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 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 provide the capacity to calculate the impedance of the tissue or organs to which the vessels are 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 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.
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).
Perivascular measurement of blood volume flow with ultrasound has been a standard technique which has been used since the 1980'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's, and may be used as an alternate flow measurement approach for the disclosed invention.
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 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 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.
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
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 preferred embodiment perivascular system of the present invention for measuring flow and pressure;
FIG. 2 is a full raised view of one type of preferred embodiment of flow pressure sensor perivascular probe of the present invention;
FIG. 2A is a side view of probe head 49 of FIG. 2 along line IIA with a vessel inserted into the head;
FIG. 3 is a front view of another variation of a preferred embodiment of a flow-pressure sensor perivascular probe of the present invention;
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 lines 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 lines 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 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.
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.
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.
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.
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.
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 81A and 81B that are inside housing 74. Ultrasound transducers 81A and 81B 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.
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's law for pressure the gradient across a vessels wall which is expressed in the following equation:
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:
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.
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 81A and 81B as well as tonometric sensor 85 up through electrical line conduit 80 to the flow-pressure meter not shown.
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 111 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.
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.
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.
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.
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:
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 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.
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 160A, 160B, 160C, 160D and 160E, 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.
Patent Application
20080021325
