Patent Publication Number: US-6209764-B1

Title: Control of externally induced current in implantable medical devices

Description:
This is a divisional of application Ser. No. 08/847,642, filed Apr. 30, 1997, for which priority is claimed. 
    
    
     BACKGROUND OF THF INVENTION 
     1. Field of the Invention 
     This invention relates generally to implantable medical devices, and more particularly to a method and apparatus for limiting unwanted current flow through electrically excitable tissue resulting from application of an external signal on an implanted medical device. 
     2. Description of the Related Art 
     The use of implantable medical devices for electrical stimulation of electrically excitable tissue is well known in the medical arts. Electrical stimulation of brain tissue has been used for tremor suppression. Moreover, electrical stimulation of peripheral nerve tissue has been used to promote blood circulation in patients having peripheral vascular disease. Additionally, electrical stimulation of the brain and nerve tissue of the spinal cord has been used for pain management. In such devices, electrodes deliver the stimulation signal to the electrically excitable tissue. The electrodes are operatively connected to an implantable pulse generator which 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 implanted 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. Such systems detect theft and are found in the exit doorways of many stores and libraries. EAS systems typically emit an AC electromagnetic field for detecting theft of articles that have an attached electromagnetic tag. 
     When the user having an implanted medical device walks through an EAS system that emits a significant level of AC electromagnetic field, current may be induced in the electrically excitable tissue by the Faraday effect. According to the Faraday effect, when a conductive loop is disposed within a time-alternating electromagnetic field, a current is induced in that conductive loop. With the implanted medical device within the user, the conductive loop consists of the stimulation electrodes, the lead wire, the implantable pulse generator having a case, and the conductive medium of the body including the electrically excitable tissue between the stimulation electrodes and the case. The case in some implantable medical devices acts as a reference return electrode with respect to the stimulation electrodes and is composed of electrically conductive material. 
     The user may experience unwanted physiological effects from the current that is induced in the electrically excitable tissue of that conductive loop. This current may cause uncontrolled excitation of electrically excitable tissue which can lead to pain sensations and unwanted motor responses for the user. Thus, means for controlling that induced current is desired. 
     OBJECTS OF THE INVENTION 
     Accordingly, a primary object of the present invention is to limit the current that may be induced within the conductive loop formed by an implanted medical device when a significant level of external signal such as an electromagnetic field is present. The induced current is limited automatically upon sensing a significant level of an external signal or manually with the user controlling the opening of the conductive loop when the user is about to enter an area having a significant level of external signal. 
     SUMMARY OF THE INVENTION 
     In a principal aspect, the present invention takes the form of an apparatus and method for limiting current flow, induced when the level of an external signal is greater than an external signal threshold level, in a conductive loop formed by a medical device implanted within a living organism having electrically excitable tissue. The present invention includes a switch that is adapted to be operatively connected within the conductive loop which includes the implanted medical device and the electrically excitable tissue. Additionally, a control circuit controls the switch to turn the switch on when the medical device is stimulating the electrically excitable tissue to achieve a therapeutic effect if the level of the external signal is less than the external signal threshold level. Moreover, the control circuit controls the switch to turn off whenever the level of the external signal is greater than the external signal threshold level, to thereby introduce a high impedance into the conductive loop. 
     The present invention can be applied to particular advantage when used with a sensor for sensing when the level of the external signal is greater than the external signal threshold level. The control circuit would automatically turn the switch off when the sensor senses that the level of the external signal is greater than the external signal threshold level. Alternatively, the living organism, having the implanted medical device, manually initiates the opening of the conductive loop. In this manner, the conductive loop can be opened manually to limit unwanted current that can result from application of a significant level of external signal on a closed conductive loop. 
