Patent Publication Number: US-8983606-B2

Title: Enhanced sensing by an implantable medical device in the presence of an interfering signal from an external source

Description:
TECHNICAL FIELD 
     The disclosure relates generally to implantable medical devices and, in particular, to sensing by implantable medical devices in the presence of an interfering signal from an external source. 
     BACKGROUND 
     A wide variety of medical systems are implanted within patients to provide a therapy to and/or monitor a physiologic condition of a patient. These implantable medical systems may include an implantable medical device (IMD) and one or more implantable medical leads to deliver therapy to or monitor conditions of a number of organs, nerves, muscles or tissues of the patient, such as the heart, brain, stomach, spinal cord, pelvic floor or the like. 
     Occasionally, patients that have implantable medical systems may benefit from a medical procedure that may generate an interfering signal. For example, a patient may benefit from a magnet resonance image being taken of a particular area of his or her body. Magnetic resonance imaging (MRI) is a technique for imaging portions of the body of the patient for purposes of medical diagnosis. During an MRI procedure, the patient is exposed to magnetic and radio frequency (RF) fields to obtain images of a portion of the body. In particular, the patient is exposed to a strong static (i.e., non-varying) magnetic field that is typically always present around the MRI device whether or not a procedure is in progress. In the presence of the strong static magnetic field, a number of gradient (i.e., time-varying) magnetic fields and RF fields are applied during the MRI procedure to obtain the desired images. The magnitude, frequency or other characteristic of the various fields applied during the MRI procedure may vary based on the type of device producing the fields or the type of scan being performed. 
     Exposure of the implantable medical system to the various fields generated by the MRI device may result in undesirable operation of the implantable medical system. In some instances, the gradient magnetic fields or the RF fields may induce a current or voltage on the leads of the implantable medical system. The current or voltage induced on the leads may interfere with the ability of the IMD to properly sense cardiac signals of the heart of the patient. For example, current or voltage induced on the lead by the gradient magnetic or RF fields may cause the IMD to incorrectly sense a cardiac signal when one is not present or to fail to sense a cardiac signal when one is present. Such interference may result in the IMD delivering therapy when it is not desired or withholding therapy when it is desired. 
     SUMMARY 
     In general, this disclosure relates to techniques to improve sensing by an IMD during exposure to an interfering signal. The IMD may adjust one or more sensing parameters or may utilize different sensing components of a sensing module of the IMD prior to or immediately subsequent to entering an environment having an external source that generates the interfering signal. The IMD may, for example, adjust a sampling frequency, resolution, input range, gain, bandwidth, filtering parameters, or a combination of these or other sensing parameters of the sensing module. These adjustments enable the sensing module to obtain a more detailed representation of the sensed signals, including the noise components of the sensed signals caused by the interfering signal. Without having an adequate representation of the noise components of the sensed signal, it is difficult to separate the noise components of the sensed signal from the cardiac electrical signal. 
     By increasing the sampling frequency and/or resolution of the sensing module, the sensing module may obtain a more detailed digital representation of the sensed signal, including non-linearities, spikes, or other shapes of the sensed signal. As another example, the IMD may increase the input range and/or decrease the gain of the sensing module to reduce the likelihood of clipping of the sensed signal to again obtain a more detailed representation of the sensed signal. As a further example, the IMD may increase the bandwidth and/or decrease the filtering of the sensing module to capture the spectral components of the sensed signal that are outside the usual frequency bands of interest to obtain a more accurate representation of the sensed signal. The IMD may subsequently perform any of a number of signal processing techniques to the enhanced sensed signal to reduce the distortion induced on the cardiac electrical signal by the interfering signal. 
     In one example, this disclosure is directed to an implantable medical device comprising a sensing module to sense electrical signals and a processor. The processor receives an input associated with the presence of an environment having an external source that generates an interfering signal and adjusts a sensing configuration of the sensing module to reduce the likelihood of clipping of sensed signals that include noise caused by the interfering signal. The sensing module senses signals using the adjusted sensing configuration while in the presence of the environment having the external source that generates the interfering signal. 
     In another example, this disclosure is directed to a method comprising receiving an input associated with the presence of an environment having an external source that generates an interfering signal, adjusting a sensing configuration of a sensing module of an implantable medical device to reduce the likelihood of clipping of sensed signals that include noise caused by the interfering signal, and sensing signals using the adjusted sensing configuration while in the presence of the environment having the external source that generates the interfering signal. 
     In a further example, this disclosure is directed to a computer-readable medium comprising instructions that, when executed by a processor, cause the processor to receive an input associated with the presence of an environment having an external source that generates an interfering signal, adjust a sensing configuration of a sensing module of an implantable medical device to reduce the likelihood of clipping of sensed signals that include noise caused by the interfering signal, and sense signals using the adjusted sensing configuration while in the presence of the environment having the external source that generates the interfering signal. 
