A flow-velocity sensor probe includes a thermistor for generating heat which is placed in a fluid, and a waterproof resin and a metal piece for easily radiating the heat generated by the thermistor. The flow velocity of the fluid is measured continuously from a change of temperature in the thermistor, and the thermistor is provided in close proximity to the metal piece for transmitting the heat to the fluid.

BACKGROUND OF THE INVENTION 
This invention relates to a flow-velocity sensor probe used in 
flow-velocity measurement such as measurement of cardiac output. 
In measurement of cardiac output which is essential in managing critically 
ill patients with cardiac failure, conventionally use is made of methods 
that rely upon ultrasound, dye dilution and radioisotopes, etc. Owing to 
its simplicity and accuracy, wide use is made of a thermodilution method 
based upon the right-heart catheter method, in which a catheter is held in 
the pulmonary artery. 
However, the information obtained in the thermodilution method is 
discontinuous. In addition, when cardiac output is measured, an infusion 
fluid must be injected each time a measurement is taken. Owing to such 
problems as the complexity of surgery, infection which accompanies the 
repeated injection of a cold saline solution, a drop in body temperature 
and increased load on the heart, the number of times measurements can be 
taken is limited, especially in case of a seriously ill patient whose 
cardiac output needs to be ascertained. 
One method of continuously measuring cardiac output very precisely is a 
method using the CCOM system (continuous cardiac output monitoring system) 
developed by the present inventors (U.S. Pat. No. 4,685,470). This 
monitoring system includes a catheter probe and a monitoring unit. By 
continuously measuring the amount of heat loss, which is due to blood 
flow, using a thermistor (referred to as a CFT) self-heated by passage of 
a current therethrough, cardiac output is monitored continuously without 
the intermittent injection of a cold saline solution. 
Cardiac output (CO) is the amount of blood expelled from the heart in a 
unit of time and usually is expressed by a value per minute. Ordinarily, 
if the heart or a major blood vessel is not short-circuited, the amount of 
blood expelled from the right heart and that expelled from the left heart 
are equal, and cardiac output CO (L/min) is obtained in accordance with 
the following equation from flow velocity (cm/sec) in the pulmonary artery 
and the cross-sectional area s (cm.sup.2) of the pulmonary artery: 
EQU CO=0.06.multidot.s.multidot.v (1) 
The principle of continuous measurement of cardiac output will now be 
described. 
A thermistor is used as an ordinary temperature sensor and operates on the 
basis of a change in resistance value in dependence upon a change in 
temperature. 
Since a thermistor is also a resistor, the thermistor itself emits heat 
when a large amount of current is passed through it. Accordingly, when 
such a thermistor is placed in the bloodstream, the temperature of the 
thermistor becomes that at which equilibrium is established between the 
amount of heat produced by the electric current and the amount of heat 
carried away by the flow of blood. Since this equilibrium temperature Tt 
varies in dependence upon the flow velocity, the thermistor can be 
utilized as a flow-velocity sensor. 
The relationship between equilibrium temperature Tt (C.degree.) and blood 
flow velocity (cm/sec) can be expressed by the following equation, which 
is based upon experimentation: 
EQU log Tt=A.multidot.log v+B (2) 
where A and B are constants dependent upon the fluid and the 
characteristics of the thermistor, etc. 
In order to measure cardiac output CO continuously, it is necessary to 
obtain a relation between the equilibrium temperature Tt and CO. 
Therefore, cancelling the flow velocity v from Eqs. (1) and (2) gives us 
the following equation: 
EQU log Tt=A.multidot.log CO+B-A.multidot.log (0.06.multidot.s)(3) 
However, Eq. (3) includes an unknown parameter, namely the cross-sectional 
area s of the pulmonary artery, and cannot be used as is in measuring 
cardiac output. Accordingly, if cardiac output and equilibrium temperature 
are measured at least once and the measured values are substituted into 
Eq. (3) as calibration values CO.sub.CAL and Tt.sub.CAL, we have 
EQU log Tt.sub.CAL =A.multidot.log CO.sub.CAL +B-A.multidot.log 
(0.06.multidot.s) (4) 
When the cross-sectional area s of the pulmonary artery is cancelled from 
Eqs. (3) and (4), we obtain 
EQU log (Tt/Tt.sub.CAL)=A.multidot.log (CO/CO.sub.CAL) (5) 
Accordingly, cardiac output CO can be expressed by the following equation 
as a function of equilibrium temperature Tt: 
EQU CO=CO.sub.CAL .multidot.(Tt/Tt.sub.CAL).sup.1/A ( 6) 
This makes it possible to measure cardiac output continuously using a 
self-heating thermistor. 
