatic sinus bradycardia, atrial pacing at a rate of 60 eliminates symptoms of bradycardia. Note the pacing spike which triggers atrial contraction prior to each P-wave. The AV interval is programmed long to preserve sequential AV contraction, which is preferable to ventricular pacing.
The most common cause of slow heart rhythm is sinus bradycardia, characterized by a prolonged interval between each cardiac cycle and a P-wave before every QRS-complex. Sinus bradycardia is common at night among normal persons when they are fast asleep. Athletes are frequently in sinus bradycardia during the day and often have heart rates in the range of 40 to 60/min because their large powerful hearts generate a large stroke volume with each heart beat which compensates for the slow heart rate.
Abnormally slow sinus bradycardia resulting in reduced cardiac output is most common in elderly patients. Elderly patients with mild degrees of sinus bradycardia and heart rates in the range of 45 to 60/min are often asymptomatic, and these patients do not require pacemaker implantation. However, elderly patients with marked sinus bradycardia causing heart rates slower than 45/min are frequently symptomatic. These patients may note the gradual onset of lethargy, generalized weakness, lack of stamina, episodic confusion, and tend to avoid exertion because of their lower cardiac output. Implantation of a permanent pacemaker in these patients may restore their alertness, energy levels, and ability to return to normal activities. Furthermore, enhancing the cardiac output by elevating the heart rate may relieve congestive heart failure, improve renal function and enhance exercise duration including ability to walk distances.
The other major cause of slow heart rates in elderly patients is conduction disease. Elderly patients often develop fibrosis in the specialized conduction cells which comprise the AV-node and the right and left bundles. Why this fibrosis gradually infiltrates the conduction cells blocking the transmission of electrical signals in the heart is unknown. Often patients first develop either right or left bundle branch block and usually remain asymptomatic. Partial degrees of right or left bundle branch block may be noted, or the bundle branch block initially may occur only during tachyarrhythmias, which is referred to as rate dependent bundle branch block. As fibrosis of the conduction system progresses, atrioventricular (A-V) block may ensue; that is the AV-conduction system linking the atriae and the ventricles may become blocked by fibrosis infiltrating the AV-node, Hiss bundle, or both right and left ventricles, eventually resulting in A-V dissociation or electrical interruption of the atrial and ventricles, causing the atriae and ventricles to beat independently. This is a condition called heart block.
Heart block is classified into first, second and third degree heart block. In first degree heart block the AV interval is prolonged greater than 0.20 sec due topartial interruption of AV-conduction. First degree AV block is usually harmless and requires no medical treatment. Occasionally, if the duration of the AV-interval is markedly prolonged, the contribution of atrial contraction to LV-filling is diminished and the cardiac output may be reduced slightly.
In second degree heart block, QRS-complexes are intermittently dropped completely. For example, in 2:1 AV block, every other heart beat is dropped and the heart rate is halved. Patients with second degree heart block are often symptomatic and generally require implantation of a permanent pacemaker.
Wenckebach atrio-ventricular block is a subset of second degree AV-interval block in which the PR-interval becomes progressively prolonged until a QRS-complex is dropped resulting in a brief pause in the heart rate. This subset of second degree AV block usually causes no symptoms and does not require implantation of a pacemaker.
Third degree or complete heart block results when electrical conduction between the atrial and ventricles is totally interrupted, and the atrial and ventricles beat independently. This results in a condition referred to as A-V dissociation in which the P-waves and QRS-complexes on the electrocardiogram are unrelated. In complete heart block, a slow ventricular rate ensues with wide QRS-complexes, referred to an idioventricular rhythm. This slow idioventricular rhythm arises from the ventricles and provides a heart rate of typically 15 to 45/min. This markedly reduced heart rate is usually enough to maintain blood pressure, but patients with complete heart block nearly always feel weak, lethargic and lightheaded. Often these patients experience transient loss of consciousness and may abruptly fall to the floor suffering traumatic injuries, occasionally including hip fractures, concussions, or subarachnoid hemorrhages. The life span of patients with complete heart block not treated with a pacemaker averages six months.
Patients with complete heart block always benefit from implantation of dual chamber pacemaker in which both the atriae and neutrides are paced in synchrony. With restoration of normal heart rates and AV synchrony, patients instantly feel improved. Due to higher cardiac output resulting from AV pacing, patients with complete heart block feel stronger, become more alert and may resume normal activities. With continued A-V pacing for years, they may live normal life spans.
Modern dual chamber pacemakers have revolutioned the treatment of elderly patients with the second degree or third degree heart block. Dual chamber pacemakers pace both the atriae and the ventricles. Atrial pacing occurs first followed by ventricular pacing, identical to the normal sequence of the heart. The interval between atrial and ventricular pacing is referred to as the AV interval and is similar to the P-R interval of the normal heart rhythm. A-V paced rhythm can be identified on the electrocardiogram by pacing spikes before each P-wave and QRS-complex. Restoration of the normal synchronized pattern of atrial contraction followed quickly by ventricular contraction considerably augments the performance of the heart.
