Abstract:
A system and method for measuring fluid flow in a conduit and pressure in a conduit is disclosed. A preferred embodiment of the system and method uses an ultrasound sensor for determining volume of flow and a tonometric system for determining pressure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application claims priority under 35 USC §119 (e) from U.S. provisional application Ser. No. 60/812,845 filed Jun. 12, 2006 with the title of “System and Method of Perivascular Pressure and Flow Measurement”. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not applicable.  
       REFERENCE TO A “SEQUENCE LISTING” 
       [0003]     Not applicable.  
       BACKGROUND OF THE INVENTION  
       [0004]     1. Field of the Invention  
         [0005]     The present invention relates to a system and method for measuring pressure and flow of blood, more particularly it is related to the perivascular measurement of blood flow and pressure at the same location on a blood vessel.  
         [0006]     2. Background of the Invention  
         [0007]     Blood flow and blood pressure measurement provide useful physiological information in biological systems. If flow and pressure are measured at the same location of a blood vessel, the measurement can provide the capacity to calculate the impedance of the tissue or organs to which the vessels are supplying blood.  
         [0008]     At present localized pressure measurement in a blood vessel is commonly made with a sensor placed at the end of a catheter tip which is inserted into the blood stream. Because of the invasive nature of the catheter, and the possible change in flow and pressure that can result from introducing a foreign object into the blood stream, use of a catheter has its limitations. Also chronic or long term measurements can not be made with a catheter since prolonged insertion of the catheter into the blood vessel causes the patients immune system to treat it as a foreign body and tissue will form around the catheter degrading its ability to measure flow and pressure.  
         [0009]     Another pressure measurement principle is the tonometric approach, where a pressure sensor is pressed against the outside of a vessel. If certain conditions are met, the pressure sensed in this manner will be equal to the blood pressure inside the vessel. Although the tomometric principle of blood pressure measurement has been know for years and has found use for the non-invasive measurement of intra-arterial pressure (see for instance U.S. Pat. No. 5,284,150) it has not been adopted as an implantable method for measuring the localized blood pressure of a vessel due to a number of technical problems. A discussion of the general theory behind the technique appears in the article “Arterial Tonometry: Review and Analysis” by Drzewiecki, Melbin and Noordergraaf in the J. Biomechanics Vol. 16 No. 2 pp. 141-152 (1983).  
         [0010]     Perivascular measurement of blood volume flow with ultrasound has been a standard technique which has been used since the 1980&#39;s. U.S. Pat. No. 4,227,407, describes a perivascular system and method of ultrasound measurement that has proved very successful. The principles described in this patent have been applied in the development of transit time flow sensors by Transonic Systems Inc. of Ithaca, N.Y. Doppler flow velocity measurements have equally been well documented since the 1970&#39;s, and may be used as an alternate flow measurement approach for the disclosed invention.  
         [0011]     Thus, what is needed is a system and method to obtain in real time pressure and flow readings in a blood vessel or other type of flexible conduit. There is also a need for a system and method to obtain continuous readings of flow and pressure in a blood vessel or other type of flexible conduit over an extended period of time without loss of accuracy in the readings.  
       SUMMARY  
       [0012]     Thus, it is an objective of the present invention to provide a system and method of obtaining at the same location on a blood vessel or other flexible conduit in real time volume flow and pressure measurements. It is a further objective to obtain such readings using a single perivascular sensor without penetration of the vessel wall. It is yet still a further objective to be able to make these readings in real time over an extended period of time.  
         [0013]     The present invention achieves these and other objectives by providing: a method for determining fluid flow and pressure of a fluid flowing in a flexible conduit having the steps of: a) making a volume flow or flow velocity measurement using an ultrasound wave beam passed into a conduit at an oblique angle to the a fluid flowing in the conduit; b) flattening a portion of the conduit; e) obtaining a pressure reading at some or all of the flattened portion of the flexible conduit.  
