Abstract:
Procedures, implantable wireless sensing devices, and sensor assemblies suitable for monitoring physiological parameters within living bodies. Such sensor assembly includes a sensing device and an anchor for securing the sensing device within a living body. The sensing device comprises a housing having at least one internal cavity and a transducer and electrical circuitry within the at least one internal cavity. The sensing device further comprises an antenna that is within the at least one internal cavity or outside the housing. The housing has at least one additional housing portion in which the transducer, the electrical circuitry, and the antenna are not located. The anchor has a metal portion that surrounds the at least one additional housing portion so as not to surround the transducer, the electrical circuitry, or the antenna.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application Nos. 62/391,742 and 62/391,743, both filed May 9, 2016. The contents of these prior applications are incorporated herein by reference. 
         [0002]    Various patents pertain to procedures, systems, and implantable sensing devices suitable for monitoring physiological parameters within living bodies, as nonlimiting examples, U.S. Pat. Nos. 8,744,544, 8,715,300, 8,696,693, 8,512,252, 8,322,346, 8,267,863, 8,014,865, 7,860,579, 7,686,762, 7,634,319, 7,615,010, 7,317,951, and 6,968,743. Some of the inventions disclosed in these patents particularly pertain to anchoring and delivery of implantable wireless hermetically-sealed sensing devices by percutaneous methods or minimally invasive surgery (MIS). While the innovations disclosed in these patents can be applied to many different body organs and systems, of particular interest has been placement in the cardiovascular system and especially within or in the vicinity of a heart chamber to monitor one or more physiological parameters within the chamber. 
         [0003]    Some of the inventions disclosed in the above-noted patents note the ability of using metallic devices, for example vascular closure devices, atrial septum defect occluder devices (ASD and PFO occluders), and closure paravalvular leak devices, to anchor implantable wireless sensing devices. Nonlimiting examples of such devices include the CELT ACD® produced by Vasorum Ltd. (http://vasorum.ie/) or various devices produced by Occlutech International AB (http://www.occlutech.com/index.php/en/products). However, various problems can be encountered when attempting to anchor an implantable wireless sensing device using such devices, for example, metallic vascular closure devices can have a Faraday-cage effect on a wireless sensing device that can adversely effect the range and quality of tele-powering or wireless communications of the sensing device, and stresses induced in the sensing device caused by its attachment to the closure device can adversely effect the performance of the sensing device, for example, by inducing drift. 
     
    
     BACKGROUND OF THE INVENTION 
       [0004]    The present invention generally relates to procedures, implantable wireless sensing devices, and sensor assemblies suitable for monitoring physiological parameters within living bodies. 
         [0005]    According to one aspect of the invention, a sensor assembly includes a sensing device and an anchor for securing the sensing device within a living body. The sensing device comprises a housing having at least one internal cavity and a transducer and electrical circuitry within the at least one internal cavity. The sensing device further comprises an antenna that is within the at least one internal cavity or outside the housing. The housing has at least one additional housing portion in which the transducer, the electrical circuitry, and the antenna are not located. The anchor has a metal portion that surrounds the at least one additional housing portion so as not to surround the transducer, the electrical circuitry, or the antenna. 
         [0006]    According to another aspect of the invention, a method of measuring a physiological parameter in a vein utilizes a sensor assembly comprising the elements described above. Such a method may place the sensor assembly in a hepatic vein, hepatic portal vein, or inferior vena cava. 
         [0007]    Other aspects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIGS. 1 through 5  schematically represent anchors of types that can be used to anchor implantable wireless sensing devices in accordance with nonlimiting embodiments of the invention. 
           [0009]      FIG. 6  schematically represents a conventional sensing device. 
           [0010]      FIGS. 7 through 11  schematically represent sensing devices in accordance with nonlimiting aspects of the invention. 