Dual chamber pacemaking required is that an electrode be positioned at the apex of the right ventricle to pace the ventricles and a second electrode be placed in the right atrium to pace the atriae. These two electrodes transmit electricity from the pacemaker to the atriae and ventricles to initiate atrial and ventricular contraction and also record the electrical signals arising from normal spontaneous contraction of the atriae and ventricles.
All modern pacemakers are demand pacemakers; that is they pace only on demand as needed to prevent bradycardia. When the atriae and ventricles beat spontaneously, the resulting P-waves and QRS-complexes are transmitted by the two electodes to the pacemaker which is then inhibited from pacing. Once the atrial and ventricular contraction rates fall below preset minimum rates, the pacemaker reinitiates pacing. The demand function of pacing serves two functions. First, pacing on demand only as needed conserves battery life. Secondly, demand pacing permits normal spontaneous contraction of the atriae and ventricles to occur as much as possible. Contraction of the ventricles triggered normally by conduction through the right and left bundles results in a slightly more effective ventricular contraction pattern, compared to the asymmetrical ventricular contraction pattern resulting from pacing the apex of the right ventricle.
After the introduction of the first implantable pacemaker in 1958, all pacemakers were initially fix-rate; they all paced at a rate of 70/min. Ten years later the concept of the programmable pacemaker was introduced. With an external magnetic device applied over the pacemaker, the pacing rate could be changed, but the pacemaker continued to pace at only one rate. Fixed-rate pacing considerably limited activity levels for most patients. While these patients felt comfortable at rest, they were unable to sustain exercise for more than brief periods because fixed heart rates limited increases in cardiac outputs considerably during exercise. In 1980, an important new advance in pacing was introduced: a method for increasing the pacing rate progressively as a patient exercised to higher levels. All modern pacemakers today incorporate this important feature, which is referred to rate adaptive pacing. Pacemakers now contain a sensor, usually an accelerometer, which detects movement by the patient and provides faster heart rates for higher levels of activity. Furthermore, the rate of acceleration of the heart rate with exercise, the increase in heart rate at different levels of exercise, and the rapidity of deceleration of the heart rate as the patient stops exercising are all programmable. Thus, the peak pacing heart rate for patients who participate in vigorous sports can be set as high as 140/min. The rates of acceleration and deceleration of the heart rate with exercise can be adjusted also to the activity level of the patient.
Modern dual chamber pacemakers feature other specialized pacing modes which can benefit patients. Most pacemakers available today have a sleep mode feature. When programmed on, the sleep mode provides patients with a slower heart rate at night. At a pre-set time in the evening, the pacing rate falls 10 to 15 beats per minute to provide the heart with a slower rate at night to permit the heart to rest better. At a pre-set time in the morning, the pacemaker increases the pacing rate to the higher baseline rate used throughout the day. Other pacemakers are programmed to switch to the slower rate when the patient becomes motionless after falling asleep and revert back to the faster daytime rate in the morning when the patient becomes more active.
Another feature of modern pacemakers is mode switching for rapid atrial rates. The earliest dual chamber pacemakers often paced the ventricles at excessively rapid rates during atrial tachycardias. For example, when the atriae converted from normal sinus rhythm to atrial fibrillation or atrial flutter, the atrial electrode would track rapid atrial activity and cause a correspondingly rapid ventricular paced rate, perceived by the patient as an uncomfortable tachycardia. To eliminate this problem, pacemaker manufacturers now provide a mode switching algorhythm in most pacemakers which converts the ventricular paced rhythm to a fixed rate mode when the atrial rhythm becomes too fast due to an abnormal atrial tachycardia.
Another valuable feature of modern pacemakers is their ability to count the numbers of paced atrial and ventricular beats and to count the number of normally conducted spontaneous atrial and ventricular beats. Thus, one knows the exact number of normal and paced beats, which typically total approximately one hundred thousand beats daily.
Another helpful feature of more advanced dual chamber pacemakers is that some pacemakers can identify and store in memory electrocardiographc recordings of abnormal heart rhythms. This telemetry feature is particularly useful in patients who have complex tachyarrhythmias for determining the efficacy of antiarrhythmic medications for suppressing tachyarrhythmias.
Battery life for the earliest pacemakers was very limited and necessitated replacement of pacemakers at intervals of less than two years. Modern lithium batteries last much longer, typically from 5 to 10 years. Individual batteries vary considerably in longevity, however, just as some flashlight batteries last longer than others.