     This summary is intended to provide an overview of the subject matter described in this disclosure. It is not intended to provide an exclusive or exhaustive explanation of the techniques as described in detail within the accompanying drawings and description below. Further details of one or more examples are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the statements provided below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a conceptual diagram illustrating an environment in which an implantable medical system is exposed to an interfering signal from an external source. 
         FIG. 2  is a conceptual diagram illustrating the implantable medical system of  FIG. 1  in more detail. 
         FIG. 3  is a functional block diagram of an example configuration of components of an IMD of the implantable medical system. 
         FIG. 4  is a functional block diagram illustrating components of an example sensing module of an IMD. 
         FIG. 5  is a flow diagram illustrating example operation of an IMD adjusting sensing parameters in accordance with the techniques of this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a conceptual diagram illustrating an environment  10  in which an implantable medical system  13  is exposed to an interfering signal  11  from an external source. Implantable medical system  13  of  FIG. 1  includes an implantable medical device (IMD)  14  and one or more leads (e.g., leads  15 A and  15 B, collectively “leads  15 ”) implanted within patient  12 . IMD  14  and leads  15  are adapted to provide therapy to and/or to monitor a physiological condition of patient  12 . The techniques, however, are not limited to devices implanted within patient  12 . For example, the techniques may be used in conjunction with an external medical system that is adversely affected by interfering signal  11 . 
     Environment  10  includes an external energy source  16  that generates interfering signal  11  to which implantable system  13  is exposed. In the example illustrated in  FIG. 1 , the external energy source is an MRI device  16 . Although the techniques of this disclosure are described with respect to interfering signal  11  generated by MRI device  16 , the techniques may be used to control operation of IMD  14  within environments in which other types of interfering signals are present. For example, IMD  14  may operate in accordance with the techniques of this disclosure in environments in which interfering signal  11  is generated by other sources, such as electrocautery devices, diathermy devices, ablation devices, radiation therapy devices, electrical therapy devices, magnetic therapy devices, radio frequency identification (RFID) readers, or any other environment with devices that radiate energy to produce magnetic, electromagnetic, electric or other disruptive energy fields. 
     MRI device  16  uses magnetic and RF fields to produce images of body structures for diagnosing injuries, diseases and/or disorders. In particular, MRI device  16  generates a static magnetic field, gradient magnetic fields and/or RF fields. The static magnetic field is a non-varying magnetic field that is typically always present around MRI device  16  whether or not an MRI scan is in progress. Gradient magnetic fields are time-varying magnetic fields that are typically only present while the MRI scan is in progress. RF fields are pulsed RF fields that are also typically only present while the MRI scan is in progress. The magnitude, frequency or other characteristic of interfering signal  11  may vary based on the type of MRI device producing the field. A 1.5 Tesla MRI device, for example, generates a static magnetic field at approximately 15,000 Gauss, generates gradient magnetic fields up to approximately 45 mT/m at 200 T/m/s, and generates RF pulses at approximately 64 MHz. 
     Some or all of the various types of fields produced by MRI device  16  may interfere with the operation of IMD  14 . In other words, one or more of the various types of fields produced by MRI device  16  may make up interfering signal  11 . For example, the gradient magnetic fields or RF fields produced by MRI device  16  may interfere with sensing by IMD  14 . In particular, the gradient magnetic fields and RF fields produced by MRI device  16  may induce currents or voltages on implantable leads  15  coupled to IMD  14 . In some instances, IMD  14  inappropriately detects the induced current or voltage on leads  15  as physiological signals, which may in turn cause IMD  14  to deliver undesired therapy or withhold desired therapy. In other instances, the induced current or voltage on leads  15  result in IMD  14  not detecting physiological signals that are actually present, which may again result in IMD  14  delivering undesired therapy or withholding desired therapy. 
     This disclosure describes techniques to improve sensing during exposure to interfering signal  11 . IMD  14  may be configured into an MRI-compatible operating mode prior to or immediately subsequent to entering environment  10 . The MRI-compatible operating mode may include changing the operating parameters of sensing components or utilizing different sensing components of IMD  14  to provide enhanced sampling of sensed signals. The MRI-compatible operating mode may, for example, adjust a sampling frequency, resolution, input range, gain, bandwidth, filtering parameters, or a combination of these or other sensing parameters of the sensing module to obtain more details about components of the sensed signal that are outside the usual frequency or amplitude ranges of interest. In other words, the adjustments to the sensing module enable IMD  14  to obtain a more detailed representation of the sensed signals, including the noise components of the sensed signals caused by interfering signal  11 . With a more accurate representation of the noise components of the sensed signal, IMD may more easily perform signal processing techniques to the enhanced sensed signal to reduce the distortion induced on the cardiac electrical signal by the interfering signal. 
