Abstract:
One aspect of the invention involves a possible fault scenario due to a breach of an inner layer of insulation of an elongated medical electrical lead which couples an active electrical circuit for an active implantable medical device (AIMD)—typically within a conductive AIMD housing—to a sensor disposed within a sensor capsule. In one form, the AIMD provides physiological sensing of a patient parameter, such as endocardial pressure via a chronically implanted absolute pressure sensor. In such a physiological monitor, a high impedance connection is established between the active electrical circuit and the conductive AIMD housing. In a therapy delivering AIMD, a high impedance connection is established between therapy electrodes and the active electrical circuit. As a result, any errant electrical current(s) will be shunted directly to the reference-ground of the sensor-bearing lead in lieu of traveling through a patient&#39;s tissue or conductive body fluid.

Description:
CROSS REFERENCE AND INCORPORATION BY REFERENCE  
       [0001]     This patent disclosure relates to provisional patent application filed on even date hereof; namely, application Ser. No. 60/745,789 (Atty Dkt. P-24201.00) entitled, “FAULT TOLERANT SENSORS AND METHODS FOR IMPLEMENTING FAULT TOLERANCE IN IMPLANTABLE MEDICAL DEVICES,” the entire contents, including exhibits appended thereto, are hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates generally to fault tolerant sensors and related components that couple to active implantable medical devices (AIMDs).  
       BACKGROUND OF THE INVENTION  
       [0003]     Active implantable medical devices are used to monitor, diagnose, and deliver therapies to patients suffering from a variety of conditions. Exemplary AIMDs include implantable pulse generators (IPGs) including pacemakers, gastric, nerve, brain and muscle stimulators, implantable drug pumps, implantable cardioverter-defibrillators (ICDs) and the like.  
         [0004]     Due in part to the fact that an AIMD resides in a difficult environment and can be exposed to vibratory, tensile stresses, forces and caustic materials, there exists a need for a modicum of fault tolerance against a variety of possible device, component and system failures and improper operation. Among other things, certain forms, aspects and embodiments of the present invention provide improved and more predictable performance of an AIMD when subjected to a variety of failure modes.  
         [0005]     There are many situations in which a patient requires long-term monitoring and when it may be desirable to implant a sensor for monitoring within the body of the patient. One such monitor is a pressure monitor, which can measure the pressure at a site in the body, such as a blood vessel or a chamber of the heart. When implanted in a vessel or a heart chamber, the sensor responds to changes in blood pressure at that site. Blood pressure is measured most conveniently in units of millimeters of mercury (mm Hg) (1 mm Hg=133 Pa).  
         [0006]     The implanted pressure sensor is coupled to an implanted medical device, which receives analog signals from the sensor and processes the signals. Signals from the implanted pressure sensor may be affected by the ambient pressure surrounding the patient. If the patient is riding in an airplane or riding in an elevator in a tall building, for example, the ambient pressure around the patient may change. Changes in the ambient pressure affect the implanted pressure sensor, and may therefore affect the signals from the pressure sensor.  
         [0007]     A typical implanted device that employs a pressure sensor is not concerned with total pressure, i.e., blood pressure plus ambient pressure. Rather, the device typically is designed to monitor blood pressure at the site of the internal sensor. To provide some compensation for changes in ambient pressure, some medical devices take additional pressure measurements with an external pressure sensor. The external pressure sensor, which may be mounted outside the patient&#39;s body, responds to changes in ambient pressure, but not to changes in blood pressure. The blood pressure is a function of the difference between the signals from the internal and external pressure sensors.  
         [0008]     Although the internal pressure sensor may generate analog pressure signals as a function of the pressure at the monitoring site, the pressure signals are typically converted to digital signals, i.e., a set of discrete binary values, for digital processing. An analog-to-digital (A/D) converter receives an analog signal, samples the analog signal, and converts each sample to a discrete binary value. In other words, the pressure sensor generates a pressure signal as a function of the pressure at the monitoring site, and the A/D converter maps the pressure signal to a binary value.  
         [0009]     The A/D converter can generate a finite number of binary values. An 8-bit A/D converter, for example, can generate 256 discrete binary values. The maximum binary value corresponds to a maximum pressure signal, which in turn corresponds to a maximum pressure at the monitoring site. Similarly, the minimum binary value corresponds to a minimum pressure signal, which in turn corresponds to a minimum site pressure. Accordingly, there is a range of pressure signals, and therefore a range of site pressures, that can be accurately mapped to the binary values.  
         [0010]     In a patient, the actual site pressures are not constrained to remain between the maximum and minimum monitoring site pressures. Due to ambient pressure changes or physiological factors, the pressure sensor may experience a site pressure that is “out of range,” i.e., greater than the maximum monitoring site pressure or less than the minimum monitoring site pressure. In response to an out-of-range pressure, the pressure sensor generates an analog signal that is greater than the maximum pressure signal or less than the minimum pressure signal. An out-of-range pressure cannot be mapped accurately to a binary value.  
