Abstract:
A system and surgical procedure for monitoring physiological parameters within an internal organ of a living body. The procedure entails making a first incision to expose the organ and a second incision through an external wall of the organ and into an internal cavity. A sensing unit is placed in the second incision such that a proximal end thereof remains outside the organ. The unit includes a sensing device having a sensing element for sensing the physiological parameter within the organ, and an anchor to which the sensing device is secured. The unit occludes the second incision and a distal end of the unit does not extend more than one centimeter into the cavity. The anchor is then secured to the wall of the organ, after which the first incision is closed and a readout device telemetrically communicates with the sensing device to obtain a reading of the physiological parameter.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application Nos. 60/926,713 filed Apr. 30, 2007, 60/937,323 filed Jun. 28, 2007, and 61/009,190 filed Dec. 26, 2007. The contents of these prior patent applications are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention generally relates to implantable medical devices, monitoring systems and associated procedures. More particularly, this invention relates to an implantable medical sensing unit, a sensing system, and a procedure for monitoring physiological properties of a living body, such as pressure, temperature, flow, acceleration, vibration, composition, and other properties of biological fluids within an internal organ. 
         [0003]    Following open heart surgery in high risk patients, postoperative hemodynamic monitoring has been performed by pulmonary artery catheterization (PAC), which involves the insertion of a catheter into a pulmonary artery. The pulmonary artery catheter, often referred to as a Swan-Ganz catheter, allows for the measurement of pressures in the right atrium, right ventricle, pulmonary artery, and the filling (“wedge”) pressure of the left atrium. However, a significant drawback of PAC is that the catheter is invasive, expensive, and carries morbidity. 
         [0004]    More recently, various implantable devices have been developed to monitor and wirelessly communicate physiological parameters of the heart, as well as physiological parameters of other internal organs, including the brain, bladder and eyes. Such predicate wireless devices can generally be divided into two functional categories: large-sized (pacemaker-type) and smaller-sized telemetric devices. An example of a pacemaker-type wireless pressure sensor is the LVP-1000 Left Ventricular Pressure Monitoring System under development by Transoma Medical, Inc. The LVP-1000 comprises a sensor adapted to be implanted into an external wall of the heart, a wireless transmitting unit adapted to be located elsewhere within the patient, and wiring that physically and electrically connects the sensor and transmitting unit. The sensor of the LVP-1000 is adapted to be secured with sutures to the left side of the heart during an open-chest surgical procedure. 
         [0005]    Smaller telemetric sensors include batteryless pressure sensors developed by CardioMEMS, Inc., Remon Medical, and the assignee of the present invention, Integrated Sensing Systems, Inc. (ISSYS). For example, see commonly-assigned U.S. Pat. Nos. 6,926,670 and 6,968,734 to Rich et al., and N. Najafi and A. Ludomirsky, “Initial Animal Studies of a Wireless, Batteryless, MEMS Implant for Cardiovascular Applications,” Biomedical Microdevices, 6:1, p. 61-65 (2004). With such technologies, pressure changes are typically sensed with an implant equipped with a mechanical capacitor (tuning capacitor) having a fixed electrode and a moving electrode, for example, on a diaphragm that deflects in response to pressure changes. The implant is further equipped with an inductor in the form of a fixed coil that serves as an antenna for the implant, such that the implant is able to receive radio frequency (RF) signals from outside the patient and transmit the frequency output of the circuit. The implant can be placed with a catheter, for example, directly within the heart chamber whose pressure is to be monitored, or in an intermediary structure such as the atrial or ventricular septum. 
         [0006]      FIGS. 1   a  and  1   b  represent two types of wireless pressure sensing schemes disclosed in the Rich et al. patents. In  FIG. 1   a , an implant  10  is shown as operating in combination with a non-implanted external reader unit  20 , between which a wireless telemetry link is established using a resonant scheme. The implant  10  contains a packaged inductor coil  12  and a pressure sensor in the form of a mechanical capacitor  14 . Together, the inductor coil  12  and capacitor  14  form an LC (inductor-capacitor) tank resonator circuit that has a specific resonant frequency, expressed as 1/(LC) 1/2 , which can be detected from the impedance of the circuit. At the resonant frequency, the circuit presents a measurable change in magnetically-coupled impedance load to an external coil  22  associated with the reader unit  20 . Because the resonant frequency is a function of the capacitance of the capacitor  14 , the resonant frequency of the LC circuit changes in response to pressure changes that alter the capacitance of the capacitor  14 . Based on the coil  12  being fixed and therefore having a fixed inductance value, the reader unit  20  is able to determine the pressure sensed by the implant  10  by monitoring the resonant frequency of the circuit. 
