The heart is composed of four chambers enclosed within a wall of muscle. The upper two chambers (atria) will help fill the lower two chambers (ventricles) with blood. The ventricles are larger and more heavily muscled than the atria. They pump blood to the lungs and throughout the rest of the body.
The heart's natural pacemaker, located in the upper part of the heart, normally regulates your heart rate. This natural pacemaker, called the sino-atrial node (S-A node), is a small cluster of specialized cells that produce electrical signals at regular intervals. The S-A node automatically increases your heart rate in response to your body's needs - for example, during exercise, when a faster blood flow is required.
The electrical impulses sent out by the S-A node travel to the atrioventricular node (A-V node), a second cluster of cells located near the center of the heart. The A-V node then transmits the electrical signals out to the walls of the ventricles.
This natural electrical pathway is very important because when the signals reach the lower heart, both ventricles contract, pumping blood throughout the body. Any problem with this natural electrical pathway may cause a change in heart rate and/or rhythm and effect circulation of blood from the heart to the rest of the body. Sometimes the heart is not able to keep up with the body's need for blood, and your cardiologist may recommend pacemaker implantation.
A pacemaker alters the heart rate to help meet your body's needs. It does this by providing pacing signals that are much like the heart's normal signals. Depending on the particular situation, a pacemaker may:
- Replace S-A node signals that are delayed or get lost along the pathway between the upper and lower heart
- Help maintain a normal timing sequence between the upper and lower heart
- Make sure the critical lower chambers of the heart always contract at an adequate rate
Every pacemaker system has two parts; the pulse generator, which produces the pacing impulses and the lead or leads which deliver these impulses to the heart. The same leads also carry signals back from the heart. By "reading" these signals, the pulse generator is able to monitor the heart's activity and respond appropriately.
The pulse generator is usually implanted below the collarbone just beneath the skin. The leads are threaded into the heart through a vein located near the collarbone; the tip of each lead is then positioned inside the heart. However, sometimes the pulse generator is positioned in the abdomen and the pacemaker leads are attached to the outside of the heart. The decision, where and how to implant the pacemaker system, is made by your cardiologist based on individual needs.
Following implantation, periodic pacemaker evaluations are scheduled to monitor pacemaker function. Evaluations will include office visits as well monitoring by telephone. The object of these assessments is to check to see if your pacemaker is working properly. Your cardiologist determines the frequency of pacemaker evaluations. During office visits your cardiologist may analyze pacemaker function using a computer and adjust programmable functions to help the pacemaker work better and to preserve battery life. Monitoring by the telephone allows you to have your pacemaker checked in the comfort of your home using a specialized telephone transmitter which relays important information regarding your pacemaker function to your cardiologist's office.