         [0011]     For example, the pressure sensor may experience a high pressure at the monitoring site that exceeds the maximum site pressure. In response, the pressure signal generates a pressure signal that exceeds the maximum pressure signal. The pressure signal is sampled and the data samples are supplied to the A/D converter. When the A/D converter receives a data sample that is greater than the maximum pressure signal, the A/D converter maps the data sample to a binary value that reflects the maximum pressure signal, rather than the true value of the data sample. In other words, the data sample is “clipped” to the maximum binary value. Similarly, when the A/D converter receives a data sample that is below the minimum pressure signal, the converter generates a binary value that reflects the minimum pressure signal rather than the true value of the data sample.  
         [0012]     Because of changes in ambient pressure, pressures sensed by the internal pressure sensor may be in range at one time and move out of range at another time. When the pressures move out of range, some data associated with the measured pressures may be clipped, and some data reflecting the true site pressures may be lost. In such a case, the binary values may not accurately reflect the true blood pressures at the monitoring site.  
         [0013]     To avoid clipping, the implanted device may be programmed to accommodate an expected range of site pressures. Estimating the expected range of site pressures is difficult, however, because ambient pressure may depend upon factors such as the weather, the patient&#39;s altitude and the patient&#39;s travel habits. Pressures may be in range when the patient is in one environment, and out of range when the patient is in another environment.  
         [0014]     The risk of clipping can further be reduced by programming the implanted device with a high maximum site pressure that corresponds to the maximum binary value and with a low minimum site pressure that corresponds to the minimum binary value. Programming the device for a high maximum and a low minimum creates a safety margin. The price of safety margins, however, is a loss of sensitivity. Safety margins mean that pressures near the maximum and minimum site pressures are less likely to be encountered. As a result, many of the largest and smallest binary values are less likely to be used, and the digital data is a less precise representation of the site pressures.  
       BRIEF SUMMARY OF THE INVENTION  
       [0015]     The present invention provides one or more structures, techniques, components and/or methods for avoiding or positively resolving one or more possible failure modes for a chronically implanted medical device that couples to one or more sensors.  
         [0016]     In one embodiment of the invention, a possible fault scenario includes a breach of an inner layer of insulation of an elongated medical electrical lead having a coaxial pair of conductors which couple a circuitry for an active AIMD—which is typically disposed within a substantially hermetic housing—and to a sensor disposed within a sensor capsule. In one embodiment the AIMD provides only physiological sensing of a patient parameter, such as endocardial pressure. In one form of the invention, the sensor comprises an absolute pressure sensor adapted for chronic implantation within a portion of a right ventricle (RV) of a patient. The portion could include the RV outflow tract (RVOT) which is a region of relatively high-rate blood flow which correspondingly requires a robust sensor capsule and coupling to a medical electrical lead coupled thereto.  
         [0017]     One type of mitigation for this embodiment involves placing a high impedance electrical connection between the electrically conductive AIMD housing and the active circuitry within the AIMD housing. The result is that electrical current will flow directly to the reference-ground of the AIMD (e.g., for the lead itself which couples via a so-called set screw to the device) and not through the body of a patient to the conductive AIMD housing.  
         [0018]     One aspect of the invention involves the ability to maintain AIMD and sensor functionality and avoid the possibility of having to explant the AIMD from the patient as well as the oftentimes accompanying possibility of complications due to an explant procedure.  
         [0019]     In another embodiment, an AIMD is configured to sense a physiologic parameter of a patient (e.g., blood pressures, acceleration, pH levels, lactate, saturated oxygen, blood sugar or glucose, calcium, potassium, sodium, etc.) and provide a therapy such as cardiac pacing, high-energy cardioversion/defibrillation therapy and/or a drug or substance delivery regimen or the like. For example, in an AIMD configured to chronically measure blood pressure, provide cardiac pacing therapy and, as appropriate, deliver high-energy defibrillation therapy, an inner insulation breach of a medical electrical lead could cause a short-circuit condition (to the conductive body fluid of a patient) requiring explant of the AIMD. According to the invention, a fault mitigation for this particular embodiment involves providing a high impedance connection between the pace electrode(s), sense electrode(s), high energy electrode(s) (e.g., for providing a ventricular defibrillation therapy), and the operative device circuitry. It should be noted that the cardiac pacing, sensing and defibrillation electrodes normally are fabricated with very high impedance characteristics as is well known and used in the art, so adding a high impedance current path therebetween results in no electrical current flowing to (or through) the patient, while maintaining device functionality which in this embodiment includes delivery of potentially life-saving high-energy defibrillation therapy. This form of the invention can include an AIMD having a single medical electrical lead including at least one pace/sense pair of electrodes, a high-energy defibrillation electrode (e.g., a metallic coil-type electrode) and a sensor capsule (e.g., a pressure sensing device). Such a device can operate in a single chamber pacing mode as is well known in the art. In another form of this embodiment of the invention the AIMD includes two medical leads each having pacing electrodes configurable to pace in a unipolar and/or bipolar manner. For example, at least one electrode and a sensor capsule couples to a first lead and is disposed in electrical and fluid communication with a ventricular chamber and at least one other electrode couples to an atrial chamber.  