     These and other features and advantages of the present invention will be better understood by considering the following detailed description of the invention which is presented with the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A shows an electrical stimulation system which forms a closed conductive loop when implanted within a human body; 
     FIG. 1B shows the electrical stimulation system of FIG. 1A with the further inclusion of a switch for opening the conductive loop formed by the implanted electrical stimulation system, according to a preferred embodiment of the present invention; 
     FIG. 2 shows simple switch devices and the cross sections of those switch devices fabricated with semiconductor integrated circuit technology; 
     FIG. 3 shows a current limiting circuit according to a preferred embodiment of the present invention; and 
     FIG. 4 shows an external signal sensor circuit according to a preferred embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1A, an electrical stimulation system  100  comprises stimulation electrodes  102  implanted near electrically excitable tissue  104  of a human body  106  of a user. The electrically excitable tissue in FIG. 1A is nerve tissue within the spinal column. However, electrodes of implanted medical devices are also commonly implanted near other types of nerve tissue such as peripheral nerve tissue and brain tissue. The electrical stimulation system  100  also includes an implantable pulse generator  108  which provides the stimulation signals to be applied on the stimulation electrodes  102  via a conductive lead wire  110 . 
     A programmer  112  sends messages to the implantable pulse generator  108  to modify the stimulation signals to be applied on the electrodes  102 . The programmer controls the pulse generator by radio frequency communication to the pulse generator via an antenna  114  and corresponding receiver in the generator  108 . An example of such an electrical stimulation system is the ITREL II system available from Medtronic, Inc., Minneapolis, Minn. While the preferred system employs fully implanted elements, systems employing partially implanted generators and radio-frequency coupling may also be used in the practice of the present invention. Such systems are also available from Medtronic, Inc. under the trademarks X-trel and Mattrix. 
     The electrical stimulation system  100  of FIG. 1A is implanted to cause a therapeutic effect of pain reduction by inducing an action potential in the electrically excitable tissue  104 . The implantable pulse generator generates the stimulation signals to be applied on the electrodes  102 . For example, the induced action potential in the nerve fibers of the spinal cord can reduce pain experienced by the human body  106 . 
     The preferred embodiment of the present invention is described for the electrical stimulation system  100  used to achieve the therapeutic effect of pain reduction in the human body  106 . However, it should be appreciated that the present invention can be used with other implantable medical devices for achieving other therapeutic effects aside from just pain management. Additionally, it should be appreciated that the present invention can be used for medical devices implanted in other living organisms aside from just a human body. 
     In the implanted medical device of FIG. 1A, a conductive loop is formed by the stimulation electrodes  102 , the lead wire  110 , the implantable pulse generator  108  having a conductive case, and a conductive medium of the human body  106  between the stimulation electrodes and the conductive case of the implantable pulse generator. The case of the implantable pulse generator can act as a reference electrode with respect to the stimulation electrodes. The conductive medium of the human body includes the electrically excitable tissue  104 . 
     When the user having such an implanted medical device enters an area having a significant level of time-alternating electromagnetic field, a current may be induced in the conductive loop formed by the implanted medical device and by the electrically excitable tissue in accordance with the Faraday effect. According to the Faraday effect, when a time-alternating electromagnetic field is applied on a conductive loop, a current is induced in that conductive loop. Common sources of time-alternating electromagnetic field are Electronic Article Surveillance (EAS) systems installed on exit doorways of many stores and libraries. Other sources of time-alternating electromagnetic fields include Nuclear Magnetic Resonance Imaging (NMRI) systems, security screening corridors found in airports, and cellular telephones. 
     When such an external signal induces a current in the conductive loop and in turn the electrically excitable tissue, the user may experience pain and uncontrollable motor responses. Thus, this induced current is unwanted, and the present invention limits this unwanted current. 
     In the implanted medical device system  100  of FIG. 1A, the implantable pulse generator  108  generates stimulation signals to be applied on the stimulation electrodes  102 . The conductive loop formed by the implanted medical device and by the electrically excitable tissue is closed when stimulation signals are applied on the stimulation electrodes to induce an action potential in the electrically excitable tissue  104 . That action potential creates a current in the electrically excitable tissue to cause a therapeutic effect. In that case, the conductive loop is closed such that current can flow in the conductive loop. 
