Platinum in Medical Technology
Platinum in Cardiac Stimulation
Heart failure is one of the most common causes of death in the western world. Approximately 21 % of all people die here as a result of heart failure. Men are more frequently affected than women.
For many years it has now been possible to treat patients with cardiac arrhythmias by means of implantable medical devices. Both implantable cardiac pacemakers and implantable defibrillators are employed. Whereas pacemakers provide a supportive stimulation of the heart when it beats too slowly (bradycardia), defibrillators are applied when ventricular fibrillation occurs, i.e. the heart is stimulated so rapidly (tachycardia) that the heart muscle cannot mechanically keep up with the electrical impulses and thus the blood circulation comes to a standstill. Sudden death by cardiac arrest would ensue. Sudden death by ventricular fibrillation can however be averted by defibrillation.
The first patient who had a cardiac pacemaker implanted was Arne Larsson. The implantation was carried out on 8th October 1958 by the cardiac surgeon Åke Senning and Rune Elmqvist, an engineer with the Siemens company. Currently about 1 million cardiac pacemakers are implanted worldwide each year, about 70,000 in Germany.
The first defibrillator was implanted early in the 1980s. Currently about 400,000 defibrillators are implanted in patients worldwide each year. Besides cardiac pacemakers and defibrillators, devices for cardiac resynchronization are now also applied. For this purpose, electrodes are implanted in the right ventricle and right atrium and also a further one on the outside of the left ventricle. These devices are implanted in patients whose cardiac pumping function is greatly reduced because of an asynchronous cardiac contraction sequence.
A cardiac pacemaker – and also an implantable defibrillator – comprises an pulse generator and the so-called electrodes which are anchored in the heart and transfer the electrical stimulation pulses generated by the pulse generator to the tissue. The electrically active part of these electrodes, which delivers the electrical pulse to the heart muscle, is located at their distal end. The components found here can have a variety of different designs (Abb. 2) and these are then fixed in the heart tissue by correspondingly numerous methods.
Platinum alloys are employed for these components. The properties that predestine platinum for this application are again the outstanding biocompatibility, the corrosion resistance and the electrical conductivity. The corrosion resistance in the body under the simultaneous influence of electrical potentials is particularly important, because electrical currents flow through these components.
As platinum itself does not fulfill the mechanical properties required for this application, platin-iridium alloys are generally used (with 10, 20 or 30 wt.-% iridium). The individual components are then manufactured either by turning or from a coiled wire. Typically they have an external diameter of 1 to 2 .
It is furthermore important to optimize the electrical properties at the interface between the electrode and the heart muscle during cardiac stimulation. Coatings are used for this purpose, which reduce the electrochemical impedance at the interface. Among other materials, platinum is used which can be deposited by PVD or electroplating to give a morphology with a particularly large physical surface area (Abb. 3) and thus can give a particularly low interfacial impedance.