Patent Publication Number: US-9839382-B2

Title: Device and method for sensing blood glucose

Description:
BACKGROUND OF THE INVENTION 
     Embodiments of the present disclosure generally relate to blood glucose and more particularly to methods and devices that utilize electromagnetic principles to detect changes in the transmembrane electrolyte balance due to glucose induced fluid shift. 
     Congestive heart failure (CHF) is emerging as a major public health concern, representing a significant cause of hospitalization for individuals aged 65 years and older. Two of the most prominent risk factors for heart failure are hypertension (high blood pressure) and diabetes. Not only do people with diabetes tend to have a cluster of risk factors for heart diseases, including hypertension, obesity, insulin resistance, and abnormal blood lipid levels, but diabetes itself is also an independent risk factor for the condition. People with diabetes are more likely to develop heart disease than the general population. 
     For people with diabetes, there is growing evidence that controlling blood glucose levels is very important in preventing heart disease. According to the American Diabetes Association, self-monitoring of blood glucose (SMBG) has a positive impact on the outcome of therapy and helps to achieve specific glycemic goals. However, the inconvenience, expense, pain, and complexity involved in invasive measurement methods commercially available lead to underutilization, mainly in people with type II diabetes. 
     Today, certain noninvasive (NI) methods have been proposed for determining blood glucose levels. The convention NI methods fall into two categories. The first category of methods is based on the measurement of glucose using one or more of the intrinsic molecular properties of glucose, such as the near-infrared or mid-infrared absorption coefficient, optical rotation, Raman shifts, and photo-acoustic absorption, as well as others. The second category of methods measures the effects of glucose on the physical properties of blood and tissue. The second category of methods is based on an assumption that glucose is a dominant (highly fluctuating) blood analyte and, as such, contributes significantly to the change in the relevant physical parameters of the tissue. Hence, measurement of such parameters can lead indirectly to evaluation of the blood glucose (BG) level. The measured parameters are evaluated relatively to calibration, performed through correlation of the NI signal to a reference BG value. Therefore, the relative change of glucose in blood or interstitial fluid (ISF) plays the major role, as other blood analytes, which are less fluctuating, are fully or at least partially eliminated through calibration. 
     However, conventional methods for measuring blood glucose experience certain limitations. For example, non-invasive methods may require a blood sample to be taken before testing and/or may be available only when the patient visits a doctor&#39;s office. For example, at least some conventional methods use external devices that have sensors attached to the skin. The skin sensors present the potential for variations in the sensor-skin interface, changes in microcirculation, and metabolic rate etc. Those variables cause some challenges in measurement accuracy and sophisticated calibration is required for achieving required accuracy. Further, conventional methods may require the patient to take certain actions to perform the test at various times throughout the day. 
     A need remains for a blood glucose monitoring device and method that are accurate, painless, and easy-to-operate in order to encourage more frequent testing, leading to tighter glucose control and a delaying/decreasing of long-term complications and the associated health care costs. 
     SUMMARY 
     In accordance with one embodiment, a blood glucose sensing device is provided that comprises a house having of an exterior surface and that defines an interior space. The housing is configured to be located within a cardiovascular pathway of a patient. An inductor-capacitor (LC) circuit is located within the interior space defined by the housing. The inductor-capacitor circuit comprises an inductive circuit and a capacitive circuit electronically coupled to one another. The inductive and capacitive circuit has inductance and capacitance values that define a blood glucose sensitive resonant frequency such that a resonant frequency of the LC resonant circuit varies in response to changes in blood glucose levels within the blood in the cardiovascular pathway surrounding the housing. 
     Optionally, the device comprises a microprocessor that is configured to process signals received from the LC resonant circuit to generate date representative of the blood glucose level. The microprocessor may process the signals from the LC resonant circuit to determine the resonant frequency of the LC resonant circuit; and based on the resonant frequency, determines the blood glucose level. The device may comprise an implantable medical device (IMD) coupled to a proximal end of the lead, the IMD including the microprocessor, the lead having a distal end configured to be implanted within a heart of the patient, the LC resonant circuit located within the lead proximate to the distal end such that the LC resonant circuit is located within a chamber of the heart. 
     Optionally, the device further comprises a battery circuit, the LC resonant circuit further comprising an excitation circuit electrically coupled to receive power from the battery circuit and configured to excite the inductor-capacitor resonant circuit by generating an excitation signal. The LC resonant circuit is configured to be excited by an excitation signal from an external source. A microcontroller is configured to build a map of the relations between resonant frequencies, permittivities and glucose levels. 
     Optionally, the device further comprises an LC circuit that includes plates that have fringe flex components projecting from ends of the plates into the blood pool to be sensitivity to permittivity changes in the blood pool. The LC resonant circuit comprises curved plates arranged to face one another with ends of the plates adjacent one another. Optionally, further comprises a microcontroller configured to build a model of a relation between resonant frequencies, and permittivities. 
     Optionally, the device may comprise a microcontroller configured to collect measurements indicative of the resonant frequency of the LC resonant circuit and calculate data indicative of the resonant frequency. The microcontroller may be further configured to repeat the collecting and calculating operation during at lease first and second test intervals and to determine whether the glucose levels changes between the first and second test intervals. 
     In accordance with an embodiment herein, a method is provided that comprises implanting a blood glucose sensing device within a cardiovascular pathway of a patient, the sensing device includes an inductor-capacitor (LC) circuit that has a blood glucose sensitive resonant frequency that varies in response to changes in blood glucose levels of the blood. The method generates an excitation signal to excite the LC resonant circuit. The method collects measurements indicative of the resonant frequency of the LC resonant circuit and calculates data indicative of a glucose level of the blood based on the measurements indicative of the resonant frequency. 
     Optionally, the generating, collecting and calculating operations are repeated during at least first and second test intervals, the method further comprising determining whether the glucose level changes between the first and second test intervals. Optionally, the method calculates a permittivity of the blood based on the measurements and identifying a change in the glucose level based on a change in the permittivity of the blood over time. The calculating operation includes comparing the change in the permittivity to a threshold, and wherein the identifying operation identifies that the glucose level has changed when the change in permittivity exceeds the threshold. Optionally, the method determines the resonant frequency based on the measurements and determining a permittivity of the blood proximate to the LC resonant circuit based on the resonant frequency. 
     Optionally, the generating operation includes generating the excitation signal from an external source outside of the patient. The generating operation includes generating the excitation signal from an implantable device within the patient. Optionally, the method may comprise an implantable device which may include the blood glucose sensing device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  illustrates a graphical model of a relation between relative permittivity and self-resonant frequency of an LC resonant circuit utilized in accordance with embodiments herein. 
         FIG. 1B  illustrates a side sectional view of a sensing device formed in accordance with an embodiment herein. 
         FIG. 1C  illustrates a side sectional view of a sensing device formed in accordance with an alternative embodiment. 
         FIG. 1D  illustrates an embodiment of a leadless glucose sensing device formed in accordance with an alternative embodiment. 
         FIG. 1E  illustrates a capacitive circuit formed in accordance with an alternative embodiment. 
         FIG. 2A  illustrates a simplified block diagram of an embodiment of a leadless glucose sensing device that comprises a passive LC resonant circuit glucose sensor. 
         FIG. 2B  illustrates a method for building a map of permittivity to SRF relations for different glucose concentrations in accordance with embodiments herein. 
         FIG. 2C  illustrates a process for monitoring a patient&#39;s blood glucose level in accordance with embodiments herein. 
         FIG. 3  illustrates a diagram of an embodiment of a leadless glucose sensing device that comprises an active LC resonant circuit glucose sensor. 
         FIG. 4  illustrates an embodiment of a system where an implanted leadless glucose sensing device communicates with an external device and an external device. 
         FIG. 5  illustrates an embodiment of a system where an implanted leadless glucose sensing device communicates with an external device. 
         FIG. 6  illustrates a cross section of a leadless intra-cardiac medical device formed in accordance with an embodiment. 
         FIG. 7  illustrates an example of how a leadless intra-cardiac medical device may be implanted in a chamber of a heart H. 
         FIG. 8A  illustrates an implantable medical device coupled to at least three leads implanted in or proximate a patient&#39;s heart for delivering multi-chamber stimulation according to an embodiment. 
         FIG. 8B  illustrates a cross section of a portion of a lead configured to be implanted into a chamber of the heart. 
         FIG. 9  illustrates sample components of an implantable leadless intra-cardiac medical device in accordance with embodiments herein. 
         FIG. 10  illustrates a process for measuring glucose concentration. 
         FIG. 11  is a simplified diagram of a glucose sensing device that communicates with a device that is located external to the patient P. 
     
    
    
     DETAILED DESCRIPTION 
     The description that follows sets forth one or more illustrative embodiments. It will be apparent that the teachings herein may be embodied in a wide variety of forms, some of which may appear to be quite different from those of the disclosed embodiments. Consequently, the specific structural and functional details disclosed herein are merely representative and do not limit the scope of the disclosure. For example, based on the teachings herein one skilled in the art should appreciate that the various structural and functional details disclosed herein may be incorporated in an embodiment independently of any other structural or functional details. Thus, an apparatus may be implemented or a method practiced using any number of the structural or functional details set forth in any disclosed embodiment(s). Also, an apparatus may be implemented or a method practiced using other structural or functional details in addition to or other than the structural or functional details set forth in any disclosed embodiment(s). 
     In accordance with embodiments herein, methods and devices utilize the interaction between blood cellular membrane potential and electromagnetic principles to detect changes in glucose levels. Blood exhibits a certain electrolyte balance that is based in part on cellular membrane potential. When glucose levels change, the transmembrane electrolyte balance is disrupted due to a glucose-induced fluid shift. The fluid shift causes changes in the cell membrane potential. Variations of the metabolically active enantiomer D-glucose affect the permittivity and conductivity of the cellular membranes. Hence, glucose-induced water and ion transport across the cellular membrane leads to changes in the electrical properties of the cellular and consequently extracellular compartments. 