     When the user enters an area with a significant level of external signal such as a time-alternating electromagnetic field, that external signal may induce unwanted current in that conductive loop. In that case, the conductive loop is opened to limit the induced unwanted current. The general embodiment of the present invention opens that conductive loop by introducing a switch into that conductive loop. 
     Referring to FIG. 1B, the electrical stimulation system  100  of FIG. 1A is shown with similar elements having the same number label. The electrical stimulation system  100  of FIG. 1B further includes a switch  116  operatively connected between the case of the pulse generator  108  and the stimulation electrodes  102 . However, the switch  116  can be inserted within any part of the conductive loop formed by the implanted medical device for proper operation of the present. invention. In the preferred embodiment of the present invention, the switch  116  and a sensor circuit  118  used with that switch are disposed inside the implantable pulse generator  108 . 
     The switch  116  is opened to prevent flow of unwanted current induced in the conductive loop by an external signal. The switch can be opened automatically upon sensing a significant level of external signal. The sensor circuit  118  is operatively connected to the switch  116  to open that switch when the sensor circuit senses a significant level of the external signal. 
     The sensor circuit  118  can sense the level of the external signal directly, and in that case, the sensor circuit can be located anywhere on or near the human body  106 . In an alternative embodiment, the sensor can detect the level of unwanted current within the conductive loop formed by the medical device. A significant level of this unwanted current is an indirect indication of the presence of a significant level of external signal. Alternatively, the switch  116  can be a electromagnetically sensitive switch that opens when the level of the external signal is greater than an external signal threshold level, and in that case, the sensor circuit  118  is not required. 
     The switch closes when the level of the external signal is less than the external signal threshold level. External signals such as radio frequency signals are prevalent in the surroundings of a user. However, when the level of external signal exceeds the external signal threshold level, that external signal may induce enough unwanted current in the closed loop to cause undesirable physiological effects. 
     In an alternative embodiment, the user having the implanted medical device can actively open the switch  116 . For example, if the switch were magnetically sensitive, then when the user is about to enter an area where the level of the external signal is greater than the external signal threshold level, the user brings a magnet near the switch  116  in order to manually open the switch  116 . When the user perceives that a significant level of external signal is no longer present, the user again brings the magnet near the switch  116  to close the switch  116 . In that embodiment also, the sensor circuit  118  is not required. 
     Referring to FIG. 2, two common means for implementing the switch  116  in CMOS IC technology are the NMOSFET  200  and the PMOSFET  250 . The NMOSFET (N-Channel Metal Oxide Semiconductor Field Effect Transistor) has a cross section  200 A, and the PMOSFET (P-Channel Metal Oxide Semiconductor Field Effect Transistor)  250  has a cross section  250 A. 
     The cross section  200 A of the NMOSFET shows a N-type doped drain region  202 A that forms a first drain terminal  202 , a conductive gate region  204 A that forms a first gate terminal  204 , and a N-type doped source region  206 A that forms a first source terminal  206 . The conductive gate region  204 A sits on top of a silicon dioxide layer  210 . Also, a P-well region  208 A that forms a P-well terminal  208  is also shown. The P-well is a P-type doped region and sits within a N-type substrate region  212 . 
     Similarly, the cross section  250 A of the PMOSFET shows a P-type doped source region  252 A that forms a second source terminal  252 , a conductive gate region  254 A that forms a second gate terminal  254 , and a P-type doped drain region  256 A that forms a second drain terminal  256 . The conductive gate region  254 A sits on top of a silicon dioxide layer  260 . Those P-type doped drain and source regions sit within a N-type doped substrate region  258 A that forms a N-substrate terminal  258 . With the use of such devices, the switch  116  and the sensor circuit  118  for controlling the switch are located within the implantable pulse generator  108 . 
     Either the NMOSFET or the PMOSFET can be inserted as the switch  116  of FIG.  1 B. The gate terminal of such a switch device is controlled to close the conductive loop when the electrically excitable tissue is stimulated for therapeutic effect. Also the gate terminal of such a switch device is controlled to open the conductive loop when the user is about to enter an area having a significant level of external signal. 