         [0020]     In yet another embodiment of the invention, an AIMD configured with three or more discrete medical electrical leads that each independently couple to relatively low power AIMD circuitry disposed within the AIMD housing can be rendered highly robust vis-à-vis a breach in a portion of the inner insulation of the lead coupling the sensor (sensor capsule) to the AIMD circuitry. In one form of this embodiment, the AIMD can comprise a triple-chamber IPG configured to delivery cardiac resynchronization therapy (CRT) to a patient suffering from cardiac dysfunction, including symptoms of mild to advanced heart failure. In one form of this embodiment, the sensor capsule can be adapted to sense left lateral wall acceleration from a medical electrical pacing lead disposed within a portion of the great vein or an epicardial location for activation of the left ventricle (LV). Another pacing lead is adapted to couple to one of the atrial chambers (RA,LA) and yet another pacing lead is adapted to couple to an activation site of the RV. In this form of the invention a fault mitigation structure involves coupling the outer conductor to the sensor capsule and the AIMD electrical reference or ground. Again, this configuration results in no electrical current flowing to the patient in the event of a breach in the inner insulation of the lead coupled to the sensor capsule.  
         [0021]     In summary, this invention deals with a possible fault scenario due to a breach of an inner layer of insulation of an elongated medical electrical lead which couples an active electrical circuit for an active implantable medical device (AIMD)—typically within a conductive AIMD housing—to a sensor disposed within a sensor capsule. In one form, the AIMD provides physiological sensing of a patient parameter, such as endocardial pressure via a chronically implanted absolute pressure sensor. In such a physiological monitor, a high impedance connection is established between the active electrical circuit and the conductive AIMD housing. In a therapy delivering AIMD, a high impedance connection is established between therapy electrodes and the active electrical circuit. As a result, any errant electrical current(s) will not be conducted to those grounded electrodes (e.g., adapted for cardioversion-defibrillation and/or pacing therapy delivery) for outer insulation breach AND for shunted directly to the reference-ground of the sensor-bearing lead in lieu of traveling through a patient&#39;s tissue or conductive body fluid. Acceptable current leakage level is 10 microamps per the European Committee for Electrotechnical Standardization (CENELEC) for devices such as an IPG that directly contacts the heart. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]      FIG. 1  is a diagram of a human body with an implanted medical device and pressure sensors.  
         [0023]      FIG. 2  is a simplified block diagram illustrating an exemplary system that implements the an embodiment of the invention wherein a physiologic sensor provides chronic monitoring and diagnostic for a patient.  
         [0024]      FIG. 3  is an illustration of an exemplary implantable medical device (AIMD) connected to monitor a patient&#39;s heart.  
         [0025]      FIG. 4  is a block diagram summarizing the data acquisition and processing functions appropriate for practicing the invention.  
         [0026]      FIGS. 5A and 5B  are elevational side views depicting a pair of exemplary medical electrical leads wherein in  FIG. 5A  a pair of defibrillation coils are disposed with a sensor capsule intermediate the coils and in  FIG. 5B  the sensor capsule is disposed distal the coils.  
         [0027]      FIG. 6  is a cross sectional view of a coaxial conductor adapted for use with an implantable sensor.  
         [0028]      FIG. 7  is a schematic illustration of a sensor capsule coupled to a housing of an AIMD and a source of reference potential.  
         [0029]      FIG. 8  is a schematic view of a sensor capsule coupled to a electrical current detector and operative circuitry housed within an AIMD.  
         [0030]      FIG. 9  is a schematic view of an AIMD having a proximal lead-end set screw for mechanically retaining the proximal end of a medical electrical lead within a connector block, wherein said set screw couples to a source of reference potential. 
     
    
     DETAILED DESCRIPTION  
       [0031]      FIG. 1  is a diagram of a body of a patient  10  having an implantable medical device (AIMD)  12  according to one embodiment of the present invention. As depicted in  FIG. 1  lead  14  operatively couples to circuitry (not shown) within the AIMD  12  and extends into the right ventricle  16  of the heart  18 . A chronically implantable pressure sensor  20  is show disposed within a portion of a right ventricle (RV)  16  and couples to lead  14 . The pressure sensor  20  monitors and measures changes in blood pressure in the RV  16 . The blood pressure in RV  16  is a function of factors such as the volume of RV  16 , the pressure exerted by the contraction of heart  18  and the ambient pressure around patient  10  and the blood pressure varies throughout the cardiac cycle as is well known in the art. While a pressure sensor  20  is depicted in  FIG. 1  diverse other sensors can directly benefit from the teaching of the present invention as noted hereinabove.  
         [0032]     In one form of the invention the AIMD  12  receives analog signals from the implanted pressure sensor  20  via lead  14  although digital sensors and/or circuitry can be utilized in conjunction with the invention. As noted, in the depicted embodiment the signals are a function of the pressure sensed by implanted pressure sensor  20  at the monitoring site (e.g. RV  16 ) which can of course include myriad different locations on or about the heart and other muscles, circulatory system, nervous system, digestive system, skeleton, brain, diverse organs, and the like. In the depicted embodiment, patient  10  carries or otherwise provides or maintains access to an external pressure sensor or reference  22  which is used to correct the readings of the implanted absolute-type pressure sensor  20 .  FIG. 1  depicts external pressure sensor  22  coupled to a belt or strap  24  coupled to the arm of patient  10 , but this is but one of many possible sites for external pressure sensor  22 . The external pressure sensor  22  responds to changes in ambient pressure, and is unaffected by blood pressure in the RV  16 . The AIMD  12  receives signals from external pressure sensor  22  via communication such as radio frequency (RF) telemetry. Alternatively, the AIMD  12  need not communicate with external pressure sensor  22  in any way.  