     In accordance with embodiments herein, variations in electromagnetic waves are used to measure blood glucose levels. The mechanism of using electromagnetic waves for glucose concentration is based on the changes of the dielectric properties of blood. By way of example, a blood glucose sensing device may be incorporated into various structures that are located within the cardiovascular system. For example, cardiac rhythm management (CRM) devices (e.g., pacers, implantable cardioverter defibrillators, cardiac rhythm management devices or leadless pacers) are positioned within the blood flow system. The glucose device may be implemented within a CMEMS circuit which represent a micro-electromechanical system (MEMS) merged with a complementary metal oxide semiconductor (CMOS) circuit. The glucose sensing device may be implanted within various chambers of the heart (e.g. RV, RA and LV or RA) which enable the elimination of several variables associated with external devices. Measuring glucose concentration by a sensor on a lead or CMEMs or leadless pacer (LP) provides continuous, highly accurate glucose monitoring. 
     Embodiments herein provide methods and devices that utilize electromagnetic technology based glucose sensors on a pacing lead, a CRM device, a CMEMS sensor or LP that would detect glucose concentration in the blood surrounding the lead or the sensor. Embodiments herein provide non-invasive glucose monitoring for better management of heart failure co-morbidities therefore reducing hospitalization and health care costs. 
       FIG. 1A  illustrates a graphical model of a map between permittivity and self-resonant frequency of an LC resonant circuit utilized in accordance with embodiments herein. The relation between permittivity and resonant frequency may be determined through modeling, in vitro testing or otherwise. The model  10  (also referred to as a P-SPF model) assumes that the LC resonant circuit is surrounded with or located proximate to a fluid (e.g., blood) that exhibits shifting permittivity due to glucose concentration. The model  10  plots relative permittivity along the vertical axis (normalized to no unit) and self-resonant frequency (SRF) along the horizontal axis (in GHz). The model  10  includes an example permittivity-SRF graph  12  that shows permittivity of the surrounding fluid in the presence of an LC resonant circuits. 
     The graph  12  corresponds to a changing glucose level and illustrates the relation between various resonant frequencies of an LC resonant circuit and a corresponding amount of permittivity exhibited by surrounding blood. By way of example, the permittivity-SRF graph  12  illustrates that, for a given LC circuit, when the permittivity within the blood is relatively high (e.g. 65) the LC resonant circuit exhibits a low SRF (e.g. near approximately 1 GHz). When the glucose concentration changes, this reduces blood permittivity, and thus the LC resonant circuit exhibits a higher SRF (e.g., the SRF is approximately 8 GHz, when the blood exhibits a permittivity of approximately 50). As additional examples, the graph  12  shows that a blood pool having a corresponding glucose concentration and a permittivity of 55 will cause the LC resonant circuit to exhibit an SRF of about 6 GHz. When the permittivity is 45, the LC resonant circuit will exhibit an SRF of approximately 10 GHz. The graph  12  may vary based on the particular LC resonant circuit. The graph  12  represents one example of how the SRF varies for a particular LC resonant circuit relative to the amount of glucose in the blood pool. 
     The model  10  may be determined for individual patients, for each LC resonant circuit, for types of LC resonant circuits and the like. As explained herein, once the model  10  is determined, changes in the glucose concentration may be determined by monitoring changes in SRF, such as relative to baseline SRF characteristics. Embodiments herein maintain a permittivity glucose relation between various permittivity levels and glucose levels and P-SPF model  10 . The permittivity-glucose (PG) relation and P=SPF model  10  may be predetermined through laboratory work, while calibration can be done periodically though operation or at other times. One or more PG relations and P-SPF models  10  may be stored in a data store in an LIMD, sensor device, external device, network system, locally or remotely. The PG relation and P-SRF model  10  are used to determine glucose levels. 
       FIG. 1B  illustrates a side sectional view of a sensing device  50  formed in accordance with an embodiment herein. The device  50  includes a housing  52  that hermetically encloses an interior space  54 . The interior space  54  includes a base member  56  that extends into the interior space  54  from one wall toward an opposing wall  58 . The base member  56  encloses a first LC resonant circuit  60  and supports a second LC resonant circuit  70  upon an exterior of the base member  56 . The LC resonant circuit  60  is located within the interior space  54  of the housing  52 . 
     The first LC resonant circuit  60  includes a capacitive circuit  62  comprising one or more parallel plates  64  that are separated by a dielectric material  66 . The parallel plates  64  are held in a fixed position and relation to one another. The first LC resonant circuit  60  further includes an inductive circuit  68  that comprises a coil conductor wound within the base member  56  about the capacitive circuit  62 . Other structures may be employed for the capacitive circuit  62  and inductive circuit  68 . The capacitive and inductive circuits  62  and  68  may be electrically coupled to one another in parallel or in series. The capacitive and inductive circuits  62  and  68  have capacitance and inductance values that define a blood glucose sensitive resonant frequency such that a resonant frequency of the LC resonant circuit  60  varies in response to changes in blood glucose level in the cardiovascular pathway surrounding the housing  52 . More specifically, the resonant frequency changes in response to changes in permittivity of the blood (caused by changes in the blood glucose level). The resonant frequency is sufficiently high (along the patterns in  FIG. 1B ) that changes in permittivity are distinguishable from one another based on changes in the resonant frequency. 
     Optionally, the plates  64  of the capacitive circuit  62  may be oriented longitudinal bottom and/or perpendicular to the top exterior surfaces of the housing  52  or in other orientations to enhance the sensitivity of fringe flex fields to blood permittivity. 
     The second LC resonant circuit  70  includes a capacitive circuit  72  that comprises one or more parallel plates  74  that are separated by an open area  76 . The parallel plates  74  may be moved closer to or further from one another based upon a pressure difference experienced between the interior space  54  and exterior of the housing  52 . For example, as the pressure against the exterior surface  59  increases, the wall  58  moves in the direction of arrow  61  towards the upper surface  63  of the base member  56 . When the wall  58  moves toward the base member  56 , the spacing between the plates  74  reduces thereby changing the capacitance value of the capacitive circuit  72 . Similarly, as the pressure against the exterior surface  59  decreases, the wall  58  moves in a direction opposite to arrow  61  away from the upper surface  63  of the base member  56 , thereby changing the capacitance value of the capacitive circuit  72  in an opposite direction. 
     The second LC resonant circuit  70  further includes an inductive circuit  78  that comprises a coil conductor wound about an exterior of the base member  56 . Other structures may be employed for the capacitive circuit  72  and inductive circuit  78 . The capacitive and inductive circuits  72  and  78  may be electrically coupled to one another in parallel or in series. The capacitive and inductive circuits  72  and  78  have capacitive and inductive values that define a pressure sensitive device such that the resonant frequency of the second LC resonant circuit  70  varies in response to changes in pressure in the cardiovascular pathway surrounding the device  50 . For example, the pressure sensitive LC resonant circuit  70  maybe included similar to the pressure sensitive LC resonant circuits described in Patent Application Publication US 2014/010627, the complete subject matter of which is expressly incorporated herein by reference in its entirety. 
       FIG. 1C  illustrates a side sectional view of a blood glucose sensing device  150  formed in accordance with an alternative embodiment. The device  150  includes a housing  152  that hermetically seals an interior space  154 . The housing  152  includes opposed longitudinal walls  158  and  159 . A base member  156  is located along an interior surface of wall  158 . The base member  156  encloses a first capacitive circuit  162  that is formed from capacitor plates  164  and  165 . The capacitor plates  164  and  165  are separated by a dielectric material defined by the base member  156 . The plates  164 ,  165  are held in a fixed position and relation to one another. The capacitor plates  164  and  165  include lateral edges  163  and  167  that are directed toward, and located proximate to, the wall  158  of the device  150 . A spacing between the plates  164 ,  165  extends in a direction (generally denoted at  171 ) that is parallel to, and located relatively proximate to, the wall  158 . 
     During operation, an electric field is generated between the plates  164  and  165 . The electric field is comprised of various field flux components. For example, the electric field includes a main flux component  174  that is located within the dielectric material of the base member  156  in the space  171  and is positioned immediately between facing surfaces of the plates  164  and  165 . The electric field also includes fringe flux components  176  that extend between the lateral edges  163  and  167  of the plates  164  and  165 . The fringe flex components extend laterally in opposite directions from the plates  164 ,  165 ; however, for simplicity only the fringe flex components  176  are illustrated. The fringe flux components  176  project along field lines that extend through the walls  158  and  159  into the surrounding blood. As the glucose concentration in the blood various, the permittivity similarly various. The fringe flux component  176  passes through the surrounding blood and thus is exposed to the corresponding blood permittivity. 
     The first LC resonant circuit  160  further includes an inductive circuit  168  that comprises a coil conductor wound about a perimeter of the base member  156  and about the capacitive circuit  162 . Other structures may be employed for the capacitive circuit  162  and inductive circuit  168 . The capacitive and inductive circuits  162  and  168  may be electrically coupled to one another in parallel or in series. The capacitive and inductive circuits  162  and  168  have capacitance and inductance values that define a blood glucose sensitive resonant frequency such that a resonant frequency of the LC resonant circuit  160  varies in response to changes in blood glucose level in the cardiovascular pathway surrounding the housing  152 . 
     Optionally, additional physical configurations may be provided to locate larger portions of the fringe flux component  176  within the blood. For example, the plates  164 ,  165  may be positioned such that lateral edges on opposite sides of each plate  164  and  165  are located immediately adjacent the exterior surfaces of the walls  158 ,  159  of the housing  152 . By locating the lateral edges on both sides of the plates  164  and  165  immediately adjacent to the exterior of the housing  152 , a substantial portion of the fringe flux components  176  on both sides of the capacitive circuit  162  are largely exposed to blood. 
     Additionally or alternatively, the housing  152  may be constructed such that a portion of the main flux component  174  passes through blood. For example, a hollow passage (similar to a doughnut hole) may be created through the core of the base member  156  and housing  152  such that blood may pass through the open core without being exposed to the interior  154 . As another example, a notch may be formed in a side of the wall  158  that extends into the portion of the base member  156  located between the plates  164  and  165 . The notch may create a cavity (e.g. a U-shaped cavity, a C-shaped cavity, etc.) that is exterior to the housing  152 , but located between the plates  164  and  165 . 
     Optionally, the device  150  may include a second LC resonant circuit  170  having a capacitive circuit  72  comprises one or more parallel plates  175  that are separated by an open area. The parallel plates  175  may be moved closer to or further from one another based upon a pressure difference experienced between the interior space  154  and exterior of the housing  152 . For example, as the pressure against the exterior surface increases, the wall  159  moves towards the upper surface of the base member  156 . The second LC resonant circuit  170  further includes an inductive circuit  178  that comprises a coil conductor wound about an exterior of the base member  156 . The inductive circuits  158  and  178  are wound concentrically with one another as one example. 