         [0007]      FIG. 1   b  shows another wireless pressure sensor implant  30  operating in combination with a non-implanted external reader unit  50 . A wireless telemetry link is established between the implant  30  and reader unit  50  using a passive, magnetically-coupled scheme, in which on-board circuitry of the implant  30  receives power from the reader unit  50 . In the absence of the reader unit  50 , the implant  30  lays passive and without any internal means to power itself. When a pressure reading is desired, the reader unit  50  must be brought within range of the implant  30 . 
         [0008]    In  FIG. 1   b , the implant  30  contains a packaged inductor coil  32  and a pressure sensor in the form of a mechanical capacitor  34 . The reader unit  50  has a coil  52  by which an alternating electromagnetic field is transmitted to the coil  32  of the implant  30  to induce a voltage in the implant  30 . When sufficient voltage has been induced in the implant  30 , a rectification circuit  38  converts the alternating voltage on the coil  32  into a direct voltage that can be used by electronics  40  as a power supply for signal conversion and communication. At this point the implant  30  can be considered alert and ready for commands from the reader unit  50 . The implant  30  may employ the coil  32  as an antenna for both reception and transmission, or it may utilize the coil  32  solely for receiving power from the reader unit  50  and employ a second coil  42  for transmitting signals to the reader unit  50 . Signal transmission circuitry  44  receives an encoded signal generated by signal conditioning circuitry  46  based on the output of the capacitor  34 , and then generates an alternating electromagnetic field that is propagated to the reader unit  50  with the coil  42 . 
         [0009]    The implant  30  is shown in  FIG. 1   b  without a battery, and therefore its operation does not require occasional replacement or charging of a battery. Instead, the energy required to perform the sensing operation is entirely derived from the reader unit  50 . However, the implant  30  of  FIG. 1   b  could be modified to use a battery or other power storage device to power the implant  30  when the reader unit  50  is not sufficiently close to induce a voltage in the implant  30 . 
         [0010]    Small telemetric sensors of the types described above are adapted for implantation within the heart using a catheter or other minimally invasive outpatient technique, and not through the exterior wall of the heart during surgery. 
       BRIEF SUMMARY OF THE INVENTION 
       [0011]    The present invention provides a system and surgical procedure for monitoring one or more physiological parameters within an internal organ of a living body. The system and procedure are particularly well-suited for performing short-term monitoring of organs such as the heart, brain, kidneys, lungs, bladder, etc., with a miniature implantable sensing device placed in an external wall of the organ, such as a wall of the heart, blood vessel, kidneys, lungs, bladder, etc., or a wall surrounding the organ, such as the abdominal wall or the meninges surrounding the brain. A particular but nonlimiting example is monitoring the left heart filling pressures for post-operative care of patients following open chest surgery, such as bypass surgery, heart valve surgery, and heart transplant surgery. In this regard, the present invention is intended to replace the need for postoperative pressure monitoring using such traditional invasive PAC techniques, subarachnoid devices used to monitor intracranial pressures, and other similar sensors. 
         [0012]    A system and surgical procedure make use of at least one sensing unit adapted to be implanted in the living body and attached to an organ therein, and a readout device that is not adapted to be implanted in the living body. The sensing unit includes a sensing device having a sensing element adapted to sense the physiological parameter within the organ, and an anchor to which the sensing device is secured. The sensing unit is adapted for placement in an incision in an external wall of the organ so that a proximal end of the sensing unit is outside the organ, an oppositely-disposed distal end of the sensing unit does not extend more than one centimeter into the internal cavity, and the sensing unit occludes the incision. The readout device telemetrically communicates with the sensing device to obtain a reading of the physiological parameter. 