         [0033]     The AIMD  12  optionally includes a digital processor. Thus, the analog signals from implanted pressure sensor  20  are converted to digital signals for processing. Referring briefly to  FIG. 2 , the analog signals are first amplified by an amplifier  32  and are sampled and are mapped to discrete binary values by an A/D converter  34 . Each binary value corresponds to a pressure signal that in turn corresponds to a site pressure. The A/D converter  34  maps each sample to a binary value that corresponds most closely to the actual pressure signal and site pressure reflected by the sample.  
         [0034]     The sensitivity of AIMD  12  to changes in pressure is a function of the range of pressures that map to a single binary value. The smaller the pressure change represented by consecutive binary values, the more sensitive implanted medical device  12  is to changes in pressure. For example, an 8-bit A/D converter may be configured to map pressures between a minimum site pressure of 760 mm Hg and a maximum site pressure of 860 mm Hg to discrete binary values. In this example, a one-bit increase represents a pressure increase of about 0.4 mm Hg.  
         [0035]     In a conventional implanted medical device, there may be a tradeoff between range and sensitivity. When the number of possible discrete binary values is fixed, expanding the range of site pressures that are represented by the binary values results in a decrease in sensitivity, because a one-bit change represents a larger pressure change. Similarly, decreasing the range results in an increase in sensitivity because a one-bit change represents a smaller pressure change.  
         [0036]     In an illustrative example, an 8-bit A/D converter may be configured to map pressures between 760 mm Hg and 860 mm Hg to discrete binary values, with a one-bit increase representing a pressure increase of about 0.4 mm Hg. When the same 8-bit A/D converter is configured to map pressures between 746 mm Hg and 874 mm Hg to discrete binary values, the overall range of site pressures that can be mapped to binary values expands by 28 mm Hg. The sensitivity, however, decreases. A one-bit increase represents a pressure increase of 0.5 mm Hg.  
         [0037]     Not all changes to range affect sensitivity. In some circumstances, a range may be offset without affecting sensitivity. In an offset, the minimum site pressure and the maximum site pressure are increased or decreased by the same amount. For example, a 8-bit A/D converter may be configured to map pressures between 760 mm Hg and 860 mm Hg to discrete binary values, with a one-bit increase representing a pressure increase of about 0.4 mm Hg. When the pressure range is shifted downward to pressures between 740 mm Hg and 840 mm Hg, the range is offset but not expanded. When the range is offset, sensitivity is not affected. A one-bit increase still represents a pressure increase of about 0.4 mm Hg.  
         [0038]     Implanted medical device  12  implements techniques for automatically adjusting mapping parameters in response to changes in pressure conditions. In particular, implanted medical device  12  periodically evaluates the digital pressure data to determine whether pressure data may be going out of range, and expands and/or offsets the range to avoid having data go out of range. In addition, implanted medical device  12  determines whether the range can be decreased so that sensitivity can be enhanced.  
         [0039]      FIG. 2  is a block diagram of an exemplary system  30  that implements the invention. Pressure sensor  20  supplies an analog pressure signal to amplifier  32 . The analog pressure signal is a function of the site pressure, where pressure sensor  20  is disposed. The analog pressure signal may be, for example, a voltage signal. Amplifier  32  amplifies the signal by, for example, amplifying the voltage. Amplifier  32  may perform other operations such as serving as an anti-aliasing filter. Amplifier  32  has an adjustable gain and an adjustable offset. The gain and offset of amplifier  32  are adjustable under the control  42  of a controller, which may take the form of a microprocessor  36 . The controller may take other forms, such as an application-specific integrated circuit (ASIC), a field programmable gate array (FPGA), or any other circuit including discrete and/or integrated components and that has control capabilities.  
         [0040]     Amplifier  32  supplies the amplified analog signal to A/D converter  34 . The range of pressure signals supplied to A/D converter  34  is a function of the gain of amplifier  32  and the offset of amplifier  32 . By adjusting the gain and/or offset of amplifier  32 , microprocessor  36  regulates the mapping parameters; that is, the correspondence between site pressures and binary values. A/D converter  34  samples the pressure signals from amplifier  32  and converts the samples into discrete binary values, which are supplied to microprocessor  36 . In this way, microprocessor  36 , amplifier  32  and A/D converter  34  cooperate to map the site pressures to binary values.  
         [0041]     The number of possible discrete binary values that can be generated by A/D converter  34  is fixed. When there is a risk of data out of range, it is not feasible to increase the number of binary values that represent the site pressures. As will be described in more detail below, microprocessor  36  adjusts the gain and/or the offset of amplifier  32  so that the data remain in range and so that the digital pressure data generated by A/D converter  34  accurately reflect the site pressures sensed with pressure sensor  20 .  
         [0042]     Microprocessor  36  processes the digital pressure data according to algorithms embodied as instructions stored in memory units such as read-only memory (ROM)  38  or random access memory (RAM)  40 . Microprocessor  36  may, for example, control a therapy delivery system (not shown in  FIG. 2 ) as a function of the digital pressure data.  