Furthermore patients who require dual chamber pacing of both the atriae and ventricles constantly will consume battery life more rapidly than a patient who requires only occasional pacing. Therefore, interrogation of pacemakers at regular intervals is necessary to approximate the amount of battery lif remaining. This is done routinely with a magnet device connected to a computer which provides a full interrogation of all the pacing parameters previously programmed for the pacemaker, including pacing and sensing thresholds, pacing voltage outputs, pacing modes, lower and upper pacing rates, activity sensor settings, telemetry intra-cardiac recordings, and estimates of remaining battery life based on battery voltage. Battery voltage diminishes as the battery is depleted. At a certain voltage replacement of the battery-depleted pacemaker is indicated and must be done before the battery is completely depleted causing the pacemaker to stop pacing. Pacemakers are designed so that several months of pacing remain when the indication for pacemaker replacement is reached. When the pacemaker battery is nearing elective pacemaker replacement indicator, increasing frequent office visits are required at intervals of one or two months so that the pacemaker can be replaced soon after the replacement indicator is reached.
Occasionally elderly patients with high grade AV block and no underlying ventricular escape rhythms are pacemaker dependent. These patients die immediately when their pacemaker fails to pace. For these patients replacement of the pacemaker before complete battery depletion is essential.
AV Sequential Pacing
If a patient with a dual chamber pacemaker, full capture of both the atria and ventricles is shown. Each pacing spike from the right atrial electrode triggers atrial contraction, and each pacing spike from the right ventricular electrode triggers left ventricular contraction.
Modern pacemakers often greatly alleviate symptoms in patients with slow heart rates and in some patients are life saving. The decision to proceed with implantation of a permanent pacemaker however, is a major consideration in the care of a patient for several reasons. First, pacemaker implantation requires a surgical operation which is occasionally associated with major complications, especially risk of infection. Secondly, once a permanent pacemaker is placed, the patient is obligated to attend check up evaluations for the remainder of his or her life. Thirdly, pacemaker implantation is expensive. Typically, the cost of a pacemaker and two electrodes exceeds $5,000. The large majority of patients undergoing pacemaker implantation in the United States are over 65 years of age and are insured by Medicare. Medicare law requires that patients receiving permanent pacemakers have documentation of their slow heart rates with electrocardiographic rhythm recordings and that they have symptoms directly related to their slow heart rates. Implantation of a permanent pacemaker is virtually never indicated in a patient who is totally asymptomatic.
Younger patients often experience transient slow heart rates during fainting spells caused by the sight of blood referred to as vasovegal syncope. Pacemaker insertion is rarely ever indicated in these patients. Occasionally family members make a decision against pacemaker insertion in an elderly relative who has far advanced dementia and is unable to speak, feed themselves or recognize family members.
The decision to proceed with pacemaker implantation is best made by the patient and his or her cardiologist in discussion with the patient’s family. The risks and benefits of pacing should be discussed with the patient and family, including the need for regular pacemaker surveillance for the remainder of the patients life and the need for pacemaker replacements for battery depletion in the future.
Selection of Pacemaker Model
The three manufacturers of pacemakers in the United States each offer several models of pacemakers with different features. Selecting an appropriate pacemaker to meet the individual features. Selecting an appropriate pacemaker to meet the individual patient’s specific needs is important, and the patients and family members may wish to discuss this issue with their cardiologist.
In a patient with chronic atrial fibrillation, a dual chamber pacemaker is inappropriate because the atrial electrode will not be able to pace the atriae. For these patients, a single chamber pacemaker which paces only the right ventricle will be appropriate. However, dual chamber AV pacing is appropriate for many patients with brief transient atrial fibrillation. In these patients restoration of a more rapid atrial rate with atrial pacing may help to reduce the likelihood of further atrial fibrillation. Selecting a pacemaker with an algorithm for suppressing atrial fibrillation may also be helpful for these patients.
In patients with sick sinus syndrome and complex tachyarrhythmias, selecting a pacemaker with advanced telemetry features may be helpful for detecting and diagnosing tachyarrhythmic medications. For patients with a history of atrial tacharrhythmias, a pacemaker with mode switching capability should be selected to prevent excessively rapid ventricular pacing resulting from atrial tracking of atrial tachyarrhythmias.
For very small patients with little subcutaneous tissue and thin skin, small sized pacemakers are available. These smaller pacemakers are also useful for women patients who have undergone mastectomy operation previously. They are essential for children and infants. Although the smaller pacemakers provide many of the features of larger pacemakers, they have smaller lithium batteries and therefore have shortened battery life spans, typically 3 to 5 years instead of 5 to 10 years.
Technique of pacemaker implantation
Implanting a permanent pacemaker with electrodes in the right ventricle and right atrium requires a small operation which typically requires approximately 1 hour. Light sedation with intravenous sedative medications is commonly used and general anesthesia is virtually never necessary.