           [0011]      FIGS. 12 through 20  schematically represent various sensor assemblies comprising the anchors of  FIGS. 1 through 5  and the sensing devices of  FIGS. 7 through 11  in accordance with nonlimiting aspects of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    Illustrated in the drawings are components of monitoring systems that include the implementation of an implantable wireless sensing device configured to be placed within a living body, including internal organs thereof, for monitoring one or more physiological parameters. Physiological parameters of particular interest include but are not limited those relating to the function of the circulatory, respiratory, urinary, and nervous systems, and organs of particular interest include but are not limited to the heart, blood vessels, liver, brain (e.g., intracranial), kidneys, lungs, and bladder. Notable particular examples relating to the heart include any of the four heart chambers (particularly the left ventricle and left atrium), and notable examples relating to blood vessels include the hepatic veins and the inferior vena cava. 
         [0013]    The placement of an implantable sensing device, such as implantable hemodynamic monitoring (IHM) sensors or other types of sensors, in a hepatic vein allows for both monitoring the health of the heart and the liver. Hepatic veins are blood vessels which transport the liver&#39;s deoxygenated blood and blood which has been filtered by the liver (blood from the pancreas, colon, small intestine, and stomach) to the inferior vena cava. The hepatic veins originate in the liver lobule&#39;s central vein. Hepatic veins are unusual in that they do not have valves. As such, pressure within a hepatic vein directly relates to pressure within the right side of the heart. Furthermore, cardiac pulsations within the hepatic veins are very small and therefore the risk of implant migration is low. 
         [0014]    There are also advantageous aspects relating to the placement of an implantable sensing device, such as an IHM sensor or other type of sensor, in the inferior vena cava. The largest vein in the human body, the inferior vena cava collects blood from veins serving the tissues inferior to the heart and returns this blood to the right atrium of the heart. Although the vena cava is very large in diameter, its walls are thin due to the low pressure exerted by venous blood. The inferior vena cava forms at the superior end of the pelvic cavity when the common iliac veins unite to form a larger vein. From the pelvis, the inferior vena cava ascends through the posterior abdominal body wall just to the right of the vertebral column. Along its way through the abdomen, blood from the internal organs joins the inferior vena cava through a series of large veins, including the gonadal, renal, suprarenal and inferior phrenic veins. The hepatic vein provides blood from the digestive organs of the abdomen after it has passed through the hepatic portal system in the liver. Blood from the tissues of the lower back, including the spinal cord and muscles of the back, enters the vena cava through the lumbar veins. Many smaller veins also provide blood to the vena cava from the tissues of the abdominal body wall. Upon reaching the heart, the inferior vena cava connects to the right atrium on its posterior side, inferior to the connection of the superior vena cava. The inferior vena cava and its tributaries drain blood from the feet, legs, thighs, pelvis and abdomen and deliver this blood to the heart. Many one-way venous valves help to move blood through the veins of the lower extremities against the pull of gravity. Blood passing through the veins is under very little pressure and so must be pumped toward the heart by the contraction of skeletal muscles in the legs and by pressure in the abdomen caused by breathing. Venous valves help to trap blood between muscle contractions or breaths and prevent it from being pulled back down towards the feet by gravity. 
         [0015]    There are also advantageous aspects relating to the placement of an implantable sensing device, such as an IHM sensor or other type of sensor, in other veins, such as in the hepatic portal vein to monitor and manage liver diseases and conditions such as portal hypertension, hyperdynamic circulation, portal vein thrombosis, liver fibrosis, liver cirrhosis, etc. Though the portal vein is not a true vein because it conducts blood to capillary beds in the liver and not directly to the heart, it is a major component of the hepatic portal system, one of only two portal venous systems in the body, with the hypophyseal portal system being the other. Hepatic vein pressure can be used to also monitor the hepatic portal pressure. The measurement of wedged hepatic venous pressure (WHVP) is the most common procedure used to indirectly evaluate portal pressure. The concept is the same as using the wedged pulmonary arterial pressure as a surrogate measurement of pulmonary capillary and left atrial pressure. 
