Platinum in Cardiac Pacemakers
Platinum in Cardiac Pacemakers
Materials for Implantation in the Human Body
Articificial pacing of the heart is one of the earliest examples of electronic equipment applied to the long term treatment of patients rather than to its usual role of diagnosis. The heart pacemaker is a device which is totally implanted in the body and which provides a small electric shock to the heart to make it beat at the correct rate. This is required in patients who have a clinical condition in which the heart rate falls to 30 to 40 pulses per minute instead of the required 70 to 80 pulses per minute.
The standard method of pacing the heart is by an endocardial electrode, which is an insulated wire with a contact tip of platinum, inserted down the external jugular vein into the right ventricle of the heart. The pacemaker unit itself is inserted either over the patient’s pectoral fold or in the right upper quadrant of the abdomen.
The development of a totally implanted pacemaker depends upon the ability to produce a small sterilisable unit which is inert in body tissue. The electronic circuitry presents few problems apart from the power source, which takes up about two-thirds of the volume of a pacemaker and for which mercury cells are commonly used. The only British manufacturer of an implanted pacemaker is Devices Implants Limited of Welwyn Garden City in Hertfordshire, a firm which has been developing and manufacturing them for many years. For their pacemakers, all components must pass strict mechanical, electrical and body compatibility tests.
The only metallic materials which have proved completely safe when implanted in the severe environment of the human body are platinum and 10 per cent iridium-platinum alloy, because of their remarkable resistance to corrosion. The latter is used on the pacemaker as the indifferent or earth return electrode, which can be clearly seen in the illustration.
The major problem still facing the manufacturer of any implanted active system is that of the power supply. The mercury cell produces 1.35 volts and will run for approximately to 3 years in a pacemaker. This means that every years the patient has to have the pacemaker removed and a new one inserted.
There are many interesting new approaches to the power supply problem. One of these is a biogalvanic cell utilising the oxygen available in the body. This oxygen is reduced in the cell using platinum black on a platinum grid as the catalyst. Another approach is the isotope battery, in which the heat generated by the radioactive isotope plutonium 238 is converted to electrical energy using a thermopile.
It is hoped that, by new methods of powering pacemakers, patients will be able to have trouble-free pacing for a period of at least five years. After discharge from hospital following an implantation the patient can lead a normal life, returning to hospital for checks from time to time to make sure that all is well with the system. Thousands of people who would otherwise be totally incapacitated are working and enjoying life again thanks to cardiac pacemakers for which the platinum and 10 per cent iridium-platinum electrode materials were supplied by Johnson Matthey Metals Limited.