       FIG. 1D  illustrates, in a simplified sectional side view, an embodiment of a leadless glucose sensing device  102 . For purposes of illustration, the device  102  is depicted with a hypothetical opening  104  to show several interior components of the device  102 . The device  102  includes a housing  106  comprising an external surface  108  and defining an interior space  110 . The sensing device  102  is described in connection with a first LC resonant circuit sensitive to blood glucose. Optionally, the sensing device  102  may also include a second LC resonant circuit sensitive to changes in blood pressure similar to the pressure sensitive LC resonant circuits in the &#39;627 application. 
     The device  102  comprises an LC resonant circuit glucose sensor including a capacitive circuit  112  and an inductive circuit  114 . In the example of  FIG. 1D , the capacitive circuit  112  comprises one or more concentric plates  116  separated by dielectric material and the inductive circuit  114  comprises a coil inductor. The device  102  includes circuitry to detect changes in a resonant frequency of the LC resonant circuit (e.g., circuit  62  and/or circuit  70  in  FIG. 1B ) and based thereon, generate data indicative of glucose levels. For example, an integrated circuit  120  (having one or more processors), one or more associated electrical conductors  122 , and a battery circuit  124  (comprising a battery) may be coupled to the LC-resonant circuit(s) for detecting the operating frequency of the LC resonant circuit(s), identifying permittivity calculating glucose levels, identifying pressure levels, and/or for exciting the LC resonant circuit(s). The integrated circuit  120 , the electrical conductors  122 , and the battery circuit  124  are located within the interior space  110  of the housing  106 . 
     In a typical implementation, one terminal of the inductive circuit  114  is coupled via a conductor to a plate of the capacitive circuit  112 , while another terminal of the inductive circuit  114  is coupled via another conductor to another plate of the capacitive circuit  112 . Thus, the inductive circuit  114  and the capacitive circuit  112  are coupled in parallel, thereby forming a resonant circuit that is capable of being excited by an externally applied electromagnetic field or an internally applied signal. Alternatively, the circuits  112  and  114  may be coupled in series. 
     Optionally, the device  102  may include components for sensing cardiac signals and/or stimulating (e.g., pacing) cardiac tissue. For example, the device may operate in one or more of the following modes: VDD, DDD, VVIR, CRT, or some other suitable mode. These components may include, for example, an electrode  126  (e.g., a helical electrode), an electrode  128  (e.g., a ring electrode), one or more additional electrodes (represented by electrode  130 ), and other circuitry. This other circuitry may include, for example, corresponding functionality of the integrated circuit  120 , one or more of the electrical conductors  122 , and the battery circuit  124 . 
       FIG. 1E  illustrates a configuration for a capacitive circuit  190  formed in accordance with an alternative embodiment. The capacitive circuit  190  includes curved capacitor plates  192  and  194 . The capacitor plate  192  includes opposed lateral ends  196  and  198 . The capacitor plate  194  includes opposed lateral ends  195  and  193 . The capacitor plates  192  and  194  are curved or semicircular in shape and positioned in a non-concentric configuration such that the ends  196  and  193  are located proximate to one another and face one another, while the ends  198  and  195  are similarly located proximate to and facing one another. The curved capacitor plates  192  and  194  collectively define an overall circular shape with each plate  192  and  194  forming a curved open portion of the circular shape. The plates  192  and  194  are spaced apart from one another, such that circumferential gaps  197  and  191  are located between the ends  193 ,  195 ,  196  and  198 . The gaps  191 ,  197 , as well as a core region  199  may be filled with a dielectric material to maintain the plates  192  and  194  in a fixed relation to one another, but spaced apart from one another. 
     During operation, an electric field is formed between the plates  192  and  194 , where the electric field includes a main flux component in the core region  199  and fringe flux components in the regions  189  that flare radially outward from the gaps  191 ,  197 . At least the regions  189  are located within the blood pool surrounding the device such that the fringe flux components are exposed to changes in the permittivity of the blood. 
     Optionally, a portion of the core region  199  may be rendered hollow such that blood may pass through the region  199  (such as denoted by the circular dashed line  187 ). By permitting blood to pass through the region  199  through which the main flux components pass, the capacitive circuit  190  may become more sensitive to permittivity changes. Optionally, more than two capacitor plates may be utilized. For example, more than a single capacitor may be provided within the capacitive circuit  190  (or any of the lead or device described herein). 
     The capacitive circuit  190  may be provided within the device  102  in addition to, or in place of, the capacitive circuit  112 . The capacitive circuit  190  is coupled to the corresponding inductive circuit (e.g.  114  in  FIG. 1D ). 
       FIG. 2A  is a simplified block diagram of an embodiment of a leadless glucose sensing device  202  that comprises a passive LC resonant circuit glucose sensor. The housing is represented by a dashed line  204 . The LC resonant circuit  206  is located at the left-most section of the device  202 , while the circuit  208  to the right of the LC resonant circuit  206  performs certain glucose level sensing-related operations as well as cardiac sensing and/or pacing operations. For purposes of illustration, the circuit  208  is depicted as including a signal processing circuit  220  (including one or more processors), a memory circuit  222 , a sensing/pacing circuit  224 , a battery circuit  226 , and two electrodes  228  and  220 . It should be appreciated that different combinations of these components may be employed in other embodiments constructed in accordance with the teachings herein. 
     The LC resonant circuit  206  comprises a capacitive circuit  210  and an inductive circuit  212 . The inductance and capacitance values of these components are selected to cause the LC resonant circuit  206  to resonate at a select reference frequency when in the presence of blood having a corresponding reference or select glucose level. In this case, the LC resonant circuit  206  is excited (e.g., induced with a signal that causes the LC resonant circuit  206  to resonate) by an externally generated radiofrequency (RF) signal  214 . As one example, the RF signal  214  maybe generated by an external antenna that may be help against the patient or provided within another common structure used by the patient. For example, the antenna may be provided under a bed, within a pillow, within a chair back, automobile seat back, handheld and the like. The antenna is connected to an external device such as a bedside monitor, a portable computer, smart phone, smart watch and the like. Upon excitation, an oscillating signal  216  (represented, for convenience, by a dashed line) at the resonant frequency is established in the LC resonant circuit  206 . 
     The capacitance and inductance values for the capacitive circuit  210  and inductive circuit  212  are set to define a blood glucose sensitive resonant frequency such that a resonant frequency of the LC resonant circuit  206  varies in response to changes in blood glucose levels within the blood in the cardiovascular pathway surrounding the housing. A change in the blood glucose level will result in a change in a permittivity characteristic (and/or other electrical characteristic) of the blood. The change in permittivity, in turn, causes a change in the resonant frequency of the LC resonant circuit  206 . Thus, a change in glucose level will correspond to a change in the frequency of the oscillating signal  216 . 
     The circuit  218  collects measurements indicative of the resonant frequency. For example, the oscillating signal  216  is detected by the circuit  218  (e.g., a high impedance sense amplifier, a low impedance current sensing circuit, or some other suitable circuit) and provided to the signal processing circuit  220 . The signal processing circuit  220  processes the measurements of the received signal to determine at least one frequency of the signal. The signal processing circuit  220  utilizes the frequency to calculate data representative of the glucose level of the blood surrounding the device  202 . 
     The signal processing circuit  220  may then store this data in the memory circuit  222  for subsequent use. For example, as discussed below, the device  202  may periodically collect data over a period of time, and send the stored data to an external device at some later point in time. As another example, one or more operating parameters (e.g., pacing parameters) of the device  202  may be adjusted based on the glucose data. To facilitate receiving the oscillating signal  216 , the signal processing circuit  220  and/or the circuit  218  may comprise one or more of: a sensing circuit, an amplifier, a filter, a switching circuit, or other suitable circuits. These circuits may perform one or more of: detecting, filtering, or amplifying the oscillating signal  216 . 
     As represented by corresponding lines in  FIG. 2A , the battery circuit  226  is electrically coupled to one or more of the circuits  218 - 224  and any other circuits (not shown) that require power from the battery circuit  226 . It should be appreciated that the battery circuit  226  may be implemented using any suitable implantable power source. 
     The signal processing circuit  220  is also electrically coupled to each electrode  228  and  230  (e.g., via the circuit  224 ) for sensing cardiac activity and/or stimulating cardiac tissue. Thus, in some cases, the electrodes  228  and  230  are used for stimulating cardiac tissue. In some cases, one or more of the electrodes  228  and  230  may be used for sensing cardiac activity (e.g., for near-field sensing and/or far-field sensing). For example, the electrodes  228  and  230  may correspond to the electrodes  126  and  128  of  FIG. 1B  (or some other combination of the electrodes of  FIG. 1B ). 
     The sensing/pacing circuit  224  is electrically coupled to the electrodes  228  and  230  to receive electrical signals indicative of cardiac activity and/or to output cardiac stimulation signals (e.g., pacing pulses). To facilitate interfacing with these components, the sensing/pacing circuit  224  may comprise one or more of: a sensing circuit, an amplifier, a filter, a signal generator, a signal driver, a switching circuit, or other suitable circuits. Thus, the sensing/pacing circuit  224  may filter, amplify, and detect signals received from the electrodes  228  and  230 . In addition, the sensing/pacing circuit  224  may generate, filter, and amplify signals sent to the electrodes  228  and  230 . 
     The signal processing circuit  220  may process cardiac signals received via the sensing/pacing circuit  224  to identify cardiac events. For example, a microprocessor of the signal processing circuit  220  may be configured to acquire intra-cardiac electrogram data (and/or other cardiac related signal data) and identify P waves, R waves, T waves and other cardiac events of interest. Based on analysis of these cardiac events, the processing circuit may selectively generate stimulation signals (e.g., pacing pulses) to be delivered to cardiac tissue via one or more electrodes. The signal processing circuit  220  also may control stimulation operations by controlling the signals generated by the sensing/pacing circuit  224 . For example, a microprocessor of the signal processing circuit  220  may be configured to trigger the generation of pacing signals, specify pacing signal characteristics (e.g., energy level and duration), and inhibit pacing signals. 
     It should be appreciated that the signal processing circuit  220  may take various forms in different embodiments. For example, in some implementations, a single circuit (e.g., a microprocessor) may be employed to handle processing for both glucose sensing and cardiac operations. In other implementations, however, different circuits may be employed to provide the processing for these different operations. 