         [0016]    Preferred aspects of the present invention include the ability to provide implantable wireless sensing devices suitable for monitoring one or more physiological parameters within veins, including in any of the veins mentioned above. The physical footprint of such an implantable wireless sensing device is preferably limited to the sensing device, an anchor that secures the sensing device to or within the vein, and optionally a separate antenna that wirelessly transmits data and other communications to a remote device, such as a readout unit, which may also tele-power the sensing device. The physical footprint of such an implantable wireless sensing device can be far smaller than monitoring systems that must be physically connected to a relatively large remote transmitting device, for example, as in the case of the LVP-1000 Left Ventricle Pressure Monitoring System offered by Transoma Medical, Inc. Implantable wireless sensing device utilized by the invention may employ resonant, passive, or active communication schemes described in prior patents, including but not limited to those disclosed in U.S. Pat. Nos. 8,744,544, 8,715,300, 8,696,693, 8,512,252, 8,322,346, 8,267,863, 8,014,865, 7,860,579, 7,686,762, 7,634,319, 7,615,010, 7,317,951, and 6,968,743. The drawings schematically represent various implantable wireless sensing devices as comprising a single hermetically-sealed housing that contains a transducer and electronic circuitry, for example, an application specific integrated circuit (ASIC), which operate in combination with an antenna to transmit and receive data. 
         [0017]    In the drawings, the antenna is represented as comprising a coil (e.g., copper windings) wrapped around a core (e.g., ferrite), though other antenna configurations and materials are foreseeable. The transducer, which is located at a proximal end of the housing, 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 a suitable transducer for hemodynamic monitoring of various blood pressures within the cardiovascular system is a MEMS capacitive pressure sensor for sensing pressure, though other materials and any variety of sensing elements, e.g., capacitive, inductive, resistive, piezoelectric, etc., could be used. For example, the transducer 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. 
         [0018]    The sensing device may be powered with a battery or other power storage device, but in preferred embodiments is powered entirely by a remote device that is not configured for implantation, such as a readout unit. Such a readout unit may be configured to receive an output signal from the sensing device, process the signal, and relay the processed signal as data in a useful form to a user. Because the sensing device is equipped with a built-in antenna, the device requires only an anchor for implantation and does not require 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. 
         [0019]    In the drawings, consistent reference numbers are used to identify functionally equivalent structures of various implantable wireless sensing devices  10  and to identify functionally equivalent structures of various anchors  50  that are adapted to secure the sensing devices  10 . As noted above, the drawings schematically represent the sensing devices  10  as comprising a single hermetically-sealed housing  12  that contains a transducer  14  and electronic circuitry  16 , for example, an application specific integrated circuit (ASIC), which operate in combination with an antenna  18  to transmit and receive data. The sensing devices  10  differ from each other by the placement of the antenna  18  within or outside the housing  12 . The anchors  50  may be chosen in part on the basis of the placement of the antenna  18  relative to the housing  12 . The housings  12  of the sensing devices  10  are configured to comprise a housing portion  20  that is in addition to portions of the housing  12  in which one or more internal cavities  22  are located that contain the transducer  14  and antenna  18 . As such, the additional housing portion  20  is not required to contain, and preferably does not contain, any component relating to the operation of the transducer  14  and the transmission of data to and from the sensing device  10  via the antenna  18 , and therefore a cavity is not required to be present in the additional housing portion  20 . Furthermore, such an additional housing portion  20  may form a distal end  24  of the housing  12 , i.e., opposite of the proximal end  26  of the housing  12  where the transducer  14  is located such that the antenna  18  is located between the transducer  14  and the additional housing portion  20 , or may be located between the transducer  14  and the antenna  18 , or may be a combination of both (i.e., the additional housing portion  20  may comprise two spaced-apart portions  20 ) that are connected together only through the housing  12  or by the anchor  50 . The additional housing portion  20  may be integrally formed as an indiscrete region of the housing  12 , or separately formed and directly attached to the housing  12 , or separately formed and indirectly attached to the housing  12  with the anchor  50 . 
         [0020]    Instead of containing components relating to the operations of the transducer  14  and antenna  18  or to data transmission, the additional housing portion  20  can be dedicated to the attachment of an anchor  50  to the sensing device  10 . In particular, the additional housing portion  20  is particularly well suited for enabling the sensing device  10  to be secured with metallic anchors  50 , including but not limited to vascular closure devices, atrial septum defect occluder devices (ASD and PFO occluders), and closure paravalvular leak devices, such that the transducer  14  and antenna  18  are sufficiently remote from the anchor  50  that metallic portions of the anchor  50  do not interfere with their operations. As an example, the additional housing portion  20  preferably creates a spacial axial distance between the antenna  18  and a metallic anchor  50  (or metallic portions thereof) to reduce Faraday-cage effects otherwise caused by metal, and creates a spacial axial distance between a metallic anchor  50  and the transducer  14  to reduce if not avoid stresses that could mechanically interfere with the operation of the transducer  14 , for example, cause a drift in its signal output. 
