The human heart beats 3000 million times in an average lifetime, pumping the equivalent of 48 million gallons (218 million litres) of blood around the body.
The regular rhythm — on average, 72 beats each minute — is controlled by the sino-atrial node, a tiny rounded organ located in the top left corner inside the heart. This is the heart’s natural pacemaker, which sends electrical impulses to the tissues. The heart contracts and heart’s expands in response to these impulses, producing the heartbeat.
Occasionally, the heart’s electrical con-ducting system can be disturbed by illness, such as angina or a heart attack. Sometimes it just fails completely. If this happens, the heart can be stimulated electrically to continue beating regularly.
If the heart stops it can sometimes be restarted with an electrical shock from a machine called a defibrillator. If the normal beat does not resume immediately, some-times a temporary pacemaker can be fitted outside the body — it is usually strapped to the waist. For those suffering from other irregularities of the heart beat a pacemaker is surgically placed inside the body, implanted in the chest.
All pacemakers, inside and outside the body, work in the same way. An electrode on wire, called the end of a pacing lead, is attached to the wall of the heart’s right ventricle (chamber), either directly through the chest, or threaded through a vein. The electrode is powered by the pacing box, a miniature generator operated by lithium batteries. Modern pacemaker batteries last at least five years, and some last up to 12 years.
Powered by the pacing box, the electrode produces electrical impulses which stimulate the sino-atrial node and make the heart beat. The pacing box is set to maintain the intervals of the impulses at a given rate, usually one beat per second, which is a little slower than the average heart rate. However, the box functions only when the heart is not producing its own electrical impulses at the correct intervals. It is sensitive enough to detect these delays and by filling in the gaps, maintains a normal rhythm. Some models include a radio transmitter and receiver, which means that a doctor can adjust the rate of the pacemaker from outside the patient’s body.
The first successful pacemakers were used by Dr Walter Lillehei, a cardiac specialist at the University of Minnesota, USA, in the late 1950s. They consisted of an electrode on a wire fed to the heart through the chest and attached to a battery pack strapped around the waist. The pack was about the size of a cigarette packet. Although the system was convenient because no surgery was needed to replace the batteries, the opening in the chest for the wire repeatedly became infected. External pacemakers are now used for temporary heart problems only, or until an internal pacemaker can be fitted.
The pacing box of the most commonly used internal pacemaker is about the size of a matchbox and weighs no more than 25g. It is usually made of lightweight titanium.
The box is implanted in the body, usually just inside the skin of the chest wall. It must be in the best position for threading the tube through the large vein to the heart and attaching the electrode, which is the size of a match head, to the heart wall. The body does not reject it because it is not living material.
The implanting operation is done while the patient is under general anaesthetic, but surgery to replace the batteries can usually be done with only a local anaesthetic.
A person wearing a pacemaker needs to be examined by a doctor frequently to make sure that it is functioning properly. Also, some wearers have to take care that their pacemakers are not affected by certain electrical circuits, such as magnetic detectors in airports or libraries.
New electronic technology may produce even smaller pacemakers which can be attached to the heart wall, eliminating, wires and large battery packs, although!they are still powered by batteries.
Another development is the rate-responsive pacemaker, which is sensitive to the patient’s activity. Instead of providing at impulse once a second, it will increase the impulses when he is active and slow them down when he is resting — like the heart, natural pacemaker.
Since the First successful pacemaker developed, more than 5 million people with serious heart disease have been helped to live more comfortable and active lives.
Picture Credit : Google