Abstract:
An implantable device used to monitor and maintain at least one physiologic function, which is capable of operating in the presence of damaging electromagnetic interference. The implantable device includes primary and secondary modules, each independently protected from EMI damage via at least one shielding and/or filtering, and a non-electrical communication device for communicating in at least one direction between the primary and the secondary modules. The primary module, in response to input from electrical sensing leads, activates the secondary module in a failsafe mode. In the failsafe mode, the secondary module carries out a physiologic function upon activation and in the presence of electromagnetic interference.

Description:
PRIOR PROVISIONAL APPLICATION 
     This application claims the benefit under 35 U.S.C. § 119( e ) of U.S. Provisional Patent Application Ser. No. 60/198,631, filed on Apr. 20, 2000. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an MRI-resistant implantable device. The implantable device of the present invention permits satisfactory performance in the presence of the electromagnetic fields emanated during magnetic resonance imaging (MRI) procedures. Patients provided with the present invention can undergo MRI procedures, and gain the benefits therefrom, while maintaining the use of the diagnostic and therapeutic functions of the implantable device. 
     BACKGROUND INFORMATION 
     Implantable devices such as implantable pulse generators (IPGs) and cardioverter/defibrillator/pacemaker (CDPs) are sensitive to a variety of forms of electromagnetic interference (EMI). These devices include sensing and logic systems that respond to low level signals from the heart. Because the sensing systems and conductive elements of these implantable devices are responsive to changes in local electromagnetic fields, they are vulnerable to external sources of severe electromagnetic noise, and in particular to electromagnetic fields emitted during magnetic resonance imaging (MRI) procedures. Therefore, patients with implantable devices are generally advised not to undergo MRI procedures. 
     With the exception of x-ray procedures, MRI procedures are the most widely applied medical imaging modality. Significant advances occur daily in the MRI field, expanding the potential for an even broader usage. There are primarily three sources of energy that could lead to the malfunction of an implantable device, during an MRI procedure. First, a static magnetic field is generally applied across the entire patient to align proton spins. Static magnetic field strengths up to 7 Tesla for whole body human imaging are now in use for research purposes. The increase in field strength is directly proportional to the acquired signal to noise ratio (SNR) which results in enhanced MRI image resolution. Consequently, there is impetus to increase static field strengths, but with caution for patient safety. These higher field strengths are to be considered in the development of implantable devices. 
     Second, for image acquisition and determination of spatial coordinates, time-varying gradient magnetic fields of minimal strength are applied in comparison to the static field. The effects of the gradients are seen in their cycling of direction and polarity. With present day pulse sequence design and advances in MRI hardware, it is not uncommon to reach magnetic gradient switching speeds of up to 50 Tesla/sec (this is for clinical procedures being used presently). Additionally, fast imaging techniques such as echo-planar imaging (EPI) and turbo FLASH are in use more frequently in the clinic. Non-invasive magnetic resonance angiography uses rapid techniques almost exclusively on patients with cardiovascular disease. Previous research evaluating the effects of MRI on pacemaker function did not include these fast techniques. Therefore, the use of MRI for clinical evaluation for individuals with implantable cardiac devices may be an issue of even greater significance. Rapid MRI imaging techniques use ultra-fast gradient magnetic fields. The polarities of these fields are switched at very high frequencies. This switching may damage implantable devices or cause them to malfunction. 
     Lastly, a pulsed RF field is applied for spatial selection of the aligned spins in a specimen during an MRI procedure. FDA regulations relative to the power limits of the RF fields are in terms of a specific absorption rate (SAR), which is generally expressed in units of watts per kilogram. These limits may not consider the effects on implantable devices as the deleterious effects of transmission of RF fields in the MRI system may no longer be the primary concern in their design parameters. 
     While advancements in techniques used to protect implantable devices from MRI fields have been made, the techniques described mainly concern incorporating additional protective circuitry in the implantable devices or providing alternative modes of operation in response to electromagnetic insult. For example, U.S. Pat. No. 5,217,010 to Tsitlik et al. describes the use of inductive and capacitive filter elements to protect internal circuitry; U.S. Pat. No. 5,968,083 to Ciciarelli et al. describes switching between low and high impedance modes of operation in response to EMI insult; and U.S. Pat. No. 6,188,926 to Vock concerns a control unit for adjusting a cardiac pacing rate of a pacing unit to an interference backup rate when heart activity cannot be sensed due to EMI. 