         [0021]    The attachment of the sensing device  10  to the anchor  50  can be accomplished in different ways, including but not limited to one or more of the following: attachment by a third material (e.g., glue, epoxy, etc.), mechanical grips, threads (e.g., the housing  12  is threaded into the anchor  50 ), using a discrete attachment member (e.g., made from PEEK or NiTi material), compression, thermal compression, or a mechanical attachment feature of the sensing device  10  or anchor  50  (e.g., fingers, loops, spirals, etc.). 
         [0022]      FIGS. 1 through 5  schematically represent anchors  50  that have a generally cylindrical outline and define an axis  52  that may be an axis of rotational symmetry. Each of the anchors  50  has distal and proximal ends  54  and  56 , at which axial spaced flanges or rings  58  and  60  are present. The anchors  50  are solid between their distal and proximal ends  54  and  56 , i.e., lack an internal hole or cavity ( FIG. 1 ), or have a through-hole  62  ( FIG. 2 ), or have a blind hole  64  ( FIG. 3 ), or comprise two discrete rings  58  and  60  that are not connected to each other but define two separate through-holes  62  ( FIG. 4 ), or comprise two discrete rings  58  and  60  that are interconnected to each other by longitudinal legs  66  to define a through-hole  62  within the anchor  50  ( FIG. 5 ). In the embodiments of  FIGS. 2 through 5 , the anchors  50  are sized to accommodate a sensing device  10  within their respective through-hole  62 , blind hole  64 , rings  58  and  60 , and/or legs  66 . In the embodiment of  FIG. 1 , a sensing device  10  may be attached to either axial end  54  or  56  of the anchor  50 . Other anchor  50  configurations are also within the scope of the invention. The anchors  50  may be formed or fabricated from a variety of materials, including but not limited to metals including stainless steels and shape-memory alloys (e.g., NiTi alloys), and/or polymers including PEEK. 
         [0023]      FIGS. 6 through 11  schematically represent sensing devices  10  that each defines an axis  28  that may be an axis of rotational symmetry. The housings  12  of the sensing devices  10  are sized and shaped for attachment to one or more of the anchors  50  shown in  FIGS. 1 through 5 , for example, by attachment to either axial end  54  or  56  of the anchor  50  of  FIG. 1 , or placement in the through-hole  62 , blind hole  64 , rings  58  and  60 , or legs  66  of the anchors  50  of  FIGS. 2 through 5 . The sensing device  10  of  FIG. 6  will be referred to herein as a conventional sensing device  10 , in that its housing  12  is represented as being sized to accommodate the transducer  14 , antenna  18 , and electronic circuitry  16  within an internal cavity  22  defined by the housing  12 , with little additional cavity space not occupied by the transducer  14 , antenna  18 , and electronic circuitry  16 . The sensing devices  10  of  FIGS. 7 through 11  differ from the conventional sensing device  10  of  FIG. 6  as well as from each other by the placement of the antenna  18  within ( FIGS. 6 through 9 and 11 ) or outside ( FIG. 10 ) their respective housings  12 , and the inclusion of at least one additional housing portion  20  that is in addition to the internal cavity (or cavities)  22  that contain(s) the transducer  14 , antenna  18 , and electronic circuitry  16 . The placement of the antenna  18  outside of the housing  12  in  FIG. 10  allows for the diameter of the antenna  18  to be larger from the diameter of the housing  12 . Because the diameter of the antenna  18  greatly affects the tele-powering and tele-communication range of the sensing device  10 , the wider diameter antenna  18  of  FIG. 10  may eliminate the requirement for a ferrite coil, so that the sensing device  10  requires only a coil. 
