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

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     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 
     Not applicable. 
     REFERENCE TO A “SEQUENCE LISTING” 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a system and method for measuring, a 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. 
     2. Background of the Invention 
     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 allow a determination of the impedance of the tissue or organs to which the vessel is supplying blood. 
     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 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 patient&#39;s immune system to treat the catheter as a foreign body and tissue will form around the catheter thus degrading the ability of the catheter to measure flow and pressure. 
     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 tonometric principle of blood pressure measurement is known and has found use for the non-invasive measurement of intra-arterial pressure (see for instance U.S. Pat. No. 5,284,150) tonometrics 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). 
     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. 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 been documented since the 1970&#39;s, and may be used as an alternate flow measurement approach. 
     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 
     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 or conduit wall. It is yet still a further objective to be able to make these readings in real time over an extended period of time. 
     The present disclosure 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; and c) obtaining a pressure reading at some or all of the flattened portion of the flexible conduit. 
     Yet another aspect of the disclosure 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, the first transducer being positioned upstream of the second transducer to transmit ultrasound beams between the transducers that illuminate and pass through a full cross sectional area of the conduit; b) a meter operatively connected to the first transducer and the second transducer to control operation of and receive signals from the transducers representative of the characteristics of the ultrasound beam before and after transmission of the ultrasound beam through the conduit to thereby calculate volume flow; c) a pressure transducer detachably positioned on the same location of the conduit against an outside surface of the conduit such that the pressure transducer shapes the adjacent surface of the flexible conduit into a flat surface, and d) operatively connecting to the meter to control operation of and to receive signals from the pressure transducer, which signals are representative of a pressure inside the conduit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood by an examination of the following description, together with the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram of a perivascular system for measuring flow and pressure; 
         FIG. 2  is a full raised view of one embodiment of flow pressure sensor perivascular probe; 
         FIG. 2A  is a side view of a probe head of  FIG. 2  along line  2 A with a conduit inserted into the probe head; 
         FIG. 3  is a front view of another variation of a flow-pressure sensor perivascular probe; 
         FIG. 4  is a cut away cross sectional view of the probe in  FIG. 3  along line IV-IV; 
         FIG. 5  is a detailed cut away cross sectional view of a portion of the probe of  FIG. 3  along line V-V; 
         FIG. 6  is a front view of an implantable probe; 
         FIG. 7  is a cross sectional cut away view of the probe in  FIG. 6  along line VII-VII; 
         FIG. 8  provides a cross sectional review of another variation of an implantable probe; 
         FIG. 9  is an exploded view of the probe and cuff of  FIG. 8 ; 
         FIG. 10  is a view of the top of the cuff of  FIGS. 8 and 9 ; 
         FIG. 11  is a schematic diagram of a Doppler ultrasound system; and 
         FIG. 12  is a crossectional view of  FIG. 11  along line XII. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  is a schematic diagram of the major functional components of the present flow and pressure measurement system  21 . System  21  includes a probe  23  that measures both blood flow and pressure at a common location on a blood vessel  25  to which probe  23  has been attached. Probe  23  attaches by an electrical lead  27  to a combined flow and pressure meter  29 . Probe  23  includes ultrasound transducers  81 A and  81 B to measure flow and a tonometric pressure measurement sensor which will be described in detail below. 
     The present system can use a perivascular ultrasound system similar to the one described in U.S. Pat. No. 4,227,407, which is expressly incorporated herein by reference as if set out herein, and discloses the basic features of this type of perivascular ultrasound measurement system. 
     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 printing device  41 . Knob  37  directs the printer to print pressure, flow or a combination of both on graph printing paper  39 . 
       FIG. 2  provides a raised view of one variation of a flow and pressure sensor probe  43 . Probe  43  has a handle  45  that has an electrical lead  47  that passes through the handle and connects with a probe head  49 . Probe head  49  includes a combination clip and ultrasound reflector  53  which attaches to a housing  55 , which includes both ultrasound transducers  81 A and  81 B (not shown in  FIG. 2 ) and a tonometric pressure sensor. Probe  43  has a flexible neck  59  to allow for the positioning of probe head  49  such as 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  81 A and  81 B located in housing  55 . The interior of housing  55  will be discussed in more detail. As shown in  FIG. 2A  clip  53  holds a vessel  25  securely but detachably against housing  55 . 
     As noted above, the probe  43  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 . A housing  74  contains ultrasonic transducers (not shown in  FIG. 3 ) and a tonometric pressure sensor  75  that projects out of housing  74  and abuts against conduit or blood vessel  77 . Clip  79  also projects out of housing  74  to securely hold conduit or blood vessel  77  against housing  74 . Electrical lead  80  carries electrical signals between the ultrasonic transducers (not shown in  FIG. 3 ) and the tonometric pressure sensor  75  and the flow and pressure meter  29 . 
       FIG. 4  is a cut away cross-sectional view of probe head  73  and vessel  77  along line IV-IV in  FIG. 3 .  FIG. 4  shows the position of ultrasound transducers  81 A and  81 B that are located inside housing  74 . Ultrasound transducers  81 A and  81 B are positioned to exchange ultrasound transmissions that are reflected off of an interior surface  87  of clip  79 . Readings of flow volume of the blood in vessel  77  are taken from the ultrasound transmissions produced by the transducers and analyzed as indicated above. Tonometric pressure sensor  75  has a flat sensing surface  89  that shapes the portion of vessel  77  that the surface 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 . 
       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  shape of the 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  75 , the sensing surface of the tonometric sensor must always conform or shape 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 a pressure gradient across a vessel&#39;s 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:
 