     Furthermore, in some embodiments, the resonant frequency of the LC-resonant circuit  206  may be determined by an external device. In such a case, the leadless glucose sensing device  202  need not employ the circuit  218  or the capability of generating data representative of cardiac glucose. Rather, based on the resonant frequency determination made by the external device, the external device will determine the blood glucose level. 
     Embodiments herein calculate a self-resonant frequency of the LC resonant circuits and calculate corresponding blood permittivity, glucose levels and pressure in various manners. As one example, a template may be constructed that maps various SRF to permittivity levels, where the map or template includes multiple graphs plotting functional relations between permittivity and SRF, each graph associated with a corresponding glucose level. Next, methods are described in connection with  FIGS. 2B and 2C  for building the maps and utilizing the maps during glucose testing. 
       FIG. 2B  illustrates a method for building a map of the permittivity to SRF relation or pattern for a given LC resonant circuit and a blood pool that experienced different select levels of glucose concentration. The method may be performed before implant at the time a device is manufactured and/or after device implant. The map may be based on models, lab work or otherwise, where each LC circuit need not be test. At  232 , a glucose sensing (GS) device is implanted at a location proximate to a blood pool to be monitored. For example, the GS device may be implanted proximate to a chamber of the hear, such as in the RV, LV, RA, LA, and/or in the great vessels of the heart that enter and leave the heart (e.g., the superior and inferior vena cava, the pulmonary artery, the pulmonary vein, and the aorta). The GS device may be located proximate to other portions of the vascular system provided that a sufficient amount of blood surrounds or is located at a proximity to the LC resonant circuit such that the permittivity of the blood pool affects the resonant frequency of the LC resonant circuit. The amount of blood and/or the available distance between the blood pool and the LC resonant circuit will vary depending on a construction of the LC resonant circuit, a sensitivity of the LC resonant circuit, the type of excitation source utilized and other design factors. 
     At  234 , the system transmits a reference excitation signal. The excitation signal may be generated by an external RF transmitter source, such as an antenna located near the patient. Optionally, the excitation signal may be generated by an implanted transmitter source, such as within an IMD, a leadless pacemaker or other implanted device. The reference excitation signal may be generated by the same implanted device (e.g., IMD or LP) that includes the glucose sensing device. Optionally, the LC resonant circuit may be used to generate the reference excitation signal, such as when the LC resonant circuit is driven by an oscillator. The reference excitation signal may be maintained at a constant amplitude and frequency through a select test period of time. Alternatively, the amplitude and/or frequency of the reference excitation signal may be varied within a select range during individual transmit intervals over the select test period of time. 
     At  236 , the LC resonant circuit is monitored to collect measurements related to one or more characteristics indicative of the resonant behavior of the LC resonant circuit. For example, the measurements may represent impedance, voltage and/or current measurements across input terminals of the LC resonant circuit. When the LC resonant circuit is implemented within an IMD or LP, a processor or microcontroller therein may collect the impedance, voltage and/or current measurements across the LC resonant circuit. Additionally or alternatively, the measurements may be collected by an external device that measures impedance, voltage and/or current measurements across terminals of a separate transmit or receive antenna external to the patient. The measurements may correspond to an amount of reflection experienced as a transmit antenna and/or the amount of reflection experienced at the LC resonant circuit. 
     The operations at  234  and  236  may be performed once when using an excitation pulse with multiple frequency components or may be repeated in an iterative manner in connection with multiple reference signals transmitted at various select frequencies to obtain a collection of measurements indicative of the resonant behavior of the LC resonant circuit at the select frequencies. 
     At  238 , the method determines the resonant frequency of the LC resonant circuit based on the measurements collected at  236 . The resonant frequency is determined from the measurements utilizing various methods. At  238 , the method saves the measurements (collected at  236 ), the resonant frequency (determined at  238 ) and, optionally, the parameters defining the reference signals (transmitted at  234 ). 
     At  240 , the method determines the permittivity of the blood pool that is surrounding or located proximate to the LC resonant circuit. The permittivity may be determined based on the resonant frequency determined at  238 . Optionally, the permittivity maybe based on known constant properties of the capacitive circuit and inductive circuit within the LC resonant circuit. By way of example only, one method for determining permittivity for a medium surrounding an LC resonant circuit is described in U.S. Pat. No. 6,014,029, the complete subject matter of which is incorporated herein by reference in its entirety. More than one permittivity level may be determined at  240  based upon corresponding different resonant frequencies, measurements and reference signals collected at  234 - 238 . The permittivity value or values determined at  240  are stored for the present measurement session along with the additional information determined at  234 - 238 . 
     At  242 , the method measures the glucose level of the blood pool proximate to the LC resonant circuit utilizing a secondary measurement device. The glucose level may be measures in various manners, such as by measuring the glucose level directly from a blood sample or other test method. For example, the glucose level may be determined through fluid chemical analysis, breath chemical analysis, infrared spectroscopy, optical coherence tomography, thermal spectroscopy, ocular spectroscopy, fluorescence or impedance spectroscopy. Alternatively, conventional ultrasound, electromagnetic or thermal techniques may be utilized to measure the blood glucose level. At  242 , the glucose level is saved in connection with the information generated and saved during the operations at  232 - 240 . 
     At  244 , the method determines whether to repeat the process. For example, the operations at  232 - 242  may be repeated at different times of day, and/or while a patient is experiencing different levels of physical or other types of stress. The operations at  232 - 244  may be repeated before and after meals, as well as based on other criteria that may affect a patient&#39;s blood glucose level. When it is determined to repeat the process, flow returns to  234 . Otherwise, the process ends. Optionally, the operations at  232 - 244  may be repeated at the time of device implant or during checkups, where the patient&#39;s blood glucose is temporarily changed while in the hospital or clinic. Optionally, the operations  232 - 244  may be performed in a test lab when the device is manufactured before implant. 
     Each time the operations at  234 - 242  are repeated, the saved information is updated based on a present measurement/data collection session and stored, to create a map defining patterns between SRF, permittivity and glucose levels for the patient. By way of example only, a map may be recorded that includes all or a portion of the data points illustrated in  FIG. 1A . For example, after multiple collection sessions, the data may be determined in connection with a particular patient and LC resonant circuit, where the blood exhibits permittivities of 35 to 65 in connection with corresponding different glucose levels and while the LC resonant circuit exhibits a resonant frequencies of 1 to 15 GHz. 
       FIG. 2C  illustrates a process for monitoring a patient&#39;s blood glucose level in accordance with embodiments herein. At  254 , the system transmits a base excitation signal. The excitation signal may be generated by an external RF transmitter source, such as an antenna located near the patient. Optionally, the base excitation signal may be generated by an implanted transmitter source, such as within an IMD, a leadless pacemaker or other implanted device. Optionally, the base excitation signal may be generated by the same device (e.g., IMD or LP) that includes the glucose sensing device. Optionally, the LC resonant circuit may be used to generate the base excitation signal. The base excitation signal may be maintained at a constant amplitude and frequency for a select test period of time. Alternatively, the amplitude and/or frequency of the base excitation signal may be varied within a select range during an individual transmit interval of the base excitation signal. 
     At  256 , the LC resonant circuit is monitored to collect measurements related to one or more characteristics concerning the resonant behavior of the LC resonant circuit. As noted herein, the measurements may represent impedance, voltage and/or current measurements across the input terminals of the LC resonant circuit. When the LC resonant circuit is implemented within an IMD or LP, a processor or microcontroller therein may collect the impedance, voltage and/or current measurement across the LC resonant circuit. Additionally or alternatively, the measurements may be collected by an external device that measures impedance, voltage and/or current measurements across terminals of a separate transmit or receive antenna. 
     The operations at  254  and  256  may be performed once with a multi-frequency excitation signal or may be repeated in an iterative manner in connection with multiple base excitation signals transmitted at various select frequencies to obtain a collection of measurements indicative of the resonant behavior of the LC resonant circuit at various frequencies. 
     At  258 , the method determines the resonant frequency of the LC resonant circuit based on the measurements collected at  256 . The resonant frequency may be determined from the measurements utilizing various methods. 
     At  260 , the method determines the permittivity of the blood pool that is surrounding or located proximate to the LC resonant circuit. The permittivity may be determined based on the resonant frequency determined at  238 , as well as based on the known constant properties of the capacitive circuit and inductive circuit within the LC resonant circuit. By way of example, the measurements may be compared to one or more previously stored P-SPF models and PG relations associated with the current patient or a collection of patients (such as generated in connection with  FIG. 2B  and illustrated in  FIG. 1A ). The P=SPF models and PG relations designates permittivity to resonant frequency patterns. The measurements collected at  256  are compared to the P-SPF model to identify a corresponding data point having an associated permittivity of the blood pool and resonant frequency of the LC resonant circuit. The data point corresponds to a glucose level that is derived from the P-SPF model. 
     At  262 , the method compares the current blood permittivity with previously recorded permittivity level or levels to identify an amount and nature of change in the blood permittivity. 
     At  264 , the method determines whether the change in permittivity exceeds a threshold, also referred to as a common glucose level threshold. The threshold corresponds to an amount of change in permittivity that may be experienced without representing a change in blood glucose level. It is recognized that blood permittivity may change for other reasons within certain limits, other than due to glucose level changes. The threshold at  264  is set to differentiate between permittivity changes due to blood glucose and other reasons. When the permittivity change exceeds the threshold, flow moves to  266 . Otherwise, flow returns to  254 . 
     At  266 , the method identifies a new glucose level associated with the current permittivity value. For example, the new glucose level may be determined based on one or more templates or other information prerecorded in connection with the present patient, and/or in connection with a control group of patients. 
     Thereafter, the process of  FIG. 2C  may be repeated or the method may enter an idle state until it becomes desirable to repeat the blood glucose test of  FIG. 2C . 