         [0013]    The surgical procedure generally entails making a first incision in a living body to expose at least a portion of an internal organ of the living body. A second incision is then made through an external wall of the organ and into an internal cavity within which an internal physiological parameter is desired to be sensed. A sensing unit is then placed in the second incision such that a proximal end of the sensing unit is outside the organ. The sensing unit includes a sensing device having a sensing element adapted to sense the physiological parameter within the organ, and an anchor to which the sensing device is secured. The sensing unit is placed in the second incision so that a distal end of the sensing unit does not extend more than one centimeter into the internal cavity. The anchor is then secured to the external wall of the organ such that the sensing device is secured within the second incision by only the anchor and the second incision is occluded by only the sensing unit. The first incision can then be closed, after which a readout device located outside the living body is used to telemetrically communicate with the sensing device to obtain a reading of the physiological parameter. 
         [0014]    The monitoring system and procedure are intended to be particularly well-suited for providing safe, fast, detailed, real-time, and continuous cardiac pressure measurements for short-term applications, such as during an operation (intra-operative) and postoperative monitoring over a period of hours, days or weeks in an emergency room or hospital. In cases where the patient is moved to a rehabilitation facility, the implant can be utilized for much longer periods and data relating to the physiological parameter(s) being monitored can be wirelessly sent to a physician or nurse in order to provide diagnostic tailored treatment of the patient. For patients that need even longer term monitoring, at-home monitoring can be easily accomplished by tying the readout device to the Internet, telephone, or other long-distance communication system. The wireless, batteryless operation of the sensing unit allows the unit to potentially function for a patient&#39;s lifetime with no maintenance or need for sensor replacement after initial implantation. 
         [0015]    Miniaturization of the sensing unit can be effectively achieved by fabricating the sensing element as a miniature MEMS (micro-electromechanical system) sensor, combined with custom electronics and a telemetry antenna. Another preferred aspect of the invention is to limit the volume protrusion of the sensing unit into the cavity being monitored. In the case of the heart, the risk of thrombogenesis can be significantly reduced by limiting protrusion of the sensor unit into the blood stream within a heart chamber, in terms of distance into the cavity as well as shape and size of the protruding portion of the sensing unit. For this purpose, the sensing device is preferably configured so that the sensing element is located on a distal surface (relative to insertion direction) of the device, such that only the distal surface of the sensing device need contact the biological fluid being monitored. 
         [0016]    Other objects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIGS. 1   a  and  1   b  are block diagrams of wireless pressure monitoring systems that utilize resonant and passive sensing schemes, respectively, which can be utilized by monitoring systems of this invention. 
           [0018]      FIGS. 2   a  and  2   b  are schematic representations of a wireless sensing device and a readout device suitable for use in wireless monitoring systems of this invention. 
           [0019]      FIG. 3  schematically represents internal components of processing circuitry suitable for use in the sensing device of  FIG. 2   a.    
           [0020]      FIG. 4  represents a perspective view of a cylindrical self-contained sensing device of the type represented in  FIG. 2   a.    
           [0021]      FIG. 5  schematically represents the sensing device of  FIG. 4  assembled with a dome-type anchor to form a sensing unit ready for implantation. 
           [0022]      FIG. 6  schematically represents the sensing unit of  FIG. 5  implanted in a wall of an internal organ in accordance with an embodiment of the invention. 
           [0023]      FIG. 7  schematically represents an exploded view of the sensing device of  FIG. 5 . 
           [0024]      FIG. 8  schematically represents a cross-sectional view of the sensing unit of  FIG. 5  implanted in a wall of an internal organ. 
           [0025]      FIG. 9  schematically represents the sensing device of  FIG. 4  assembled with a bolt-type anchor in accordance with another embodiment of the invention. 
           [0026]      FIGS. 10 and 11  schematically represent sensing units equipped with alternative bolt-type anchors implanted in a wall of an internal organ. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    Illustrated in  FIGS. 2   a  through  11  are monitoring systems and components thereof that implement one or more implantable sensors configured to be placed through an external wall of an internal organ for monitoring one or more physiological parameters within an internal cavity of the organ. Organs of particular interest include but are not limited to the heart, brain, kidneys, lungs, and bladder. Each system preferably makes use of a sensing unit that consists essentially of a sensing device and anchor, meaning that the sensing units lack a wire, cable, tether, or other physical component that conducts the output of the sensing device to a separate location where another component utilizes the output of the sensing device and/or transmits the output of the sensing device to a location outside the body of the patient. As such, the physical footprint of the implanted portion of the monitoring system can be limited to the sensing device and its anchor, such that the sensing unit is far smaller than, for example, the Transoma Medical, Inc., LVP-1000 Left Ventricle Pressure Monitoring System, which must be physically connected to a relatively large remote transmitting device. 