     IMD  14  may receive an input associated with the presence of environment  10  having an external source that generates interfering signal  11  and adjust the sensing configuration of the sensing module in response to the input. IMD  14  may, for example, be automatically configured into the MRI-compatible operating mode in response to detecting one or more conditions indicative of the presence of MRI device  16 , e.g., existence of a strong magnetic field detected by a Hall sensor or other magnetic sensor. In this example, the detection of the one or more conditions indicative of the presence of MRI device  16  is the input associated with the presence of environment  10 . In other instances, IMD  14  is manually programmed into the MRI-compatible operating mode prior to entering environment  10 . For example, an external device (not illustrated) may wirelessly communicate with IMD  14  to send one or more commands that cause IMD  14  to transition to the MRI-compatible operating mode. In this example, the one or more commands received via wireless communication are the input associated with the presence of environment  10 . 
     Although the techniques of this disclosure are described in the context of environment  10  including an MRI device  16  as the external source, the techniques may be used in other environments in which in which the standard sampling configuration does not allow for accurate interpretation of cardiac electrical signals due to interfering signals, including but not limited environments during electrocautery procedures, diathermy procedures, ablation procedures, radiation therapy procedures, electrical therapy procedures, and magnetic therapy procedures. 
       FIG. 2  is a conceptual diagram illustrating implantable medical system  13  in more detail. Implantable medical system  13  includes an IMD  14  and leads  15 A and  15 B that extend from IMD  14 . In the example illustrated in  FIG. 2 , IMD  14  is an implantable cardiac device that senses electrical activity of a heart  38  of patient  12  and provides electrical stimulation therapy to heart  38  of patient  12 . The electrical stimulation therapy to heart  38 , sometimes referred to as cardiac rhythm management therapy, may include pacing, cardioversion, defibrillation and/or cardiac resynchronization therapy (CRT). 
     In the illustrated example, lead  15 A is a right ventricular (RV) lead that extends through one or more veins (not shown), the superior vena cava (not shown), and right atrium  40 , and into right ventricle  42  of heart  38 . Lead  15 A includes electrodes  44  and  46  located along a distal end of lead  15 A. In the illustrated example, lead  15 B is right atrial (RA) lead that extends through one or more veins and the superior vena cava, and into the right atrium  40  of heart  38 . Lead  15 B includes electrodes  50  and  52  located along a distal end of lead  15 B. 
     Electrodes  44  and  50  may take the form of extendable helix tip electrodes mounted retractably within an insulative electrode head (not shown) of respective leads  15 . Electrodes  46  and  52  may take the form of ring electrodes. In other embodiments, electrodes  44 ,  46 ,  50  and  52  may be other types of electrodes. For example, electrodes  44 ,  46 ,  50  and  52  may all be ring electrodes located along the distal end of the associated leads  15 . Additionally, either or both of leads  15  may include more than two electrodes or only a single electrode. 
     Each of the electrodes  44 ,  46 ,  50  and  52  may be electrically coupled to a respective conductor within the body of its associated lead  15 . The respective conductors may extend from the distal end of the lead to the proximal end of the lead and couple to circuitry of IMD  14 . For example, leads  15  may be electrically coupled to a stimulation module, a sensing module, or other modules of IMD  14  via connector block  54 . In some examples, the proximal ends of leads  15  may include electrical contacts that electrically couple to respective electrical contacts within connector block  54 . In addition, in some examples, leads  15  may be mechanically coupled to connector block  54  with the aid of set screws, connection pins or another suitable mechanical coupling mechanism. 
     Electrodes  44 ,  46 ,  50  and  52  may be used to sense cardiac electrical signals attendant to the depolarization and repolarization of heart  38 . The cardiac electrical signals are conducted to IMD  14  via one or more conductors of respective leads  15 . IMD  14  may use any combinations of the electrodes  44 ,  46 ,  50 ,  52  or the housing electrode for unipolar or bipolar sensing. As such, the configurations of electrodes used by IMD  14  for sensing and pacing may be unipolar or bipolar depending on the application. IMD  14  may analyze the sensed signals to monitor a rhythm of heart  38  or detect an abnormal arrhythmia of heart  38 , e.g., tachycardia, bradycardia, fibrillation or the like. In some instances, IMD  14  provides electrical stimulation therapy based on the cardiac signals sensed within heart  38 . For example, IMD  14  may trigger or inhibit delivery of the electrical stimulation therapy based on the sensed cardiac signals. In other words, the electrical stimulation therapy may be responsive to the sensed events. 
     As described above with respect to  FIG. 1 , exposure of IMD  14  to an interfering signal  11  may introduce noise on the signals received by the sensing components of IMD  14 . This noise may cause IMD  14  to inappropriately detect cardiac events not actually present (i.e., oversense) or to not detect cardiac events that are actually present (i.e., undersense). In either case, the oversensing or undersensing may cause IMD  14  to deliver undesired therapy or withhold desired therapy. 
     Configuring IMD  14  to operate in accordance with adjusted sensing capabilities may enhance the representation of the sensed signal and thus allow IMD  14  to more easily reduce, and possibly eliminate, the noise components of the signal using signal processing techniques. As such, IMD  14  may be configured to an MRI-compatible operating mode that includes adjustments to one or more sensing parameters or the utilization of different sensing components prior to or immediately subsequent to entering environment  10  in which the interfering signal  11  is present. 