         [0014]     In yet another aspect of the present invention it provides a system for measuring flow volume and pressure in a flexible conduit having: a) a first ultrasound transducer and a second ultrasound transducer detachably positioned adjacent to said location of the flexible conduit, said first transducer being positioned upstream of said second transducer to transmit ultrasound beams between said transducers that illuminate and pass through the full cross sectional area of said conduit; b) a meter operatively connected to said first transducer and said second transducer to control operation of and receive signals from said transducers representative of the characteristics of the ultrasound beam before and after transmission of the ultrasound beam through the conduit thereby calculate volume flow; c) a pressure transducer detachably positioned on the same location of the flexible conduit against its outside surface such that it forms the adjacent surface of the flexible conduit into a flat surface, and d) operatively connecting to said meter to control operation of and to receive signals from said pressure transducer, which signals are representative of the pressure inside the flexible conduit. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The invention will be better understood by an examination of the following description, together with the accompanying drawings, in which:  
         [0016]      FIG. 1  is a schematic diagram of a preferred embodiment perivascular system of the present invention for measuring flow and pressure;  
         [0017]      FIG. 2  is a full raised view of one type of preferred embodiment of flow pressure sensor perivascular probe of the present invention;  
         [0018]      FIG. 2A  is a side view of probe head  49  of  FIG. 2  along line IIA with a vessel inserted into the head;  
         [0019]      FIG. 3  is a front view of another variation of a preferred embodiment of a flow-pressure sensor perivascular probe of the present invention;  
         [0020]      FIG. 4  is a cut away cross sectional view of the probe in  FIG. 3  along line IV-IV;  
         [0021]      FIG. 5  is a detailed cut away cross sectional view of a portion of the probe of  FIG. 3  along lines V-V;  
         [0022]      FIG. 6  is a front view of an implantable probe;  
         [0023]      FIG. 7  is a cross sectional cut away view of the probe in  FIG. 6  along lines VII-VII;  
         [0024]      FIG. 8  provides a cross sectional review of another variation of an implantable probe;  
         [0025]      FIG. 9  is an exploded view of the probe and cuff of  FIG. 8 ;  
         [0026]      FIG. 10  is a view of the top of the cuff of  FIGS. 8 and 9 ;  
         [0027]      FIG. 11  is a schematic diagram of a Doppler ultrasound system; and  
         [0028]      FIG. 12  is a crossectional view of  FIG. 11  along line XII. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0029]      FIG. 1  is a schematic diagram of the major functional components of flow and pressure measurement system  21  of the present invention. The system includes a probe  23  that measures both blood flow and pressure at a common location on a blood vessel  25  to which it has been attached. Probe  23  attaches by an electrical lead  27  to a combined flow and pressure meter  29 . Probe  23  includes ultrasound transducers to measure flow and a tonometeric pressure measurement senor which will be described in detail below.  
         [0030]     The present invention would use a perivascular ultrasound system similar to the one described in U.S. Pat. No. 4,227,407, which is incorporated herein by reference as if set out herein at length, discloses the basic features of this type of perivascular ultrasound measurement system.  
         [0031]     Meter  29  is a standard Transonic HT314 surgical meter made by Transonic Systems Inc. that has the added capability of measuring blood pressure as well as blood flow. Screen  31  can display mean volume of flow, flow messages or signal quality information as directed by knob  33 . Screen  35  displays pressure, pressure massages or information on signal quality as directed by knob  36 . Knob  37  controls the graph  39  printing device  41 . Knob  37  directs the printer to print pressure, flow or a combination of both on graph printing paper  39 .  
         [0032]      FIG. 2  provides a raised view of one variation of a flow and pressure sensor probe of the present invention. Probe  43  has a handle  45  that has an electrical lead  47  that passes through the handle and connects with probe head  49 . Probe head  49  includes a combination clip and ultrasound reflector  53  which attaches to housing  55 , which includes both ultrasound transducers and a tonometric pressure sensor. Probe  43  has a flexible neck  59  to allow for the positioning of probe head  49  around a vessel in a patient.  FIG. 2A  provides a side view of probe head  49 . The inner surface  61  of clip  53  acts as a reflective surface for the ultrasound transducers located in housing  55 . The interior of housing  55  will be discussed in more detail. As shown in  FIG. 2A  clip  53  holds vessel  25  securely but detachably against housing  55 .  