         [0043]     Microprocessor  36  may further compile statistical information pertaining to the digital pressure data. In one embodiment, microprocessor  36  generates a histogram of the digital pressure data. The histogram, which may be stored in RAM  40 , reflects the distribution of pressures sensed by pressure sensor  20 .  
         [0044]     The histogram includes a plurality of “bins,” i.e., a plurality of numbers of digital data samples of comparable magnitude. For example, a histogram that stores the number of digital values corresponding to pressures between 760 mm Hg and 860 mm Hg may include twenty bins, with each bin recording the number of data samples that fall in a 5 mm Hg span. The first bin holds the number of values between 760 mm Hg and 765 mm Hg, while the second bin holds the number of values between 765 mm Hg and 770 mm Hg, and so on. More or fewer bins may be used.  
         [0045]     The distribution of values in the bins provides useful information about the pressures in right ventricle  16 . Data accumulates in the histogram over a period of time called a “storage interval,” which may last a few seconds, a few hours or a few days. At the end of the storage interval, microprocessor  36  stores in RAM  40  information about the distribution of pressures, such as the mean, the standard deviation, or pressure values at selected percentiles. Microprocessor  36  may then clear data from the histogram and begin generating a new histogram.  
         [0046]     When microprocessor  36  adjusts the mapping parameters, the new histogram may be different from the preceding histogram. In particular, the new histogram may record the distribution of an expanded range of pressure data, or a reduced range of pressure data, or a range that has been offset up or down. In general, the adjustments to the mapping parameters tend to center the distribution in the histogram, and tends to reduce the number of values in the highest and lowest bins. Microprocessor  36  adjusts the mapping parameters based upon the distribution of digital pressure data in the preceding histogram. Microprocessor  36  may make the adjustments to avoid data out of range, to avoid having unused range, or both.  
         [0047]     In one embodiment of the invention, microprocessor  36  senses the possibility of out-of-range data or unused range by sensing the contents of the boundary bins of the histogram, for example by checking whether the data distribution has assigned values to the bins that accumulate the lowest values and the highest values of the histogram. As a result of checking the bins, microprocessor  36  may automatically adjust the gain, or the offset, or both of amplifier  32 .  
         [0048]      FIG. 3  is an illustration of an exemplary AIMD  100  configured to deliver bi-ventricular, triple chamber cardiac resynchronization therapy (CRT) wherein AIMD  100  fluidly couples to monitor cardiac electrogram (EGM) signals and blood pressure developed within a patient&#39;s heart  120 . The AIMD  100  may be configured to integrate both monitoring and therapy features, as will be described below. AIMD  100  collects and processes data about heart  120  from one or more sensors including a pressure sensor and an electrode pair for sensing EGM signals. AIMD  100  may further provide therapy or other response to the patient as appropriate, and as described more fully below. As shown in  FIG. 3 , AIMD  100  may be generally flat and thin to permit subcutaneous implantation within a human body, e.g., within upper thoracic regions or the lower abdominal region. AIMD  100  is provided with a hermetically-sealed housing that encloses a processor  102 , a digital memory  104 , and other components as appropriate to produce the desired functionalities of the device. In various embodiments, AIMD  100  is implemented as any implanted medical device capable of measuring the heart rate of a patient and a ventricular or arterial pressure signal, including, but not limited to a pacemaker, defibrillator, electrocardiogram monitor, blood pressure monitor, drug pump, insulin monitor, or neurostimulator. An example of a suitable AIMD that may be used in various exemplary embodiments is the CHRONICLE® implantable hemodynamic monitor (IHM) device available from Medtronic, Inc. of Minneapolis, Minn., which includes a mechanical sensor capable of detecting a pressure signal.  
         [0049]     In a further embodiment, AIMD  100  comprises any device that is capable of sensing a pressure signal and providing pacing and/or defibrillation or other electrical stimulation therapies to the heart. Another example of an AIMD capable of sensing pressure-related parameters is described in commonly assigned U.S. Pat. No. 6,438,408B1 issued to Mulligan et al. on Aug. 20, 2002.  
         [0050]     Processor  102  may be implemented with any type of microprocessor, digital signal processor, application specific integrated circuit (ASIC), field programmable gate array (FPGA) or other integrated or discrete logic circuitry programmed or otherwise configured to provide functionality as described herein. Processor  102  executes instructions stored in digital memory  104  to provide functionality as described below. Instructions provided to processor  102  may be executed in any manner, using any data structures, architecture, programming language and/or other techniques. Digital memory  104  is any storage medium capable of maintaining digital data and instructions provided to processor  102  such as a static or dynamic random access memory (RAM), or any other electronic, magnetic, optical or other storage medium.  