A method of calculating cardiac output using the CCOM system will now be 
described. 
In a CCOM system, the above-mentioned calibration is performed by the 
thermodilution method, and two thermistors are attached to a catheter 
probe. One of these thermistors is a self-heating CFT thermistor for 
measuring equilibrium temperature and a PAT thermistor for measuring blood 
temperature using the thermodilution method. 
CFT thermistor temperature Tt.sub.R is dependent upon a change in blood 
flow velocity but is also dependent upon a change in blood temperature TB. 
Accordingly, a correction in Tt.sub.R which accompanies a change in blood 
temperature from the time of calibration is carried out in accordance with 
the following equation: 
EQU Tt=Tt.sub.R -K.multidot.(TB-TB.sub.CAL) (7) 
where 
Tt.sub.R : CFT thermister temperature at time of measurement 
K: blood temperature correction coefficient 
TB: blood temperature 
TB.sub.CAL : blood temperature at time of calibration 
If the temperature correction of Eq. (7) is applied to Eq. (6), the 
following equation is obtained: 
EQU CO=CO.sub.CAL .multidot.{[Tt.sub.R -K.multidot.(TB-TB.sub.CAL)]/Tt.sub.CAL 
}.sup.1/A ( 8) 
Thus, as set forth above, cardiac output CO can be calculated in accordance 
with Eq. (8) from the continuously measured CFT-thermistor temperature 
Tt.sub.R and blood temperature TB. However, when the full range (0-12 
L/min) of cardiac output is calculated with the value of the constant A in 
Eq. (8) being a simple value, there is a decline in precision. Therefore, 
the range over which cardiac output is measured is divided into two parts. 
Specifically, cardiac output is calculated using arithmetic expressions 
for a case where the value of A when the cardiac output is in a high 
flow-rate region is AH and a case where the value of A when the cardiac 
output is in a low flow-rate region is AL. It should be noted that the 
constant A is an index of temperature with regard to flow velocity and 
shall be referred to as the "A value" hereinafter. 
(1) Processing in a case where the calibration value CO.sub.CAL of cardiac 
output is greater than 2.75 L/min: 
Initially, calibration of cardiac output is carried out by the thermal 
dilution method. Next, the CFT-thermistor temperature Tt.sub.2.75 when the 
cardiac output is 2.75 L/min is calculated from the calibration values 
(CO.sub.CAL and Tt.sub.CAL). That is, when Eq. (6) is transformed into an 
equation which obtains Tt and CO=2.75 L/min is substituted into the 
equation with the A value serving as AH, we have 
EQU Tt.sub.2.75 =Tt.sub.CAL .multidot.(2.75/CO.sub.CAL).sup.AH ( 9) 
At the time of measurement, cardiac output is obtained in accordance with 
the following arithmetic expressions where the measurement range is 
divided into two parts: 
##EQU1## 
(2) Processing in a case where the calibration value CO.sub.CAL of cardiac 
output is less than 2.75 L/min: 
As in the case of (1) above, the CFT-thermistor temperature Tt.sub.2.75 
when the cardiac output is 2.75 L/min is calculated from the calibration 
values (CO.sub.CAL and Tt.sub.CAL). That is, the A value is adopted as AL 
as the following is obtained from Eq. (6): 
EQU Tt.sub.2.75 =Tt.sub.CAL .multidot.(2.75/CO.sub.CAL).sup.AL ( 12) 
At the time of measurement, cardiac output is obtained in accordance with 
the following arithmetic expressions where the measurement range is 
divided into two parts: 
##EQU2## 
FIG. 1 illustrates the structure of a conventional flow-velocity sensor 
probe (catheter probe). The probe includes a catheter tube 1 and a balloon 
inflating line 9, a pressure measuring line 10, an infusion fluid 
injecting line 11 and a thermistor connecting line 12, all of which are 
connected to the catheter tube 1 via a manifold 6. 
The structure of the catheter tube 1 is such that a pressure measuring 
aperture 4, a CFT thermistor 2 and a PAT thermistor 3 are disposed at the 
tip of the catheter tube. The CFT thermistor 2 and PAT thermistor 3 are 
electrically connected to a CFT connector 7 and a PAT connector 8, 
respectively. 
FIG. 2 illustrates the structure of the CFT thermistor mount in the 
conventional flow-velocity sensor probe. 
As shown in FIG. 2, the CFT thermistor 2 is dipped in a waterproof epoxy 
resin 34 in order to assure a waterproof condition and is then inserted 
into a tube 31 made of polyimide. The CFT thermistor 2 inserted into the 
tube 31 is fitted into a side aperture 29 in the catheter tube 1, and the 
thermistor 2 is then bonded into place by an epoxy bonding agent 36 in 
order to fix the thermistor to the catheter tube 1. 