       FIG. 3  is a simplified schematic and block diagram of an embodiment of a leadless glucose sensing device  302  that comprises an active LC resonant circuit glucose sensor. Similar to the device  202  of  FIG. 2A , the device  302  comprises a housing  304 , an LC resonant circuit  306  and a circuit  308  that performs certain glucose sensing-related operations as well as cardiac sensing and/or pacing operations. The LC resonant circuit  306  comprises a capacitive circuit  310  and an inductive circuit  312 . A circuit  318  is configured to sense an oscillating signal  316  of the LC resonant circuit  306 . The circuit  308  comprises a signal processing circuit  320  (which includes one or more microprocessors), a memory circuit  322 , a sensing/pacing circuit  324 , a battery circuit  326 , and electrodes  328  and  330 . Different combinations of these components may be employed in other embodiments constructed in accordance with the teachings herein. 
     In this embodiment, the LC resonant circuit  306  is excited by an internal excitation circuit  314  instead of by external excitation signals. The excitation circuit  314  generates a signal (e.g., a single pulse, a set of pulses, or a periodic pulse signal) that serves to excite the LC resonant circuit  306  and, if applicable, maintain oscillations in the LC resonant circuit  306 . To this end, the excitation circuit  314  may include an oscillator  332  that generates an excitation signal or some other suitable excitation signal generator circuit. 
     In some implementations, the signal processing circuit  320  (or some other suitable circuit of the device  302 ) includes a processor that, among other things, controls the operation of the excitation circuit  314 . For example, upon receipt of a suitable command from an external device (e.g., an external monitoring device) at the signal processing circuit  320 , the excitation circuit  314  may be controlled to commence excitation of the LC resonant circuit  306 . Alternatively, the signal processing circuit  320  may be configured to initiate excitation at certain times (e.g., periodically). Concurrent with either of the above operations, the processor of the signal processing circuit  320  may commence processing of the received oscillating signal  316  and generating data representative of the glucose level in the surrounding blood. 
     A leadless glucose sensing device may communicate with external devices in different ways in different embodiments.  FIGS. 4 and 5  depict two examples illustrating how a leadless glucose sensing device may communicate with different types of external devices. 
       FIG. 4  illustrates an embodiment of a system  400  where a leadless glucose sensing device  402  that is implanted in a patient (not shown) communicates with an external device  404  and an external device  406 . In this example, the passive LC resonant circuit is excited by RF signals  422  generated by the external device  404 , and resulting oscillating signals in the excited LC resonant circuit may be received by the external device  404 . In addition, the device  402  communicates with the external device  406  (e.g., programmer, a home monitor, etc.) to, for example, upload and download information. The external device  404  may collect measurements related to the resonant behavior of the LC resonant circuit  408 . 
     The device  402  includes an LC resonant circuit  408 , a signal processing circuit  410  (having one or more processors), a memory circuit  412 , and a battery circuit  414  that are electrically coupled with one another, if applicable. Several other circuits that would be included in the device  402  are not shown to reduce the complexity of  FIG. 4 . The external device  404  includes an antenna  416  (e.g., a coil) that may be much larger than an effective antenna (e.g., the coil of the inductive circuit) for the LC resonant circuit  408 . For example, the antenna  416  may have dimensions of 12-20 centimeters in diameter while the coil of the inductive circuit may have dimensions of 3-4 millimeters in diameter. In this way, an RF circuit  420  of the external device  404  is able to more effectively couple relatively high frequency RF signals  422  through the tissue of a patient (not shown) to excite the LC resonant circuit  408 . The frequency of RF signals  422  may be at or near the resonant frequency of the LC resonant circuit  408 . 
     The device  402  also includes a telemetry circuit  424  and associated antenna  426  for communicating with the external device  406  via RF signals  428 . For example, the external device  406  may communicate with the device  402  to initiate glucose sensing operations, to upload data generated by the glucose sensing operations, to control cardiac-related operations, and so on. Of note, the external device  406  may employ a smaller antenna (not shown) than the antenna  416  since less RF energy may be required to communicate with the device  402  than is required to excite the LC resonant circuit  408  due to the use of lower frequency RF signals for this communication. 
       FIG. 5  illustrates an embodiment of a system  500  where a leadless glucose sensing device  502  that is implanted in a patient (not shown) communicates with an external device  504 . Similar to the device  402  of  FIG. 4 , the device  502  communicates with the external device  504  (e.g., programmer, a home monitor, etc.) to, for example, upload and download information. In addition, the device  502  includes an LC resonant circuit  506 , a signal processing circuit  508 , and a memory circuit  510  that are electrically coupled in a suitable manner. Several other circuits that would be included in the device  502  are not shown to reduce the complexity of  FIG. 5 . 
     The configuration of  FIG. 5  may be employed in cases where the external device  504  also includes the capability to excite a passive LC resonant circuit glucose sensor of the device  502 . For example, the external device  504  may include a communication circuit  512  that communicates via at least one antenna  514  with a telemetry circuit  516  of the device  502  (as represented by RF signals  518 ) and that excites the LC resonant circuit  506  via RF signals  520 . The use of the single external device  504  for both operations is enabled based on the teachings herein because relative large reactive components may be employed for the LC resonant circuit  506 . The configuration of  FIG. 5  also may be employed in cases where the device  502  employs an active LC resonant circuit glucose sensor. In such a case, the communication circuit  512  would not transmit the RF signals  520  to excite the LC resonant circuit  506 . Rather, the communication circuit  512  would simply communicate with the telemetry circuit  516  via RF signals  518 . 
       FIG. 6  illustrates a cross section of a leadless intra-cardiac medical device formed in accordance with an embodiment. The device  602  includes a housing  604  that houses LC-resonant circuit glucose sensing components and other cardiac-related components (e.g., for sensing and/or pacing). As discussed herein, an LC-resonant circuit includes a capacitive circuit (comprising plates  606  and a dielectric material  608 ) and an inductive circuit (comprising a multi-layer coil  610 ). 
     The plates  606  of the capacitive circuit take the form of a cylinder or a partial cylinder. Here, each cylinder is oriented in a longitudinal direction along the longitudinal axis of the housing  604 . That is, the longitudinal axis of each cylinder is parallel with (or, in some cases, the same as) longitudinal axis of the housing  604 . 
     In other embodiments (not shown in  FIG. 6 ), the capacitive circuit may be housed entirely within (but located adjacent to) the housing  604 . As shown in  FIG. 6 , the inductive circuit may take the form of a cylindrical coil or some other coil-like structure. The inductive circuit may be constructed in various ways. In some embodiments, the inductive circuit is constructed with DFT wire (41% AG or less) or copper wire. The wire may be coated with, for example, ETFE or some other insulation material. In some embodiments, the wire may be relatively thin (e.g., 100 micrometers to 2 mils) so that the coil may have large number of turns, thereby providing a higher value of inductance for a given size coil. The physical properties of the inductive circuit (e.g., the number of coil turns) and the capacitive circuit (e.g., size and distance between the plates  606 ) are selected to provide a desired resonant frequency for the LC resonant circuit. In some embodiments, the resonant circuit has a resonant frequency of 10 GHz or less. 
     The device  604  also includes a circuit  614  (e.g., comprising an integrated circuit and/or discrete circuits) for performing glucose sensing-related operations as taught herein. The circuit  614  is powered by a battery circuit  616 . The device  604  also includes components for performing other cardiac-related operations. In this example, the device  602  includes electrodes  618 ,  620 ,  622 , and  624 . The circuit  614  also may include circuitry for acquiring and processing signals indicative of cardiac activity and for applying stimulation signals to cardiac tissue. For example, for sensing operations, at least one sensing circuit is coupled to one or more of the electrodes  618 - 624  for measuring cardiac electrical activity. In addition, for stimulation operations, at least one signal generator circuit is coupled to one or more of the electrodes  618 - 624  for stimulating cardiac tissue. 
     Electrodes of the device  602  may be configured in different ways for different stimulation operations. In some implementations, the electrode  618  acts as a cathode and the electrode  620  acts as an anode. In other implementations, the electrode  618  acts as the cathode and the housing  604  (e.g., comprising a conductive biocompatible material) acts as the anode. 
     Electrodes of the device  602  may be configured in different ways for different cardiac sensing operations. For example, in some implementations, the electrodes  618  and  620  are used for acquiring near-field signals, while the electrodes  622  and  624  are used for acquiring far-field signals. For sensing, the housing  604  (e.g., comprising a conductive material) and/or another electrode may act as a reference electrode (e.g., ground). As used herein, the term near-field signal refers to a signal that originates in a local chamber (i.e., the same chamber) where the corresponding sense electrodes are located. Conversely, the term far-field signal refers to a signal that originates in a chamber other than the local chamber where the corresponding sense electrodes are located. 
     To facilitate long-term implant within a patient, all external surfaces and materials of the leadless intra-cardiac medical device  602  comprise biocompatible materials. For example, the housing  604  may be constructed of titanium, a ceramic material, or some other suitable biocompatible material. The electrodes  618 - 624  may be constructed of titanium or some other suitable conductive and biocompatible material. In addition, the flexible material may be constructed of polyurethane, silicone or some other suitable flexible and biocompatible (and, optionally electrically insulating) material. Furthermore, in embodiments that employ insulators, the insulators may be constructed of ceramic, polyurethane, silicone or some other suitable electrically insulating and biocompatible material. 
     To facilitate long-term implant, the leadless intra-cardiac medical device  602  is hermetically sealed. To this end, hermetic sealing techniques may be employs to attach the flexible material  612  to the housing  604 . In addition, hermetically sealed feedthroughs may be employed in some embodiments to electrically couple the electrodes  618 - 624  to internal conductors of the leadless intra-cardiac medical device  602 . Alternatively, feedthroughs may not be employed in embodiments where the electrodes  618 - 624  are part of a hermetic housing  604 . In such a case, an electrical connection may be made to an interior surface of these electrodes. 
     In some embodiments, the leadless intra-cardiac medical device  602  is sized to facilitate venous-based implant to a single cardiac chamber (e.g., the RV). For example, the housing  604  may have a cross-sectional width (e.g., outer diameter) of 12 French or less in some embodiments. In addition, to accommodate the internal circuitry (in particular, the battery of the battery circuit  616 ), the housing  604  may have a length of at least 30 millimeters in some embodiments. It should be appreciated; however, that different dimension may be employed in other embodiments. For example, the outer diameter of the housing  604  may be 7 or 8 French of less in some embodiments. Also, the length of the housing  604  may be 30 millimeters or less in some embodiments. 
       FIG. 7  illustrates an example of how a leadless intra-cardiac medical device  702  may be implanted in a chamber of a heart H. In this example, the leadless intra-cardiac medical device  702  is implanted at the apex of the right ventricle (RV) of the heart H. In accordance with the teachings herein, the device  702  includes an LC resonant circuit glucose sensor  712  for measure RV glucose. 