         [0051]     As further shown in  FIG. 3 , AIMD  100  may receive one or more cardiac leads for connection to circuitry enclosed within the housing. In the example of  FIG. 3 , AIMD  100  receives a right ventricular endocardial lead  118 , a left ventricular coronary sinus lead  122 , and a right atrial endocardial lead  120 , although the particular cardiac leads used will vary from embodiment to embodiment. In addition, the housing of AIMD  100  may function as an electrode, along with other electrodes that may be provided at various locations on the housing of AIMD  100 . In alternate embodiments, other data inputs, leads, electrodes and the like may be provided. Ventricular leads  118  and  122  may include, for example, pacing electrodes and defibrillation coil electrodes (not shown) in the event AIMD  100  is configured to provide pacing, cardioversion and/or defibrillation. In addition, ventricular leads  118  and  122  may deliver pacing stimuli in a coordinated fashion to provide biventricular pacing, cardiac resynchronization, extra systolic stimulation therapy or other therapies. AIMD  100  obtains pressure data input from a pressure sensor that is carried by a lead such as right ventricular endocardial lead  118 . AIMD  100  may also obtain input data from other internal or external sources (not shown) such as an oxygen sensor, pH monitor, accelerometer or the like.  
         [0052]     In operation, AIMD  100  obtains data about heart  120  via leads  118 ,  120 ,  122 , and/or other sources. This data is provided to processor  102 , which suitably analyzes the data, stores appropriate data in memory  104 , and/or provides a response or report as appropriate. Any identified cardiac episodes (e.g. an arrhythmia or heart failure decompensation) can be treated by intervention of a physician or in an automated manner. In various embodiments, AIMD  100  activates an alarm upon detection of a cardiac event or a detected malfunction of the AIMD. Alternatively or in addition to alarm activation, AIMD  100  selects or adjusts a therapy and coordinates the delivery of the therapy by AIMD  100  or another appropriate device. Optional therapies that may be applied in various embodiments may include drug delivery or electrical stimulation therapies such as cardiac pacing, resynchronization therapy, extra systolic stimulation, neurostimulation.  
         [0053]      FIG. 4  is a block diagram summarizing the data acquisition and processing functions appropriate for practicing the invention. The functions shown in  FIG. 4  may be implemented in an AIMD system, such as AIMD  100  shown in  FIG. 3 . Alternatively, the functions shown in  FIG. 4  may be implemented in an external monitoring system that includes sensors coupled to a patient for acquiring pressure signal data. The system includes a data collection module  206 , a data processing module  202 , a response module  218  and/or a reporting module  220 . Each of the various modules may be implemented with computer-executable instructions stored in memory  104  and executing on processor  102  (shown in  FIG. 3 ), or in any other manner.  
         [0054]     The exemplary modules and blocks shown in  FIG. 4  are intended to illustrate one logical model for implementing an AIMD  100 , and should not be construed as limiting. Indeed, the various practical embodiments may have widely varying software modules, data structures, applications, processes and the like. As such, the various functions of each module may in practice be combined, distributed or otherwise differently-organized in any fashion across a patient monitoring system. For example, a system may include an implantable pressure sensor and EGM circuit coupled to an AIMD used to acquire pressure and EGM data, an external device in communication with the AIMD to retrieve the pressure and EGM data and coupled to a communication network for transferring the pressure and EGM data to a remote patient management center for analysis. Examples of remote patient monitoring systems in which aspects of the present invention could be implemented are generally disclosed in U.S. Pat. No. 6,497,655 issued to Linberg and U.S. Pat. No. 6,250,309 issued to Krichen et al., both of which patents are incorporated herein by reference in their entirety.  
         [0055]     Pressure sensor  210  may be deployed in an artery for measuring an arterial pressure signal or in the left or right ventricle for measuring a ventricular pressure signal. In some embodiments, pressure sensor  210  may include multiple pressure sensors deployed at different arterial and/or ventricular sites. Pressure sensor  210  may be embodied as the pressure sensor disclosed in commonly assigned U.S. Pat. No. 5,564,434, issued to Halperin et al., hereby incorporated herein in its entirety.  
         [0056]     Data sources  207  may include other sensors  212  for acquiring physiological signals useful in monitoring a cardiac condition such as an accelerometer or wall motion sensor, a blood flow sensor, a blood gas sensor such as an oxygen sensor, a pH sensor, or impedance sensors for monitoring respiration, lung wetness, or cardiac chamber volumes. The various data sources  207  may be provided alone or in combination with each other, and may vary from embodiment to embodiment.  
         [0057]     Data collection module  206  receives data from each of the data sources  207  by polling each of the sources  207 , by responding to interrupts or other signals generated by the sources  207 , by receiving data at regular time intervals, or according to any other temporal scheme. Data may be received at data collection module  206  in digital or analog format according to any protocol. If any of the data sources generate analog data, data collection module  206  translates the analog signals to digital equivalents using an analog-to-digital conversion scheme. Data collection module  206  may also convert data from protocols used by data sources  207  to data formats acceptable to data processing module  202 , as appropriate.  
         [0058]     Data processing module  202  is any circuit, programming routine, application or other hardware/software module that is capable of processing data received from data collection module  206 . In various embodiments, data processing module  202  is a software application executing on processor  102  of  FIG. 3  or another external processor.  
         [0059]     Reporting module  220  is any circuit or routine capable of producing appropriate feedback from the AIMD to the patient or to a physician. In various embodiments, suitable reports might include storing data in memory  204 , generating an audible or visible alarm  228 , producing a wireless message transmitted from a telemetry circuit  230 .  
