Cardiac Pacemakers: How They Work and Save Lives
The Cardiac Pacemaker
Normally, the heart carries out its job of pumping blood by means of a built-in system of rhythmic electrical impulses. These impulses are carried through the heart by nerves that lead to its muscle fibers, causing them to contract. Two main nerves lead to the ventricles, which are responsible for pumping blood. If one of these nerves fails to function properly, the heart beats irregularly. If both nerves fail for a few seconds, the brain receives insufficient blood, and the patient faints. As a rule, the system soon begins to work again, but there is always the danger that the brain may be left without its blood supply for a few minutes, suffering permanent damage, or even death. The heart has a secondary, stand-by impulse system which may take over in an emergency, but it produces only half the necessary heartbeats per minute, not enough to keep the whole organism in action.
Early Stimulation Attempts
The first serious suggestion of using ‘Faradaic’ (inductive) electric stimulation in cases of cardiac arrest was made by an English surgeon, W. H. Walshe, in a treatise published in 1862. Ten years later, his French colleague, Duchenne de Boulogne, described in his paper Électrisation localisée, some successful experiments with his apparatus which he called le main électrique, the ‘electric hand.’ One electrode was placed on the skin of the patient suffering from cardiac arrest; the other was held by the doctor in his right hand while he placed his left hand rhythmically on the patient’s thorax. This produced contractions of the heart muscle.
Hyman’s Artificial Cardiac Pacemaker
It was, however, an American heart specialist working for the US Navy, A. S. Hyman, who developed the first effective cardiac stimulator for clinical use in 1932. He called the apparatus, which weighed 7.2 kg, an ‘artificial cardiac pacemaker,’ thereby introducing the term into medical language. Technical development during and after the Second World War made it possible to reduce the size of the pacemaker so much that the patient could carry it permanently in his body, and after 1950 pacemakers of almost a dozen different types were rapidly developed.
How Pacemakers Work
The pacemaker is not an artificial heart, nor does it take over the job of pumping blood. It merely generates electric impulses. Some types do this all the time; others only when the natural system fails. The pacemaker is a miniature electronic unit implanted, as a rule, immediately under the skin of the chest so that it can easily be taken out and replaced. It has a battery and one or more transistors which amplify the weak current from the battery. As with Dr. Hyman’s original large-scale model, a wire runs from the pacemaker to the surface of the heart, or through a vein leading to its interior, ending in the right ventricle. Thanks to the amplifying action of the transistors, pacemaker batteries have to supply very little current and therefore last for years before they have to be renewed. The latest development is nuclear batteries in which the heat from pellets of the radioisotope plutonium 238 generates an electric current. These batteries are expected to have a lifespan of 10 years.