Cardiac Pacemakers

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Abstract
The development of miniaturized implantable electronic devices for artificial cardiac pacing has now passed the experimental stage, although much development is still possible. Generally, the battery life has not been as long as was predicted and it is common to find the units becoming exhausted after 1 to 2 years. In most cases this is the result of too much power being delivered per impulse. Now that the required levels are known, modifications can be made and it should be possible to extend the useful life of the units to the original predictions. After the early troubles with leads and electrodes, which sometimes had welded joints and dissimilar metals in them, the current practice is to make the electrode part of the lead, but there are exceptions. These leads have proved to be very reliable and a broken wire is extremely rare, although many have been in use over 3 years and some exceed 4. To an extent the reliability of the electronic unit depends upon its simplicity and the aim has been to keep the number of components to a minimum. There is, however, an advantage in arranging for a series-parallel combination of components, if this will ensure that a part failure is not detrimental to the patient. The aim of all circuit designs should be such that any malfunctioning of the electronic section will lead to a slowing down in rate or a diminished power output, but not to an appreciable rise in rate. A problem, however, that still needs investigation is the question of tissue reaction to the implant and the method of sterilization. A number of pacemakers have been rejected by the body after varying periods of time ranging from 3 weeks to 10 months. There is difficulty in deciding whether this is due to a tissue reaction to the foreign body, or to a low grade infection introduced at the time of implantation. It is possible, also, that the siting of the pacemaker within the body may have some bearing on the problem of the reaction. Pacemakers have, however, proved useful devices in the treatment of heart block and many of the design principles which have evolved can be usefully applied to the provision of stimulations for other muscles.