The use of implantable medical devices for electrical stimulation of electrically excitable tissue is well known in the medical arts. For example, electrical stimulation of the brain, the spinal cord, or a peripheral nerve may be used to treat any number of medical conditions. In such devices, electrodes deliver the stimulation of signal to the electrically excitable tissue. The electrodes are operatively connected to an implantable pulse generator (IPG) that is packaged in a case that is adapted to be implantable. Those electrodes are coupled to that pulse generator by a conductive lead wire.
A user having such an implantable medical device during normal life activities may be forced to go through a time-alternating electromagnetic field. Prevalent examples of sources of electromagnetic field are Electronic Article Surveillance (EAS) systems and Metal Detectors (MD). Such systems detect theft of articles that have an attached electromagnetic tag and are found in the exit doorways of many stores and libraries.
EAS/MD systems work by generating an electromagnetic field that retail customers must pass through at the entrance and/or exit from a business establishment or other protected area. EAS systems detect the presence of antitheft tags on merchandise as they pass through the electromagnetic field if they have not been deactivated. Similarly, metal detectors (MD) generate an electromagnetic field that is perturbed by the presence of metal objects that might be carried by a person passing through the metal detector. Both these systems can induce voltages, via Faradays Law, on the lead system of implantable stimulators. The induced voltage is proportional to the area formed from one end of the lead to the other end of the lead (e.g. from the IPG case to the end of the lead). More specifically, according to Faraday's law, the voltage induced on a wire (lead system) is proportional to: (A) the area X number of turns described by the wire; (B) the flux density of the electromagnetic field; C) the frequency of the magnetic field; and (D) the cosine of the angle between the electromagnetic field and the vector normal to the area described by the wire. For dual lead systems, an induced voltage may also appear across the distal ends of the leads and is proportional to the area formed between the two leads.
These induced voltages may be of sufficient voltage and pulse duration to cause undesired tissue stimulation in the patient. This may result in shocking sensations to the patient, pacing of the heart, IPG sensing irregularities or other undesired stimulation effects. Additionally, in some medical devices with sensing, the induced voltage may also cause a false “sensing” response.
One technique for minimizing the effects of induced voltages is to keep the lead length of the implanted system to a minimum and/or to keep the lead in a straight line to minimize the loop area generated by the lead system. However, it may not always be possible to minimize the loop area or lead length. Another technique for minimizing the effects of induced voltages is to restrict the IPG to bipolar use only since this disconnects the case from the lead. However, this is not entirely effective since the electronic switch to the case may become forward biased (conductive) if the induced voltage is sufficiently high. Another problem with this approach is that it requires feedthrough capacitors on each of the IPG outputs to reduce the susceptibility to high frequency RF. Feedthrough capacitors can act as a low impedance connection between the lead and the IPG case at EAS/MD frequencies, thereby negating the benefit of bipolar stimulation. See U.S. Pat. Nos. 5,751,539 and 5,905,627.
It is therefore desirable to provide a way to improve electromagnetic compatibility (EMC) with EAS/MD systems that overcomes the disadvantages of the prior art. Moreover, it is desirable to improve EMC with EAS/MD systems while still allowing the use of unipolar stimulation.