Pout=Pin  [3]
 
Thus as can be seen the pressure differential across the vessel wall at the flattened portion goes to zero ΔP→0.
 
     The tonometric sensing surface  89  is flat to thereby conform or dispose 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 flat 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  29  (not shown). 
       FIG. 6  provides an enlarged view of another variation of a flow-pressure sensor probe  101 . The variation of the disclosure in  FIG. 6  would be implanted into a test subject such as a laboratory rat, sheep, horse etc. for chronic, long term measurements. The probe  101  would naturally be placed around a blood vessel  103  by inserting the blood vessel through a gap  105  formed by housing  107  and clip  109 . Since vessel  103  is flexible and easily deformable the vessel  103  may be inserted through gap  105 . Probe  101  is sized such that a sensing surface  113  of a tonometric sensor  111  abuts firmly up against an outside wall of vessel  103  and forms the flat surface described previously that allows for the direct tonometric measurement of pressure. Alternatively, an insert sized to fit into the probe  101  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  29  (not shown) 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  29  or computer (not shown). Alternately, electrical conduit  115 , may connect to a fully implanted signal telemetry device (not shown) in the subject. 
       FIG. 7  is a cross sectional cut away view along line VII-VII of  FIG. 6 . In  FIG. 7  a flattened portion  117  of vessel  103  wall can be seen. When a probe, such as probe  101  is chronically implanted, overtime tissue  119  may grow around probe  101  and between probe housing  107  and vessel  103 . However, tissue  119  does not negatively affect tonometric sensor  111  because at flattened surface  117  the tissue  119  atrophies and relies on sensing surface  113  for support. This reliance by tissue  119  on the sensing surface  113  enhances the operation of the tonometric sensor  111 , as the interposed fibrous tissue  119  becomes passive and thus incapable of altering pressure in vessel  103 . Additionally, the tissue growth  119  between housing  107  and vessel  103  forms a uniform transition between ultrasound transceivers  81 A and  81 B located in housing (not shown in  FIG. 7 ) and vessel  103 , which will reduce motion artifacts. 
       FIG. 8  is another variation of chronically implantable type of probe. In this variation numbering of the various parts disclosed in  FIGS. 6 and 7  has been retained. The added feature is an insert or cuff  121 . Cuff  121  is made of an acoustically compatible, flexible and reliant material. Cuff  121  is sized to fit into housing  107  of probe  123 . As depicted in  FIG. 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  123 . Biological compatibility reduces rejection of the cuff  121  by the body. A material that meets this criteria is Pebax® (Elf-Autchem). A detailed discussion of the insert or cuff  121  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. 
       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  121  has a hole  133  in a top of the cuff to receive sensor  111 . 
     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.
 
     One preferred embodiment of the disclosure employs a transit time ultrasound sensor, which fully illuminates the cross sectional area of the vessel  103  with bidirectional beams of ultrasound. It is within the spirit of the disclosure to employ other sensors for the measurement of flow. 
     In another variation, 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  111  and Doppler sensor  149  adjacent vessel  103 . 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 functions, publications and textbooks known by those of ordinary skill in the art adequately sets forth the level of those skilled in the art. In one variation the Doppler ultrasound sensor could be limited to reading flow velocity and not volume flow. However, by taking a series of readings over a cross sectional area of the vessel  103 , the internal diameter of the vessel  103  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,  1608 ,  160 C,  160 D and  160 E of vessel  103  in order to thereby estimate volume flow. 
     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.