     The configuration of implantable medical system  13  illustrated in  FIGS. 1 and 2  is merely an example. In other examples, implantable medical system  13  may include more or fewer leads extending from IMD  14 . For example, IMD  14  may be coupled to three leads, e.g., a third lead implanted within a left ventricle of heart  38 . In another example, IMD  14  may be coupled to a single lead that is implanted within either an atrium or ventricle of heart  38 . As such, IMD  14  may be used for single chamber or multi-chamber cardiac rhythm management therapy. 
     In addition to more or fewer leads, each of the leads may include more or fewer electrodes. In instances in which IMD  14  is used for therapy other than pacing, e.g., defibrillation or cardioversion, the leads may include elongated electrodes, which may, in some instances, take the form of a coil. IMD  14  may deliver defibrillation or cardioversion shocks to heart  38  via any combination of the elongated electrodes and housing electrode. As another example, implantable medical system  13  may include leads with a plurality of ring electrodes, e.g., as used in some implantable neurostimulators. 
     The techniques of this disclosure are described in the context of cardiac rhythm management therapy for purposes of illustration. The techniques of this disclosure, however, may be used to operate an IMD that provides other types of electrical stimulation therapy. For example, the IMD may be a device that provides electrical stimulation to a tissue site of patient  12  proximate a muscle, organ or nerve, such as a tissue proximate a vagus nerve, spinal cord, brain, stomach, pelvic floor or the like. As such, description of these techniques in the context of cardiac rhythm management therapy should not be limiting of the techniques as broadly described in this disclosure. 
       FIG. 3  is a functional block diagram of an example configuration of components of IMD  14 . In the example illustrated by  FIG. 3 , IMD  14  includes a control processor  60 , sensing module  62 , stimulation module  66 , telemetry module  70 , memory  72  and power source  74 . The components of IMD  14  are illustrated as being interconnected by a data bus  76 , but may be connected by one or more direct electrical connections in addition to or instead of data bus  76 . 
     One or more electrodes  44 ,  46 ,  50 , or  52  (or the housing electrode) senses electrical signals attendant to the depolarization and repolarization of heart  38 . In this manner, the electrodes may be viewed as sensors. Thus, the term “sensor” as used herein may include an electrode. The electrical signals sensed by electrodes  44 ,  46 ,  50 , or  52  are conducted to sensing module  62  of via one or more conductors of leads  15 . In other instances, leads  15  may include one or more sensors dedicated for sensing. In further examples, sensing module  62  is coupled to one or more sensors that are not included on leads  15 , e.g., via a wired or wireless coupling. Other types of sensor besides electrodes may include, but are not limited to, pressure sensors, accelerometers, flow sensors, blood chemistry sensors, activity sensors or other type of physiological sensor. 
     Sensing module  62  includes sensing components used to process signals received from the one or more sensors. The components of sensing module  62  may be analog components, digital components or a combination thereof. Sensing module  62  may include multiple sensing channels each having associated sensing components. Each sensing channel may, for example, include one or more sense amplifiers, filters, rectifiers, threshold detectors, analog-to-digital converters (ADCs) or the like. Some sensing channels may convert the sensed signals to digital form and provide the digital signals to processor  60  for processing or analysis. For example, sensing module  62  may amplify signals from the sensing electrodes and convert the amplified signals to multi-bit digital signals by an ADC. Processor  60  may store the digitized versions of signals as EGMs in memory  72 . Other sensing channels may compare processed signals to a threshold to detect the existence of P- or R-waves and indicate the existence of the P- or R-waves to processor  60 . 
     Exposure of IMD  14  to an interfering signal  11  may introduce noise on the cardiac signals received by sensing module  62  of IMD  14 . The noise on the sensed signals may result in sensed signals that are incoherent. Additionally, the noise on the sensed signals may cause IMD  14  to inappropriately detect cardiac events not actually present (i.e., oversense) or to not detect cardiac events that are actually present (i.e., undersense). The oversensing or undersensing may cause IMD  14  to deliver undesired therapy or withhold desired therapy. 
     The spectral components of the noise induced by interfering signal  11  on the sensed signal typically differs in frequency and amplitude relative to the spectral components of cardiac electrical signals. Filtering can reduce the effects of the induced noise, but the filtering may be inadequate for accurately interpreting cardiac electrical signals due to the potentially large amplitude of the noise and the energy components of the noise that overlap with and extend beyond the frequency band of the cardiac electrical signals. To allow for more effective reduction of the noise induced by interfering signal  11 , it may be advantageous to reduce the amount of information that is lost by temporarily implementing a sampling configuration capable of more accurately recording the induced noise, without affecting the integrity of the cardiac electrical signals. 