         [0028]    While the resonant and passive schemes described in reference to  FIGS. 1   a  and  1   b  are also within the scope of the invention, sensing devices of this invention are more preferably intended to translate a physiologic parameter into a frequency tone and modulate the impedance of an antenna with the frequency tone to communicate the physiologic parameter to an external readout unit.  FIG. 2   a  represents a wireless implantable sensing device  60  as comprising a transducer  62 , electronic circuitry  64  (e.g., an application specific integrated circuit, or ASIC), and an antenna  66 . The antenna  66  is shown as comprising windings  68  (e.g., copper wire) wrapped around a core  70  (e.g., ferrite), though other antenna configurations and materials are foreseeable. The transducer  62  is preferably a MEMS device, more particularly a micromachine fabricated by additive and subtractive processes performed on a substrate. The substrate can be rigid, flexible, or a combination of rigid and flexible materials. Notable examples of rigid substrate materials include glass, semiconductors, silicon, ceramics, carbides, metals, hard polymers, and TEFLON. Notable flexible substrate materials include various polymers such as parylene and silicone, or other biocompatible flexible materials. A particular but nonlimiting example of the transducer  62  is a MEMS capacitive pressure sensor for sensing pressure, such as various blood pressures within the heart, intracranial pressure, intraocular pressure, etc., though other materials and any variety of sensing elements, e.g., capacitive, inductive, resistive, piezoelectric, etc., could be used. For example, the transducer  62  could be configured to sense temperature, flow, acceleration, vibration, pH, conductivity, dielectric constant, and chemical composition, including the composition and/or contents of a biological fluid, for example, oxygen, carbon dioxide, glucose, gene, hormone, or gas content of the fluid. The sensing device  60  may be powered with a battery or other power storage device, but in preferred embodiments is powered entirely by a readout device, such as a readout unit  80  represented in  FIG. 2   b.    
         [0029]    In addition to powering the sensing device  60 , the readout unit  80  is represented as being configured to receive an output signal from the sensing device  60 , process the signal, and relay the processed signal as data in a useful form to a user. The readout unit  80  is shown equipped with circuitry  82  that generates a high-frequency (e.g., 13.56 MHz), high-power signal for an antenna  84  to create the magnetic field needed in communicate with the sensing device  60 . The readout unit  80  contains additional circuitry  86  to receive and demodulate a backscattered signal from the sensing device  60 , which is demodulated and then processed with a processing unit  88  using calibration coefficients to quantify the physiological parameter of interest. The readout unit  80  is further shown as being equipped with a user interface  90 , by which the operation of the readout unit  80  can be controlled to allow data logging or other user control and data examination. 
         [0030]      FIG. 3  represents a block diagram showing particularly suitable components for the electronic circuitry  64  of  FIG. 2   a . The circuitry  64  includes an oscillator  92 , for example a relaxation oscillator, connected to a resistor  93  and a MEMs mechanical capacitor  94 . A preferred MEMS capacitor  94  comprises a fixed electrode and a moving electrode on a diaphragm that deflects relative to the fixed electrode in response to pressure, such that the capacitor  94  is able to serve as a pressure sensing element for the transducer  62 . A nonlimiting example of a preferred MEMS capacitor  94  has a pressure range of about −100 to about +300 mmHg, with an accuracy of about 1 mmHg. Alternatively, a variable resistor transducer could be used with a fixed capacitance, or an inductor could be substituted for the transducer or fixed circuit element. Based on the RC or other time constant (1/(LC) 1/2 ), the oscillator  92  produces a frequency tone that directly relates to the capacitive value of the capacitor  94  and, therefore, the physiologic parameter of interest. 