         [0060]     In a further embodiment, the particular response provided by reporting module  220  may vary depending upon the severity of the hemodynamic change. Minor episodes may result in no alarm at all, for example, or a relatively non-obtrusive visual or audible alarm. More severe episodes might result in a more noticeable alarm and/or an automatic therapy response.  
         [0061]     When the functionality diagramed in  FIG. 4  is implemented in an AIMD, telemetry circuitry  230  is included for communicating data from the AIMD to an external device adapted for bidirectional telemetric communication with AIMD. The external device receiving the wireless message may be a programmer/output device that advises the patient, a physician or other attendant of serious conditions (e.g., via a display or a visible or audible alarm). Information stored in memory  204  may be provided to an external device to aid in diagnosis or treatment of the patient. Alternatively, the external device may be an interface to a communications network such that the AIMD is able to transfer data to an expert patient management center or automatically notify medical personnel if an extreme episode occurs.  
         [0062]     Response module  218  comprises any circuit, software application or other component that interacts with any type of therapy-providing system  224 , which may include any type of therapy delivery mechanisms such as a drug delivery system, neurostimulation, and/or cardiac stimulation. In some embodiments, response module  218  may alternatively or additionally interact with an electrical stimulation therapy device that may be integrated with an AIMD to deliver pacing, extra systolic stimulation, cardioversion, defibrillation and/or any other therapy. Accordingly, the various responses that may be provided by the system vary from simple storage and analysis of data to actual provision of therapy in various embodiments.  
         [0063]     The various components and processing modules shown in  FIG. 4  may be implemented in an AIMD  100  (e.g., as depicted in FIGS.  1  or  3 ) and housed in a common housing such as that shown in  FIG. 3 . Alternatively, functional portions of the system shown in  FIG. 4  may be housed separately. For example, portions of the therapy delivery system  224  could be integrated with AIMD  100  or provided in a separate housing, particularly where the therapy delivery system includes drug delivery capabilities. In this case, response module  218  may interact with therapy delivery system  224  via an electrical cable or wireless link.  
         [0064]     FIGS.  5 A-B are plan views of medical electrical leads according to alternate embodiments of the present invention.  FIG. 5A  illustrates a lead  10  including a lead body  11  having a proximal portion  12  and a distal portion  13 ; distal portion  13  includes a distal tip  14 , to which a fixation element  15  and a cathode tip electrode  16  are coupled, a defibrillation electrode  19  positioned proximal to distal tip  14  and a sensor  17  positioned proximal to defibrillation electrode  19 .  FIG. 5B  illustrates a lead  100  also including lead body  11 , however, according to this embodiment, sensor  17  is positioned distal to defibrillation electrode  19  and distal tip  14  further includes an anode ring electrode  18  and cathode tip electrode  16  is combined into fixation element  15 . Appropriate cathode electrode, anode electrode and defibrillation electrode designs known to those skilled in the art may be incorporated into embodiments of the present invention. Although FIGS.  5 A-B illustrate proximal portion  12  including a second defibrillation electrode  20 , embodiments of the present invention need not include second defibrillation electrode  20 . For those embodiments including defibrillation electrode  20 , electrode  20  is positioned along lead body such that electrode  20  is located in proximity to a junction between a superior vena cava  310  and a right atrium  300  when distal portion  13  of lead body  11  is implanted in a right ventricle  200  ( FIG. 3 ). Additionally, tip electrode  16  and ring electrode  18  are not necessary elements of embodiments of the present invention.  
         [0065]     FIGS.  5 A-B illustrate fixation element  15  as a distally extending helix, however element  15  may take on other forms, such as tines or barbs, and may extend from distal tip  14  at a different position and in a different direction, so long as element  15  couples lead body  11  to an endocardial surface of the heart in such a way to accommodate positioning of defibrillation electrode  19  and sensor  17  appropriately, as will be described in conjunction with  FIGS. 2-5 .  
         [0066]     According to alternate embodiments of the present invention, sensor  17  is selected from a group of physiological sensors, which should be positioned in high flow regions of a circulatory system in order to assure proper function and long term implant viability of the sensor; examples from this group are well known to those skilled in the art and include, but are not limited to oxygen sensors, pressure sensors, flow sensors and temperature sensors. Commonly assigned U.S. Pat. No. 5,564,434 describes the construction of a pressure and temperature sensor and means for integrating the sensor into an implantable lead body. Commonly assigned U.S. Pat. No. 4,791,935 describes the construction of an oxygen sensor and means for integrating the sensor into an implantable lead body. The teachings U.S. Pat. Nos. 5,564,434 and 4,791,935, which provide means for constructing some embodiments of the present invention, are incorporated by reference herein.  
         [0067]     FIGS.  5 A-B further illustrates lead body  11  joined to connector legs  2  via a first transition sleeve  3  and a second transition sleeve  4 ; connector legs  2  are adapted to electrically couple electrodes  15 ,  16 ,  19  and  20  and sensor  17  to an AIMD in a manner well known to those skilled in the art. Insulated electrical conductors, not shown, coupling each electrode  15 ,  16 ,  19  and  20  and sensor  17  to connector legs  2 , extend within lead body  11 . Arrangements of the conductors within lead body  11  include coaxial positioning, non-coaxial positioning and a combination thereof; according to one exemplary embodiment, lead body  11  is formed in part by a silicone or polyurethane multilumen tube, wherein each lumen carries one or more conductors.  