         [0033]     As noted above the present invention also measures blood pressure of blood flowing in a vessel with a tonometric blood pressure sensing device.  FIG. 3  is a close up view of the front of a probe head  73 . Housing  74  contains ultrasonic transducers (not shown in this figure) and a tonometeric pressure sensor  75  that projects out of housing  74  and abuts against blood vessel  77 . Clip  79  also projects out of housing  74  to securely hold vessel  77  against housing  74 . Electrical lead  80  carries electrical signals between the ultrasonic transducers and the tonometric sensor and the Flow and Pressure meter.  
         [0034]      FIG. 4  is a cut away cross-sectional view of probe of head  73  and vessel  77  a along line IV-IV in  FIG. 3 .  FIG. 4  shows the position of ultrasound transducers  81 A and  81 B that are inside housing  74 . Ultrasound transducers  81 A and  81 B are positioned to exchange ultrasound transmissions that are reflected off of the interior surface of  87  of clip  79 . Readings of flow volume of the blood in vessel  77  are taken from the ultrasound transmissions of transducers and analyzed as indicated above. Tonometric pressure sensor  75  has a flat sensing surface  89  that forms the portion of vessel  77  it abuts against into a flat surface to obtain the necessary readings. Blood flow in the cut away view of vessel  77  is indicated by arrows  93 .  
         [0035]      FIG. 5  is a detailed cut away view of vessel  77 , tonometric sensor  75  and clip  79  along lines V-V of  FIG. 3 . In  FIG. 5  the flat surface  91  formed on vessel wall  96  by the flat sensing surface  89  of tonometric sensor  75  can be seen. In order to make the pressure measurements with a tonometric sensor the sensing surface of the tonometric sensor must always form the adjacent portion of the blood vessel into a flat surface. Tonometric sensing of pressure is based on the principle that when a portion of the surface of a flexible conduit is flattened, the pressure outside and inside the vessel at the flattened portion of the blood vessel will be equal. Thus, a sensor taking a pressure reading at the flattened portion of the surface of the blood vessel will be reading the pressure in the adjacent interior portion of the blood vessel. This concept is based on Laplaces&#39;s law for pressure the gradient across a vessels wall which is expressed in the following equation:  
               Pout   -   Pin     =     T   r             [   1   ]               
 In this equation Pout is the pressure outside the wall of the vessel and Pin is the pressure on the inside of vessel. T is the vessel wall tension and r is the radius of the vessel. Equation  1  can be modified as follows by simple algebraic manipulation:  
               P   ⁢           ⁢   out     =       T   r     +     P   ⁢           ⁢   i   ⁢           ⁢   n               [   2   ]               
 If the wall of the vessel is then flattened in effect then the radius r goes to infinity r=∞. Thus substituting this value for r in the above equation results in T/r going to zero so the above equation can be reduced to the following: 
 out=Pin   [3] 
 Thus as can be seen the pressure differential across the vessel wall at the flattened portion goes to zero ΔP→0. 
 
         [0036]     The tonometeric sensing surface  89  of the present invention is flat to thereby form the adjacent vessel wall into a flat and rigid surface necessary for the pressure measurement. Various types of semiconductor sensing elements could be embedded in the surface  89  to make the pressure measurements at the flattened surface  91 . These could be capacitive type of pressure sensors, strain gauges, etc. These devices are typically made of piezoelectrical active types of materials that are naturally sensitive to the application of mechanical stress. As can be seen in  FIG. 4  electrical connections  97  run from the ultrasonic transducers  81 A and  81 B as well as tonometric sensor  85  up through electrical line conduit  80  to the flow-pressure meter not shown.  
         [0037]      FIG. 6  provides an enlarged view of another variation of the flow-pressure sensor probe  101  of the present invention. The variation of the invention in  FIG. 6  would be implanted into a test subject such as a laboratory rat, sheep, horse etc. for chronic long term measurements. It would naturally be placed around a blood vessel  103  by inserting the blood vessel through the gap  105  formed by housing  107  and clip  109 . Since vessel  103  is flexible and easily deformable it can be inserted through gap  105  with no problem. Probe  101  is sized such that the sensing surface  113  of tonometeric sensor  1   11  abuts firmly up against the outside wall of vessel  103  and forms the flat surface described previously that allows for the direct measurement of pressure. Alternatively, an insert sized to fit into the probe could be use to hold the vessel, this will be discussed below. Electrical conduit  115  passes out through the skin of the test animal and directly attaches to a flow-pressure meter by a long lead or alternatively attaches to a telemetric pack attached to the outside of the animal and the readings are conveyed by wireless transmission to the flow-pressure sensor meter or computer running appropriate software not shown. Alternately, electrical conduit  115 , may connect to a fully implanted signal telemetry device. This would be implanted in the animal or human subject.  