Thermistor leads 32 are passed through the interior of the catheter tube 1 
and are electrically connected to the CFT connector 7 of the thermistor 
connecting line 12. 
A number of problems are encountered in the prior art. Specifically, in the 
conventional CCOM system described above, the change in the temperature of 
the CFT thermistor regarding blood flow is small and the sensitivity 
needed in order to measure cardiac output is unsatisfactory. More 
specifically, in the CFT thermistor mount of the flow-velocity sensor 
probe (catheter probe), the structure is such that a resin having poor 
thermal conductivity located between the CFT thermistor and the outside 
(blood) blocks the efficient transfer of heat, which is emitted by the CFT 
thermistor, to the outside, and therefore cooling by the blood cannot be 
carried out sufficiently. 
In addition, the conventional flow-velocity probe (catheter probe) allows 
escape of heat along the thermistor leads, and this has an affect upon the 
temperature of the PAT thermistor. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide a 
flow-velocity sensor probe which solves the aforementioned problems 
encountered in the prior art. 
According to the present invention, the foregoing object is attained by 
providing a flow-velocity sensor probe comprising heat generating means 
placed in a fluid, temperature detecting means for detecting temperature 
produced by the heat generating means, and thermally conductive heat 
radiating means for radiating heat, which is generated by the heat 
generating means, by transmitting the heat to the fluid, the thermally 
conductive heat radiating means being provided in close proximity to the 
heat generating means. 
In such an arrangement, a change in the temperature of the CFT thermistor 
with respect to blood is enlarged to raise the sensitivity of flow 
velocity measurement. 
Other features and advantages of the present invention will be apparent 
from the following description taken in conjunction with the accompanying 
drawings, in which like reference characters designate the same or similar 
parts throughout the figures thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A preferred embodiment of the present invention will now be described in 
detail with reference to the accompanying drawings. 
FIG. 3 is a diagram showing a flow-velocity sensor probe according to an 
embodiment of the present invention and illustrating the structure of a 
CFT thermistor mounting portion in the side aperture of a catheter. 
As illustrated in FIG. 3, the CFT thermistor 2 is dipped in the waterproof 
epoxy resin 34 in order to assure a waterproof condition and is then 
bonded to a small metal piece 35. Resin 34 also is an electrical 
insulator. The small metal piece 35 consists of gold (Au), which has a 
high thermal conductivity. Boron nitride (BN), having a high thermal 
conductivity, is mixed with the waterproof epoxy resin 34 for the purpose 
of enhancing its thermal conductivity. Silicon nitride, alumina and 
diamond may be used for the purpose of enhancing the thermal conductivity 
of the epoxy resin 34. 
The CFT thermistor 2 affixed to the small metal piece 35 is bonded into 
place by the epoxy bonding agent 36 in order to fix the thermistor in the 
side aperture 29 to the catheter tube 1. In order to maintain a waterproof 
seal between the side aperture 29 of the catheter and the small metal 
piece 35, the thermistor and the catheter tube (made of vinyl chloride) 
are bonded by an epoxy bonding agent 33, which exhibits an excellent 
adhesive property. A material having excellent thermal conductivity is 
selected and used as the waterproof epoxy resin 34. 
The thermistor leads 32 are electrically connected to the CFT connector 7 
of the thermistor connecting line 12 through the interior of the catheter 
probe 1. Passing an electric current via the thermistor leads 32 causes 
the thermistor 2 to produce heat, which is transmitted to the outside via 
the resin 34 and the metal piece 35. 
The sensitivity of the flow-velocity sensor probe to flow velocity can be 
expressed using the abovementioned A value as an index. 
FIG. 4 illustrates the relationship between the A value and a change in CFT 
thermistor temperature plotted against flow velocity calculated from Eq. 
(2). 
As apparent from FIG. 4, the CFT temperature decreases, and therefore the 
sensitivity of the flow-velocity probe is improved, with an increase in 
the A value. It should be noted that the A values shown in FIG. 4 are 
values obtained by multiplying the A in Eq. (2) by -1000. 
A system for measuring the A value of the flow-velocity sensor probe will 
now be described. 
FIG. 5 is a block diagram showing a system for measuring the A value of the 
flow-velocity sensor probe. The system is constituted by a acrylic tube 
having a diameter of 20 mm and a tube made of vinyl chloride. A saline 
solution 20 is circulated through this tubing instead of blood. The arrows 
in FIG. 5 indicate the direction in which the saline solution 20 
circulates. 