     A distal section of the leadless intra-cardiac medical device  702  comprises a helix electrode  704  that is actively affixed to an inner wall of the RV. The helix electrode  704  in combination with a ring electrode  706  may be used for near-field sensing of RV events. In addition, bipolar electrodes  708  and  710  at a proximal section of the leadless intra-cardiac medical device  702  may be employed for far-field sensing of RA events and/or other cardiac events. Here, the electrodes  708  and  710  may be optimized for such far-field sensing based on, for example, one or more of: placement of the electrodes  708  and  710  at a proximal section of the leadless intra-cardiac medical device  702 , increased spacing between the electrodes  708  and  710 , or increased sizing of the electrodes  708  and  710 . 
       FIG. 8A  illustrates a simplified diagram of an implantable medical device  870  in electrical communication with at least three leads  820 ,  824  and  830  implanted in or proximate a patient&#39;s heart  12  for delivering multi-chamber stimulation (e.g. pacing, ATP therapy, high voltage shocks and the like) according to an embodiment. The stimulation includes defibrillation shocks that are delivered along one or more defibrillation shocking vectors, such as between an RV electrode and a CAN electrode or between RV, SVC and CAN electrodes. The device  870  is also configured to perform ULV based estimation of the DFT based on local conduction information collected along a defibrillation vector and utilizing LV electrodes. As explained below, the leads  820 ,  824  and  830  are used to sense VT and VF and to deliver, among other things, antitachycardia and defibrillation shocks. The device  870  is programmable, by an operator, to set certain operating parameters, as well as therapy-related parameters. The device  870  is configured to operate with various configurations of leads. Exemplary lead configurations are shown in the Figures. The device  870  is configured to deliver various types of therapies. 
     To sense atrial cardiac signals and to provide right atrial chamber stimulation therapy, the medical device  870  is coupled to an implantable right atrial lead  820  having at least an atrial tip electrode  822 , which typically is implanted in the patient&#39;s right atrial appendage. The implantable medical device  870  may be a pacing device, a pacing apparatus, a cardiac rhythm management device, an implantable cardiac stimulation device, an implantable cardioverter/defibrillator (ICD) and/or a cardiac resynchronization therapy (CRT) device. 
     To sense left atrial and ventricular cardiac signals and to provide left chamber pacing therapy, the medical device  870  is coupled to an LV lead  824 . The LV lead  824  may receive atrial and ventricular cardiac signals and deliver left ventricular pacing therapy using a left ventricular (LV) tip electrode  826 , and intermediate LV electrodes  823 ,  825  and  829 . Left atrial pacing therapy uses, for example, first and second left atrial (LA) electrodes  827  and  828 . The LV and LA electrodes  823 - 29  may represent sensing sites, where cardiac signals are sensed, and/or may represent pacing and/or shock therapy sites. A right ventricular lead  830  includes an RV tip electrode  832 , an RV ring electrode  834 , an RV coil electrode  836 , and a superior vena cava (SVC) coil electrode  838  (also known as a RA coil electrode). The right ventricular lead  830  is capable of sensing cardiac signals, and delivering stimulation in the form of pacing and shock therapy to the SVC and/or right ventricle. 
       FIG. 8B  illustrates a cross section of a portion of one or more of the leads  820 ,  824  and  830  in  FIG. 8A  configured to be implanted into a chamber of the heart. A proximal end of the lead is connected to the IMD  870 . The lead  800  includes a lead body  802  extending along a length of the lead  800 . The lead body  802  includes one or more lumen that are configured to receive corresponding conductors (for example helical conductor  804  and  806 ) that electrically connect one or more electrodes  808  on the lead  800  to an IMD (not shown) which may be located proximate to the heart (such as in a sub clavicle pocket). In the example of  FIG. 8 , the electrode  808  represents a ring electrode, although it is understood that other types of electrodes may be provided within the illustrated portion of the lead  800 , as well as at other locations along the length thereof. 
     The lead  800  includes an LC resonant circuit  810  that includes an inductive circuit and a capacitive circuit. The inductive circuit comprises one or more inductor coils  812  wound about a central lumen  813 . The capacitive circuit comprises conductive plates  814  and  816 . The inductive and capacitive circuits are electrically coupled to one another, in series or parallel. Additionally, or alternatively, the capacitive circuit  162  ( FIG. 1B ) or  190  ( FIG. 1E ) or another may be used. 
     Optionally, a second LC resonant circuit may be provided on the lead  800 , where the second LC resonant circuit is constructed to exhibit a resonant frequency that varies in response to pressure changes. In this alternative embodiment, one LC resonant circuit is set to have a resonant frequency sensitive to glucose changes in the blood, while the other LC resonant circuit is configured to have a resonant frequency that varies with pressure change in the surrounding area. The first and second LC resonant circuits have separate and distinct resonant frequencies. As one example, the inductance value for the second LC resonant circuit (responsive to pressure change) may be larger than the inductance value of the LC resonant circuit responsive to glucose concentration. 
     Representative operations relating to glucose sensing by an embodiment of a leadless intra-cardiac medical device in accordance with the teachings herein will be described in more detail in conjunction with the flowchart of  FIG. 10 . For convenience, the operations of  FIG. 10  (or any other operations discussed or taught herein) may be described as being performed by specific components. It should be appreciated, however, that these operations may be performed by other types of components and may be performed using a different number of components. It also should be appreciated that one or more of the operations described herein may not be employed in a given implementation. 
       FIG. 9  illustrates sample components of an embodiment of an implantable leadless intra-cardiac medical device  900  (e.g., a stimulation device such as an implantable cardioverter defibrillator, a pacemaker, etc., or a monitoring device) that may be configured in accordance with the various embodiments that are described herein. It is to be appreciated and understood that other cardiac devices can be used and that the description below is given, in its specific context to assist the reader in understanding, with more clarity, the embodiments described herein. 
     In various embodiments, the device  900  may be adapted to treat both fast and slow arrhythmias with stimulation therapy, including cardioversion, defibrillation, and pacing stimulation. While a particular multi-chamber device is shown, it is to be appreciated and understood that this is done for illustration purposes. Thus, the techniques and methods described below can be implemented in connection with any suitably configured or configurable device. Accordingly, one of skill in the art could readily duplicate, eliminate, or disable the appropriate circuitry in any desired combination to provide a device capable of treating the appropriate chamber(s) with, for example, cardioversion, defibrillation, and pacing stimulation. A housing  905  for the device  900  is often referred to as the “can”, “case” or “case electrode”, and may be programmably selected to act as the return electrode for all “unipolar” modes. The housing  905  may further be used as a return electrode alone or in combination with one or more coil electrodes (not shown) for shocking purposes. As discussed herein, the housing  905  may be constructed of a biocompatible material (e.g., titanium) to facilitate implant within a patient. 
     The device  900  further includes a plurality of terminals that connect the internal circuitry of the device  900  to electrodes  901 ,  902 ,  912 , and  914  of the device  900 . Here, the name of the electrodes to which each terminal is connected is shown next to that terminal. The device  900  may be configured to include various other terminals depending on the requirements of a given application. Thus, it should be appreciated that other terminals (and associated circuitry) may be employed in other embodiments. 
     To achieve right atrial sensing and pacing, a right atrial tip terminal (A.sub.R TIP) is adapted for connection to a right atrial tip electrode  902 . A right atrial ring terminal (A.sub.R RING) may also be included and adapted for connection to a right atrial ring electrode  901 . To achieve right ventricular sensing and pacing, a right ventricular tip terminal (V.sub.R TIP) and a right ventricular ring terminal (V.sub.R RING) are adapted for connection to a right ventricular tip electrode  912  and a right ventricular ring electrode  914 , respectively. 
     At the core of the device  900  is a programmable microcontroller  920  that controls the various modes of stimulation therapy. As is well known in the art, microcontroller  920  typically includes a microprocessor, or equivalent control circuitry, designed specifically for controlling the delivery of stimulation therapy, and may further include memory such as RAM, ROM and flash memory, logic and timing circuitry, state machine circuitry, and I/O circuitry. Typically, microcontroller  920  includes the ability to process or monitor input signals (data or information) as controlled by a program code stored in a designated block of memory. The type of microcontroller is not critical to the described implementations. Rather, any suitable microcontroller  920  may be used that carries out the functions described herein. The use of microprocessor-based control circuits for performing timing and data analysis functions are well known in the art. 
     Representative types of control circuitry that may be used in connection with the described embodiments can include the microprocessor-based control system of U.S. Pat. No. 4,940,052 (Mann et al.), the state-machine of U.S. Pat. No. 4,712,555 (Thornander et al.) and U.S. Pat. No. 4,944,298 (Sholder), all of which are incorporated by reference herein. For a more detailed description of the various timing intervals that may be used within the device and their inter-relationship, see U.S. Pat. No. 4,788,980 (Mann et al.), also incorporated herein by reference. 
       FIG. 9  also shows an atrial pulse generator  922  and a ventricular pulse generator  924  that generate pacing stimulation pulses for delivery by the right atrial electrodes, the right ventricular electrode, or some combination of these electrodes via an electrode configuration switch  926 . It is understood that in order to provide stimulation therapy in each of the four chambers of the heart, the atrial and ventricular pulse generators  922  and  924  may include dedicated, independent pulse generators, multiplexed pulse generators, or shared pulse generators. The pulse generators  922  and  924  are controlled by the microcontroller  920  via appropriate control signals  928  and  930 , respectively, to trigger or inhibit the stimulation pulses. 
     Microcontroller  920  further includes timing control circuitry  932  to control the timing of the stimulation pulses (e.g., pacing rate, atrioventricular (AV) delay, inter-atrial conduction (A-A) delay, or inter-ventricular conduction (V-V) delay, etc.) or other operations, as well as to keep track of the timing of refractory periods, blanking intervals, noise detection windows, evoked response windows, alert intervals, marker channel timing, etc., as known in the art. 
     Microcontroller  920  further includes an arrhythmia detector  934 . The arrhythmia detector  934  may be utilized by the device  900  for determining desirable times to administer various therapies. The arrhythmia detector  934  may be implemented, for example, in hardware as part of the microcontroller  920 , or as software/firmware instructions programmed into the device  900  and executed on the microcontroller  920  during certain modes of operation. 