         [0024]    The additional housing portions  20  are not required to contain any component relating to the operation of the sensing device  10  or its transducer  14 , antenna  18 , or electronic circuitry  16 , and therefore a cavity is not required to be present in the additional housing portions  20 . As such, the representations of the additional housing portions  20  in the drawings do not necessarily (though may) indicate a cavity, but instead more generally indicate regions of their housings  12  that may entirely be a cavity-free solid. The additional housing portion  20  is represented in  FIGS. 7 and 8  as located in or forming the distal end  24  of the housing  12  opposite its proximal end  26  where the transducer  14  and electronic circuitry  16  are located, such that the antenna  18  is located between the additional housing portion  20  and the transducer  14  and electronic circuitry  16 . The additional housing portion  20  is represented in  FIGS. 9 and 10  as located within a midsection of the housing  12  between the antenna  18  at the distal end  24  of the housing  12  and the transducer  14  and electronic circuitry  16  at the proximal end  26  of the housing  12 . The additional housing portion  20  is represented in  FIG. 11  as comprising two spaced-apart portions  20 , a first between the antenna  18  located within the midsection of the housing  12  and the transducer  14  and electronic circuitry  16  at the proximal end  26  of the housing  12 , and a second between the antenna  18  and the distal end  24  of the housing  12 . The additional housing portions  20  of  FIGS. 7 and 9 through 11  are integrally formed as an indiscrete region of the housing  12 , whereas the additional housing portion  20  of  FIG. 8  is separately formed and directly attached to the distal end  24  of the housing  12 . 
         [0025]      FIG. 12  represents a sensor assembly  40  comprising the sensing device  10  of  FIG. 7  assembled with the anchor  50  of  FIG. 2 , and  FIG. 13  represents the sensor assembly  40  of  FIG. 12  implanted in a wall  30  of an organ such that the anchor  50  secures the sensing device  10  to the organ wall  30 . As evident from  FIG. 13 , the distal end  24  of the sensing device  10  sufficiently extends through the through-hole  62  of the anchor  50  so that the anchor  50  contacts and surrounds or circumscribes only that part of the housing  12  that is formed by the additional housing portion  20  or otherwise in which the additional housing portion  20  is present, such that the anchor  50  does not have a negative effect or has a minimal negative effect on the functions of the internal antenna  18  and transducer  14 . In  FIGS. 12 and 13 , the anchor  50  is radially aligned with the additional housing portion  20  with respect to the axis  28  of the housing  12  ( FIG. 7 ). 
         [0026]      FIG. 14  represents a sensor assembly  40  comprising the sensing device  10  of  FIG. 7  assembled with the anchor  50  of  FIG. 3 , and shows the sensor assembly  40  implanted in a wall  30  of an organ with the anchor  50 . As evident from  FIG. 14 , the distal end  24  of the sensing device  10  is received in the blind hole  64  of the anchor  50  so that the anchor  50  contacts and surrounds or circumscribes only the distal end  24  of the housing  12 , which is formed by or contains the additional housing portion  20 .  FIG. 15  represents a sensor assembly  40  similar to that of  FIG. 14  but whose anchor  50  has been modified to have fingers or legs  66  that extend from the axial end  56  of the anchor  50  in which the blind hole  64  is formed and capture the proximal end  26  of the housing  12  to better secure the sensing device  10 . 
         [0027]      FIG. 16  represents a sensor assembly  40  comprising the sensing device  10  of  FIG. 9  assembled with the anchor  50  of  FIG. 2 , and shows the sensor assembly  40  implanted in a wall  30  of an organ with the anchor  50 . The distal and proximal ends  24  and  26  of the sensing device  10  are located outside of the through-hole  62  of the anchor  50 , which contacts and surrounds or circumscribes only the midsection of the housing  12  that is formed by the additional housing portion  20  or otherwise in which the additional housing portion  20  is present. In  FIG. 16 , the anchor  50  is radially aligned with the additional housing portion  20  with respect to the axis  28  of the housing  12  ( FIG. 9 ). 