         [0068]      FIG. 6  is a cross sectional view of a coaxial conductive lead body  11  adapted for operative coupling proximal of a sensor capsule taken along the line  6 - 6  of  FIG. 5B  according to the invention. In  FIG. 6 , an inner conductor  50  is spaced from an outer conductor  52  with an insulative material  54  disposed therebetween. The exterior of the biocompatible outer insulation  56  of the lead body  11  shields the conductors  50 , 52  from contact with conductive body fluid. One aspect of the instant invention involves failure of the outer insulation  56  and ways to render such a failure essentially innocuous to a patient.  
         [0069]      FIG. 7  is a schematic illustration of a sensor capsule  17  coupled to a housing  100  of an AIMD and a source of reference potential  53  according to certain embodiments of the invention described herein.  
         [0070]      FIG. 8 . is a schematic view of a sensor capsule  17  coupled to a electrical current detector  55  and operative circuitry housed within an AIMD  100 . As described herein in the event that excess current is detected energy for the sensor capsule  17  can be interrupted, either permanently or temporarily.  
         [0071]      FIG. 9  is a schematic view of an AIMD  100  having a proximal lead-end set screw  13  for mechanically retaining the proximal end of a medical electrical lead  11  within a connector block  57 , wherein said set screw couples to a source of reference potential  53 . The set screw can also promote electrical communication between conductors on the proximal end of the lead  11  and corresponding conductive portions of the connector block  57 . The conductive portions connect via hermetically sealed conductive feedthrough pins to operative circuitry within the AIMD  100 .  
         [0072]     In one embodiment, an AIMD configured to chronically monitor venous pressure in the RV continuously applies 2.2 volts to the pressure sensor via the lead and monitors the resulting current pulse waveform to determine the pressure and temperature of the sensor in the RV. If an increase in electrical current appears, the pressure sensor is switched off to prevent the possibility of DC current flowing to the heart. This particular AIMD is adapted to detect R waves and monitor pressure and temperature (used to calibrate the pressure sensor). The R wave detector indicates the beginning of each cardiac cycle, which is used in the algorithm to determine various parameters from the pressure waveform throughout the cardiac cycle.  
         [0073]     In one exemplary embodiment of the invention the sensor lead has a coaxial configuration of two conductors. The outer one of the pair of elongated conductors is commonly coupled to the housing of the optionally conductive sensor capsule, to an exposed portion of the distal portion of the lead, and to the ground-reference connection of the integrated circuit (IC), or equivalent, operatively disposed within the sensor capsule. The inner one of the pair of coaxial conductors is connected to the electrical supply connection of the IC and the sensor capsule. The outer conductor of the lead couples to the ground-reference of the AIMD and the inner conductor of the lead is maintained at +2.2 volts. The conductive housing of the AIMD couples through a high impedance electrical pathway to a high impedance input of the sense amplifier (i.e., a connector block having a conductive set screw adapted to couple to and mechanically retain the lead outer conductor. This outer conductor thus couples to the ground-reference. As stated, the inner conductor is electrically couples to the electrical supply of the AIMD, nominally +2.2 volts.  
         [0074]     Among others, the present invention provides for a robust, fault tolerant AIMD via some or all of the following.  
         [0075]     Outer coil of the lead connected to lead tip and sensor capsule and to device ground so if the lead outer insulation fails at any point on the outer portion of the lead body, no DC voltage appears across the body tissue and thus, no net flow of electrical current.  
         [0076]     In certain embodiments, the inner conductor of the coaxial-conductor lead is completely enclosed by the lead outer coil. Thus, in addition to the electrical shunt effect, the grounded outer coil creates an electrical shield for the inner conductor. Since the inner conductor is employed to transmit analog pressure data (i.e., from the sensor to the AIMD) the resulting system has enhanced tolerance for electromagnetic interference (EMI) and for certain EMI signals, essentially EMI immunity.  
         [0077]     Connection of the set screw to the lead outer conductor and to the ground-reference provides the following advantages; namely, it ensures that no net DC voltage appears between the setscrew and the lead tip. In contrast, if the setscrew was connected to the lead inner conductor (maintained at +2.2 volts rather than ground) and the self-healing grommets (or septum) on the connector block were not completely sealed, an electrical current path can couple the setscrew, the body, and the lead tip. This situation could result in net DC current traveling through the heart which would not be advantageous for a patient. In addition, over time the DC current could also cause corrosion of the setscrew thereby avoiding yet another possible failure mode.  
         [0078]     Thus, a system and method have been described which provide methods and apparatus for mitigating possible failure mechanisms for AIMDs coupled to chronically implantable sensors. Aspects of the present invention have been illustrated by the exemplary embodiments described herein. Numerous variations for providing such robust structures and methods can be readily appreciated by one having skill in the art having the benefit of the teachings provided herein. The described embodiments are intended to be illustrative of methods for practicing the invention and, therefore, should not be considered limiting with regard to the following claims.  
         [0079]     While exemplary embodiments have been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that these exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide a convenient road map for implementing an exemplary embodiment of the invention. Various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims and their legal equivalents.