         [0038]      FIG. 7  is a cross sectional cut away view along line VII-VII of  FIG. 6 . In  FIG. 7  the flattened portion  117  of vessel  103  wall can be seen. When a device like this is chronically implanted overtime tissue  119  grows around probe  101  and between probe housing  107  and vessel  103 . However, this does not negatively affect tonometric sensor  111  because at flattened surface  117  the tissue actually atrophies and relies on sensor wall  113  for support. This in fact enhances the operation of the tonometric sensor, as the interposed fibrous tissue becomes passive and incapable of altering pressure. Additionally, the tissue growth between housing  107  and between it and vessel  103  form a uniform transition between ultrasound transceivers located in housing (not shown in  FIG. 7 ) and vessel  103  which will eliminate motion artifacts.  
         [0039]      FIG. 8  is another variation of chronically implantable type of probe. In this variation numbering of the various parts used on that disclosed in  FIGS. 6 and 7  has been retained. The added feature is an insert or cuff  121 . Cuff  121  is made of a flexible and reliant material. It is sized to fit into housing  107  of probe  123 . As depicted in FIGS.  8  cuff  121  has an opening  125  sized conformal to vessel  103 , and is designed to fit securely but detachably in housing  107  of probe  123 . Cuff  121  is made of a material that is acoustically compatible and biocompatible with vessel  103 . Being acoustically compatible with the vessel and blood, the material will not deform the ultrasound fields that derive flow readings from vessel  103 . This increases the accuracy of the probe. Biological compatibility present rejection of the cuff by the body. A material that meets this criteria is Pebax® (Elf-Autchem). A detailed discussion of the inert or cuff appears in Copending provisional application Ser. No. 60/881,826 filed Jan. 23, 2007 and titled “Disposable Insert for a Perivascular Probe Head, which is incorporated herein by reference.  
         [0040]      FIG. 9  provides an exploded view of cuff  121  and housing  107  into which Cuff  121  is inserted in a secure but detachable fashion.  FIG. 10  provides a top view of cuff  121  along line X-X of  FIG. 9 . As can be seen cuff  12  has a hole  133  in its top to receive sensor  111 .  
         [0041]     Since volume flow and pressure can be measured on the same location of a blood vessel, these measurements make it possible to calculate the impedance of the tissue or organ(s) supplied by the vessel being measured. Impedance Z can be calculated by dividing pressure by flow, the equation would be as follows where P is pressure and Q is flow:  
             Z   =     P   Q             [   4   ]             
 
 Values for impedance can be determined with either flow volume, as is the case with the use of transit time ultrasound or with flow velocity as is the case with back scattered Doppler ultrasound system that are discussed below. 
 
         [0042]     One preferred embodiment of the invention employs a the transit time ultrasound sensor which fully illuminate the cross sectional area of the vessel with its bidirectional beams of ultrasound. It is within the spirit of the invention, to employ other sensors for the measurement of flow.  
         [0043]     In another variation of the invention Doppler ultrasound sensors could be used in place of transit time flow sensors.  FIG. 11  provides a schematic diagram of a Doppler ultrasound system with the combined tonometric sensor and Doppler sensor  149  adjacent vessel  103 . As is well known in Doppler ultrasound systems ultrasound  151  is directed into the vessel  103  at an oblique angle. For a detailed discussion of how a Doppler ultrasound sensor works we refer to publications and textbooks known by those of ordinary skill in the art. In one variation of the invention the Doppler ultrasound sensor could make only a reading flow velocity and not volume flow. However, by taking a series of readings over a cross sectional area of the vessel, the internal diameter of the vessel may be determined as well and volume flow may be measured.  FIG. 12  provides a cross sectional view of the system of  FIG. 11  along line XII, where Doppler ultrasound sensor  149  takes readings of the flow speed at several different cross sectional points  160 A,  160 B,  160 C,  160 D and  160 E, to thereby estimate volume flow.  
         [0044]     While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and detail may be made to it without departing from the spirit and scope of the invention.