     In accordance with the techniques of this disclosure, processor  60  may change the sensing configuration of sensing module  62  prior to or immediately subsequent to entering environment  10  to improve the sensing capability of IMD  14  and thus better distinguish between noise induced by interfering signal  11  and the cardiac electrical signals. Processor  60  may receive an input associated with the presence of environment  10  having an external source that generates interfering signal  11  and adjust the sensing configuration of sensing module  62  in response to the input. The aspects of the sensing configuration of sensing module  62  that may be adjusted include a sampling frequency, resolution, input range, gain, bandwidth, filtering parameters, or a combination thereof. By increasing the sampling frequency and/or resolution, for example, sensing module  62  may obtain a more detailed digital representation of the sensed signal (noise+cardiac electrical signal), including non-linearities, spikes, or other shapes of the sensed signal. Without having an adequate representation of the sensed signal, it may be difficult to separate the noise signal from the cardiac electrical signal. As another example, processor  60  may increase the input range and/or decrease the gain of sensing module  62  to reduce the likelihood of clipping of the sensed signal to again obtain a more detailed representation of the sensed signal. As a further example, processor  60  may increase the bandwidth and/or decrease the filtering of sensing module  62  to capture the spectral components of the sensed signal that are outside the usual amplitude or frequency bands of interest, thus obtaining a more detailed sensed signal. 
     Processor  60  may implement these adjustments alone or in combination. In other words, processor  60  may adjust only a single sensing parameter of sensing module  62  or a combination of sensing parameters of sensing module  62 . In instances in which processor  60  adjusts more than one sensing parameter, processor  60  may adjust the sensing parameters concurrently and independently. In other words, the adjustments of the various parameters are not dependent on one another. In another example, the adjustment of one parameter may be dependent on the signal sensed using a previously adjusted parameter. For example, processor  60  may adjust the sampling frequency to obtain a more detailed version of the sensed signal and, based on the more detailed sensed signal, processor  60  may further adjust the gain, input range, resolution, bandwidth, and/or filtering parameters of sensing module  62 . In this manner, adjustment of some of the parameters may be made based on analysis of the sensed signal using a previously adjusted parameter. 
     Processor  60  may employ digital signal analysis techniques to more effectively distinguish between the induced noise and cardiac electrical signals. Processor  60  may perform additional filtering or subtraction to reduce the amount of noise within the sensed signal. Alternatively, processor  60  may perform one or more transform techniques (e.g., Fast Fourier Transform (FFT), wavelet transform, or other transform technique) to distinguish between the induced noise and the cardiac electrical signals. Processor  60  may also obtain an independent noise signal representative of the noise without the cardiac electrical signal and adaptively filter the noise signal from the sensed signal to obtain the cardiac electrical signal. Processor  60  may, for example, obtain the independent noise signal using a telemetry antenna and associated telemetry components (e.g., amplifier, ADC, etc). As another example, processor  60  may obtain the independent noise signal from a noise detector within IMD  14  or otherwise coupled to IMD  14 . As a further example, processor may obtain the independent noise signal from a conductor within the lead that is associated with a non-sensing electrode. 
     Processor  60  may control stimulation module  66  to provide electrical stimulation therapy based on the cardiac electrical signals extracted from the sensed signals by processor  60 . When IMD  14  is configured to generate and deliver therapy to heart  38 , control processor  60  controls stimulation module  66  to deliver electrical stimulation therapy to heart  38  via one or more of electrodes  44 ,  46 ,  50 ,  52  and/or the housing electrode. Stimulation module  66  is electrically coupled to electrodes  44 ,  46 ,  50  and  52 , e.g., via conductors of the respective leads  15 , or, in the case of the housing electrode, via an electrical conductor disposed within the housing of IMD  14 . 
     Control processor  60  controls stimulation module  66  to generate and deliver electrical pacing pulses with the amplitudes, pulse widths, rates, electrode combinations or electrode polarities specified by a selected therapy program. For example, electrical stimulation module  66  may deliver bipolar pacing pulses via ring electrodes  46  and  52  and respective corresponding helical tip electrodes  44  and  50  of leads  15 . To this end, stimulation module  66  may include a pulse generator or other components needed to generate electrical stimulation signals. Stimulation module  66  may deliver one or more of these types of stimulation in the form of other signals besides pulses or shocks, such as sine waves, square waves, or other substantially continuous signals. In addition to pacing pulses, stimulation module  66  may, in some instances, deliver other types of electrical therapy, such as defibrillation and/or cardioversion therapy. 
     Control processor  60  may include any one or more of a microprocessor, a controller, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or equivalent discrete or integrated circuitry, including analog circuitry, digital circuitry, or logic circuitry. The functions attributed to control processor  60  herein may be embodied as software, firmware, hardware or any combination thereof. 