     Microcontroller  920  may include a morphology discrimination module  936 , a capture detection module  937  and an auto sensing module  938 . These modules are optionally used to implement various exemplary recognition algorithms or methods. The aforementioned components may be implemented, for example, in hardware as part of the microcontroller  920 , or as software/firmware instructions programmed into the device  900  and executed on the microcontroller  920  during certain modes of operation. 
     The electrode configuration switch  926  includes a plurality of switches for connecting the desired terminals (e.g., that are connected to electrodes, coils, sensors, etc.) to the appropriate I/O circuits, thereby providing complete terminal and, hence, electrode programmability. Accordingly, switch  926 , in response to a control signal  942  from the microcontroller  920 , may be used to determine the polarity of the stimulation pulses (e.g., unipolar, bipolar, combipolar, etc.) by selectively closing the appropriate combination of switches (not shown) as is known in the art. 
     Atrial sensing circuits (ATR. SENSE)  944  and ventricular sensing circuits (VTR. SENSE)  946  may also be selectively coupled to the right atrial electrodes and the right ventricular electrodes through the switch  926  for detecting the presence of cardiac activity in each of the four chambers of the heart. Accordingly, the atrial and ventricular sensing circuits  944  and  946  may include dedicated sense amplifiers, multiplexed amplifiers, or shared amplifiers. Switch  926  determines the “sensing polarity” of the cardiac signal by selectively closing the appropriate switches, as is also known in the art. In this way, the clinician may program the sensing polarity independent of the stimulation polarity. The sensing circuits (e.g., circuits  944  and  946 ) are optionally capable of obtaining information indicative of tissue capture. 
     Each sensing circuit  944  and  946  preferably employs one or more low power, precision amplifiers with programmable gain, automatic gain control, bandpass filtering, a threshold detection circuit, or some combination of these components, to selectively sense the cardiac signal of interest. The automatic gain control enables the device  900  to deal effectively with the difficult problem of sensing the low amplitude signal characteristics of atrial or ventricular fibrillation. 
     The outputs of the atrial and ventricular sensing circuits  944  and  946  are connected to the microcontroller  920 , which, in turn, is able to trigger or inhibit the atrial and ventricular pulse generators  922  and  924 , respectively, in a demand fashion in response to the absence or presence of cardiac activity in the appropriate chambers of the heart. Furthermore, as described herein, the microcontroller  920  is also capable of analyzing information output from the sensing circuits  944  and  946 , a data acquisition system  952 , or both. This information may be used to determine or detect whether and to what degree tissue capture has occurred and to program a pulse, or pulses, in response to such determinations. The sensing circuits  944  and  946 , in turn, receive control signals over signal lines  948  and  950 , respectively, from the microcontroller  920  for purposes of controlling the gain, threshold, polarization charge removal circuitry (not shown), and the timing of any blocking circuitry (not shown) coupled to the inputs of the sensing circuits  944  and  946  as is known in the art. 
     For arrhythmia detection, the device  900  utilizes the atrial and ventricular sensing circuits  944  and  946  to sense cardiac signals to determine whether a rhythm is physiologic or pathologic. It should be appreciated that other components may be used to detect arrhythmia depending on the system objectives. In reference to arrhythmias, as used herein, “sensing” is reserved for the noting of an electrical signal or obtaining data (information), and “detection” is the processing (analysis) of these sensed signals and noting the presence of an arrhythmia. 
     Timing intervals between sensed events (e.g., P-waves, R-waves, and depolarization signals associated with fibrillation) may be classified by the arrhythmia detector  934  of the microcontroller  920  by comparing them to a predefined rate zone limit (e.g., bradycardia, normal, low rate VT, high rate VT, and fibrillation rate zones) and various other characteristics (e.g., sudden onset, stability, physiologic sensors, and morphology, etc.) in order to determine the type of remedial therapy that is needed (e.g., bradycardia pacing, antitachycardia pacing, cardioversion shocks or defibrillation shocks, collectively referred to as “tiered therapy”). Similar rules may be applied to the atrial channel to determine if there is an atrial tachyarrhythmia or atrial fibrillation with appropriate classification and intervention. 
     Cardiac signals or other signals may be applied to inputs of an analog-to-digital (A/D) data acquisition system  952 . The data acquisition system  952  is configured (e.g., via signal line  956 ) to acquire intra-cardiac electrogram (“IEGM”) signals or other signals, convert the raw analog data into a digital signal, and store the digital signals for later processing, for telemetric transmission to an external device  954 , or both. For example, the data acquisition system  952  may be coupled to the right atrial electrodes and the right ventricular electrodes through the switch  926  to sample cardiac signals across any pair of desired electrodes. 
     The data acquisition system  952  also may be coupled to receive signals from other input devices. For example, the data acquisition system  952  may sample signals from a physiologic sensor  970  or other components shown in  FIG. 9  (connections not shown). 
     The microcontroller  920  is further coupled to a memory  960  by a suitable data/address bus  962 , wherein the programmable operating parameters used by the microcontroller  920  are stored and modified, as required, in order to customize the operation of the device  900  to suit the needs of a particular patient. Such operating parameters define, for example, pacing pulse amplitude, pulse duration, electrode polarity, rate, sensitivity, automatic features, arrhythmia detection criteria, and the amplitude, waveshape and vector of each shocking pulse to be delivered to the patient&#39;s heart H within each respective tier of therapy. One feature of the described embodiments is the ability to sense and store a relatively large amount of data (e.g., from the data acquisition system  952 ), which data may then be used for subsequent analysis to guide the programming of the device  900 . 
     Advantageously, the operating parameters of the implantable device  900  may be non-invasively programmed into the memory  960  through a telemetry circuit  964  in telemetric communication via communication link  966  with the external device  954 , such as a programmer, transtelephonic transceiver, a diagnostic system analyzer or some other device. The microcontroller  920  activates the telemetry circuit  964  with a control signal (e.g., via bus  968 ). The telemetry circuit  964  advantageously allows intra-cardiac electrograms and status information relating to the operation of the device  900  (as contained in the microcontroller  920  or memory  960 ) to be sent to the external device  954  through an established communication link  966 . 
     The device  900  includes one or more physiologic sensors  970 . At least one sensor  970  comprises a glucose sensor as taught herein. In some embodiments, the device  900  may include a “rate-responsive” sensor that may provide, for example, information to aid in adjustment of pacing stimulation rate according to the exercise state of the patient. One or more physiologic sensors  970  (e.g., a glucose sensor) may be used to detect changes in cardiac output, changes in the physiological condition of the heart, or diurnal changes in activity (e.g., detecting sleep and wake states). Accordingly, the microcontroller  920  may respond to this sensing by adjusting the various pacing parameters (such as rate, A-V Delay, V-V Delay, etc.) at which the atrial and ventricular pulse generators  922  and  924  generate stimulation pulses. 
     While shown as being included within the device  900 , it is to be understood that a physiologic sensor  970  may also be external to the device  900 , yet still be implanted within or carried by the patient. Examples of physiologic sensors that may be implemented in conjunction with the device  900  include sensors that sense respiration rate, pH of blood, ventricular gradient, oxygen saturation, blood glucose and so forth. Another sensor that may be used is one that detects activity variance, wherein an activity sensor is monitored diurnally to detect the low variance in the measurement corresponding to the sleep state. For a more detailed description of an activity variance sensor, the reader is directed to U.S. Pat. No. 5,476,483 (Bornzin et al.), which patent is hereby incorporated by reference. 
     The one or more physiologic sensors  970  may optionally include one or more of components to help detect movement (via, e.g., a position sensor or an accelerometer) and minute ventilation (via an MV sensor) in the patient. Signals generated by the position sensor and MV sensor may be passed to the microcontroller  920  for analysis in determining whether to adjust the pacing rate, etc. The microcontroller  920  may thus monitor the signals for indications of the patient&#39;s position and activity status, such as whether the patient is climbing up stairs or descending down stairs or whether the patient is sitting up after lying down. 
     The device  900  additionally includes a battery  976  that provides operating power to all of the circuits shown in  FIG. 9 . For a device  900  which employs shocking therapy, the battery  976  is capable of operating at low current drains (e.g., preferably less than 10 .mu.A) for long periods of time, and is capable of providing high-current pulses (for capacitor charging) when the patient requires a shock pulse (e.g., preferably, in excess of 2 A, at voltages above 200 V, for periods of 10 seconds or more). The battery  976  also desirably has a predictable discharge characteristic so that elective replacement time can be detected. Accordingly, the device  900  preferably employs lithium or other suitable battery technology. 
     The device  900  can further include magnet detection circuitry (not shown), coupled to the microcontroller  920 , to detect when a magnet is placed over the device  900 . A magnet may be used by a clinician to perform various test functions of the device  900  and to signal the microcontroller  920  that the external device  954  is in place to receive data from or transmit data to the microcontroller  920  through the telemetry circuit  964 . 
     The device  900  further includes an impedance measuring circuit  978  that is enabled by the microcontroller  920  via a control signal  980 . The known uses for an impedance measuring circuit  978  include, but are not limited to, electrode impedance surveillance during the acute and chronic phases for proper performance, electrode positioning or dislodgement; detecting operable electrodes and automatically switching to an operable pair if dislodgement occurs; measuring respiration or minute ventilation; measuring thoracic impedance for determining shock thresholds; detecting when the device  900  has been implanted; measuring stroke volume; and detecting the opening of heart valves, etc. The impedance measuring circuit  978  is advantageously coupled to the switch  926  so that any desired electrode may be used. 
     In the case where the device  900  is intended to operate as an implantable cardioverter/defibrillator (ICD) device, it detects the occurrence of an arrhythmia, and automatically applies an appropriate therapy to the heart aimed at terminating the detected arrhythmia. To this end, the microcontroller  920  may include a shocking circuit (not shown). The shocking circuit generates shocking pulses of low (e.g., up to 0.5 J), moderate (e.g., 0.5 J to 10 J), or high energy (e.g., 11 J to 40 J), as controlled by the microcontroller  920 . Such shocking pulses may be applied to the patient&#39;s heart H through, for example, two or more shocking electrodes (not shown). 
