Science Dimension volume 1 issue 4 October 1969
There are now more than 2,000 Canadians whose hearts, from injury through disease, have lost the natural ability to maintain a regular beat and must be stimulated electrically by an artificial pacemaker.
Pacemakers now in use consist of an array of five mercury batteries and a transistor. The pacer unit, about the size of a hockey puck, is embedded in the abdomen. The beat of a damaged heart is stimulated by electrical impulses which are transmitted along two platinum wires embedded and stitched into the heart wall. To stimulate the heart, 16 to 20 microjoules of energy/pulse are required. This energy is usually transferred to the heart in the form of a pulse with a duration of two milliseconds. The stimulus rate is usually set to produce 60 to 70 heartbeats per minute. This is sufficient to restore to near-normal activity a person whose heartbeat becomes incoordinated to the point where the output of blood by the heart is insufficient to maintain normal life. This is known as heart block.
With cardiac pacemakers, people can live nearly normal lives. However, they must be prepared to undergo minor operations to replace batteries that run down about every 15 months. This battery failure has led experimenters to consider various substitutes.
|Above: O.Z. Roy conducting tests with heart pacer in saline solution. (NRC)|
In 1963, the National Research Council of Canada undertook a research program to determine if this hazard could be removed by having the human body itself supply its own lifesaving electrical power. This continuing NRC research program produced the world's first biological pacemaker in 1965. The prototype device generated enough power to stimulate the rhythmic beat of a dog's heart for 48 hours. Today, improvements at NRC have led to development of biological energy sources which have operated in animals for more than one year, and with an estimated operational life of approximately 10 years.
O. Z. Roy, a medical electronics engineer with the Instrument Section of NRC's Radio and Electrical Engineering Division, began initial experiments along lines first laid down by Allessandra Volta in the early 1800's. Volta immersed silver and zinc in jars of salt water and caused current to flow, forming the first galvanic cell.
The principle he discovered still remains in use. All galvanic cells consist of an anode, a cathode and an electrolyte. The cathodes are characterized by the ease with which they accept electrons. In so doing they are reduced to a lower state of oxidation. Anodic metals part easily with electrons, dissolving to form positively charged ions in the electrolyte. This is an oxidation process. Both oxidation and reduction processes are accompanied by chemical changes in accordance with Faraday's Law, which gives an index of how much material will be used to convert chemical energy to electrical energy for any given battery capacity.
NRC investigations were directed mainly at a biogalvanic system in which the electrodes are the fuel and are consumed while the tissue in which they are implanted provides the electrolytes and in some cases a depolarizer as well. (Polarization occurs if ions produced at the cathode do not migrate rapidly enough through the electrolyte and, by accumulating at the cathode, set up a potential opposing that of the energy source, thereby reducing or stopping the output of current.) The electrical characteristics of galvanic cells depend on such factors as the types of anodic and cathodic materials, their size, as well as the electrolyte composition. The cathodic materials are either those that supply their own depolarizer, such as silver-silver chloride, or those such as platinum black which use the oxygen available in the body as the depolarizer. Zinc has been used as the anode mainly because it produces large potentials at high current densities and seems to be well tolerated by the body.
In 1967 - following successful experiments in which dogs with heart block were maintained for 180 day periods - the biogalvanic pacemaker research efforts focussed on converting it into a biological fuel cell system whose electrodes would be made of two inert metals which would not go into solution, but would act as a catalyst to produce electrons which by flowing through external circuits would produce electrical energy. Experiments were conducted using albino New Zealand rabbits and mongrel dogs. The dogs were used primarily to assess the galvanic pacemakers while the rabbits were used to investigate the various cells.
|Above: A close-up of NRC's experimental heart pacer. (NRC)|
More efficient stimulating methods have lowered the potential requirements from 6 to 2 1/2 volts and enabled the use of inert platinum black as the cathode. However, at the anode, zinc is still necessary, says Mr. Roy.
"The need today, for human patients, is for a pacemaker that would run for at least 10 years. We have not been successful, as yet, in eliminating zinc and achieving a completely inert system that will break down some component of the body such, as glucose and in this way release the needed electrons. However, our indications are that the platinum black cathode of our half. fuel cell might withstand the stresses of 10 years' use and the efficiency of zinc consumption is such that 30 grams of this material would be required over this 10-year period."
A parallel research program, involving the effects of constant pulsing on the heart itself, has been conducted since 1967 in collaboration with Dr. H. A. Heggtveit and Dr. W. G. Waddell, respectively, of the University of Ottawa's Department of Pathology and Department of Experimental Surgery.
"We are trying to determine what happens to the subject electrically, physiologically and histologically, all with the view to lowering the amount of energy required to pace a heart. This is not only necessary to reduce the energy requirements of the pacemaker, but also to minimize the tissue damage which occurs in the biological system. We may find that we can stimulate with a far lower amount of energy or find a much less damaging wave form," Mr. Roy says.
Reprinted courtesy of National Research Council of Canada