         [0028]      FIG. 17  represents a sensor assembly  40  comprising the sensing device  10  of  FIG. 11  assembled with the anchor  50  of  FIG. 5 , and shows the sensor assembly  40  implanted in a wall  30  of an organ with the anchor  50 . The distal and proximal ends  24  and  26  of the sensing device  10  are located at or adjacent opposite ends of the through-hole  62  of the anchor  50 , which surrounds or circumscribes the housing  12  between its distal and proximal ends  24  and  26 . As a result, the internal antenna  18  (located at the midsection of the housing  12 ) is not surrounded by either ring  58  and  60  of the anchor  50 , but instead is only surrounded or circumscribed by the longitudinal legs  66  that interconnect the rings  58  and  60 . In this embodiment, the rings  58  and  60  may be metallic, but the legs  66  are preferably formed of a nonmetallic material so as to not have a Faraday-cage effect on the antenna  18 .  FIG. 18  represents a sensor assembly  40  comprising the anchor  50  of  FIG. 5  similar to  FIG. 17 , but assembled with the sensing device  10  of  FIG. 7 . The distal and proximal ends  24  and  26  of the sensing device  10  are again located at opposite ends of the through-hole  62  of the anchor  50 , and the internal antenna  18  (located near the proximal end  26  of the housing  12 ) is only surrounded or circumscribed by the longitudinal legs  66  and is not surrounded by either ring  58  or  60 . 
         [0029]      FIG. 19  represents a sensor assembly  40  in which the sensing device  10  of  FIG. 7  assembled with a modified version of the anchor  50  of  FIG. 5 , in which the longitudinal legs  66  axially extend and protrude beyond both rings  58  and  60  of the anchor  50 . The internal antenna  18  (located at the midsection of the housing  12 ) is predominantly (though not exclusively) surrounded or circumscribed by the longitudinal legs  66  that interconnect rings of the anchor  50 . Similar to  FIG. 17 , though the rings  58  and  60  may be metallic, the legs  66  are preferably formed of a nonmetallic material so as to not have a Faraday-cage effect on the antenna  18 . 
         [0030]      FIG. 20  represents a sensor assembly  40  comprising the sensing device  10  of  FIG. 10  assembled with the anchor  50  of  FIG. 2 , and shows the sensor assembly  40  implanted in a wall  30  of an organ with the anchor  50 . The distal and proximal ends  24  and  26  of the sensing device  10  are located outside of the through-hole  62  of the anchor  50 , which contacts and surrounds or circumscribes the region of the housing  12  formed by the additional housing portion  20  or otherwise in which the additional housing portion  20  is present and predominant. As a result, the external antenna  18  of the sensing device  10  is not surrounded or circumscribed by any portion of the anchor  50 . 
         [0031]    A notable advantage of sensor assemblies  40  of the types described above include the capability of effective long-term monitoring of the cardiovascular system and the liver. Data obtained with the sensing devices  10  can be used for multiple purposes, including but limited to management of cardiac diseases, such as congestive heart failure, arrhythmia, structural heart diseases, congenital heart diseases, patients with single functioning ventricle, hypotension, hypertension, etc., management of liver diseases and conditions such as portal hypertension, hyperdynamic circulation, portal vein thrombosis, liver fibrosis, liver cirrhosis, etc., monitoring hepatic encephalopathy, hepatrorenal syndrome, hepatocellular carcinoma, etc., prevention of liver problems including failure, and long-term management of patients. Data from the sensing devices  10  may be sampled at home, at a doctor&#39;s office, in a surgery room, during post-op stay including ICU, and during hospital stay. 
         [0032]    Sensor assemblies  40  of the types represented in  FIGS. 12 through 20  can be implanted in various ways. For example, if implanted in a vein, one of the assemblies may be implanted in the wall  30  of the vein so that the proximal end  26  of the sensor housing  12  slightly protrudes into the vein, with the result that the sensor assembly  40  has little or no effect on blood flow through the vein. Alternatively, it is foreseeable that the entire sensor assembly  40  may be placed inside a vein, in which case an anchor  50  may be used to secure the sensing device  10  so that it is centrally located within the vein but is spaced apart from the walls  30  of the vein by legs or arms of the anchor  50  so as to have little if any effect on blood flow. For example, the anchor  50  can be equipped with one or more loops, fingers, spirals, screws, etc., that secure the sensing device  10  to oppositely-disposed walls  30  of the vein. Alternatively, the anchor  50  may be stitched to the wall  30  of the vein, such as with an anchor  50  disclosed in U.S. Pat. No. 9,168,005 
         [0033]    Alternatively, the sensor assemblies  40  may be implanted to intentionally block (obstruct) blood flow through a vein. With this approach, the sensor assembly  40  may be implanted in a hepatic vein so that its proximal end  26  (containing the transducer  14 ) faces the right atrium via the inferior vena cava or faces the liver. Still another approach is a combination of these approaches, but with multiple sensing assemblies  40  being implanted with at least one assembly  40  facing the right atrium via the inferior vena cava and at least another facing the liver. To effectively block blood flow, the external shape of the anchor  50  is preferably cylindrical shaped. 