     Cardioversion level shocks are generally considered to be of low to moderate energy level (so as to minimize pain felt by the patient), be synchronized with an R-wave, pertain to the treatment of tachycardia, or some combination of the above. Defibrillation shocks are generally of moderate to high energy level (i.e., corresponding to thresholds in the range of 5 J to 40 J), delivered asynchronously (since R-waves may be too disorganized), and pertaining to the treatment of fibrillation. Accordingly, the microcontroller  920  is capable of controlling the synchronous or asynchronous delivery of the shocking pulses. 
     The device  900  thus illustrates several components that may provide the implantable intra-cardiac medical device functionality described above. For example, the microcontroller  920  may control and perform the operations of  FIGS. 2B and 2C . The microcontroller  920  is configured to collect measurements indicative of the resonant frequency of the LC resonant circuit; and calculate data indicative of a glucose level of the blood based on the measurements indicative of the resonant frequency. The microcontroller is further configured to repeat the collecting and calculating operations during at least first and second test intervals, and to determine whether the glucose level changes between the first and second test intervals. For example, the microcontroller  920  (e.g., a processor providing signal processing functionality) may implement or support at least a portion of the processing functionality discussed above. Also, one or more of the switch  926 , the sense circuits  944 ,  946 , and the data acquisition system  952  may acquire cardiac signals that are used in the signal acquisition operations discussed above. Similarly, one or more of the switch  926  and the pulse generator circuits  922 ,  924  may be used to provide stimulation signals that are used in the cardiac stimulation operations discussed above. The data described above (e.g., glucose data and/or cardiac data) may be stored in the data memory  960 . The physiologic sensors  970  may comprise the glucose sensor(s) discussed above. Thus, in general, the processing circuitry described herein (e.g., the circuit  208 ,  308 , or  614 , etc.) may correspond to one or more of the illustrated components of the device  900 . 
       FIG. 10  illustrates a process for measuring glucose concentration. As represented by block  1002  of  FIG. 10 , at some point in time, the leadless intra-cardiac medical device commences glucose sensing. For example, the device may receive a message (e.g., a command from an external monitor device) or some other type of signal (e.g., an RF signal from an external device that provides an RF excitation signal) from an external device. As a result of the receipt of the message or signal, the leadless intra-cardiac medical device may commence processing the oscillating signal from the LC resonant circuit and generating data representative of glucose levels. As another example, the leadless intra-cardiac medical device may be configured to periodically conduct glucose sensing operations. 
     As represented by block  1004 , in embodiments where the leadless intra-cardiac medical device comprises an active LC resonant circuit, the device may trigger an excitation circuit to excite the LC resonant circuit. As discussed herein, this trigger may be based on receipt of a message or signal, based on a glucose sensing schedule (e.g., periodic sensing) implemented at the device, or based on some other factor(s). 
     As represented by block  1006 , the leadless intra-cardiac medical device processes signal produced by the excited LC resonant circuit to determine at least one frequency of the signals. For example, the device may monitor the frequency of the signals over a period of time to determine how the frequency varies over that period of time. 
     As represented by block  1008 , the leadless intra-cardiac medical device generates data representative of the glucose level based on the at least one frequency determined at block  1006 . For example, the device may generate data indicative of how the measured cardiac glucose varies over a designated period of time. 
     As represented by block  1010 , the leadless intra-cardiac medical device transmits the data generated at block  1008  to an external device (e.g., an external monitoring device) via RF signaling. For example, the leadless intra-cardiac medical device may send this information on-demand (e.g., in response to a message), according to a schedule (e.g., periodically), or in some other suitable manner. 
       FIG. 11  is a simplified diagram of a device  1102  (implanted within a patient P) that communicates with a device  1104  that is located external to the patient P. The implanted device  1102  and the external device  1104  communicate with one another via a wireless communication link  1106  (as represented by the depicted wireless symbol). 
     In the illustrated example, the implanted device  1102  is a leadless intra-cardiac medical device including an LC resonant circuit glucose sensor (not shown) in accordance with the teachings herein. For example, the implanted device  1102  may be a pacemaker, an implantable cardioverter defibrillator, or some other similar device. It should be appreciated, however, that the implanted device  1102  may take other forms. 
     The external device  1104  also may take various forms. For example, the external device  1104  may be a base station, a programmer, a home safety monitor, a personal monitor, a follow-up monitor, a wearable monitor, or some other type of device that is configured to communicate with the implanted device  1102 . The communication link  1106  may be used to transfer information between the devices  1102  and  1104  in conjunction with various applications such as remote home-monitoring, clinical visits, data acquisition, remote follow-up, and portable or wearable patient monitoring/control systems. For example, when information needs to be transferred between the devices  1102  and  1104 , the patient P moves into a position that is relatively close to the external device  1104 , or vice versa. 
     The external device  1104  may send information it receives from an implanted device to another device (e.g., that may provide a more convenient means for a physician to review the information). For example, the external device  1104  may send the information to a web server (not shown). In this way, the physician may remotely access the information (e.g., by accessing a website). The physician may then review the information uploaded from the implantable device to determine whether medical intervention is warranted. 
     It should be appreciated that various modifications may be incorporated into the disclosed embodiments based on the teachings herein. For example, the structure and functionality taught herein may be incorporated into types of devices other than the specific types of devices described above. In addition, glucose sensors for a leadless intra-cardiac medical device may be implemented in different ways in different embodiments based on the teachings herein. Different types of structural members and mechanical support structures may be employed in conjunction with a leadless intra-cardiac medical device as taught herein. Also, various algorithms or techniques may be employed to monitor glucose in various cardiac chambers (e.g., RA, RV, LA, and LV chambers) in accordance with the teachings herein. In some aspects, an apparatus or any component of an apparatus may be configured to provide functionality as taught herein by, for example, manufacturing (e.g., fabricating) the apparatus or component so that it will provide the functionality, by programming the apparatus or component so that it will provide the functionality, or through the use of some other suitable means. 
     In some embodiments, code including instructions (e.g., software, firmware, middleware, etc.) may be executed on one or more processing devices to implement one or more of the described functions or components. The code and associated components (e.g., data structures and other components used by the code or used to execute the code) may be stored in an appropriate data memory that is readable by a processing device (e.g., commonly referred to as a computer-readable medium). 
     Moreover, some of the operations described herein may be performed by a device that is located externally with respect to the body of the patient. For example, an implanted device may send raw data or processed data to an external device that then performs the necessary processing. 
     The signals discussed herein may take various forms. For example, in some embodiments a signal may comprise electrical signals transmitted over a wire, light pulses transmitted through an optical medium such as an optical fiber or air, or RF waves transmitted through a medium such as air, and so on. In addition, a plurality of signals may be collectively referred to as a signal herein. The signals discussed above also may take the form of data. For example, in some embodiments an application program may send a signal to another application program. Such a signal may be stored in a data memory. 
     Moreover, the recited order of the blocks in the processes disclosed herein is simply an example of a suitable approach. Thus, operations associated with such blocks may be rearranged while remaining within the scope of the present disclosure. Similarly, the accompanying method claims present operations in a sample order, and are not necessarily limited to the specific order presented. 
     Also, it should be understood that any reference to elements herein using a designation such as “first,” “second,” and so forth does not generally limit the quantity or order of those elements. Rather, these designations may be used herein as a convenient method of distinguishing between two or more different elements or instances of an element. Thus, a reference to first and second elements does not mean that only two elements may be employed there or that the first element must precede the second element in some manner. Also, unless stated otherwise a set of elements may comprise one or more elements. In addition, terminology of the form “at least one of A, B, or C” or “one or more of A, B, or C” or “at least one of the group consisting of A, B, and C” used in the description or the claims means “A or B or C or any combination of these elements.” For example, this terminology may include A, or B, or C, or A and B, or A and C, or A and B and C, or 2A, or 2B, or 2C, and so on. 
     As used herein, the term “determining” encompasses a wide variety of actions. For example, “determining” may include calculating, computing, processing, deriving, investigating, looking up (e.g., looking up in a table, a database or another data structure), ascertaining, and the like. Also, “determining” may include receiving (e.g., receiving information), accessing (e.g., accessing data in a memory), and the like. Also, “determining” may include resolving, selecting, choosing, establishing, and the like. 
     While certain embodiments have been described above in detail and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive of the teachings herein. In particular, it should be recognized that the teachings herein apply to a wide variety of apparatuses and methods. It will thus be recognized that various modifications may be made to the illustrated embodiments or other embodiments, without departing from the broad scope thereof. In view of the above, it will be understood that the teachings herein are intended to cover any changes, adaptations or modifications that are within the scope of the disclosure. 
     The (module/controller) may include any processor-based or microprocessor-based system including systems using microcontrollers, reduced instruction set computers (RISC), application specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), logic circuits, and any other circuit or processor capable of executing the functions described herein. Additionally or alternatively, the (module/controller) represent circuit modules that may be implemented as hardware with associated instructions (for example, software stored on a tangible and non-transitory computer readable storage medium, such as a computer hard drive, ROM, RAM, or the like) that perform the operations described herein. The above examples are exemplary only, and are thus not intended to limit in any way the definition and/or meaning of the term “controller.” The (module/controller) may execute a set of instructions that are stored in one or more storage elements, in order to process data. The storage elements may also store data or other information as desired or needed. The storage element may be in the form of an information source or a physical memory element within the (module/controller) The set of instructions may include various commands that instruct the (module/controller) to perform specific operations such as the methods and processes of the various embodiments of the subject matter described herein. The set of instructions may be in the form of a software program. The software may be in various forms such as system software or application software. Further, the software may be in the form of a collection of separate programs or modules, a program module within a larger program or a portion of a program module. The software also may include modular programming in the form of object-oriented programming. The processing of input data by the processing machine may be in response to user commands, or in response to results of previous processing, or in response to a request made by another processing machine. 
     It is to be understood that the subject matter described herein is not limited in its application to the details of construction and the arrangement of components set forth in the description herein or illustrated in the drawings hereof. The subject matter described herein is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. 
     It is to be understood that the above description is intended to be illustrative, and not restrictive. For example, the above-described embodiments (and/or aspects thereof) may be used in combination with each other. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. While the dimensions, types of materials and coatings described herein are intended to define the parameters of the invention, they are by no means limiting and are exemplary embodiments. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. Further, the limitations of the following claims are not written in means—plus-function format and are not intended to be interpreted based on 35 U.S.C. §112(f), unless and until such claim limitations expressly use the phrase “means for” followed by a statement of function void of further structure.