     However, the techniques described do not provide a fail-safe system in the case that the protective circuitry or the alternative modes of the implantable device fails to protect the implantable device from malfunction due to exposure to electromagnetic fields. What is needed is a modular backup system that is resistant to electromagnetic insult and can support the basic functionality of the implantable device, so that if the device fails to function for a duration, such as during an MRI procedure, the backup system can provide the necessary assistance functions. 
     SUMMARY OF THE INVENTION 
     The present invention provides an implantable device that is resistant to electromagnetic interference comprising first and second modules and a non-optical arrangement for communication between the first module and the second module. During a normal operating mode the first module performs physiologic functions and the second module is deactivated. When electromagnetic interference is detected, the second module, which is resistant to EMI insult, is activated and the first module is deactivated to further protect its components from EMI. 
     The present invention also provides an implantable device used to monitor and maintain at least one physiologic function, which is capable of operating in the presence of damaging electromagnetic interference. The implantable device includes primary and secondary modules, each independently protected from EMI damage via at least one shielding and/or filtering, and a non-electrical communication device for communicating in at least one direction between the primary and the secondary modules. The primary module, in response to input from electrical sensing leads, activates the secondary module in a failsafe mode. In the failsafe mode, the secondary module carries out a physiologic function upon activation and in the presence of electromagnetic interference. 
     In an advantageous embodiment, the physiologic function performed by the implantable device is a cardiac assist function, and the implantable device is a cardiac assist device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a cross-section of an implantable device according to an embodiment of the present invention. 
     FIG. 2 is a block diagram showing functional components of an implantable device according to an embodiment of the present invention. 
     FIG. 3 shows an embodiment of the robust pacing circuitry included in the secondary module of the implantable device according to an embodiment of the present invention. 
     FIG. 4 represents a “cordwood” construction embodiment of the pacing circuitry of the secondary module according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     A cross-section diagram of an embodiment of the implantable device according to the present invention is shown in FIG.  1 . The body of the device  10  is shown in rectangular form for illustrative purposes only and may have a rounded shape when implanted in the body to avoid tissue damage due to sharp edges. The body of the implantable device  10  includes two modules, a primary module  20  and a secondary module  30 , which are hermetically sealed from each other. As will be described further below, according to an exemplary embodiment of the present invention, the primary module is a demand pacemaker (DDD) with PCD functionality. As is known in the art, a demand (DDD) pacemaker denotes an implantable device that paces and senses both atrial and ventricle chambers of the heart and can either trigger or inhibit functions depending on detected parameters. During normal operation, the secondary module  30  is deactivated, and the primary module  20  controls the various pacing, cardioversion and defibrillation operations of the implantable device  10  via electrical pacing lead  24 . The primary module  20  also detects parameters indicating how the heart is functioning via electrical sensing lead  28 . Both the pacing leads and sensing leads are bipolar leads. 
     The primary module  20  includes a circuitry portion  21  which contains signal detection and logic circuitry for performing pacing and analysis functions and a battery portion  22 . The battery portion  22  includes either no magnetic material or non-magnetic materials. It may be, for example, a lithium-iodine battery, or its equivalent in another chemistry; e.g., it may have an anode of lithium or carbon and a cathode of iodine, carbon monofluoride, silver vanadium oxide, sulfur dioxide, SOCl 2 , or SO 2  Cl 2 . The circuitry portion  21  is separated from the battery portion  22  by a non-magnetic and non-corrosive layer  23  which, as described below, can be made from titanium or from a carbon-composite material. 
     The implantable device  10  also includes a secondary module  30  which contains independent circuitry  31  and battery portion  32  also separated by a non-magnetic and non-corrosive layer  33 . The secondary module  30  is not activated when the primary module  20  operates, but is only switched on when the primary module  20  malfunctions or detects a voltage induced by electromagnetic interference (EMI) that exceeds a certain level, such as, for example, 3 Volts. During such an occurrence, the secondary module  30  acts as a backup VOO pacemaker, which is ventricle driven, with no ventricle-sensing input nor any ventriculr triggering or inhibition. The secondary module  30  sends pacing signals via unipolar electrical lead  34  to a ventricle chamber of the heart but does not receive any detected input signals. In accordance with its backup function, the secondary module  30  is supplied with power by a separate battery  32 , which is also of a non-magnetic type, such as a lithium-iodine battery or those other kinds discussed above. 