         [0031]    The circuitry  64  is further shown as including a modulator  96 , with which the frequency tone of the oscillator  92  is encoded on a carrier frequency, placed on the antenna  66 , and then transmitted to the readout unit  80 . This is accomplished simply by opening and closing a switch  98  and adding a capacitance  100  to the antenna matching circuit, resulting in an AM (amplitude modulation) LSK (load shift keying) type modulation. This transmission approach is similar to that used in RFID (radio frequency identification) communications, except RFID does not typically encode analog information but instead encodes a few digital bits either on an AM LSK or FSK (frequency shift keying) modulation. 
         [0032]    Because the preferred embodiment of the sensing device  60  does not utilize wires to transmit data or power, nor contains an internal power source, the circuitry  64  further includes a regulator/rectifier  102  to extract its operating power from an electromagnetic (EM), generated by the readout unit  80  or another EM power source. The regulator/rectifier  102  rectifies incoming power from the inductive antenna  66  and conditions it for the other circuit components within the circuitry  64 . Finally, a matching circuit  104  is shown as comprising a trimmable capacitor bank  106  to resonate the inductor antenna  66 , which is energized by the magnetic field and backscatters data as previously described. 
         [0033]    As an alternative to the embodiment of  FIG. 3 , the modulator  96  could use a 13.56 MHz (or other frequency) magnetic field as a clock reference to create a second carrier frequency, such as one that is one-quarter or another sub-multiple or multiple of the original frequency. The second carrier frequency can then be amplitude modulated (AM) using the oscillator frequency tone and transmitted to the readout unit  80  via the same antenna  66 . In this embodiment, the readout unit  80  may or may not have a second antenna to receive the second carrier frequency-based AM signal. 
         [0034]    The communication scheme described above differs from resonate tank communication systems that use capacitive pressure transducer elements in conjunction with an inductor/antenna. In particular, the circuitry  64  allows the use of any frequency for the high power readout unit  80 , which in preferred embodiments utilizes an industrial, scientific, medical (ISM) band frequency. In contrast, the frequencies and potentially large bandwidths required of resonate tank communication systems are subject to FCC emission limitations, likely requiring the use of extra shielding or potentially other measures taken in the facilities where the sensing device  60  and readout unit  80  are to be used. Another feature of the circuitry  64  is the allowance of more combinations of oscillator elements to be used. Because resonator tank systems require an inductive element and a capacitive element in which at least one of the elements serves as a transducer, resonator tank systems do not lend themselves well to resistive-based or other based sensors. Finally, the circuitry  64  also allows for signal conditioning, such as transducer compensation, which allows for such items as removing temperature dependence or other non-idealities that may be inherent to the transducer  62 . In the preferred embodiment, a negative temperature coefficient of the MEMS capacitor  94  can be compensated with simple circuitry relying on the positive temperature coefficient of resistor elements arranged in a trimmable bank of two resistor units with largely different temperature coefficients that can be selectively added in a trimming procedure in production to select the precise level to compensate the transducer variation. 
         [0035]    In the past, the restrictive levels of energy available to small implantable medical sensing devices and the desire to maximize data rates to capture more detailed physiological parameter response has been met with a robust type of analog communication that places information on the frequency rather than amplitude of the carrier. In U.S. Pat. No. 6,929,970 to Rich et al., a secondary carrier frequency is used for communication with an interrogator unit, resulting in a technique that consumes substantially more power in the implant and requires a second external antenna to receive the signal. The greater power consumption of the implant necessitates a tradeoff between smaller size and longer communication range. In contrast, the communication scheme described above for this invention draws upon the RFID-type communications, such as those described in U.S. Pat. Nos. 7,015,826 and 6,622,567, whose contents are incorporated herein by reference. However instead of communicating digital data using a fixed rate clock, the present invention transmits analog information as the frequency of the clock to lower power consumption and enhance powering and communication range. In this way, much of the readout unit  80  can utilize hardware that is commercially available for RFID, except that a different demodulator is required. An early example of RFID can be found in U.S. Pat. No. 4,333,072. 