     Memory  72  may include computer-readable instructions that, when executed by control processor  60  or other component of IMD  14 , cause one or more components of IMD  14  to perform various functions attributed to those components in this disclosure. Memory  72  may include any volatile, non-volatile, magnetic, optical, or electrical media, such as a random access memory (RAM), read-only memory (ROM), non-volatile RAM (NVRAM), static non-volatile RAM (SRAM), electrically-erasable programmable ROM (EEPROM), flash memory, or any other computer-readable storage media. 
     The various components of IMD  14  are coupled to power source  74 , which may include a rechargeable or non-rechargeable battery. A non-rechargeable battery may be capable of holding a charge for several years, while a rechargeable battery may be inductively charged from an external device, e.g., on a daily or weekly basis. Power source  74  also may include power supply circuitry for providing regulated voltages and/or current levels to power the various components of IMD  14 . 
     Under the control of processor  60 , telemetry module  70  may receive downlink telemetry from and send uplink telemetry to programming device  18  with the aid of an antenna  78 , which may be internal and/or external to IMD  14 . As described above, for example, telemetry module  70  may receive commands from a programmer indicating that IMD  14  should transition to the MRI-compatible operating mode. Telemetry module  70  includes any suitable hardware, firmware, software or any combination thereof for communicating with another device, such as programming device  18 . For example, telemetry module  70  may include appropriate modulation, demodulation, encoding, decoding, frequency conversion, filtering, and amplifier components for transmission and reception of data. 
     The various modules of IMD  14  may include one or more processors, such as one or more microprocessors, DSPs, ASICs, FPGAs, programmable logic circuitry, or the like, that may perform various functions and operations, such as those described herein. Although illustrated in  FIG. 3  as separate modules, the functionality attributed to the modules may be performed by common hardware, firmware or software components. For example, one or more functions attributed to sensing module  62 , such as digital filtering or threshold detection, may be performed by the same processor that functions as control processor  60 . 
       FIG. 4  is a functional block diagram illustrating components of an example sensing module  62  in further detail. Sensing module  62  includes an anti-aliasing filter  79 , amplifier  80 , ADC  82 , and digital signal processor (DSP)  84 . Sensing module  62  may include other components in addition to those illustrated in  FIG. 4 , such as a rectifier, threshold detector or other components to process the sensed signal. 
     The components illustrated in  FIG. 4  may form a first sensing channel of sensing module  62 . The sensing channel processes the sensed signal and provides the processor signal or a result obtained from processing the sensed signal (e.g., existence of a P- or R-wave) to processor  60 . The first sensing channel may, for example, process signals sensed by electrodes  44  and  46  of lead  15 A. As described above, however, sensing module  62  may have more than one sensing channel, such as a sensing channel for an atrial electrical signal and a sensing channel for a ventricular electrical signal. In this case, sensing module  62  may include additional components forming additional sensing channels. The multiple sensing channels may share one or more of components or may have their own separate components. 
     Sensing module  62  receives a signal sensed by one or more electrodes  44 ,  46 ,  50  and  52 , filters, amplifies, and converts the signal to a digital format for subsequent signal processing. The signal received by sensing module  62  includes cardiac electrical signals and any noise signal induced on leads  15 , including noise signals induced by interfering signal  11 . Sensing module  62  may have one or more adjustable parameters, such as an adjustable sampling frequency, resolution, gain, input range, bandwidth, and/or filtering parameters. Sensing module  62  may also have one or more selectable components. In some instances all of these parameters or components are adjustable or selectable, while in other embodiments only one or a portion of these parameters or components are adjustable or selectable. As described above, processor  60  may adjust one or more of the parameters or may select one or more components of sensing module  62  to provide enhanced sensing when exposed to interfering signal  11 . 
     For example, processor  60  may adjust a gain of amplifier  80 . The gain of an amplifier is the ratio of output voltage to input voltage. During typical operation (e.g., when not exposed to interfering signal  11 ), the gain of the amplifier typically ranges from several tens to several hundreds. Such a gain is acceptable for amplifying cardiac electrical signals. 
     However, the noise signals induced on the lead may have amplitudes that are larger than the cardiac electrical signals. In this case, amplifying the sensed signals (i.e., cardiac electrical activity+noise) using the conventional gain may result in amplifier  80  saturating or ADC  82  clipping portions of the sensed signal. Saturation or clipping is a form of distortion that truncates a signal once it exceeds a threshold of amplifier  80  or ADC  82 . When the signal reaches the threshold, amplifier  80  or ADC  82  outputs its largest amplitude regardless of the actual amplitude of the signal. Thus, the clipping distorts the sensed signal by removing details about the portions of the sensed signal above the threshold. 
     As described above, the more detail obtained regarding the characteristics of the sensed signal, the easier it is for control processor  60  to separate the portion of the sensed signal attributed to the interfering signal from the portion of the signal attributed to the cardiac electrical signal. Therefore, processor  60  may decrease the gain of amplifier  80  (e.g., by changing the resistance of a component(s) of the amplifier or by programming the gain via software) prior to or immediately subsequent to entering environment  10  to reduce the likelihood of clipping of the sensed signal. Processor  60  may, for example, decrease the gain by a certain factor (e.g., by 50%) or dynamically adjust the gain (e.g., using a signal limiter, which will allow signals below a certain threshold to pass unaffected while attenuating signals above a certain threshold). By reducing the amount of clipping that occurs, sensing module  62  obtains more detail regarding the characteristics of the sensed signal (e.g., amplitude, shape, etc). 