     Both the primary and secondary modules  20 ,  30  are encased within shieldings  16  that protect their respective circuitry components from external electromagnetic fields. The shieldings  16  can be made from carbon-matrix composites with continuous carbon fiber filler, which is particularly effective in EMI shielding, as discussed in  Electromagnetic interference shielding using continuous carbon-fiber carbon-matric and polymer-matrix componsites , Luo, X., and Chung, D. D. L., in Composites: Part B (1999), and also suitable for injection molding to encase circuit components. The thickness of the shieldings  16  varies from approximately 1 to 3 millimeters. In addition, the batteries of the primary and secondary modules  22 ,  32  are also encased in separate shieldings  16  made of similar materials. 
     An optical window  40 , made from glass or ceramic, which may be an infrared-transmissive window, is situated between the respective circuitry portions  21  and  31  of the primary and secondary modules  20 ,  30 . The optical window  40  allows for communication to occur between the primary and secondary modules  20 ,  30 . The window  40  is transparent to a range of frequencies of visible or infrared radiation. The thickness of the window has an optimal range of between 0.3 and 1 centimeter. To maintain a hermetic seal between the modules  20 ,  30 , the optical window  40  is bound with brazing to sealing fixtures  35 ,  36  (also referred to as ferrules) that are welded to the shielding layers  16  of the respective modules  20 ,  30  in a manner that may correspond, for example to that described in, for example, U.S. Pat. No. 5,902,326 to Lessar et al. 
     To further protect the implantable device  10  from external electromagnetic fields, the entire implantable device  10 , including the electrical leads  24 ,  28 ,  34 , is coated with a non-magnetic, biocompatible layer  18  such as rolled titanium or flexible graphite. Flexible graphite has been shown to be a particularly effective shielding gasket material as discussed, for example, in  Flexible Graphite for Gasketing, Adsorption, Electromagnetic Interference Shielding, Vibration Damping, Electrochemical Applications, and Stress Sensing , Chung, D. D. L.,  Journal of Mat. Eng . and  Performance , Vol. 92 (2000), due to its resilience, chemical resistance, and shielding properties. Graphite/polymer composites may also serve as layer  18 . With both the inner  16  and outer  18  shielding layers in place, only the ends of the electrical leads  24 ,  28 ,  34 , that are in direct contact with heart tissue remain vulnerable to electromagnetic fields. Since the ends of the leads  24 ,  28 ,  34  must be exposed in order to pace the heart or detect electrical impulses, electromagnetic interference can propogate through the ends of the leads  24 ,  28 ,  34  to the circuitry of the primary and secondary modules  20 ,  30 . The circuitry described below addresses this problem. 
     FIG. 2 shows functional components of a dual-module implantable device  10  according to an embodiment of the present invention. As shown, the functional components of the primary module  20  include a power supply (from the battery  22 ) which supplies power along a main power and device communication bus  125  to the circuitry  21 . The circuitry  21  includes a processor  100  coupled to the main bus  125 , which can be implemented as a parallel processor, or as a microprocessor adapted to perform analog signal processing functions  102  in addition to error detection  104  and power reduction operations  106 . In the analog processing mode  102 , the processor  100  analyzes cardiac signals input from the sensing lead  28  and determines a QRS complex from the various properties of the input signals. The processor  100  determines from the analysis, in a manner know in the art, whether a detrimental heart condition exists, and directs a pacing circuit  140  to transmit corrective pulses to ameliorate the condition. 
     The processor  100  is also configured to detect internal errors or circuitry malfunctions. As will be described further, when such errors are detected, the processor  100 , initiates a shut down of the primary module  20  and sends a signal via optical window  40  that instructs module  30  to become activated. Furthermore, to preserve the life of the battery  22  for as long as possible, the processor  100  regulates the application of power to various circuit elements in order to reduce static power consumption, in a manner such as described, for example, in U.S. Pat. No. 5,916,237 to Schu. The processor  100  is coupled to a memory unit  170  in which instructions and data are stored for performing the functions described herein. 