         [0036]    The transducer  62  (e.g., mechanical capacitor  94 ), the electronic circuitry  64  (including chips, diodes, capacitors, etc., thereof), the antenna  66  and any additional or optional components (e.g., additional transducers  62 ) of the sensing device  60  are preferably contained in a single sealed housing.  FIG. 4  depicts a preferred example as being a cylindrical housing  110  having a flat distal face  112 , though other shapes are also possible, for example, a torpedo-shape in which the peripheral face  114  of the housing  110  immediately adjacent the distal face  112  is tapered or conical (not shown). The housing  110  can be formed of glass, for example, a borosilicate glass such as Pyrex Glass Brand No 7740 or another suitably biocompatible material. A biocompatible coating, such as a layer of a hydrogel, titanium, nitride, oxide, carbide, silicide, silicone, parylene and/or other polymers, can be deposited on the housing  110  to provide a non-thrombogenic exterior for the biologic environment in which the sensing device  60  will be placed. As can be seen in  FIG. 5 , the inductive antenna  66  (comprising a copper coil  68  surrounding a ferrite core  70 ) occupies most of the internal volume of the housing  110 . The size of the antenna  66  is governed by the need to couple to a magnetic field to enable telepowering with the readout unit  80  from outside the body, for example, a transmission distance of about 10 cm or more. The cylindrical shape of the housing  110  is convenient for the sensing device  60  to be placed with a conventional catheter, as well as anchors discussed in reference to  FIGS. 5 through 11  below. The circuitry  64  is disposed between the antenna  66  and an end of the housing  110  that preferably carries the transducer  62 . A nonlimiting example of an overall size for the housing  110  is about 3.7 mm in diameter and about 16.5 mm in length. 
         [0037]    A preferred aspect of the invention is to locate the transducer  62  on a distal surface of the sensing device  60 , for example, the flat distal face  112  of the cylindrical housing  110 , or on the peripheral face  114  of the housing  110  immediately adjacent the distal face  112 . In a preferred embodiment, the flat distal face  112  is defined by a biocompatible semiconductor material, such as a heavily boron-doped single-crystalline silicon, in whose outer surface the pressure-sensitive diaphragm (or other sensing element  94 ) is formed. In this manner, only the distal face  112  of the housing  110  need be in contact with a biological fluid whose physiological parameter is to be monitored. In the case of monitoring pressure within the heart, this aspect of the invention can be utilized to minimize protrusion of the sensing device  60  into the heart chamber, thereby reducing the risk of thrombogenesis. 
         [0038]      FIGS. 5 through 11  represent different embodiments for anchors  120 A,  120 B, and  120 C with which the sensing device  60  of  FIG. 5  can be anchored to a wall of an internal organ, for example, by making an incision in the wall from the exterior of the organ, inserting the sensing device  60  in the incision, and then securing the sensing device  60  to the wall. Access to the organ can be through any suitable surgical procedure in which the desired implantation location is made accessible, such as by an open-chest surgical procedure including but not limited to bypass surgery, heart valve surgery, and heart transplant surgery. Together, the sensing device  60  and the anchor  120 A,  120 B or  120 C form a sensing unit  150 A,  150 B, and  150 C, respectively, that has minimal protrusion into the organ. Maximum protrusion of the sensing units  150 A-C is preferably not more than one centimeter, more preferably not more than eight millimeters, for example between about 0.5 to about 2 millimeters, with a preferred volumetric protrusion of not more than about 0.02 cm 3 . The distal end of the units  150 A-C (for example, as defined by the distal face  112  of the housing  110  or the distal end of the anchor  120 A-C) may also be slightly recessed below the internal surface of the wall, for example, up to about two millimeters from the internal surface of the wall. Particularly suitable materials for the anchors  120 A-C include but are not limited to NITINOL, TEFLON, polymers such as parylene, silicone and PEEK, metals, glass, and ceramics. A nonlimiting example of an overall size for the anchors  120 A-C is a maximum outer diameter of about 9.5 mm and longitudinal length of about 9 mm. 
         [0039]    In  FIGS. 5 through 8 , the anchor  120 A is configured to have a tubular portion  122 A partially surrounded by a dome-shaped portion  124 A. The sensing device  60  is axially disposed within the tubular portion  122 A, such that the distal face  112  carrying the transducer  62  protrudes from the tubular portion  122 A. The sensing device  60  can be secured in the tubular portion  122 A by any suitable means, such as an interference fit, a biocompatible epoxy, glue or cement, or any other type of attachment method or combinations of attachment methods known to those skilled in the art. The dome-shaped portion  124 A generally joins the tubular portion  122 A at an end  125  opposite the open end  126  of the tubular portion  122 A through which the sensing device  60  is received. The dome-shaped portion  124 A defines a substantially tubular section  130  that circumscribes the tubular portion  122 A and terminates at an edge  128  short of the open end  126  of the tubular portion  122 A. Multiple oblong openings  132  are defined in the tubular section to enable the anchor  120 A to be secured to a wall of an internal organ, such as with standard surgical sutures or another suitable attachment technique that can be performed during the surgical procedure, such as nails, screws, springs, and biocompatible adhesives such as cements, glues, epoxies, etc. 