     A limited input range of ADC  82  may have a similar effect on the sensed signal, i.e., the input range of ADC  82  may cause clipping of the sensed signal when the sensed signal includes the interfering signal  11 . The input range of an ADC  82  is the range of values it is capable of converting. During typical operation (e.g., when not exposed to interfering signal  11 ), the input range of ADC  82  may be between 0 V and approximately 1 V. Such an input range is acceptable amplifying cardiac electrical signals. 
     However, the amplified noise signals induced on leads  15  by interference signal  11  may have amplitudes that exceed the input range of ADC  82 . In this case, ADC  82  may clip portions of the sensed signal. Therefore, processor  60  may increase the input range of ADC  82  (e.g., by increasing the reference voltage supplied to ADC  82 ) prior to or immediately subsequent to entering environment  10  to reduce the likelihood of clipping of the sensed signal. Processor  60  may, for example, increase the input range by a certain factor (e.g., 100%) or dynamically adjust the input range (e.g., based on the amplitude of the signals being converted). In other instances, sensing module  62  may include a plurality of ADCs, and processor  60  may select a higher resolution ADC in order to more effectively process the signals. By reducing the amount of clipping that occurs, sensing module  62  obtains more detail regarding the characteristics of the sensed signal (e.g., amplitude, shape, etc). 
     In another example, processor  60  may adjust the bandwidth of sensing module  62 . The bandwidth is the range of signal frequencies that sensing module  62  can process. Generally, a bandwidth of sensing module  62  is based on the frequency of the signal of interest, e.g., the cardiac electrical signals. Sensing module  62  may, for example, generally operate with a baseband bandwidth of approximately 100 Hz or less during typical operation of IMD  14  (e.g., when not exposed to interfering signal  11 ). 
     As described above, however, noise induced on the sensed signal by interfering signal  11  has frequencies outside of the typical frequencies of interest. Some of the frequency components of the portion of the sensed signal corresponding to the noise may be removed from the sensed signal because they are outside of the bandwidth of sensing module  62 . As a result, some of the characteristics of the sensed signal including the distortions attributed to noise may be removed. To obtain a more detailed representation of the sensed signal including the noise induced by interfering signal  11 , processor  60  may increase the bandwidth. For example, the cutoff frequency of anti-aliasing filter  79  (the input stage of sensing module  62 ) may be increased. Processor  60  may, for example, increase the baseband bandwidth of sensing module  62  to approximately 1000 Hz. In other instances, processor  60  may increase the bandwidth of sensing module  62  by decreasing the gain of amplifier  80  (gain-bandwidth product). In yet other instances, sensing module  62  may include a plurality of amplifiers and processor  60  may select a higher bandwidth amplifier in order to more effectively process the signals. 
     ADC  82  of sensing module  62  samples the amplified signal at a particular sampling frequency to convert the analog signal to a plurality of digital samples that provide a digital representation of the analog signal. ADC  82  may sample the amplified signal between approximately 200 and 300 Hertz (Hz) during typical operation (e.g., when not exposed to interfering signal  11 ) to obtain a digital representation of the cardiac electrical activity. However, such a sampling frequency may not be sufficient when the sensed signal includes distortion caused by interfering signal  11 . For example, non-linearities, spikes, or other details of the shape of the sensed signal may be lost when sampling at the typical sampling rate. Therefore, processor  60  may increase the sampling frequency of ADC  82  prior to or immediately subsequent to entering environment  10  to capture more detail about the noise signal. For example, processor  60  may provide a faster clock used by ADC  82  to sample the analog signal. Processor  60  may, in one instance, increase the sampling frequency of ADC  82  to be larger than approximately 1 kHz (e.g., 4 or 5 kHz) to provide enhanced sensing during the exposure to interfering signal  11 . 
     Digital signal processor  84  of sensing module  62  filters the sensed signal to remove frequency components of the sensed signal that are not of interest. In the example illustrated in  FIG. 4 , digital signal processor  84  is located after ADC  82  and thus would comprise a digital filter. Digital signal processor  84  may filter the sensed signal to block frequencies that do not correspond with the frequencies of interest. Digital signal processor  84  may be a band pass filter that passes signals that have a frequency between approximately 20 Hz and 80 Hz, while blocking all other signals. When the sensed signal includes distortion caused by interfering signal  11 , however, the filter may block portions of the signal attributed to interfering signal  11 . Therefore, processor  60  may decrease the filtering, e.g., by adjusting the pass band of digital signal processor  84  such that frequencies associated with interfering signal  11  are passed by digital signal processor  84 . Processor  60  may, in one instance, increase the pass band of digital signal processor  84  to include frequencies up to hundreds or thousands of Hz to provide enhanced sensing during the exposure to interfering signal  11 . Processor  60  may, in another instance, not perform any digital filtering until after the noise signal has been reduced via other signal processing routines. 
