The heart is a unique structure in the human body in that it combines both mechanical and electrical properties in the same cell. Normal electrical function in the heart includes the firing of a dominant pacemaker, an area of heart tissue that initiates heartbeat. The dominant pacemaker is usually in the area of the heart called the sinus node, which is located in the right atrium (one of four chambers of the heart: two atria and two ventricles). The pacemaker generates a resting heart rate between 60 and 100 beats per minute. This dominant impulse is then transmitted throughout the heart by a wiring system called the auriculoventricular (AV) node and His-Purkinje fibers, which carries the impulse down into the ventricles, resulting in synchronized contraction of the cardiac chambers for optimal performance. When problems occur in this system, it is possible to implant a man-made pacemaker to replace the heart’s own intrinsic pacemaker function.
Pacemakers have undergone a remarkable evolution since their initial implantation in 1958. The first pacemaker was a primitive device by today’s standards, a ventricular pacemaker used for patients with Stokes-Adams attacks (passing out or syncopal spells due to complete heart block with slow or absent pulse). Since then, pacemakers have evolved into devices of great technical sophistication. As of 2001, more than 500,000 patients have received pacemakers, and up to 115,000 new devices are implanted each year in the United States alone. Today’s permanent pacemakers can be programmed to perform complex functions, allowing clinicians to meet patients’ specific clinical needs and also to optimize hemodynamic (blood flow and blood pressure) support. Depending on the patient’s specific diagnosis, the cardiologist may elect to place a singleor dual-chamber pacemaker either in the right atrium, right ventricle, or both. It is even possible now to place a so-called biventricular pacemaker, which can stimulate both the right and the left ventricles and can improve cardiac performance in patients with heart failure under certain circumstances.
TEMPORARY AND PERMANENT PACEMAKERS
Temporary pacemakers are used only in the hospital to support patients who require temporary and intermittent support due to various degrees of heart block. The goal for the use of temporary pacemakers is the resolution of the indication (heart block) or the implantation of a permanent pacemaker. Permanent pacemakers, as the name indicates, are placed for permanent cardiac rhythm dysfunction and various degrees of heart block.
The indications for permanent pacemaker can be summarized according to the American College of Cardiology/American Heart Association published guidelines.
Complete Heart Block
Where there is no electrical communication between atria (upper chambers) and ventricles (lower
chambers). Patients will require a pacemaker in this situation regardless of whether or not they have symptoms.
Second-Degree Heart Block
Where there is some synchrony (timing compatibility) between atria and ventricles, but it is not constant (or 1:1) like the normal heart. If patients have symptoms due to this conduction disturbance, they will require a pacemaker. For asymptomatic patients, a pacemaker may be required after further evaluation by a cardiologist.
When the heart’s sinus node fails to fire at a regular and expected interval and the pause is longer than 3.0 s or the heart rate is less than 40 beats/min.
Syncope (“Passing Out”)
If patients “pass out” due to carotid sinus hypersensitivity (excessive excitability of heartbeat-generating tissue in the carotid artery) or due to recurrent cardiogenic (from the heart tissue) syncope despite optimal medical therapy, a pacemaker is indicated.
PACEMAKER IMPLANTATION AND FOLLOW-UP INSTRUCTIONS
Pacemaker implantation is done in a designated aseptic radiation theater (procedure room) by an experienced cardiac electrophysiologist (heart conduction specialist) or, in some cases, by a cardiac surgeon. The majority of pacemaker generators are placed in a subcutaneous “pocket” underneath the skin in the right or the left (depending on whether the patient is left or right handed) pectoral region. The leads are inserted via the cephalic or the subclavian veins (veins located in the chest/neck area) and are advanced under fluoroscopic (x-ray) guidance into the right atrium and/or right ventricle, depending on the pacemaker mode. The patient is treated with broad-spectrum antibiotics periprocedurally (before the procedure). The patient is subsequently monitored for 24 hr in the hospital. A chest x-ray and 12-lead electrocardiogram (EKG) are obtained postprocedure. The patient is then discharged home with restrictions on activity and follow-up appointments to check the wound site and pacemaker function.
Patients in general are recommended to stay away from electromagnetic fields, arc welding, large stereos, and chain saws. When passing through security checkpoints at airports, patients are instructed to inform the security personnel about the pacemaker, in order to limit the time spent in antitheft entrances. Scanning wands and cell phones must be kept at a distance of more than 6 in. away from the pacemaker. Patients cannot undergo magnetic resonance imaging (MRI) scans with a pacemaker, and therefore alternative imaging is required, depending on the clinical condition. Device-specific questions for home and work are made available to the patient via manufacturers’ booklets. Moreover, patients are instructed to carry a pacemaker identification card at all times.
Placing a magnet over the pacemaker resets the device. The magnet can be left in place over the pacemaker in order to provide continuous pacing without sensed input under specific circumstances, such as when the patient is undergoing surgery and a cautery device is being used.
COMPLICATIONS OF PACEMAKER IMPLANTATION
Potential complications include pneumothorax (air in the chest), hemothorax (blood in the chest), ventricular puncture leading to hemorrhage, lead displacement or fracture, infection, and problems with pacing and sensing inappropriately. Many problems due to inappropriate sensing and pacing (both underand oversensing and pacing) can be corrected by placing the pacemaker programming device over the pacemaker and changing the settings as appropriate. Excision of the pacemaker is required only for physical problems with the leads (such as displacement or fracture) and battery change at the pacemaker’s end of life. Pacemaker batteries are now extremely energy efficient and can last 10 years or more without requiring replacement.
WOMEN AND PACEMAKERS
The indications and management of pacemakers are generally the same for female patients as compared to the general population. No extra follow-up is needed during pregnancy, as long as the patient is not on any antiarrhythmic medications (medications used to treat abnormal heart rate/rhythm). Routine pacemaker checkup is strongly recommended even for asymptomatic and uneventful pregnancies.