         [0040]      FIGS. 6 and 8  depict the sensing unit  150 A of  FIG. 5  implanted in an incision in a wall  134  of an internal organ for the purpose of sensing a physiological parameter of a biological fluid within an internal cavity  136  of the organ. The wall  134  may be an exterior wall of the heart, a blood vessel, kidneys, lungs, bladder, etc., or a wall surrounding an organ, such as the abdominal wall or the meninges surrounding the brain. As evident from  FIG. 6 , only the tubular portion  122 A of the anchor  120 A is inserted into the incision, and the dome-shaped portion  124 A remains outside the incision; as such, the tubular and dome-shaped portions  122 A and  124 A are not configured to clamp the wall  134  therebetween. Furthermore, the anchor  120 A does not protrude through the wall  134 , but instead is recessed in the wall  134 , whereas the distal end  112  of the sensing device  60  protrudes into an internal cavity of the organ. As noted above, the distance the distal end  112  protrudes from the internal surface  138  of the wall  134  (e.g., the endocardium lining a chamber of the heart) is preferably not more than one centimeter, and more preferably not more than eight millimeters. As also noted above, the sensing device  60  and anchor  120 A could be configured so that the anchor  120 A, and not the sensing device  60 , protrudes beyond the wall  134 , in which case the distal end  112  of the sensing device  60  may be recessed up to about two millimeters from the internal surface  138  of the wall  134 . Again, the anchor  120 A preferably does not protrude more than one centimeter, and more preferably not more than eight millimeters, beyond the internal surface  138  of the wall  134 . Finally, it is within the scope of the invention that both the anchor  120 A and the sensing device  60  could protrude into the internal cavity  136 , or that neither the anchor  120 A nor the sensing device  60  protrudes into the internal cavity  136 , but instead are recessed in the wall  134 . 
         [0041]      FIGS. 6 and 8  further show the inclusion of a felt pad  140  between the peripheral edge  128  of the anchor  120 A and the external surface  142  of the wall  134 . A suitable material for the felt pad  140  is standard surgical grade felt. The anchor  120 A and the felt pad  140  are then simultaneously attached (e.g., sutured) to the wall  134 . Depending on the material from which it is formed, the felt pad  140  may be used to promote cell (tissue) growth and encapsulation of the incision, leading to further stabilization of the sensing unit  150 A. 
         [0042]    In  FIGS. 9 through 11 , additional anchors  120 B and  120 C are again configured to have a tubular portion  122 B/ 122 C, but with a disk-shaped portion  124 B/ 124 C at one end thereof. As before, the sensing device  60  is shown axially disposed within the tubular portion  122 B/ 122 C, such that the distal face  112  carrying the transducer  62  protrudes from the tubular portion  122 B/ 122 C. Though not shown, the disk-shaped portion  124 B/ 124 C may be formed to have multiple oblong openings to enable the anchor  120 B/ 120 C to be secured to the wall  134  of an internal organ, such as with sutures or another suitable attachment technique that can be performed during the surgical procedure. Alternatively, the sensing unit  150 C of  FIG. 11  is shown as further including a tubular insert  144  that is secured in the incision prior to placement of the remainder of the unit  150 C. The insert  144  can be attached to the wall  134  with an interference fit, or the use of a biocompatible cement, glue or epoxy, screws, springs, nails, etc. The tubular portion  122 C of the anchor  120 C can then be secured within the bore of the insert  144 . A preferred aspect of this embodiment is that the anchor  120 C is not permanently joined to the insert to facilitate exchange of the sensing unit  150 C and/or its sensing device  60 , and/or the use of a different anchor with additional features. 
         [0043]    As evident from  FIGS. 10 and 11 , both the anchors  120 B/ 120 C and the sensing devices  60  protrude into the internal cavity  136 , with the sensing devices  60  protruding farther, though any of the configurations discussed in reference to  FIGS. 5 through 8  could also be present in the embodiments of  FIGS. 9 through 11 . 