Meticulous sterility must be maintained throughout the operation to prevent infection. The anterior chest is scrubbed with an anti-septic solution and sterile drapes are applied. An intravenous line is always placed in a peripheral vein to provide access for rapid administration of intravenous medications.
A local anesthetic agent, typically lidocaine, is injected with a small gauge needle in the upper anterior chest below the clavicle or collarbone. Generally the left chest site is used for patients who are right handed, but the pacemaker can be implanted in either side. After the skin is anesthetized an incision 1½ inches in length is made, and a pocket for the pacemaker is made using sharp and blunt dissection in the layer of fat under the skin. Next, additional lidocaine is injected into the incision site and micropuncture 21 gauge needle (3½ inches in length) is advanced into the subclavian vein. After the needle tip enters the vein, blood aspirated from the needle identifies the position of the needle in the vein, and a spring wire is advanced through the needle tip into the vein. Next, the needle is withdrawn over the spring wire and removed. A polyvinylchloride vascular sheath and dilator is then advanced over the guide wire into the vein.
After the guide wire and dilator are removed, the pacing electrode is advanced through the vascular sheath into the subclavian vein and then into the right heart under fluoroscopy. The tip of the right ventricular pacing electrode is then manipulated under fluoroscopy to the RV apex where it is positioned in trabeculae in the endocardial surface.
Next, a second venipuncture is made in the left subclavian vein and a second vascular sheath dilator is advanced over another guidewire into the vein. Through this sheath, the right atrial electrode is advanced through the superior vena cava to the right atrium. The tip of the electrode is then positioned under fluoroscopy in the right atrial appendage. Located at the tip of the electrode is a tiny corkscrew which is rotated into the endocardium, affixing the tip to the endocardium to provide good electrical contact.
Next, the pacing and sensing electrical thresholds for the two electrodes are tested to ensure good contact between the tips of the two electrodes and the endocardial surface of the RV and RA. The two electrodes are then secured with silk sutures tied to the lead sleaves in the pacemaker pocket to prevent dislodgement of the electrodes. The pacemaker pocket is irrigated with an antibiotic solution to reduce the likelihood of infection.
Next, the proximal tips of the RV and RA electrodes are inserted into the pacemaker and secured with set screws. The pacemaker is placed in the pocket and the incision is closed with absorbable suture. The skin edges are then sealed together either with skin staples or with topical adhesive. A sterile dry dressing is applied.
The patient’s heart rhythm is then monitored by telemetry overnight, and typically the patient is discharged the day after the pacemaker implantation if pacing is stable. All patients are given IV-antibiotics one hour prior to the pacemaker implantation and six hours later to prevent infection. At the time of hospital discharge, all patients with newly implanted pacemakers are advised to leave the dressing intact to protect the incision site to prevent infection. In addition, patients are advised to avoid lifting their arm overhead (on the side of the pacemaker implant) because extending the arm up vertically may displace the electrode, necessitating re-operation and repositioning of the electrode tip. After approximately ten days elapses, the electrode tips become firmly attached to the endocardium of the RA and RV by scar tissue. After the first few weeks following implantation, the electrodes rarely ever become dislodged.
Patients are generally seen back in the cardiologist’s office approximately seven to ten days later to check the incision site and for skin staple removal. Six weeks later, the patient is again seen in the office for another incision check and for interrogation and reprogramming of the pacemaker. The electrical output of the pacemaker is adjusted downwards to conserve battery life since contact of the electrode tips with the endocardium improves several weeks after implantation as local inflammation subsides.
Patients with dual chamber pacemakers are routinely seen for interrogation and reprogramming of their pacemakers at intervals of six months and patients with single chamber pacemakers are seen in the office for pacemaker checks annually. All three American pacemaker manufacturers also provide equipment for automatic home monitoring of pacemakers, which is especially useful for patients living long distances from their cardiologist’s office. These monitoring devices, however, require availability of a home telephone line for transmitting pacing data to the cardiologist’s office, and are not designed for use with a cell phone. Furthermore, these home monitoring devices provide only interrogation of the pacemaker at intervals; programming the pacemaker to change pacing parameters can only be done in the cardiologist’s office with a pacemaker analyzer computer.
The reliability of modern pacemakers is extraordinary. Less than one in ten thousand pacemaker devices malfunctions electronically, failing to pace. Failure of electrode leads is more common. If an electrode becomes displaced soon after pacemaker implantation, re-operation is necessary to reposition the electrode tip, but displacement of an electrode weeks after implantation is very rare. Occasionally an electrode lead will fracture, requiring replacement of the electrode. Infection at the pacemaker implantation site invariably occurs if the pacemaker device erodes through the skin, exposing the device. This complication always necessitates removal of the pacemaker and electrode leads and implantation of a new pacemaker and new electrode leads on the contralateral side.