The saline solution 20 is heated to a temperature of 37.degree. C. in an 
isothermal bath 19, and the solution is regulated to a predetermined flow 
rate by a diffusion pump 23 and a flow-rate regulating valve 22. The 
circulating flow rate is measured continuously by an electromagnetic blood 
flowmeter 17. The value measured by the electromagnetic blood flowmeter 17 
is calibrated in advance by being compared with a value obtained using a 
method of measuring flow rate with a graduated measuring cylinder. 
A CCOM catheter probe 15, which is a flow-velocity sensor probe, is 
inserted via a check valve 16 provided in a circulating circuit 14 and is 
so arranged that a CFT thermistor is situated downstream of an agitator 
21. The agitator 21 agitates the cold saline solution when calibration in 
the thermal dilution method is carried out, and serves as a substitute for 
the heart in a living body. 
The CFT thermistor is monitored by a CCOM monitor 24, and the 
electromagnetic blood flowmeter 17 and CCOM monitor 24 are connected to a 
computer 25. 
As for the calculation of the A value, the flow-rate regulating valve 22 is 
manipulated to reduce the circulating flow rate of the circulating circuit 
14 stepwise from 12 L/min. The CFT temperature and PAT temperature (blood 
temperature) are then measured upon each stepwise reduction in flow rate, 
and the A value is found from a correlation between these temperatures and 
the flow velocity calculated from the flow rate indicated by the 
electromagnetic blood flowmeter 17. 
FIG. 6 shows the results of measuring and comparing the performance of the 
flow-velocity sensor probe of the present invention and the performance of 
the conventional probe. 
In terms of the A value which indicates the sensitivity of a probe in 
measuring flow velocity, the flow-velocity sensor of the present invention 
has an A value greater than five times that of the conventional 
flow-velocity sensor probe in both the high and low flow-rate regions. 
Furthermore, with regard also to the B parameter [the same as the constant 
B in Eq. (2)], which is an index illustrating the degree of heat radiation 
from a probe], that of the probe according to the present invention is 
lower by about three points. The number of probes was that used in 
measurement, and the A values and values of the B parameters were average 
values for the number of probes used in measurement. 
FIG. 7 is a diagram illustrating another embodiment and showing the 
flow-velocity sensor of the invention fitted into a catheter, and FIG. 8 
is an enlarged view showing the structure of this flow-velocity sensor. 
As shown in FIG. 7, a flow-velocity sensor 100 according to this embodiment 
is fitted into a catheter tube 1. 
In FIG. 8, a nichrome wire 102 is used as the heat generating means in this 
embodiment and is wound upon the catheter tube 1 from several tens of 
times to several hundred times. A thermistor 103 for detecting temperature 
is disposed so as to contact the nichrome wire 102 as much as possible and 
to be surrounded by the nichrome wire 102. The outer side of the nichrome 
wire 102 wound upon the catheter tube 1 is covered by a metal ring 104, 
and the nichrome wire 102 and metal ring 104 used as a means of radiating 
thermally conductive heat, are contacted and fixed by a waterproof bonding 
agent. In order that the tube surface of the catheter tube 1 and the outer 
surface of the metal ring will be flush, the outer diameter of the 
catheter tube is reduced beforehand by the thickness of the nichrome wire 
and metal ring. 
The nichrome wire used here has a diameter of 0.05 mm and a resistance 
value of 560.7.OMEGA./m. The wire is sheathed in polyurethane in order to 
insulate it electrically. 
Passing an electric current through the nichrome wire causes the wire to 
produce heat, which is transmitted to the outside via the metal ring. The 
extent of transmission depends upon the flow velocity of the exterior 
fluid. The flow velocity can be measured by measuring the temperature of 
the nichrome wire at such time. 
In accordance with the foregoing embodiments, as described above, heat 
generated by the CFT thermistor is transmitted to the outside efficiently, 
and the amount of heat lost along the thermistor leads is reduced. As a 
result, the sensitivity with which the flow-velocity sensor probe measures 
flow velocity can be improved. 
In addition, since the ratio of the A value in the region of high flow rate 
to that in the region of low flow rate is reduced, the relation between 
the CFT temperature and the flow velocity can be calculated very 
accurately using Eq. (2). 
Thus, in accordance with the present invention as described above, heat 
generated by the CFT thermistor is transmitted to the outside efficiently, 
and the sensitivity of flow-velocity measurement can be improved. 
As many apparently widely different embodiments of the present invention 
can be made without departing from the spirit and scope thereof, it is to 
be understood that the invention is not limited to the specific 
embodiments thereof except as defined in the appended claims.