         [0044]    In addition to the above-noted features, the anchors  120 A- 120 C could be modified to provide other features, for example, a device similar to an RFID tag can be added to the anchor such that it wirelessly transmits ID information concerning the sensing device  60 . The ID information may include an ID number, ID name, patient name/ID, calibration coefficients/information, range of operation, date of implantation, valid life of the device (operation life), etc. The anchors  120 A- 120 C may further include additional capabilities such as features for connection to a catheter, shunt, or other device (not shown). 
         [0045]    To accurately locate the distal face  112  and its transducer  62  relative to the internal surface  138  of the wall  134 , the thickness of the wall can be measured using one or more of the following procedures: an echocardiogram; a pressure-sensitive needle inserted through the desired location for the implant, wherein the pressure signal displays atrial waveforms when the needle reaches the inside of the heart; or estimation of the wall thickness by observation of the patient&#39;s size and weight. Based on the wall thickness, an appropriate combination of sensing device  60  and anchor  120 A-C can be selected to achieve a desired placement of the transducer  62  relative to the internal surface  138  of the cavity  136 . Thereafter, the incision is made at the desired location for the sensing device  60 . For example, using standard devices and procedures, a tool can be inserted into the incision and a small circular portion (for example, about 3.5 mm diameter) of the heart wall is excised. The previously assembled sensing unit  150 A-C (with the selected sensing device  60  and anchor  120 A-C) is then inserted in the resulting circular hole, after which the anchor  120 A-C and felt pad  144  (if used) are stitched to the wall  134 , for example, sutured to the myocardium and pericardial layer of the heart. 
         [0046]    In the case where the organ is the heart, the miniature sensing units of this invention are particularly useful when placed for sensing pressure in the left side of the heart (left atrium and left ventricle). For this purpose, two particular locations are especially of interest, the left atrial dome and the left ventricular apex, though other locations are also possible including the left and right atrial appendages. Of course, the sensing units of this invention are also useful for sensing pressures in other regions of the heart, including the right atrium and ventricle, in which case the sensing units may be placed in the right atrial dome, right ventricular apex, right atrial appendage, etc. 
         [0047]    In addition to the sensing units  150 A-C and reader unit  80  described above, the monitoring systems of this invention can be combined with other technologies to achieve additional functionalities. For example, the reader unit  80  can be implemented to have a remote transmission capability, such as home monitoring that may employ telephone, wireless communication, or web-based delivery of information received from the sensing units  150 A-C by the reader unit  80  to a physician or caregiver. In this manner, the reader unit  80  can be adapted for remote monitoring of the organ and patient, closed-loop drug delivery of medications to treat the organ, closed-loop pacemaker parameter tuning to treat congestive heart failure or congestive heart failure related conditions, warning of critical worsening of congestive heart failure or congestive heart failure related conditions, portable or ambulatory monitoring or diagnosis, monitoring of battery operation, data storage, reporting global positioning coordinates for emergency applications, and communication with other medical devices chosen from the group consisting of pacemakers, left ventricular assist devices (LVAD), defibrillators, cardioverter defibrillators, drug delivery systems, non-drug delivery systems, and wireless medical management systems. Furthermore, the placement of the sensing units  15 A-C can be utilized as part of a variety of different medical procedures, including early diagnosis of a heart failing due to congestive heart failure related conditions, early diagnosis of failure of the organ, early intervention in treatment of congestive heart failure related conditions, tailoring of medications, disease management, identification of complications from congestive heart failure related conditions, identification of complications from cardiovascular disease related conditions, treatment of complications from congestive heart failure related conditions, treatment of complications from cardiovascular disease related conditions, pacing adjustments to the heart, reduction in frequency and severity of hospitalizations due to cardiovascular diseases, reduction in frequency and severity of hospitalizations due to congestive heart failure, tuning of defibrillator or pacemaker parameters to improve congestive heart failure related conditions, identification of mitral valve stenosis, treatment of mitral valve stenosis, feedback regarding the impact of medication on the organ, and chronic disease management of the organ. 
         [0048]    While the invention has been described in terms of specific embodiments, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims.