U.S. patent number RE39,863 [Application Number 10/224,346] was granted by the patent office on 2007-10-02 for combined flow, pressure and temperature sensor.
This patent grant is currently assigned to Radi Medical Systems AB. Invention is credited to Leif Smith.
United States Patent |
RE39,863 |
Smith |
October 2, 2007 |
Combined flow, pressure and temperature sensor
Abstract
The invention relates to a device for measuring pressure,
temperature and/or flow velocity. It includes a sensor (6) with a
sensor support body (13) provided with a diaphragm (15) covering a
cavity (14) formed in the support body (13). A pressure sensitive
element (41) is mounted on the diaphragm, for recording pressure.
Furthermore, a temperature sensitive resistor (42) is mounted in
the vicinity of the pressure sensitive resistor and has a known
temperature dependence, for recording temperature. It also includes
an electrical circuit (43, 44, 45, 46) selectively outputting
signals from either of the pressure sensitive element and the
temperature sensitive resistor.
Inventors: |
Smith; Leif (Uppsala,
SE) |
Assignee: |
Radi Medical Systems AB
(Uppsala, SE)
|
Family
ID: |
20401201 |
Appl.
No.: |
10/224,346 |
Filed: |
January 30, 1997 |
PCT
Filed: |
January 30, 1997 |
PCT No.: |
PCT/SE97/00150 |
371(c)(1),(2),(4) Date: |
April 06, 1999 |
PCT
Pub. No.: |
WO97/27802 |
PCT
Pub. Date: |
August 07, 1997 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
09117416 |
Apr 6, 1999 |
06343514 |
Feb 5, 2002 |
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Foreign Application Priority Data
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Jan 30, 1996 [SE] |
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9600334 |
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Current U.S.
Class: |
73/719;
73/708 |
Current CPC
Class: |
A61B
5/0215 (20130101); A61B 5/028 (20130101); G01F
1/6845 (20130101); G01K 1/045 (20130101); G01L
19/0092 (20130101); G01L 19/148 (20130101); A61M
2025/0002 (20130101) |
Current International
Class: |
G01L
9/02 (20060101); G01L 19/04 (20060101) |
Field of
Search: |
;73/708,714,715,717,718,721,727,756,700,204.23,204.24,204.25,204.26,861.47,861.65
;600/549,561,585,485,488 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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42 19 454 |
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Dec 1993 |
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DE |
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0 178 368 |
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Apr 1995 |
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EP |
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Other References
Pijls, N., et al., "Fractional Flow Reserve: A Useful Index to
Evaluate the Influence of an Epicardial Coronary Stenosis on
Myocardial Blood Flow," Circulation, vol. 92, No. 11, pp. 3183-3193
(1995). cited by examiner .
Kalvesten, E., et al., "A Small-Size Silicon Microphone for
Measurements in Turbulent Gas Flows," Sensors and Actuators A
(1994). cited by examiner .
"Measurement Systems," 3rd Ed., Doebelin, pp. 506-517 (1983). cited
by examiner.
|
Primary Examiner: Oen; William
Attorney, Agent or Firm: Foley & Lardner LLP
Claims
What is claimed is:
1. A device for determining pressure, temperature and a flow
parameter of a flowing fluid, comprising i) a sensor support body
(13) mounted for insertion into a vessel of a living body and
having a diaphragm (15) covering a cavity (14) formed in said
support body (13); ii) a pressure sensitive element (41), having a
known temperature dependence, mounted on said diaphragm (15), and
providing an output indicative of a pressure; iii) a temperature
sensitive resistor (42) mounted in the vicinity of said pressure
sensitive element (41) and having a known temperature dependence,
providing an output signal indicative of a temperature, said
resistor (42) also functioning as a temperature reference means for
said pressure sensitive element (41); and iv) an electrical circuit
(43, 44, 45, 46), connected to the pressure sensitive element and
to the temperature sensitive resistor, for calculation of a flow
parameter of said flowing fluid on the basis of the temperature
signals.
2. The device of claim 1, wherein said electrical circuit comprises
a double Wheatstone bridge, including a first (42, 43, 44, 46) and
a second (41, 42, 45, 46) bridge, said two bridges having two
resistors in common, whereby the first bridge comprises said
temperature sensitive resistor (42), and the second bridge
comprises the pressure sensitive element (41) and said temperature
sensitive resistor (42).
3. The device as claimed in claim 1, comprising a Wheatstone
bridge, wherein the output from the bridge is indicative of
pressure, and the total impedance of the bridge is indicative of
temperature.
4. The device as claimed in claim 1, wherein both the pressure
sensitive element (41) and the temperature sensitive resistor (42)
are located on said diaphragm (15).
5. The device as claimed in claim 1, wherein only the pressure
sensitive element (41) is located on said diaphragm (15) and the
temperature sensitive resistor (42) is located on said sensor
support body (13).
6. The device as claimed in claim 1, wherein the temperature
sensitive resistor (42) is mounted on a dummy diaphragm, having
essentially the same properties as the diaphragm (15) on which the
pressure sensitive element is located.
7. The device as claimed in claim 1, attached at the distal end of
a guide wire having a proximal and a distal end.
8. The device as claimed in claim 1, further comprising means for
temperature compensation in the pressure measurement mode, such
that the recorded potential representing a pressure is modified by
adding or subtracting from said recorded potential, a known off set
potential value depending on temperature.
9. The device as claimed in claim 1, wherein the pressure sensitive
element (41) is of a piezoresistive type.
10. The device as claimed in claim 1, wherein the pressure
sensitive element (41) is of a capacitive type.
11. The device as claimed in claim 1, wherein the pressure
sensitive element (41) is a mechanically resonant sensor.
12. The device of claim 1, further including a selective power
supply power connected to said temperature sensitive resistor to
heat the temperature sensitive resistor to a predetermined
temperature, and a temperature deviation determinator.
13. A guide wire and sensor assembly for determining pressure,
temperature and a flow parameter of a fluid flowing in a living
body, comprising i) a guide wire (2) having a distal and a proximal
end; ii) a sensor element (6) provided at the distal end of said
guide wire, said sensor element comprising a) a sensor support body
(13) provided with a diaphragm (15) covering a cavity (14) formed
in said support body; b) a pressure sensitive element (41) having a
known temperature dependence and mounted on said diaphragm (15),
recording pressure; and c) a temperature sensitive resistor (42)
mounted in the vicinity of said pressure sensitive element (41) and
having a known temperature dependence, recording temperature, said
resistor (42) also functioning as a temperature reference for the
pressure sensitive element (41) and to provide temperature signals
for calculation of a flow parameter; and iii) an electrical circuit
comprising a double Wheatstone bridge, including a first (42, 43,
44, 46) and a second (41, 42, 45, 46) bridge, said two bridges
together comprising six resistive elements, said double Wheatstone
bridge having two resistors in common, whereby the second bridge
includes said pressure sensitive element (41) and said temperature
sensitive resistor (42), and the first bridge includes the
temperature sensitive resistor (42).
14. A guide wire and sensor assembly for determining pressure,
temperature and a flow parameter of a fluid flowing in a vessel,
comprising i) a guide wire (2) having a distal and a proximal end;
ii) a sensor element (6) provided at the distal end of said guide
wire, said sensor element comprising a) a sensor support body (13)
provided with a diaphragm (15) covering a cavity formed in said
support body (13); b) a pressure sensitive element (41) having a
known temperature dependence and mounted on said diaphragm (15),
recording pressure; and c) a temperature sensitive resistor (42)
mounted in the vicinity of said pressure sensitive element and
having a known temperature dependence, recording temperature, said
resistor also functioning as a temperature reference for the
pressure sensitive element and to provide temperature signals for
calculation of a flow parameter; and iii) an electrical circuit
selectively recording output signals from either of said pressure
sensitive element and said resistor, said circuit comprising a
Wheatstone bridge (51, 52, 53, 54), wherein the output from the
bridge is indicative of pressure, and the total impedance of the
bridge is indicative of temperature.
15. The method of claim .[.14.]. .Iadd.19.Iaddend., wherein the
flow of fluid comprises a primary and a secondary fluid, divided up
in two sequential flows having different temperatures, the primary
flow comprising blood, and the secondary flow being a fluid of
substantially lower temperature than the blood.
16. The method of claim .[.15.]. .Iadd.19.Iaddend., wherein a
temperature profile is obtained by the continuous measurement of
the temperature, and wherein the width at half height of said
temperature profile is used as .Iadd.said .Iaddend.flow
parameter.
17. The method of claim .[.15.]. .Iadd.19.Iaddend., wherein the
time for .[.the.]. cold fluid to pass the sensor .Iadd.element
.Iaddend.is used as said flow parameter.
18. The method of claim .[.15.]. .Iadd.19.Iaddend., wherein the
time of transit of .[.the.]. cold fluid from .[.the.]. .Iadd.a
.Iaddend.time of injection until it reaches said sensor
.Iadd.element .Iaddend.is used as said flow parameter.
19. A method of determining pressure, temperature and a flow
parameter of fluid flowing in vessels, comprising the following
steps: a) providing a pressure sensitive element and a resistor on
a sensor element at a measurement site in a vessel of a living
body, said pressure sensitive element and said resistor being part
of an electric circuit yielding a pressure indicative output and a
temperature indicative output, said pressure sensitive element and
said resistor having known temperature dependencies, whereby the
resistor is used as a reference for the pressure sensitive element;
b) subjecting said sensor element to flowing fluid and monitoring
the pressure and temperature of said fluid by continuously
recording said pressure indicative output and said temperature
indicative output from said electric circuit; c) subjecting said
resistor to a changed thermal environment; d) registering the
change in said temperature indicative output resulting from said
changed thermal environment; and e) calculating a flow parameter
from said change in said temperature indicative output.
20. The method of claim 19, including achieving said changed
thermal environment by said fluid causing a temperature drop in
said resistor, said flow parameter being a quantity proportional to
the volume flow.
21. The method of claim 20, wherein the volume flow is calculated
by integrating the temperature over time using the equation
.intg..times..times. .times.d.varies..intg..times..times. .times.d
##EQU00003## wherein V is the volume of injected liquid T.sub.m is
the measured temperature T.sub.1 is the temperature of injected
liquid T.sub.0 is the temperature of the blood, i.e. 37.degree. C.
Q is the volume flow t.sub.0 is the point in time where a
temperature change is detected t.sub.1 is the point in time where
the temperature is regarded as having reached normal
temperature.
22. The method of claim 19, wherein said changed thermal
environment is achieved by said resistor being heated by passing a
current through it, whereby said fluid cools said resistor such
that the actual temperature of said resistor is lower than the
expected temperature, that would have been obtained, had said
resistor not been subjected to said flowing fluid, and calculating
a flow parameter from said deviation from said expected temperature
value, said flow parameter being a quantity proportional to the
flow velocity.
23. The method of claim 22, wherein the flow velocity is calculated
using the equation V=(h-C.sub.0).sup.2/C.sub.1 wherein
h=I.sup.2R.sub.w/A(T.sub.w-T.sub.f) wherein I=wire current
R.sub.w=wire resistance T.sub.w=wire temperature
T.sub.f=temperature of a flowing fluid h=film coefficient of heat
transfer A=heat transfer area V=flow velocity.
24. A method of diagnosing small vessel disease, comprising
performing a measurement at a site in a vessel distally of a
suspected stricture according to the following steps: a) providing
a pressure sensitive element and a resistor on a sensor element at
a measurement site, said pressure sensitive element and said
resistor being part of an electric circuit yielding a pressure
indicative output and a temperature indicative output, said
pressure sensitive element and said resistor having known
temperature dependencies, whereby the resistor is used as a
reference for the pressure sensitive element; b) subjecting said
sensor element to flowing fluid and monitoring the pressure and
temperature of said fluid by continuously recording said pressure
indicative output and said temperature indicative output from said
electric circuit; c) subjecting said resistor to a changed thermal
environment; d) registering the change in said temperature
indicative output resulting from said changed thermal environment;
and e) calculating a flow parameter from said change in said
temperature indicative output; and f) comparing the calculated flow
parameter and the measured pressure with corresponding quantities
representative of a healthy patient.
25. The method of claim 24, wherein said comparing step f)
comprises comparing a flow parameter in a rest condition with a
flow parameter in a work condition, and the pressure distally of a
stenosis with the proximal pressure in a work condition.
26. A method of diagnosing small vessel disease, comprising
performing measurements at a site in a vessel distally of a
suspected stricture according to the following steps: a) providing
a pressure sensitive element and a resistor on a sensor element at
a measurement site, said pressure sensitive element and said
resistor being part of an electric circuit yielding a pressure
indicative output and a temperature indicative output, said
pressure sensitive element and said resistor having known
temperature dependencies, whereby the resistor is used as a
reference for the pressure sensitive element; b) subjecting said
sensor element to flowing fluid and monitoring the temperature of
said fluid by continuously recording said temperature indicative
output from said electric circuit; c) subjecting said resistor to a
changed thermal environment; d) registering the change in said
temperature indicative output resulting from said changed thermal
environment; e) calculating a flow parameter (Q.sub.rest) from said
change in said temperature indicative output; f) injecting a vaso
dilating drug in said vessel to simulate a work condition; g)
monitoring the pressure (P.sub.work,dist) and temperature of said
fluid by continuously recording said pressure indicative output and
said temperature indicative output from said electric circuit; h)
subjecting said resistor to a changed thermal environment; i)
registering the change in said temperature indicative output
resulting from said changed thermal environment; j) calculating a
flow parameter (Q.sub.work) from said change in said temperature
indicative output; k) determining the proximal pressure
(P.sub.prox,work); l) calculating CFR=Q.sub.work/Q.sub.rest and
FFR=P.sub.dist,work/P.sub.prox,work j) comparing the calculated CFR
and FFR with corresponding quantities representative of a healthy
patient.
27. A device for biological pressure and temperature measurements,
comprising: a guide wire; a pressure sensor mounted on the guide
wire; a temperature sensor mounted on the guide wire in the
vicinity of the pressure sensor; and an electronic circuit to
generate and output an indication of temperature based on signals
from the temperature sensor.
28. A device as set forth in claim 27, further comprising a sensor
support body and wherein the pressure sensor and the temperature
sensor are both mounted to said sensor support body.
29. A device as set forth in claim 27, further comprising a chip
and wherein the pressure sensor and the temperature sensor are both
mounted to said chip.
30. A device as set forth in claim 27, further comprising a silicon
chip and wherein the pressure sensor and the temperature sensor are
both mounted to said silicon chip.
31. A device as set forth in claim 27, further comprising a
substrate and wherein the pressure sensor and the temperature
sensor are both mounted to said substrate.
32. A device as set forth in claim 27, wherein the pressure sensor
senses pressure at the same time the temperature sensor senses
temperature.
33. A device as set forth in claim 27, wherein the pressure sensor
includes a diaphragm.
34. A device as set forth in claim 33, wherein the temperature
sensor is located off of the diaphragm.
35. A device as set forth in claim 27, wherein the electronic
circuit also generates a profile of temperature versus time.
36. A device as set forth in claim 35, wherein the electronic
circuit also generates flow information based on said profile.
Description
The present invention relates generally to pressure, temperature
and flow measurements, in particular in the medical field, and
especially to in situ measurements of the intracoronary pressure,
distally of a stricture, using a guide wire having a pressure
sensor mounted at its distal end.
In particular it concerns a combined flow, pressure and temperature
sensor.
BACKGROUND OF THE INVENTION
In order to determine or assess the ability of a specific coronary
vessel to supply blood to the heart muscle, i.e. the myocardium,
there is known a method by which the intracoronary pressure
distally of a stricture in combination with the proximal pressure
is measured. The method is a determination of the so called
Fractional Flow Reserve (see "Fractional Flow Reserve",
Circulation, Vol. 92, No. 11, Dec. 1, 1995, by Nico H. j. Pijls et
al.). Briefly FFR.sub.myo is defined as the ratio between the
pressure distally of a stricture and the pressure proximal of a
stricture, i.e. FFR.sub.myo=P.sub.dist/P.sub.prox. The distal
pressure is measured in the vessel using a micro-pressure
transducer, and the proximal pressure is the arterial pressure.
A limitation in measuring only the blood pressure and the pressure
gradient, alternatively the Fractional Flow Reserve, is that there
is no control of the flow in the coronary vessel. As an example, a
vessel having a significant stricture would not yield any pressure
drop if the myocardium is defective and has no ability to receive
blood. The diagnosis will incorrectly show that the coronary vessel
is healthy, when instead the conclusion should have been that the
myocardium and possibly the coronary vessel are ill.
A diagnosis method for diagnosing small vessel disease is performed
as follows:
The Fractional Flow Reserve is determined. If the FFR is <0.75
the coronary vessel should be treated.
If FFR is >0.75 there are two possibilities:
a) either the patient is healthy with respect to the actual
coronary vessel (the most plausible), or
b) there is a low blood flow distally of the distal pressure
measurement due to either an additional stricture or a sickly
myocardium.
In order to investigate whether alternative b) is at hand, it is
desirable to obtain knowledge regarding the health status of the
myocardium, by measuring Coronary Flow Reserve (CFR), or in the
alternative the Coronary Velocity Reserve (CVR). The idea is to
determine by how many times a patient is able to increase his/her
blood flow during work. A healthy patient should be able to
increase the blood flow by 2.5-5 times, depending on the patient's
age. Work is simulated by the addition of a so called vaso dilating
pharmaceutical/medicament, e.g. Adenosine, Papaverin or the like.
This medicament dilates the capillaries which increases the blood
flow. The same medicament is used for determining FFR.
CFV is defined as .times. .times..times. .times. ##EQU00001## (Q is
the flow).
This being a ratio and assuming that the cross sectional area is
constant during one velocity measurement, it will suffice to
measure the velocity.
CFR is defined as
CFR=Q.sub.work/Q.sub.rest=[V.sub.work*K]/[V.sub.rest*K]=V.sub.work/V.sub.-
rest
Since the desired parameter is a flow increase, it will be
sufficient to obtain it as a relative quantity
CFR=[K*V.sub.work]/[K*V.sub.rest] wherein K is a constant.
Researchers have devised methods where the pressure and flow
velocity in the coronary vessel are measured, the results being
presented as so called "pressure-velocity loops" (di Mario).
Thereby it becomes possible to distinguish patients suffering from
the so called "small vessel disease" from others. In patients with
"small vessel disease" the pressure gradient, corresponding to a
low FFR, and the velocity of flow will be low, whereas healthy
patients will have a low pressure gradient, corresponding to a high
FFR, and a high flow.
In some investigations the applicant's system for pressure
measurements in vivo, Pressure Guide.TM. (Radi Medical Systems) and
the flow sensor sold under the trade name Flowmap.TM.
(Cardiometrics) have been tested.
It is a great drawback to have to introduce two sensors into the
coronary vessel, compared to a situation where both sensors are
mounted on a "guide wire". Thus, it has been suggested to provide a
guide wire with two sensors, but this presents several technical
problems with the integration of two sensors in a thin guide
wire.
SUMMARY OF THE INVENTION
The object of the invention is therefor to make available means and
methods for carrying out such combined pressure and flow
measurements with a single unit, thus facilitating investigations
of the outlined type, and making diagnosing more reliable.
The object outlined above is achieved according to the invention
with the sensor as defined in claim 1, whereby the problems of the
prior art have been overcome. The key is to use the temperature
sensitive element for obtaining a flow parameter. Thus, there is
provided a single sensor having the ability to measure both the
pressure and to determine the velocity of flow or the volume flow.
A great advantage with such a solution is that only one electrical
circuit needs to be provided in a guide wire.
In a preferred embodiment, the sensor is an electrical sensor of a
piezoresistive type. However it is contemplated that other pressure
sensitive devices may be used, e.g. capacitive devices, or
mechanically resonating sensors.
In accordance with the invention there is also provided a method of
determining pressure, temperature and flow in a coronary vessel, as
defined in claim 20.
Further scope of applicability of the present invention will become
apparent from the detailed description given hereinafter.
However, it should be understood that the detailed description and
specific examples, while indicating preferred embodiments of the
invention are given by way of illustration only, since various
changes and modifications within the spirit and scope of the
invention will become apparent to those skilled in the art from
this detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention, will become more fully understood from the
detailed description given herein below and the accompanying
drawings which are given by way of illustration only, and thus not
limitative of the present invention, and wherein
FIGS. 1a and 1b show a microphone for recording extremely small
eddies in turbulent gas flows;
FIG. 2 shows a sensor/guide assembly to be used together with the
invention;
FIG. 3 shows a top view of a pressure sensor chip and the electric
circuitry schematically illustrated;
FIG. 4 shows schematically the circuit of a "double" Wheatstone
bridge for use in the invention;
FIG. 5 is an illustration of a Wheatstone bridge used in a second
embodiment of the invention;
FIG. 6 shows temperature profiles obtained in a thermodilution type
measurement;
FIG. 7 is a schematic illustration showing how transit time is used
to obtain the desired parameter.
DETAILED DESCRIPTION OF THE INVENTION
With reference to FIGS. 1a and 1b there is shown a prior art device
disclosed in a publication entitled "A Small-Size Microphone for
Measurements in Turbulent Gas Flows" in Sensors and Actuators A,
1994. It comprises a microphone for recording extremely small
eddies in turbulent gas flows. It is based on piezoresistive
techniques for transducing pressure fluctuations into electrical
signals.
The microphone comprises a silicon substrate 100, and a cavity 102
in said substrate. A diaphragm of polysilicon 104 covers the cavity
102. On the diaphragm a polysilicon piezoresistor 106 is attached.
Etch holes 108 and etch channels 110 are provided for manufacturing
purposes. Vent channels 112 are also provided. On the substrate 100
there are metal conductors 114 and bond pads 116 for connecting
cabling to external devices.
Now turning to FIG. 2 there is shown a sensor/guide device
comprising a solid wire 1 which is machined by so called centering
grinding, and inserted into a proximal tube portion 2. The wire 1
forms the distal portion of the guide, and extends beyond the
distal end of the proximal tube portion 2 where said tube is
connected to or integrally formed with a spiral portion 3. On the
distal end of the wire 1 there is mounted a pressure sensor 6.
Between the wire 1 and the spiral portion 3, electrical leads 4
from the electronic circuitry run parallel with said wire 1. The
sensor 6 is protected by a short section of a tube 7 having an
aperture 8 through which surrounding media act on the pressure
sensor. At the very distal end of the entire device there is a
radio opaque coil 9, e.g. made of Pt, and used for location
purposes, and a safety wire 10 for securing the distal part of the
spiral 9.
To minimize the number of electrical leads, the wire or tube may be
used as one of the electrical leads.
The proximal tubing 2 and the spiral 3 may be coupled such as to be
utilized as an electrical shield, in which case it of course cannot
be used as an electrical lead.
Now embodiments of the pressure sensor will be described with
reference to FIGS. 3-4.
The sensor is based on the small size silicon microphone mentioned
above, which is designed for detecting extremely small eddies in
turbulent gas flows. It has been fully described for that
application in said publication "Sensors and Actuators A", 1994
(incorporated herein in its entirety by reference). However, it has
been modified in accordance with the present invention in the way
described below. In order to further miniaturize the external
dimensions of the microphone to meet the requirements of the
invention, the external dimensions for accommodating the lead
pattern on the sensor should be no more than 0.18 mm.times.1.3
mm.times.0.18 mm, preferably no more than 0.14 mm.times.1.3
mm.times.0.1 mm.
An unexpected advantage of miniaturizing is that the thermal mass,
and thereby the thermal time constant, is low, i.e. the entire chip
including its resistors heats up and cools down very quickly. In
fact it is thereby possible to monitor dynamic changes in the
domain 1 Hz and faster. For the purpose of studying flow in blood
vessels, the variation of flow velocity or volume flow during a
heart cycle is easily detected, and therefor anomalies in the blood
flow may be detected.
The sensor (see FIG. 3) comprises a sensor support body in the form
of a silicon chip 13 in which there is a cavity 14 made e.g. by
etching. Across the cavity there is formed a polysilicon diaphragm
15 having a thickness of e.g. 0.4-1.5 .mu.m or possibly up to 5
.mu.m, and a side length of 100 .mu.m. Within the cavity a vacuum
of less than 1000 Pa, preferably less than 30 Pa prevails. In
contact with said diaphragm there is mounted a piezoresistive
element 41. A pressure acting on the diaphragm 15 will cause a
deflection thereof and of the piezoresistive element 41, which
yields a signal that may be detected.
In order to attach the cabling 4 to the chip, bond pads 19 are
required. These bond pads must have a certain dimension (e.g.
100.times.75 .mu.m), and must be spaced apart a certain distance,
respect distance approximately 125 .mu.m. Since the dimensional
adaptation entails narrowing the chip, the consequence is that in
order to be able to meet the mentioned requirements, the bond pads
have to be located in a row, one after the other, as shown in FIG.
3.
It is also preferred for temperature compensation purposes to have
a reference resistor 42 mounted on the sensor. This reference
resistor 42 may be located on different points on the sensor
chip.
In one embodiment it is placed on the diaphragm 15. This is
preferred since identical environments to both the active,
piezoresistive element 41 and the reference resistor 42 will be
provided. Thereby the active element, i.e. the piezoresistive
element 41, is mounted such that it will be affected by a
longitudinal tension 41 when it is subjected to a pressure. The
reference resistor 42 is preferably mounted perpendicularly with
respect to the active element 41 and along the border of the
diaphragm 15, i.e. at the periphery of the cavity 14 present
underneath the diaphragm 15.
However, it is possible to locate the reference resistor on the
silicon substrate 13 adjacent the diaphragm. This is an advantage
since the reference resistance thereby will be pressure
independent.
Another possibility is to locate the reference resistor on a
"dummy" diaphragm adjacent the real diaphragm 15, in order to
provide the same mechanical and thermal environment for the active
element 41 and the reference resistor 42.
With reference to FIG. 4, an embodiment of the electrical circuit
and its operation and function will now be described.
As schematically is shown in FIG. 4, one embodiment of the sensor
circuit comprises six resistors 41 . . . 46, two of which 41, 42
are mounted on the diaphragm, as previously mentioned (resistor 41
corresponds to resistor 41 in FIG. 3, and resistor 42 corresponds
to resistor 42 in FIG. 3). Resistor 41 is a piezoresistive element,
and resistor 42 is only temperature sensitive. The remaining
resistors 43, 44, 45, 46 are located externally of the entire
sensor/guide assembly, and do not form part of the sensor
element.
In this embodiment the resistors are coupled as a "double"
Wheatstone bridge, i.e. with resistors 42, 43, 44, 46 forming one
bridge (for temperature compensation and flow calculation),
resistors 41, 42, 42, 46 forming the second bridge for pressure
measurement. Thus, resistors 45 and 46 are shared by the bridges.
Thereby it is possible to measure the temperature (across B-C) and
pressure (across A-C) independently of each other. From the
measured temperature values the flow velocity or volume flow may be
calculated.
In another embodiment there are four resistors (51, 52, 53, 54)
connected as shown in FIG. 5, i.e. as a simple "single" Wheatstone
bridge. If at least one of the four resistors, say 51, has a
temperature coefficient .noteq.0, then temperature changes may be
measured as follows:
If the voltage V applied is maintained constant, the current I
through the circuit may be measured and is a measure of the
temperature, since the total impedance (resistance) of the circuit
will change with temperature.
Alternatively the current I may be maintained constant, and in this
case the voltage over the bridge will be temperature dependent.
By means of the shown circuit, the CFR can be determined by
registering the temperature drop due to a passing liquid having a
lower temperature than the body temperature, as will be discussed
in detail below.
For the flow determination the principle of so called hot-wire and
hot-film anemometers may be employed (reference is made to
"Measurement Systems", 3rd edition, pp 506-, by Doebelin, 1983), in
which case a flow velocity may be obtained.
Alternatively the principle of thermo-dilution may be employed in
which case the volume flow may be obtained.
Both principles will be discussed below beginning with hot-wire
anemometers.
Hot-wire anemometers commonly are made in two basic forms: the
constant current type and the constant temperature type. Both
utilize the same physical principle but in different ways. In the
constant current type, a fine resistance wire carrying a fixed
current is exposed to the fluid flowing at a certain velocity. The
wire attains an equilibrium temperature when the i.sup.2R heat is
essentially constant; thus the wire temperature must adjust itself
to change the convective loss until equilibrium is reached. Since
the convection film coefficient is a function of flow velocity, the
equilibrium wire temperature is a measure of velocity. The wire
temperature can be measured in terms of its electrical resistance.
In the constant temperature form, the current through the wire is
adjusted to keep the wire temperature (as measured by its
resistance) constant. The current required to do this then becomes
a measure of flow velocity.
For equilibrium conditions we can write an energy balance for a hot
wire as I.sup.2R.sub.w=hA(Tw-T.sub.f) where I=wire current
T.sub.w=wire temperature T.sub.f=temperature of flowing fluid
h=film coefficient of heat transfer A=heat transfer area
R.sub.w=wire resistance h is mainly a function of flow velocity for
a given fluid density.
It can be written generally on the form
h=C.sub.0+C.sub.1 {square root over (V)}
where V is the flow velocity, and C.sub.0 and C.sub.1, are
constants. For a more detailed account of the theory for hot-wire
anemometers reference is made to the cited publication.
In pressure measurement mode the resistors in the circuit (FIG. 4)
are supplied with 1-10 V (AC or DC), and the potential difference
between A and B is registered as a signal representing the
pressure. Unless the resistors 41 and 42 are identical in terms of
their temperature dependence, this potential difference will be
temperature dependent, i.e. one has to know a quantity
representative of the temperature at which the measurement takes
place in order to obtain a correct pressure value, and therefore
the bridge has to be calibrated. This is achieved by recording the
potential difference between A and B (see FIG. 4) as a function of
the potential difference between A and C at different temperatures,
e.g. in a controlled temperature oven or in a water bath. Thus, an
"off set" vs temperature dependence curve is obtained, that is used
to compensate the pressure signal (A-B) for a given temperature.
Namely, at a given temperature it is known from the calibration
curve how much should be subtracted from or added to the actual
registered signal in order to obtain a correct pressure. It would
be advantageous if resistors 41 and 42 have identical or at least a
very similar temperature dependence. This is in fact also the case,
since they are made in practice at the same time during manufacture
of the chip itself. Thus, material composition and properties are
in practice identical. Nevertheless the above outlined compensation
is necessary in most cases.
The actual compensation process is built into the software of the
electronic system, and implementation thereof requires only
ordinary skill.
The inventors have now realized that it is possible to make use of
the temperature dependent resistor in a pressure bridge as
described above, for flow measurements, using the principle of the
hot-wire anemometer.
Thus, the temperature sensitive resistor 42 (FIG. 4) having a known
temperature behavior as a function of the current supplied to it,
is fed with a current that in a static situation (i.e. no flowing
fluid surrounding it) would yield a certain temperature, as
reflected in its resistance. If there is a difference in the
measured resistance compared to what would have been expected in
the static situation (i.e. no flow), it can be concluded that a
cooling of the resistor is taking place, and thus that there is a
flow of fluid. The measurement is made over B-C in the figure. On
the basis of this information, the theory indicated above for
anemometers may be applied, and a flow velocity calculated.
The CFR value may be obtained in the following way using the
anemometer principle: 1. place a sensor distally of a suspected
stricture 2. register the flow parameter ("flow velocity") in a
rest condition, V.sub.rest*K (K is a constant) 3. inject a
medicament (e.g. Adenosin, Papaverin) for vaso dilatation 4.
register the flow parameter ("velocity") in a work condition,
V.sub.max*K (K is a constant) 5. calculate
CFR=V.sub.max/V.sub.rest
During the same procedure the FFR (Fractional Flow Reserve) may
also be obtained by measuring the distal and proximal pressures and
calculating FFR=P.sub.dist/P.sub.prox.
Now the embodiment utilizing the principle of thermodilution will
be described.
The principle of thermo-dilution involves injecting a known amount
of cooled liquid, e.g. physiological saline in a blood vessel.
After injection the temperature is continuously recorded with a
temperature sensor attached to the tip of a guide wire that is
inserted in the vessel. A temperature change due to the cold liquid
passing the measurement site, i.e. the location of the sensor, will
be a function of the flow (see FIG. 5).
There are various methods of evaluating the temperature signal for
diagnostic purposes. Either one may attempt to calculate the volume
flow, or one may use a relative measure, where the flow in a "rest
condition" is compared with a "work condition", induced by
medicaments.
The latter is the simpler way, and may be carried out by measuring
the width at half height of the temperature change profile in the
two situations indicated, and forming a ratio between these
quantities (see FIG. 6).
Another way of obtaining a ratio would be to measure the transit
time from injection and until the cold liquid passes the sensor, in
rest condition and in work condition respectively. The relevant
points of measurement are shown in FIG. 7.
The former method, i.e. the utilization of the volume flow
parameter as such, requires integration of the temperature profile
over time (see FIG. 6) in accordance with the equations given below
.intg..times..times. .times.d.varies..intg..times..times.
.times.d.intg..times..times. .times.d.varies..intg..times..times.
.times.d ##EQU00002## wherein V is the volume of injected liquid
T.sub.r,m is the measured temperature at rest condition T.sub.r,1
is the temperature of injected liquid at rest condition T.sub.0 is
the temperature of the blood, i.e. 37.degree. C. T.sub.w,m is the
measured temperature at work condition T.sub.w,1 is the temperature
of injected liquid at work condition Q is the volume flow
These quantities may then be used directly for assessment of the
condition of the coronary vessels and the myocardium of the
patient, or they may be ratioed as previously to obtain a CFR, i.e.
CFR=Q.sub.work/Q.sub.rest.
A method of diagnosing small vessel disease, using the device of
the invention comprises performing measurements at a site in a
vessel distally of a suspected stricture. Thus, a pressure
sensitive element and a resistor on a sensor element is provided at
a measurement site, by inserting through a catheter. The pressure
sensitive element and said resistor are part of an electric circuit
yielding a pressure indicative output and a temperature indicative
output, and have known temperature dependencies. The resistor is
used as a reference for the pressure sensitive element. At the site
the sensor element will be subjected to flowing fluid, i.e. blood,
and the temperature of said fluid is monitored by continuously
recording said temperature indicative output from said electric
circuit. Then said resistor is subjected to a changed thermal
environment. The change in said temperature indicative output
resulting from said changed thermal environment is registered. This
change in temperature indicative output is used to calculate a flow
parameter (Q.sub.rest). A vaso dilating drug is injected in said
vessel to simulate a work condition, and the distal pressure
(P.sub.work,dist) and temperature of said fluid is monitored by
continuously recording said pressure indicative output and said
temperature indicative output from said electric circuit. Again the
resistor is exposed to a changed thermal environment, and the
change in said temperature indicative output resulting from said
changed thermal environment is registered. A flow parameter
(Q.sub.work) is calculated from said change in said temperature
indicative output. The proximal pressure (P.sub.prox,work) is
determined, and CFR=Q.sub.work/Q.sub.rest and
FFR=P.sub.dist,work/P.sub.prox,work are calculated. Finally the
calculated CFR and FFR are compared with corresponding quantities
representative of a healthy patient.
The invention being thus described, it will be clear that the same
may be varied in many ways. Such variations are not to be regarded
as a departure from the spirit and scope of the invention, and all
such modifications as would be clear to one skilled in the art are
intended to be included within the scope of the following
claims.
In particular it may find utility in other areas of the medical
field, wherever it is desired to measure pressure, temperature and
flow with one single device. It could also be used in non-medical
fields.
* * * * *