     The primary module circuitry  21  also includes an optical source unit  150  coupled to the main bus  125 . Optical source unit  150  can be any source of visible or infrared radiation that does not consume significant amounts of power, such as a light emitting diode (LED). During normal operation of the primary module  20 , the optical source  150 , according to various implementations known in the art, turns on and off with a specific well-defined frequency or remains continually on. The optical source unit  150  is arranged in relation to the optical window  40  so that radiation emitted from the source unit  150  penetrates through the optical window  40  into the secondary module  30 . Both the processor  100  and the optical source unit  150  are situated downstream from a power-down switch  118 . 
     The primary module circuitry  21  also includes an optical sensor unit  160  similarly placed in relation to the optical window  40 , in this case, so that it can receive radiation emitted from sources within the secondary module  30 . The optical sensor unit  160  is a low-power photodetector sensitive to infrared or visible radiation of a certain frequency range. The optical sensor unit  160  is coupled to the main bus  125  upstream from the power-down switch  118 , so that it remains connected to the power supply  22  via the main bus  125  and therefore remains functional, even when the power-down switch  118  is opened. 
     Similarly, a telemetry unit  180  is also situated upstream from the power-down switch  118  so it also can function when the power-down switch  118  is opened. The telemetry unit  180  may be, for example, any well known subcutaneous near-infrared signal transmitter, for example, such as described in U.S. Pat. No. 6,192,261 to Gratton et al., that radiates through body tissues and can communicate with a near-by remote programming device (not shown) equipped with an infrared receiver, for example, during an examination at a medical facility. In another implementation, the telemetry unit may use low-power high-frequency radio signals in the Bluetooth™ range to communicate with nearby Bluetooth™-enabled network devices. In either case, the telemetry unit  180  can communicate information such as the condition of the heart, the remaining life of the implantable device batteries, and whether the primary module  20  is inoperative. 
     The processor  100  is coupled to pacing lead  24  and sensing lead  28  via respective comparators  110  and  115 . The comparator  110  compares voltage on the input lead  28  with a threshold voltage, set to, for example 3 Volts. If the input voltage exceeds the threshold voltage, the comparator  110  sends a signal to the processor  100 . The comparator  115  is reverse biased, so that it compares voltages caused by external fields, rather than the output pulse signal on the pacing lead  24 , to the threshold voltage, also set to, for example, 3 Volts. If the external voltage appearing on the pacing lead exceeds the threshold voltage, the comparator  115  sends a signal to the processor  100 . 
     When a voltage exceeds the threshold, this indicates that external EMI fields, which may be caused by an MRI device, are present, and that normal operation of the primary module  20  is to cease. To protect the primary module  20  from excessive voltage signals, a switch (not shown) is thrown to redirect lead signal through capacitive and inductive elements  114 , which filter signals on the pacing  24  and sensing  28  leads in a way known in the art before they reach the circuitry  21  of the primary module  20 . Upon receiving from either comparators  110  or  115  a signal that the threshold voltage has been exceeded, the processor  100  sends a power-down signal to open the switch  118 . Additionally, the processor  100  may send a power-down signal to open the switch  118  in response to detection of internal errors or malfunctions. U.S. Pat. No. 5,653,735 describes, for example, one way by which error detection module  104  can detect malfunctions in primary module  20  not caused by EMI. 
     When the power-down switch  118  is opened, the primary module circuitry components downstream from the switch are disconnected from the power supply  22  and no longer operate. In particular, the primary module  20  stops transmitting pacing pulses to the heart and the optical source unit  150  stops radiating through the optical window  40 . As noted above, the telemetry unit  180  and the optical sensor unit  160  of the primary module  20  continue operating. 
     When the optical source unit  150  of the primary module  20  stops emitting radiation, this event is detected by the optical detector  260  of the secondary module  30 , which is adapted to detect an absence of radiation of either a certain frequency or for a defined period of time, for example, two seconds. Upon detection, the optical detector  260  transmits a power-up signal to switch  218 , which closes and connects the secondary module circuitry  31  to the secondary power supply  32 . In this manner, the secondary module  30  is activated when the primary module  20  is deactivated. 
     The secondary module circuitry  31  includes an oscillator stage  230 , an amplifier stage  240  and a counter  245 . FIG. 3 shows an exploded view of the oscillator  230  and amplifier  240  stages, which are comprised of robust electrical components, such as bipolar transistors, that are not easily disturbed by electromagnetic insult. The oscillator  230  includes bipolar transistors  321  and  322  which are coupled in an emitter feedback arrangement. The RC circuit  310  comprised of resistor  311  and capacitor  312  sets the fixed repetition rate of the oscillator  230 . Once the secondary module  30  is turned on, a pulse is produced and sent on to an amplifier stage  240  comprising bipolar transistor  323 . A shaping RC circuit  340 , comprising capacitor  341  and resistor  342  modifies the shape of a pulse that triggers the ventricle tissues in the heart (shown as  400 ). This secondary module circuitry  31  generates an electrical pulse that stimulates the heart tissues via a lead  34  extending from the secondary module  30 , whereby it produces ventricular contraction at a fixed rate. The return path for the pulse signal is through lead  34  from the heart  400  to the secondary module  30 . Since the pacing lead  34  can conduct electromagnetic interference, a reverse biased comparator  280  switches the conducting path to capacitive and inductive filtering elements  290  when a threshold voltage is reached in a manner known in the art. The arrangement of comparator  280  and filtering elements  290  adds an extra layer of protection to the secondary module circuitry  31 , but is not necessary to the operation of circuitry  31 . 
     Because the secondary module  30  only performs basic pacing operations and does not perform diagnostic functions, if the primary module  20  shuts down in response to temporary electromagnetic interference, it is important to reactivate the primary module  20  (and deactivate the secondary module  30 ) when the implantable device  10  is no longer threatened by the electromagnetic interference. For example, since MRI procedures generally last approximately half an hour, the primary module  20  should only be deactivated for a half an hour plus an additional amount as a tolerance factor, for example. 
     To keep track of the length of time the secondary module  30  is operating, the secondary module circuitry  31  includes a counter element  245  coupled to the oscillator element  230 , that counts oscillator transitions. Once the secondary module  30  is turned on, the counter element  245  increments and can trigger a reset function to turn the primary module  20  back on when it reaches a specific count after a pre-defined length of time. In one embodiment, the counter  245  triggers an optical source  250  to transmit radiation through the optical window  40  to the primary module  20  in which the radiation is detected by optical sensor unit  160 . For example, this radiation may be a single pulse lasting for one second. In response to detection of radiation, the optical sensor unit  160  sends a trigger signal to close the power-down switch  118  and turn the primary module  20  back on. When the processor  100  of the primary module  20  detects that it is connected to the power supply  22 , it runs diagnostic tests in a power-on-reset (POR) mode, such as described, for example, in U.S. Pat. No. 6,016,448 to Busacker et al., wherein initial conditions of the heart are determined and stored in memory unit  170 . During this mode, the processor  100  also runs internal error checks, so that if the original power-down was caused by internal malfunction, and the cause of the malfunction has not been corrected, the secondary module  30  is not deactivated. 
     If the internal error checks indicate that the primary module circuitry  21  can support the PCD cardiac assist functions properly, the processor  100  sends a trigger to the pacing unit  140  to begin operation and simultaneously sends a transmission signal to the optical source unit  150 , whereupon the optical source unit  150  turns on or begins to pulse according to its pre-set frequency. The optical detector  260  of the secondary unit then detects that the optical source unit  150  of the primary unit is on, and in response, triggers the switch  218  to open, deactivating the secondary module circuitry  31 . 
     To further improve the EMI resistance of the secondary module  30 , the circuitry components  31  may be arranged, according to one embodiment of the secondary module circuitry  31 , in a “cordwood” design such as is shown in FIG.  4 . As illustrated, in this arrangement all components are laid side by side on a teflon block  415 , to avoid adherence, and a thin layer of mixed epoxy is laid onto the circuit components, which are aligned so as to minimize the wiring between the various components which reduces extraneous induced EMI pickup. When the epoxy has cured, the circuit  410  is removed from the teflon block and the components are wired as illustrated in FIG.  4 . The resistor and capacitor components  425  are shown hand-wired with very short leads, which reduces electrical pickup signals from an MRI in progress that might disturb the operation of the pacemaker circuitry. 
     In a second embodiment, the secondary module circuitry  31  comprises a custom designed integrated circuit (IC) fabricated, with the active semiconductors, resistors, capacitors and the connecting wires part of the IC. Generally speaking, a monolithic IC of this type maybe exemplified in a manner similar to that described, for example, in U.S. Pat. No. 5,649,965 to Pons et al. 
     While there has been described and pointed out the fundamental novel features of the invention as applied to the preferred embodiments, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, as well as its operation, may be made by those skilled in the art, without departing from the spirit of the invention.