     As described above, processor  60  may implement these adjustments alone or in combination. Processor  60  also analyzes the enhanced sensed signal obtained by sensing module  62  to more effectively distinguish between the induced noise and cardiac electrical signals. 
       FIG. 5  is a flow diagram illustrating example operation of IMD  14  adjusting sensing parameters in accordance with the techniques of this disclosure. Processor  60  initially determines whether to configure IMD  14  into the MRI-compatible mode (block  90 ). Processor  60  may receive an input associated with the presence of an environment having an external source that generates an interfering signal to determine whether to configure IMD  14  into the MRI-conditional mode. In one example, processor  60  may determine that the MRI-compatible mode needs to be configured in response to detecting one or more conditions indicative of the presence of MRI device  16 , e.g., existence of a strong magnetic field detected by a Hall sensor or other magnetic sensor. In this example, the detection of the one or more conditions indicative of the presence of MRI device  16  is the input associated with the presence of environment  10 . In another example, IMD  14  may determine that the MRI-compatible mode needs to be configured in response to receiving a command from an external device, e.g. via wirelessly telemetry. In this example, the one or more commands received via wireless communication are the input associated with the presence of environment  10 . 
     In response to determining that the MRI-compatible mode needs to be configured (“Yes” branch of block  90 ), processor  60  configures IMD  14  into the MRI-compatible operating mode that includes enhanced sensing (block  92 ). As described in detail above, the MRI-compatible operating mode may include changes to the operating parameters of sensing module  62  or utilizing different sensing components of sensing module  62  to provide the enhanced sensing. The aspects of the sampling configuration of sensing module  62  that may be adjusted include a sampling frequency, resolution, input range, gain, bandwidth, filtering parameters, or a combination thereof. Example adjustments are described above in detail with respect to  FIG. 4 . Processor  60  may implement these adjustments alone or in combination. Additionally, as described above with respect to  FIG. 3 , processor  60  may employ digital signal analysis techniques on the enhanced sensed signals to more effectively distinguish between the induced noise and cardiac electrical signals. 
     Processor  60  determines whether to continue operating in the MRI-conditional operating mode (block  94 ). Processor  60  may, for example, continue to monitor for a strong static magnetic field and continue to operate IMD  14  in the MRI-conditional operating mode as long as the static magnetic field is detected. As another example, processor  60  may maintain a timer that tracks the amount of time that has elapsed since entering the MRI-conditional mode and exit the MRI-conditional mode when the timer exceeds a threshold. When processor determines to continue operating in the MRI-conditional operating mode (“Yes” branch of block  94 ), sensing module  62  continues to sense signals of leads  15  using the adjusted sensing parameters or selected components and processor  60  continues to distinguish the induced noise from the cardiac electrical signals. 
     When processor determines to not continue operating in the MRI-conditional operating mode (“No” branch of block  94 ), processor  60  reconfigures IMD into the normal operating mode (block  96 ). In the normal operating mode, sensing module  62  begins to sense signals of leads  15  using the original sensing parameters or originally selected components. 
     The techniques described in this disclosure, including those attributed to one or more components of IMD  14 , may be implemented, at least in part, in hardware, software, firmware or any combination thereof. For example, various aspects of the techniques may be implemented within one or more processors, including one or more microprocessors, DSPs, ASICs, FPGAs, or any other equivalent integrated or discrete logic circuitry, as well as any combinations of such components, embodied in programmers, such as physician or patient programmers, stimulators, or other devices. The term “processor” may generally refer to any of the foregoing circuitry, alone or in combination with other circuitry, or any other equivalent circuitry. 
     Such hardware, software, or firmware may be implemented within the same device or within separate devices to support the various operations and functions described in this disclosure. In addition, any of the described units, modules or components may be implemented together or separately as discrete but interoperable logic devices. Depiction of different features as modules or units is intended to highlight different functional aspects and does not necessarily imply that such modules or units must be realized by separate hardware or software components. Rather, functionality associated with one or more modules or units may be performed by separate hardware or software components, or integrated within common or separate hardware or software components. 
     When implemented in software, the functionality ascribed to the systems, devices and techniques described in this disclosure may be embodied as instructions on a computer-readable medium such as RAM, ROM, NVRAM, SRAM, EEPROM, flash memory, magnetic data storage media, optical data storage media, or the like. The instructions may be executed to support one or more aspects of the functionality described in this disclosure. 
     Various examples have been described. However, the temporary implementation of an enhanced sampling configuration may also be applicable in other types of situations or environments in which the standard sampling configuration does not allow for accurate interpretation of cardiac electrical signals. These and other examples are within the scope of the following claims.