     However, if only a switch device such as one of the NMOSFET or the PMOSFET were inserted in the conductive loop, current may still be induced in the conductive loop even when the switch device is turned off. Parasitic current can still flow in such a switch device even when the switch device is turned off Referring to FIGS. 1B and 2, when a significant level of an external signal such as a time alternating electromagnetic field is applied on the medical device  100  implanted within the body  106 , current may be induced in the conductive loop formed by the implanted medical device. 
     When the switch is opened to limit current flow, a charge build-up can cause a voltage change at points in the conductive loop. For example, the case holding the implantable pulse generator  108  has been shown to have a voltage build-up as high as ±8 Volts when a significant level of external signal is present. Assume that the drain terminal of a NMOSFET were operatively connected to that case and that −8V were to build up on that case. Then, referring to the cross section  200 A of the NMOSFET, an unwanted parasitic current may flow from a forward-biased P-N junction formed by the P-well region  208 A and the N-type doped drain region  202 A that is operatively connected to the case. Similarly, assume that the drain terminal of a PMOSFET were operatively connected to that case and that +8V were to build up on that case. Then, referring to the cross section  250 A of the PMOSFET, an unwanted parasitic current may flow from a forward-biased P-N junction formed by the N-type substrate region  258 A and the P-type doped drain  252 A that is operatively connected to the case. 
     Thus, simply a NMOSFET or a PMOSFET may not sufficiently limit the unwanted current that may be induced by an external signal. FIG. 3 shows a current limiting circuit  300  that can limit unwanted current even when there is a voltage build-up at a node within the conductive loop. The current limiting circuit  300  of FIG. 3 comprises a switch  302  shown within dashed lines and a control circuit  304  shown within dashed lines. 
     The switch  302  includes a PMOSFET  306  as a first switch device and a NMOSFET  308  as a second switch device. The PMOSFET has a first drain terminal  310 , a first gate terminal  312 , a first source terminal  314 , and a N-type substrate terminal  316 . The NMOSFET has a second drain terminal  318 , a second gate terminal  320 , a second source terminal  322 , and a P-well terminal  324 . The PMOSFET and the NMOSFET have the same cross section as those shown in FIG.  2 . The first source terminal  314  of the PMOSFET is operatively connected to a positive power supply  326  via a first coupling capacitor  328 . The N-substrate terminal  316  of the PMOSFET is also operatively connected to the positive power supply. The first drain terminal  310  of the PMOSFET is operatively connected to the second source terminal  322  of the NMOSFET at a node  330 . 
     The second drain terminal  318  of the NMOSFET is operatively connected to the output node  332  of the current limiting circuit  300 . This output node is operatively connected to the implanted medical device. For example, in the implanted electrical stimulation system of FIG. 1B, the current limiting circuit  300  is part of the implantable pulse generator, and the output node  332  is operatively connected to the case of the implantable pulse generator  108 . The P-well terminal  324  of the NMOSFET  308  is operatively connected to the second source terminal  322 . 
     The control circuit  304  is operatively connected to the first gate terminal  312  of the PMOSFET  306  and to the second gate terminal  320  of the NMOSFET  308  to bias those switch devices to be on or off. The control circuit includes a first NOR gate  334  having a first control terminal  336  as an input and a second control terminal  338  as an input. The control circuit also includes a second NOR gate  340  having the second control terminal  338  as an input and a third control terminal  342  as an input. The output of the first NOR gate  334  and the output of the second NOR gate  340  are inputs to a third NOR gate  344 . 
     The output of the third NOR gate  344  is operatively connected to the first gate terminal  312  of the PMOSFET  306  through a first inverter  346  and a second inverter  348 . The output of the third NOR gate  344  is also operatively connected to the second gate terminal  320  of the NMOSFET  308  through a third inverter  350  and through a pair of a second coupling capacitor  352  and a third coupling capacitor  354  which are connected in parallel. 
     The control circuit also includes a biasing NMOSFET  356  having a third drain terminal  358 , a third gate terminal  360 , a third source terminal  362 , and a third P-well terminal  364 . The drain terminal of that transistor is diode connected to its gate terminal, and both its drain and gate terminals are connected to the positive power supply  326 . In addition, the P-well terminal  364  of that transistor is operatively connected to the positive power supply to form another forward biased diode connected between the P-well and the N-type doped source of NMOSFET  356 . That source terminal is operatively connected to the second gate terminal  320  of NMOSFET  308 . 
     The operation of the current limiting circuit  300  of FIG. 3 is now described. Referring to the implanted medical device  100  of FIG. 1B, the output node  332  of the current limiting circuit  300  is operatively connected to the case of the implantable pulse generator  108 . The positive power supply  326  is also part of the implantable pulse generator which is operatively coupled to the stimulation electrodes  102 . Thus, the switch  302  is connected as part of the conductive loop formed by the implanted electrical stimulation system  100 . The switch is then connected in series between the stimulation electrodes and the case of the implantable pulse generator  108  in that conductive loop. 
     The current limiting circuit operates to close the conductive loop by turning on both the PMOSFET  306  and the NMOSFET  308  of the switch  302  when the implanted medical device stimulates the electrically excitable tissue to achieve a therapeutic effect. The conductive loop is opened by introducing a high impedance at the output node  332  when the level of external signal such as a time-alternating electromagnetic field is greater than an external signal threshold level. This high impedance is generated by maintaining both of the PMOSFET  306  and the NMOSFET  308  off whenever the level of an external signal is greater than the external signal threshold level. 
     The first control terminal  336  receives a first control signal, and the third control terminal  342  receives a second control signal. The second control terminal  338  receives an over-sense control signal. The first control signal goes low when stimulation signals are applied on the electrically excitable tissue to source current into the tissue. The second control signal goes low when stimulation signals are applied on the electrically excitable tissue to sink current out of the tissue. This electrical stimulation of the tissue by the implanted medical device causes beneficial therapeutic effects. Accordingly, the conductive loop formed by the implanted medical device is closed during this stimulation process. 
     Referring to FIG. 3, for now assume that the over-sense control signal applied on the second control terminal  338  is tied low. Then, when either one of the first control signal applied on the first control terminal  336  or the second control signal applied on the second control terminal  342  goes low, the output of the third NOR gate  344  goes low. In turn, the first gate terminal  312  of the PMOSFET  306  goes low, and the second gate terminal  320  of the NMOSFET  308  goes high. The diode formed by the diode connection of the biasing NMOSFET  356  turns off. A low voltage on the gate terminal of the PMOSFET  306  turns on that PMOSFET. 
     A high voltage on the gate terminal of the NMOSFET  308  turns on that NMOSFET. When the output of the third NOR gate  344  was high, the diode formed by the biasing NMOSFET  356  was turned on, and the voltage at the gate terminal of the NMOSFET  308  was at the positive power supply voltage V DD  minus the diode drop of the NMOSFET  356  V th , (V DD −V th ). Then when the output of the third NOR gate  344  goes low, the voltage at the output of the inverter  350  goes high, the diode formed by NMOSFET  356  turns off, and the voltage on the gate terminal of NMOSFET  308  rises from V DD −V th  to V DD −V th +V DD =2V DD −V th . This high voltage on the gate terminal of NMOSFET  308  ensures that this NMOSFET remains on in the case either one of the first control signal applied on the first control terminal  336  or the second control signal applied on the second control terminal  342  goes low. 
     In this manner, both the PMOSFET and the NMOSFET within the switch  302  are on. In that case, the conductive loop is closed such that current flows within that loop to achieve a therapeutic effect. 
     The conductive loop formed by the implanted medical device is opened with a high impedance introduced into the conductive loop when the electrically excitable tissue is not being stimulated for therapeutic effect. In that condition, both the first control signal and the second control signal are set high. With those control signals, the output of the third NOR gate  344  goes high. 
     In turn, the first gate terminal  312  of the PMOSFET  306  goes high. The diode formed by the diode connection of the biasing NMOSFET  356  turns on. Thus, the second gate terminal  320  of the NMOSFET  308  is at a diode voltage drop, Vth, from the voltage of the positive power Supply, V DD . A high voltage on the gate terminal of the PMOSFET  306  turns that PMOSFET off. A voltage of a gate to source voltage drop V th  of the diode-connected transistor  356  from the voltage on the positive power supply, V DD , applied on the gate terminal of the NMOSFET  308  keeps that NMOSFET off. 
     In addition, the current limiting circuit  300  of FIG. 3 maintains a high impedance in the conductive loop formed by the implanted medical device even when there is a voltage build-up at one of the nodes in the conductive loop. In the current limiting circuit  300 , the output node  332  of the switch  302  is lied to the conductive case of the implantable pulse generator  108  of FIG.  1 B. When an external signal such as a time-alternating magnetic field is present, induced current in the conductive loop can cause a voltage build-up on the conductive case of the implantable pulse generator. This voltage build-up can result in voltages as high as ±8V at the case. In contrast to the simple switch devices of FIG. 2, the current limiting circuit  300  can maintain the high impedance within the conductive loop even with such a voltage build-up. 
     For example, with the output node  332  tied to the case of the pulse generator  108 , assume that the voltage on that case and in turn on the output node has built up to −8V. Referring to FIGS. 2 and 3, with such a high negative voltage on the output node, the NMOSFET  308  may have a forward biased PN junction between the P-well region and the N-type doped drain of that NMOSFET. Thus, unwanted current flows in that parasitic path. In addition, since MOSFET devices in CMOS technology are symmetrical, the N-type doped region of NMOSFET  308  tied to the output node can act as a source. With −8V applied on that terminal, NMOSFET  308  may turn on. 
     However, note that the PMOSFET  306  of the switching circuit is kept off with its source to gate voltage V SG  still being sufficiently low. This PMOSFET is in the path of the parasitic PN junction of the NMOSFET  308  and in the path of the drain to source terminals of that NMOSFET. Thus, the PMOSFET blocks the unwanted parasitic current caused by the high negative voltage build-up on the case of the implantable pulse generator  108 . 
     Similarly, assume that the voltage on the output node  332  has built up to a high positive voltage such as +8V. Referring to FIGS. 2 and 3, with such a high positive voltage, the PMOSFET  306  has a parasitic forward biased PN junction between the P-type doped drain region and the N-substrate region. Thus, unwanted current flows in that parasitic path. 
     However, note that NMOSFET  308  is in the path of that parasitic PN junction. The NMOSFET blocks the parasitic current in that PN junction. Note that the N-substrate  316  of the PMOSFET is tied to the positive power supply. Thus, for the parasitic PN junction to conduct that unwanted current, the drain terminal  310  of the PMOSFET must be set at a voltage higher than the voltage of the positive power supply V DD  by a threshold voltage of that PN junction, V th . 
     That drain terminal  310  of the PMOSFET  306  is operatively connected to the source terminal  322  of the NMOSFET  308 . The gate terminal  320  of the NMOSFET is set at a diode voltage drop which is typically equal to a threshold voltage of that diode junction, V th , from the positive power supply. If the drain terminal  310  of the PMOSFET (which is also the source terminal  322  of the NMOSFET) were to go above V DD +V th  such that the parasite PN junction of the PMOSFET turns on, then the gate to source voltage V GS  of the NMOSFET becomes too low for the NMOSFET to turn on. In that case, the NMOSFET turns off; and the NMOSFET which is in the path of the parasitic PN junction of the PMOSFET blocks the unwanted current flow through that parasitic PN junction. Thus, unwanted current, caused by the high positive voltage build-up on the case of the implantable pulse generator  108 , is prevented. 
     In this manner, the current limiting circuit  300  blocks any unwanted current flow that may be induced in the conductive loop formed by the implanted medical device. Thus, when this circuit is inserted in that conductive loop, the switch  302  is kept off to introduce and maintain a high impedance within that conductive loop. 
     Thus far, the over-sense control signal on the second control terminal  338  has been tied low. In that state, only the first and second control signals dictate whether the switch  302  is on or off. Those control signals close the conductive loop when stimulation signals are applied on the electrically excitable tissue for a therapeutic effect and open the conductive loop otherwise. However, with those signals alone, the conductive loop is closed to stimulate the electrically excitable tissue for therapeutic effect even when a significant level of external signal is present When an external signal is applied on that closed conductive loop, unwanted current may be induced in the conductive loop which can cause undesirable physiological effects. 
     Thus, the conductive loop is opened when an external signal is present no matter what the first and second control signals may be. The over-sense control signal applied on the second control terminal  338  allows for an override of the first and second control signals. Thus far, the over-sense control signal has been tied low such that only the first and second control signals dictate whether the switch  302  is on or off. 
     However, when the over-sense control signal is set high, this signal sets the output of the third NOR gate  344  high no matter what the first and second control signals may be. As discussed previously, when the output of the third NOR gate is high, the switch turns off and introduces a high impedance into the conductive loop to open the conductive loop. Thus, by setting the over-sense control signal applied on the second control terminal  338  high, the conductive loop is opened no matter what the first and second control signals may be. In this manner, the conductive loop is opened when a significant level of an external signal such as a time-alternating electromagnetic field is present even if the conductive loop were closed for therapeutic effect. 
     This over-sense control signal can be generated by two means. First, the user can actively set the over-sense control signal high when the user is about to enter an area where the level of external signal is likely to be greater than an external signal threshold level. For example, in the implanted medical device system  100  of FIG. 1B, the user can send a control signal via the programmer  112  to set the over-sense control signal high. In this manner, the user controls the opening of the conductive loop before the external signal having a level which is greater than the external signal threshold level causes unwanted physiological effects. 
     Alternatively, a high over-sense control signal is automatically generated when an external signal is sensed by using an external signal sensor. Such a sensor can include a field sensing coil, a chattering reed switch, or a hall effect sensing switch. These sensors are commonly available and act as a switch to toggle between two states when the level of a magnetic field is greater than the external signal threshold level and when the level of a magnetic field is less than the external signal threshold level. 
     In an alternative embodiment of the present invention, the external signal sensor is implemented using CMOS technology devices. Such an external signal sensor circuit  400  is shown in FIG.  4 . This circuit includes a first NMOSFET device  402  and a second NMOSFET device  404  that form matched constant current sources. The gate terminals of both of those devices are tied to a biasing circuit that can provide a NBIAS signal to appropriately bias those NMOSFET devices according to the common art of transistor circuit design. A first PMOSFET device  406  and a second PMOSFET device  408  form a current source mirror pair that also operates as a common gate differential current amplifier according to the common art of transistor circuit design. The source terminal  410  of the second PMOSFET  408  is operatively connected to the node  330  of FIG.  3 . 
     A NAND-gate  412  and a first inverter  414  effectively form an AND gate with the output of that gate being operatively connected to the common source node of NMOSFETs  402  and  404 . A first NAND input terminal  416  is tied to the first control terminal  336  of FIG. 3, and a second NAND input terminal  418  is tied to the third control terminal  342  of FIG.  3 . 
     The output of the differential amplifier pair of PMOSFETs is at the drain terminal of the second PMOSFET  410 . A positive feedback loop, including a second inverter  420 , a third inverter  422 , and a charging capacitor  424 , is operatively connected between the output of the differential amplifier and a over-sense node  426 . This over-sense node is operatively connected to the second control terminal  338  of the current limiting circuit  300  of FIG.  3  and provides the over-sense control signal. A current pulling NMOSFET  428  is operatively connected at the output of the differential amplifier. The gate of that device is also tied to the biasing circuit that can provide the NBIAS signal to appropriately bias that NMOSFET. 
     The operation of the external signal sensor circuit  400  is now described referring to both FIGS. 3 and 4. The first NAND input terminal  416  which is tied to the first control terminal  336  of FIG. 3 receives the first control signal. The second NAND input terminal  418  which is tied to the third control terminal  342  of FIG. 3 receives the second control signal. The overall functional purpose of the external signal sensor circuit is to automatically open the conductive loop formed by an implanted medical device when the level of an external signal is greater than the external signal threshold level. Thus, the operation of this circuit is not critical when the conductive loop is already open in the case the conductive loop is not stimulating the electrically excitable tissue for therapeutic effect. 
     In that case, the first and second control signals are both set high, and the output of the first inverter  414  is set high. Because of a high voltage at the source terminals of the NMOSFETs  402  add  404  of the matched constant current sources, those NMOSFETs are turned off which consequently turns off the pair of PMOSFETs  406  and  408 . The current pulling NMOSFET  428  is still on and discharges the output node of the differential amplifier which in turn sets the over-sense node  426  low. The over-sense signal is set low because with the first and second control signals set high, the conductive loop is already opened. 
     In contrast, in the case the conductive loop is already closed to stimulate the electrically excitable tissue for therapeutic effect, the output of the first inverter  414  is set low since either the first control signal or the second control signal is set low. With a low voltage at the source terminals of the NMOSFETs  402  and  404 , those NMOSFETs are turned on. Consequently, the pair of PMOSFETs  406  and  408  are turned on. Thus, the matched constant current sources of the NMOSFETs  402  and  404  are appropriately biased to conduct current. 
     Note that the source terminal of the second PMOSFET  408  is operatively connected to the node  330  of FIG.  3 . This node has a change in voltage if an external signal is present to cause a change in the current flowing in the conductive loop. Thus, the external signal sensor circuit  400  detects a significant level of unwanted current in the conductive loop that indirectly indicates the presence of a significant level of external signal. 
     Typically, the external signal is an AC signal, and the voltage change at the node  330  goes up and down along with the external AC signal. In the external signal sensor circuit  400  of FIG. 4, the differential amplifier senses a first positive change in the voltage at the node  330 . With that positive voltage change, the current flowing through the second PMOSFET  408  increases. This increase in current charges up the output node of the differential amplifier pair to drive the over-sense node  426  high When the level of external signal is above an external signal threshold level, the voltage on the over-sense node charges up to a high enough level which opens the conductive loop when applied on the second control terminal  338  of FIG.  3 . 
     The positive feedback loop, which includes the second inverter  420 , the third inverter  422 , and the charging capacitor  424 , provides positive feedback for coupling any positive change at the output of the differential amplifier to the over-sense node  426 . This positive feedback loop, which may include a Schmitt trigger inverter as one of the second and third inverters, can cause an immediate voltage change on the over-sense node when the voltage goes positive. The charging capacitor  424  in this loop keeps the over-sense node positive once that node has gone positive even when the AC external signal causes a negative voltage change at the node  330  of FIG.  3 . 
     In this manner, the external signal sensor circuit  400  of FIG. 4 can generate an over-sense control signal to be applied on the second control terminal  338  of the current limiting circuit  300  of FIG.  3 . The external signal sensor circuit  400  of FIG. 4 determines when a significant level of external signal is present to automatically ensure that the conductive loop is opened. Thus, unwanted current that may be induced in the conductive loop by the external signal is automatically limited using the external signal sensor circuit. 
     The advantages of the invention described herein can be generalized to any partially or fully implanted medical device that forms a conductive loop with the electrically excitable tissue. In addition, the invention can be generalized to medical devices implanted in any living organism having electrically excitable tissue that can be electrically stimulated for therapeutic effect. Moreover, the invention can be generalized to any means for introducing a high impedance into that conductive loop when an external signal such as a time-alternating electromagnetic field is present. Also, the invention can be generalized to prevent a build-up of any level of unwanted voltages in the implanted medical device. The level of unwanted voltages that can be handled by the present invention is limited by the breakdown voltage of the specific technology that is used in the implementation of the present invention. Accordingly, the forgoing description is by way of example only. The invention is limited only as defined in the following claims and equivalents thereof.