         [0034]    Sensor assemblies  40  of the types described above can also be placed in other veins, such as the hepatic portal vein. In addition, sensor assemblies  40  of the types described above can be used with balloon catheters to measure hepatic venous pressure gradient (HVPG), a very important parameter which also sheds light on the hepatic portal pressure and its health. HVPG represents the gradient between pressures in the portal vein and the intra-abdominal portion of inferior vena cava. This technique also measures hepatic venous wedge pressure (HVWP), which is a dynamic variable reflecting the hepatic sinusoidal pressure and therefore, indirectly, the portal pressure. HVWP is used to calculate HVPG, the normal value being between 1 and 5 mmHg. The current standard technique for measuring HVWP is the balloon wedge method. 
         [0035]    Sensor assemblies  40  of the types described above can also be used to monitor portal hypertension, which is a clinical syndrome defined by a pathological increase in portal pressure. The development of cirrhosis of the liver is characterized by clinical manifestations related to portal hypertension like esophageal varices, ascites, bleeding, and encephalopathy. Direct measurement of portal pressure is invasive, inconvenient, and clinically impractical. The clinical importance of measuring HVPG in portal hypertension is well documented in the literature. HVWP measurement plays an integral role in the evaluation of patients with portal hypertension. A few important applications include the assessment of portal pressure in cirrhosis to predict risk of formation of varices (HVPG&gt;12 mmHg) or the risk of rebleeding from varices (HVPG&gt;20 mmHg), to assess the response to pharmacologic agents during treatment of portal hypertension (&gt;20% fall in HVPG from baseline), and to help differentiate between presinusoidal (as in the case of noncirrhotic portal hypertension) and sinusoidal causes of portal hypertension. Hence HVPG monitoring provides important information for patients undergoing treatment for the prevention of variceal bleeding or rebleeding, allowing identification of patients who will be effectively protected against the risk of bleeding and those who, by not achieving an HVPG reduction by 20% of baseline or to mmHg, remain at a very high risk of bleeding. In one approach, an HVPG measurement is determined by the difference between the free hepatic vein pressure and the wedged hepatic vein pressure (the latter an indirect method of determining a pre-sinusoidal portal pressure, akin to a right heart catheterization measurement of left-sided heart pressures). 
         [0036]    The delivery of sensing assemblies  40  of the types described above can be accomplished by percutaneous delivery, catheter delivery (preferably through the femoral vein), minimally invasive approaches, surgical approaches, or combinations thereof. The delivery procedure may be a standalone procedure or performed as part of another procedure. 
         [0037]    While the invention has been described in terms of particular embodiments, it should be apparent that alternatives could be adopted by one skilled in the art. For example, the sensing devices  10 , anchors  50 , and sensor assemblies could differ in appearance and construction from the embodiments described herein and shown in the drawings, functions of certain components of the the sensing devices  10 , anchors  50 , and sensor assemblies could be performed by components of different construction but capable of a similar (though not necessarily equivalent) function, and appropriate materials could be substituted for those noted. In addition, the invention encompasses additional or alternative embodiments in which one or more features or aspects of different disclosed embodiments may be combined. Accordingly, it should be understood that the invention is not necessarily limited to any embodiment described herein or illustrated in the drawings. It should also be understood that the phraseology and terminology employed above are for the purpose of describing the illustrated embodiments, and do not necessarily serve as limitations to the scope of the invention. Therefore, the scope of the invention is to be limited only by the following claims.