Abstract:
An implantable medical device is provided for isolating an elongated medical lead from internal device circuitry in the presence of a gradient magnetic or electrical field. The device includes an isolation circuit adapted to operatively connect an internal circuit to the medical lead in a first operative state and to electrically isolate the medical lead from the internal circuit in a second operative state.

Description:
TECHNICAL FIELD 
   The invention relates generally to implantable medical devices, and, in particular, to a method and apparatus for electrically isolating leads coupled to an implantable medical device from circuitry in the implantable medical device. 
   BACKGROUND 
   Numerous types of implantable medical devices (IMDs), such as cardiac pacemakers, implantable cardiovertor defibrillators (ICDs), neurostimulators, operate to deliver electrical stimulation therapies to excitable body tissue via associated electrodes. The electrodes are disposed at a targeted therapy delivery site and are commonly coupled to the IMD via conductors extending through elongated leads. Patients implanted with such IMDs are generally contraindicated for undergoing MRI procedures. The gradient magnetic fields that may be applied during an MRI procedure can induce current on the elongated lead conductors, which can be large enough to cause undesired stimulation of the excitable tissue in contact with the electrode(s) carried by the lead. As the number of patients having IMDs continues to grow, it is desirable to provide IMD systems that allow patients to safely undergo MRI procedures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of an IMD coupled to a patient&#39;s heart via a cardiac lead. 
       FIG. 2  is a functional block diagram of an IMD including isolation circuitry. 
       FIG. 3  is a timing diagram illustrating IMD function during a gradient field operating mode. 
       FIG. 4  is a flow chart summarizing one method for controlling isolation circuitry included in an IMD. 
   

   DETAILED DESCRIPTION 
   In the following description, references are made to illustrative embodiments for carrying out the invention. It is understood that other embodiments may be utilized without departing from the scope of the invention. The invention is generally directed toward providing an IMD and an associated method for protecting a patient from unwanted tissue stimulation due to current induced on implanted leads in the presence of time-varying magnetic or electrical fields, such as during MRI procedures involving gradient magnetic fields or in the presence of time-varying electrical fields associated with electronic article surveillance systems (EAS). As used herein, the term “gradient field” refers to any time varying magnetic or electrical field that is strong enough to induce current on an implanted lead and potentially cause tissue stimulation. As used herein, the term “module” refers to an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, or other suitable components that provide the described functionality. 
     FIG. 1  is a schematic diagram of an IMD coupled to a patient&#39;s heart via a cardiac lead. IMD  10  is shown as a single chamber cardiac device, however it is recognized that various embodiments of the present invention may be implemented in single, dual, or multi-chamber cardiac devices or single or multi-channel neurostimulators. Embodiments of the present invention include IMDs provided as monitoring devices without therapy delivery capabilities. IMDs provided with therapy delivery capabilities may include, for example, cardiac pacemakers, cardioverter/defibrillators, drug delivery devices, and neurostimulators. 
   IMD  10  is embodied as an implantable cardioverter defibrillator (ICD) and is coupled to lead  30  for sensing cardiac signals and delivering electrical stimulation pulses to the heart in the form of cardiac pacing pulses and cardioversion/defibrillation shock pulses. Lead  30  is provided with a tip electrode  42  and ring electrode  44  which are generally used together for bipolar sensing and/or pacing functions or in combination with IMD housing  12  for unipolar sensing and/or pacing functions. Lead  30  also includes a right ventricular coil electrode  46  and a superior vena cava coil electrode  48  used in delivering high-voltage cardioversion and defibrillation shocks. 
   Each of the electrodes  42 ,  44 ,  46  and  48  are coupled to individual connectors  34 ,  36 ,  38  and  40  included in a proximal lead connector assembly  32  via conductors extending through elongated lead body  31 . The lead connector assembly  32  is adapted for insertion into a connector bore provided in connector header  14  of IMD  10 . Electrode terminals  50 ,  52 ,  54  and  56  included in connector header  14  are electrically coupled to lead connectors  34 ,  36 ,  38  and  40  when lead connector assembly  32  is fully inserted in the connector header bore. 
   Electrode terminals  50 ,  52 ,  54  and  56  are electrically coupled to internal IMD circuitry  16 , enclosed in hermetically sealed IMD housing  12 . Electrode terminals  50 ,  52 ,  54  and  56  are coupled to internal circuitry  16  via isolation circuitry  60  and protection circuitry  18 , shown schematically in  FIG. 1 . The actual physical location of isolation circuitry  60  and protection circuitry  18  may be anywhere between electrode terminals  50 ,  52 ,  54 , and  56  and any portion of the internal circuitry  16 . The functionality of isolation circuitry  60  may be implemented using dedicated components or providing dual functionality of existing switching devices included in IMD  10 . 
   Isolation circuitry  60  provides protection to the patient against unwanted tissue stimulation due to current induced on conductors carried by lead body  31 . For example, in an MRI environment involving gradient magnetic fields, current induced on lead conductors can be carried along a circuit path that includes lead  30 , the IMD housing  12 , and body tissue. Isolation circuitry  60  interrupts this circuit path by introducing a high-impedance element as will be described in greater detail herein. Protection circuitry  18  is generally grounded to IMD housing  12  thereby providing a path from electrode terminals  50 ,  52 ,  54 , and  56  to the IMD housing  12 , completing the circuit pathway through the patient&#39;s body along which induced currents may be conducted. Isolation circuitry  60  is provided to open that circuit pathway to prevent unwanted tissue stimulation in an MRI or other gradient field environment. 
   Protection circuitry  18  is provided for eliminating or minimizing electromagnetic interference (EMI) that may be encountered in normal operating environments. EMI can produce a potential between any of electrodes  42 ,  44 ,  46  and  48  and housing  12 . Circuit elements and parasitic effects provide paths for current to flow as a result of these potentials. Protection circuitry  18  prevents EMI from being coupled to the internal circuitry  16 , which may otherwise cause inappropriate IMD function. Protection circuitry  18  typically includes electrically insulated, filtered feedthroughs such that electrical connections made between electrode terminals  50 ,  52 ,  54 , and  56  and internal circuitry  16  are electrically isolated from IMD housing  12 . The filtered feedthroughs typically include capacitive elements for filtering EMI. Examples of protection circuitry included in IMDs are generally disclosed in U.S. Pat. No. 5,759,197 (Sawchuk, et al.) and U.S. Pat. No. 6,414,835 (Wolf et al.), both of which patents are hereby incorporated in their entirety. Protection circuitry  18  may include other noise-reduction and protection networks for static discharge and other transient voltages that may arise due to EMI. 
     FIG. 2  is a functional block diagram of an IMD including isolation circuitry. IMD  10  generally includes timing and control circuitry  152  and an operating system that may employ microprocessor  154  or a digital state machine for timing sensing and therapy delivery functions in accordance with a programmed operating mode. Microprocessor  154  and associated memory  156  are coupled to the various components of IMD  10  via a data/address bus  155 . IMD  10  includes therapy delivery unit  150  for delivering an electrical stimulation therapy, such as cardiac pacing therapies, under the control of timing and control  152 . Therapy delivery unit  150  is typically coupled to two or more electrode terminals  168  via switch/multiplexer  158 . Switch/MUX  158  is used for selecting which electrodes and corresponding polarities are used for delivering electrical stimulation pulses. 
   Electrode terminals  168  may also be used for receiving electrical signals from the body, such as cardiac signals or other electromyogram (EGM) signals, or for measuring impedance. In the case of cardiac stimulation devices, cardiac electrical signals are sensed for determining when an electrical stimulation therapy is needed and in controlling the timing of stimulation pulses. 
   Electrode terminals  168  are typically included in a connector header as described in conjunction with  FIG. 1 . Electrode terminals  168  may be electrically coupled to switch/MUX  158  via the isolation circuit  180  and any EMI protection circuitry  182 . The remaining functional blocks shown in  FIG. 2  are typically implemented on a hybrid circuit board having contact pads for making electrical connections to protection circuitry  182 . Isolation circuitry  180  may be implemented anywhere between electrode terminals  168  and the connections to the various components included on a hybrid circuit board. 
   Isolation circuitry  180 , shown as a functional block in  FIG. 2 , may include switching elements physically located at separate locations relative to the hybrid circuit board and IMD housing. If isolation of an associated lead from all IMD circuitry is desired, isolation circuitry  180  could be located outside the IMD housing or contained within a separate Faraday shield within the IMD housing. In other embodiments, isolation circuitry  180  may include switches used to isolate only portions of the hybrid circuitry from an associated lead and might include switching elements incorporated on the hybrid circuit board. 
   Switching elements already present in the IMD circuitry may be utilized to provide the isolation circuit functionality as well as other functions. For example, IMD  10  may be provided with switches used for protecting IMD circuitry from voltages produced by external defibrillation. Switches  185   a  through  185   d  may include such switches. In other words, any of switches  185   a  through  185   d  serving functionally as a part of isolation circuit  180  for protecting the patient from induced current in the presence of time-varying EM fields may be embodied as a switch already provided in IMD  10  for protecting the IMD circuitry from voltages produced by external defibrillation. 
   Electrodes used for sensing and electrodes used for stimulation may be selected via switch matrix  158 . When used for sensing, electrode terminals  168  are coupled to signal processing circuitry  160  via switch matrix  158 . Signal processor  160  includes sense amplifiers and may include other signal conditioning circuitry and an analog to digital converter. Electrical signals may then be used by microprocessor  154  for detecting physiological events, such as detecting and discriminating cardiac arrhythmias. 
   In some embodiments, microprocessor  154  uses signals received at electrode terminals  168  for automatically detecting induced signals associated with a gradient field, such as a time-varying magnetic field associated with MRI. Alternatively, a gradient field sensor circuit  186  may be provided for sensing external signals corresponding to a time-varying MRI or other gradient field environment. Gradient field sensor circuit  186  may be embodied according to the sensor circuit generally disclosed in U.S. Pat. No. 6,198,972 (Hartlaub et al.), hereby incorporated herein by reference in its entirety. Gradient field sensor circuit  186  may be located anywhere in a patient&#39;s body and may therefore alternatively be coupled to IMD circuitry via a sensor terminal  170 . In response to a gradient field detection signal generated by gradient field sensor circuit  186 , microprocessor  154  causes timing and control circuitry  152  to generate a signal on signal line  184  that opens switches  185   a  through  185   d  included in isolation circuitry  180 . The circuit path through the IMD housing and the patient&#39;s body is effectively opened thereby preventing unwanted tissue stimulation due to induced currents on implanted leads coupled to IMD  10 . During MRI, a sensor that detects the very strong static magnetic field may be used alone or in conjunction with other sensors to activate isolation circuitry  180   
   IMD  10  may additionally or alternatively be coupled to one or more physiological sensors. As such, physiological sensor terminals  170  are provided and are electrically coupled to a sensor interface  160  via protection circuitry  182 . Sensor terminals  170  may also be electrically coupled to IMD circuitry, or portions of IMD circuitry, through isolation circuitry  180  when terminals  170  are coupled to elongated leads that could carry induced currents to body tissue. Physiological sensors may include pressure sensors, accelerometers, flow sensors, blood chemistry sensors, activity sensors or other physiological sensors known for use with IMDs. 
   Signals received at sensor terminals  170  are received by a sensor interface  162  which provides sensor signals to signal processing circuitry  160 . Sensor signals are used by microprocessor  154  for detecting physiological events or conditions. For example, IMD  10  may monitor heart wall motion, blood pressure, blood chemistry, respiration, or patient activity. Monitored signals may be used for sensing the need for delivering a therapy under control of the operating system. 
   The operating system includes associated memory  156  for storing a variety of programmed-in operating mode and parameter values that are used by microprocessor  154 . The memory  156  may also be used for storing data compiled from sensed physiological signals and/or relating to device operating history for telemetry out on receipt of a retrieval or interrogation instruction. All of these functions and operations are known in the art, and many are generally employed to store operating commands and data for controlling device operation and for later retrieval to diagnose device function or patient condition. 
   IMD  10  further includes telemetry circuitry  164  and antenna  128 . Programming commands or data are transmitted during uplink or downlink telemetry between IMD telemetry circuitry  164  and external telemetry circuitry included in a programmer or monitoring unit. Telemetry circuitry  164  and antenna  128  may correspond to telemetry systems known in the art. In one embodiment of the invention, a gradient field mode command is transmitted to IMD telemetry circuitry  164  by a clinician or other user using an external programmer. In response to the gradient field mode command, microprocessor  154  causes timing and control circuitry  152  to generate a signal on signal line  184  to open switches included in isolation circuitry  180 . 
   During a gradient field operating mode, electrode terminals  168  (and/or sensor terminals  170 ) are electrically disconnected from IMD circuitry by introducing a high-impedance element included in isolation circuitry  180 . Isolation circuitry  180  generally includes switches  185   a  through  185   d  which may be embodied as electro-mechanical relays, semiconductor devices, or MEMS relays. Switches  185  included in isolation circuitry  180  may be implemented as generally described in the above-incorporated Hartlaub patent. It is recognized that each of switches  185   a  through  185   d  may include one or more electronic switches coupled in series to form a high-impedance element through isolation circuitry  180 . The number of switches  185  included in isolation circuitry  180  will vary between applications and will correspond to the number of electrode terminals  168  and sensor terminals  170  that need to be electrically disconnected from the IMD ground path to prevent conduction of currents induced on elongated lead conductors during MRI procedures or in the presence of other gradient EM fields. 
   When microprocessor  156  determines that an electrical stimulation therapy is needed, or if an electrical stimulation therapy is in process upon initiation of the gradient field operating mode, timing and control circuitry  152  generates a transient “close” signal on signal line  184 . The “close” signal is generated just prior to or contemporaneously with the generation of an electrical stimulation pulse by therapy delivery unit  150 . A stimulation pulse generated by therapy delivery unit  150  is delivered to electrode terminals  168  across isolation circuitry  180 . The “close” signal causes at least one switch included in isolation circuitry  180  that corresponds to a selected stimulation electrode to briefly close so that the stimulation pulse can be delivered. Other switches included in isolation circuitry  180  may remain open during stimulation pulse delivery. Accordingly, it is understood that signal line  184  may carry a multiplexed signal for operating multiple switches included in isolation circuitry  180  individually. With regard to the embodiment shown in  FIG. 1 , a switch coupled to electrode terminal  56  corresponding to tip electrode  42  may be controlled separately from a switch coupled to electrode terminal  54 , corresponding to ring electrode  44  to allow unipolar stimulation using tip electrode  42  during a gradient field operating mode. 
   IMD  10  may optionally be equipped with patient alarm circuitry  166  for generating audible tones, a perceptible vibration, muscle stimulation or other sensory stimulation for notifying the patient that a patient alert condition has been detected by IMD  10 . In some embodiments, an alarm signal may be generated upon detection of a gradient field or upon initiating a gradient field mode of operation. 
     FIG. 3  is a timing diagram illustrating IMD function during a gradient field operating mode. At a time  202 , a gradient field mode is initiated in response to a gradient field operating mode command or the automatic detection of gradient field signals, corresponding to a time-varying MRI field or other gradient EM field, by a gradient field sensor circuit. Two biasing signals  205  and  215  are provided to individual switches, for example MOSFETs, included in isolation circuitry. Initially, prior to the initiation of gradient field operating mode at time  202 , the MOSFET switches are biased with high signals  208  and  214  that maintain the switches in a closed or ON operative state. For example, a MOSFET may be biased to 5.0 volts relative to circuit common to hold the transistor in the ON state to allow normal sensing and therapy delivery functions. 
   Upon initiation of the gradient field operating mode at time  202 , biasing signals  205  and  215  are switched to low signals  210  and  216  to open the corresponding MOSFETs to an OFF operative state. For example, the MOSFETs may be biased to 0.0 volts relative to circuit common to hold the transistor in the OFF state to prevent conduction of induced currents to excitable body tissue. A feedback or bootstrap network could be used to maintain the correct state of the MOSFET. 
   Pacing pulses  206   a ,  206   b ,  206   c  through  206   n  are delivered after the initiation of gradient field mode at time  202 . Pacing therapy may have been in progress at the time of initiating the gradient field mode or a need for pacing therapy may be detected during the gradient field mode using other sensors or circuits that are not opened by isolation circuitry. In pacing dependent patients, initiation of the gradient field mode may include maintaining a predetermined pacing rate. Reference is made to U.S. Pat. App. Pub. No. 2003/0144705 (Funke), hereby incorporated herein by reference in its entirety. Intrinsic cardiac signals may be sensed during the gradient field operating mode through high impedance signal path sensing channels or utilize a gradient energy cancellation sensing method. 
   In order to deliver pacing pulses  206   a  through  206   n , at least one switch (for unipolar pacing) included in isolation circuitry is transiently closed by generating a high biasing signal  212   a ,  212   b ,  212   c ,  212   n  at appropriate times relative to pacing pulses  206   a  through  206   n . In order to deliver bipolar pacing pulses, two switches may be transiently closed during pacing pulse delivery. Timing and control module  152  ( FIG. 2 ) controls the alternation between high and low bias signal levels applied to isolation circuit switches to control the operative state of the switches. A switch is closed to close a pacing or electrical stimulation circuit at appropriate times during the gradient field mode to allow therapeutic stimulation to be performed, for example during MRI procedures. The switch(es) included in a pacing or electrical stimulation circuit are briefly closed for an interval of time starting just prior to or approximately the same time as a stimulation pulse and extending for a time at least equal to the stimulation pulse width. While the timing diagram shown in  FIG. 3  illustrates the delivery of cardiac pacing pulses, it is recognized that any type of electrical stimulation pulses may be delivered during a gradient field mode by controlling the opening and closing of switches included in isolation circuitry. For example the need for high-voltage cardioversion/defibrillation shocks may be detected based on sensing intrinsic signals using a gradient energy cancellation method and high energy therapies may be delivered based on determining a reliable sensing signal for arrhythmia detection. 
     FIG. 4  is a flow chart summarizing one method for controlling isolation circuitry included in an IMD. Flow chart  300  is intended to illustrate the functional operation of the device, and should not be construed as reflective of a specific form of software or hardware necessary to practice the invention. It is believed that the particular form of software will be determined primarily by the particular system architecture employed in the device and by the particular detection and therapy delivery methodologies employed by the device. Providing software and/or hardware to accomplish the present invention in the context of any modern IMD, given the disclosure herein, is within the abilities of one of skill in the art. 
   The IMD microprocessor initiates a gradient field operating mode at block  306 . The gradient field operating mode is initiated in response to receipt of an external command provided to the IMD using a programmer or other device enabled for telemetric communication with the IMD (block  304 ). The gradient field operating mode is alternatively initiated in response to the detection of external or internal high level signals corresponding to an MRI or other time-varying EM environment by gradient field sensor circuit at block  302 . 
   Upon initiation of the gradient field mode, switches included in isolation circuitry are opened at block  310 . A gradient magnetic field may induce currents on implanted lead conductors large enough to cause tissue stimulation. Opening of isolation circuitry switches opens the circuit path through the capacitive feedthrough elements and the IMD housing and patient&#39;s body, preventing conduction of induced currents and unwanted tissue stimulation. 
   At decision block  314 , timing and control module  152  determines if a therapeutic stimulation pulse is needed based on programmed therapy delivery mode. Upon triggering the generation of a therapy stimulation pulse, timing and control  152  generates a signal to transiently close one or more isolation circuitry switches included in a stimulation circuit path in order to allow stimulation pulse delivery at block  318 . 
   Throughout the gradient field mode, the IMD microprocessor monitors for receipt of an external command indicating that a normal operating mode should be restored at decision block  322 . Additionally or alternatively, the IMD microprocessor automatically monitors for an end to the detection of gradient signals by a gradient field sensor. In other embodiments, the gradient field mode may be maintained for a fixed interval of time after gradient field mode initiation. For example, the gradient field mode may be maintained for 30 minutes, one hour, or another interval of time that is expected to extend safely beyond the completion of an MRI procedure. As long as the gradient field mode is maintained, timing and control module  152  continues to control transient closure of stimulation circuit path switches included in isolation circuitry contemporaneously with the generation of therapeutic stimulation pulses at block  318 . 
   Upon expiration of the gradient field mode according to a predetermined time interval, receipt of an external termination command, or loss of gradient field signal detection at decision block  318 , isolation circuitry switches are closed at block  326 . Closure of isolation switches restores normally closed sensing and stimulation circuit paths for normal IMD operation. The gradient field operation mode is then terminated at block  330 . 
   Thus, an IMD and associated methods for protecting a patient from unwanted tissue stimulation during exposure to time-varying electrical or magnetic fields has been presented in the foregoing description with reference to specific embodiments. It is appreciated that various modifications to the referenced embodiments may be made without departing from the scope of the invention as set forth in the following claims.