- The most common cause of slow heart rhythm is sinus bradycardia
- Pacemakers have revolutionized the core 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 core 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.
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 rate the gradual onset of lethargy, generalized weakness, lack of stamina or confusion, and may subconsciously 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, Hissbundle, or both right and left ventricles, resulting in A-V dissociation or electrical interruption of the atrial and ventricles, causing the atrial 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. 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 dropped completely. For example, in 2:1 AV block, every other heart beat is dropped and the heart rate is halved. In 4:1 AV block, every fourth beat is dropped and the heart rate is reduced by one-fourth. 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.
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 slowed 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 AVinterval 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.
The basis of the dual chamber pacemaker is that an electrode is positioned at the apex of the right ventricle to pace the ventricles and a second electrode is 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. 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 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 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 pacemakers 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. 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 connected 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 connects 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 several million beats per minute.
Another helpful feature of more advanced dual chamber pacemakers is that some pacemakers can identify and store in memory recordings of abnormal heart rhythms. This feature is particularly useful in patients who have complex arrhythmias.
Pacemaker Batteries
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.
Modern pacemakers can greatly alleviate symptoms in patients with slow heart rates and in some patients may be life saving. The decision to proceed with implantation of a permanent pacemaker 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 $6,000. 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 hart rates during fainting spells caused by the sight of blood. Pacemaker insertion is rarely indicated in these patients. Occasionally family members make a decision against pacemaker insertion in an elderly relative who has a 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 but 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 an 18 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 into the vein. Next, the needle is withdrawn over the spring wire and removed. A polyvinylchloride tube is advanced over the guide wire into the vein.
After the guide wire is removed, the pacing electrode is advanced through the polyvinylchloride tube into the subclavian vein and then into the right heart. Underfluoroscopy the tip of the right…
The Medtronic EnRhythm® pacemaker is a small, implantable medical device prescribed for people with a slow or weak heart rhythm to stimulate the heart muscle with precisely timed pulses of electricity. The EnRhythm pacemaker, which delivers electricity to both the right atrium and right ventricle, promotes natural heart activity by significantly reducing unnecessary pacing in the heart’s right ventricle when normal heart conduction is present by automatically switching to pace the right atrium. The EnRhythm device is the first-ever pacemaker to offer this exclusive feature, called MVP® or Managed Ventricular Pacing. With MVP, the device can be programmed to deliver pacing pulses to the right ventricle less than 5 percent of the time, compared to 50 percent or more with typical dual-chamber pacing. This is an important clinical benefit since clinical studies have shown that unnecessary pacing in the right ventricle can increase the risk for heart failure and atrial fibrillation. }
