Pacemakers

The Heartbeat

Although most people consider the heart to be purely a pump, it also has an electrical system. A normally functioning, healthy electrical system guarantees that the heart will beat in a regular fashion, gradually increasing its rate when more blood should be pumped (during exercise) and slowing when less blood is required (during sleep). At rest, the heart normally beats 60 to 100 times per minute.

Arrhythmias (irregular heart beats) occur when the electrical system is abnormal, diseased, or damaged. This can cause the heart to beat inappropriately slow or fast. An abnormally slow rate is called a bradycardia, and an abnormally fast rate is called a tachycardia. Either condition can seriously impair the heart's ability to pump enough blood efficiently.

The heart has four pumping chambers. The two upper chambers are called atria; the two lower are called ventricles:
  • Atria. The atria function as collection chambers for blood returning to the heart. When they contract, they force blood into the ventricles. The left atrium collects blood from the lungs and injects it into the left ventricle. The right atrium collects it from the rest of the body and pumps it into the right ventricle.
  • Ventricles. The left ventricle is the main pumping chamber of the heart, delivering blood to the entire body. It has a big job to do and has thicker muscle than the right ventricle or the atria. The right ventricle pumps blood only to the lungs. Because the pressures in the lungs are low, the right ventricle does not need such a muscular wall.

    A normal heart beat begins with the atria pumping blood into the ventricles. After a brief delay while they fill with blood, the ventricles contract and blood is pumped into the lungs and to the rest of the body. This regular beating pattern - atrial contraction, a delay for ventricular filling, ventricular contraction-is made possible by the heart's electrical system.

    Although the muscle portion of the heart is responsible for the actual pumping of blood, the electrical system of the heart acts like a control system. It tells the atria and ventricles when to contract, and at what rate the entire heart will beat. When this control mechanism is damaged or diseased, the normal pumping function of the heart is also altered.

    The electrical activity that initiates a heart beat begins in a region of the electrical system called the sinoatrial or SA node. The SAnode is located high in the right atrium. This electrical activity spreads, like a wave, across the right and then the left atrium, causing muscular contraction. The electrical wave then enters a region of the electrical system called the atrioventricular or AV node. The AVnode is a special electrical fiber that acts as a conductor of impulses from the atria to the ventricles. Normally, this impulse conducts very slowly through the AV node. This slow conduction produces the delay that allows the ventricles to fill with blood after the atria have contracted. The electrical impulse then travels from the AV node through a specialized system of electrical tissue called the His-Purkinje system. This electrical conduction is rapid and results in the almost simultaneous contraction of the right and left ventricles.

    The Pacemaker

    What Is a Pacemaker? A pacemaker is a small, light-weight electrical device surgically placed inside the body to regulate the heart rate. The pacemaker sends electrical impulses to the heart to keep it beating properly, pumping the blood throughout the body.

    The pacemaker consists of a generator that houses the battery and circuitry, one or two insulated wires, and an electrode at the tip of these wires. The pacemaker unit (a pulse generator) uses batteries to produce an electrical signal. The wire carries this electrical impulse to the heart. The pacemaker also senses natural electrical signals from the heart. If the heart is not beating on its own, the pacemaker sends a signal to trigger the heart to beat. When the heart is beating properly on its own, the pacemaker does not try to trigger a heart beat.

    Types of Pacemaker Systems. Many types of pacemakers are available. All pacemakers will stimulate the heart to beat if the natural rate drops below the limit the pacemaker recognizes as safe. This set lower rate is determined by the doctor. Other pacemakers will speed up or slow down the heart rate in response to exercise-these pacemakers can sense when the body is working harder and needs the heart to circulate blood more rapidly to transport oxygen throughout the body. With programmable pacemakers, the doctor can place a special wand over the pacemaker to change the rate or other settings; this is a painless procedure.

    Pacemaker systems act in one or two chambers of the right side of the heart, the right atrium and the right ventricle. The insulated wires can sense the heart's natural beating as well as stimulate the heart to beat. A single-chamber pacemaker uses one insulated wire, which may go to either the right atrium or right ventricle. A dual-chamber pacemaker uses two insulated wires, one in the right atrium and the other in the right ventricle. One wire senses and stimulates the atria, while the other will sense and stimulate the ventricle, if needed.

    Temporary Pacemakers. Some patients have a temporary pacemaker before the permanent one is inserted. The temporary pacemaker battery is placed outside of the body so that a doctor can adjust the heart rate. After several days, the doctor will determine if a permanent pacemaker is needed, because some disturbances in the heart's rhythm are only temporary.

    Implanting the Pacemaker

    The permanent pacemaker implant procedure is relatively simple and lasts one to two hours. Patients are given a local anesthetic, which is administered by an injection near the collarbone to numb the area. Patients are awake throughout the procedure.

    The doctor makes a two- or three-inch incision about one inch below the collarbone and threads the pacemaker wire through the subclavian vein into the appropriate chamber of the heart. If a dual-chamber pacing system is required, a second pacemaker wire is inserted. The wire must touch the heart so it can carry electrical messages back and forth between the heart and the pacemaker battery. The pacemaker will be attached to the wire and placed under the skin. Only one incision is made for inserting both parts of the pacemaker system, so the patient will have only one small scar.

    Monitoring

    The pacemaker battery and wire should be checked regularly to make sure they are working correctly. Patients may be asked to:

  • Go to the doctor's office for regular visits.
  • Use a telephone and special equipment to transmit the heartbeat to the doctor's office, where an electrocardiogram will be recorded. Patients do not have to leave home to have these on-the-spot checkups.
  • Wear an ID bracelet or necklace that says the patient has a pacemaker, in case of an emergency.
  • Always carry a pacemaker identification card.

    The frequency of monitoring depends on the type of pacemaker, the patient's condition, and how long the pacemaker has been in place. The pacemaker may need occasional adjustments to conserve battery life and to ensure peak operating efficiency.



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    Arizona Heart Institute
    2632 N. 20th St.
    Phoenix, AZ 85006
    (602) 604-5123

    The information presented in this site should not replace the medical advice of your physician.
    You should not use this information to diagnose or treat any disease
    without first consulting with your physician or other healthcare provider.