 |
Description  |
|
|
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention pertains to the field of biomedical instrumentation. By way
of further explanation, this invention pertains to the electronic
instrumentation of the cardiovascular system of a living organism. In
still greater particularity the invention provides simultaneous
indications of a plurality of cardiovascular conditions. This invention is
further characterized by its use of thermodilution techniques to provide
an indication of the quantity of blood flow in the cardiovascular system.
Additionally this invention can provide an intracardiac electrocardiogram
tracing.
2. Description of the Prior Art
Thermodilution is an application of the calorimetric principle that, in a
mixture of fluids at different temperatures, the heat lost by one fluid
equals the heat gained by the other. For each fluid, the mathematical
product of the temperature change, specific heat and mass is equal.
A recognized method for the study of blood circulation involves producing a
temperature change in the blood at one point in the blood flow and
measuring the temperature change at a second downstream point. Assuming
that the measurement of the temperature change occurs at a point
downstream of the heat source and that the blood's heat content is
uniform, the measured change will reflect the amount of blood passing
through the blood vessel.
In thermodilution studies heat is either removed from or added to the blood
stream. One technique involves the injection of a slightly cooler saline
solution into the blood. It was introduced by Gegler in 1953 and involved
the injection of cold blood or Ringer's solution and measurement of
temperature in the pulmonary artery or aorta with thermocouples. The
resulting temperature time curve resembled the previously used dye
dilution methods of measuring cardiac output. However, this method
requires an accurate measurement of the mass and temperature of each
injection.
Methods of introducing heat to the blood flow itself have been developed.
For example, in U.S. Pat. No. 3,438,253 issued to Frederick W. Kuether et
al. on Apr. 15, 1969, a catheter with a heating coil of platinum ribbon,
whose resistance changes with temperature, is described. By measuring the
energy required to maintain the coil at a constant, elevated temperature,
the velocity and direction of blood flow may be determined. While
satisfactory for its intended purpose, this device uses continuous heating
which could raise the overall temperature of the blood thus reducing
accuracy. Furthermore, it is required to measure the cross sectional area
of the vessel, which changes during each systole and diastole, and
multiply the "velocity" by the cross section of the vessel to obtain
volume flow. The velocity of fluid inside the vessels follows a parabolic
function (Ruch & Fulten, Medical Physiology & Biphysics p. 248) and
therefore the velocity obtained will depend on the position of the
catheter inside the vessel and will change with any movement of the
catheter to or from the center of the vessel.
In some devices, thermistors, or thermally sensitive resistors, composed of
an oxidic semiconductive material whose resistance varies with
temperature, are employed as temperature measuring devices. A Wheatstone
Bridge is used to measure resistance change in the sensing element. This
temperature sensing resistance-element is used as one arm of the bridge.
If the other three resistance arm values are known, and the bridge is
balanced, then there is no current through the galvanometer and the fourth
resistance is easily calculated. Once the resistance value of the
thermally sensitive resistor is known then the actual temperature is
calculated.
Another heating method involves the introduction of heat at one point in
the blood flow and the measurement of blood temperature at a downstream
point. A device utilizing this method is shown in U.S. Pat. No. 3,359,974,
issued to Hassan H. Khalil on Dec. 26, 1967. This device uses a standard
bilumen or trilumen cardiac catheter tube, about 3 mm dia., with fine lead
wires connected to a heater winding, and a distal temperature transducer
to measure the temperature change.
The heater winding is 12 to 15 cm of six fine enameled constantan wires,
0.04 mm dia. wound in parallel and soldered to the flattened tip of a lead
wire as it emerges from a lumen of the catheter. The coil is heated with
high frequency (350 khz) current in order not to excite the myocardium.
The temperature transducer is a fine nickel or platinum resistance
thermometer in the form of a bifilar winding over the distal 16 cm of the
catheter. The windings are covered with a thin layer of flexible varnish.
The temperature transducer is connected to a three-lead thermometer bridge
and a D.C. amplifier and recorder.
The catheter is designed so that the heating coil will be in the right
atrium and superior vena cava and the temperature transducer will lie in
the pulmonary artery. While satisfactory for its intended purposes long
areas of windings are necessary to eliminate errors introduced through
incomplete blood mixing and laminar flow. In addition, the temperature
sensing unit is located on the distal or tip portion of the catheter. This
only allows for insertion of the catheter in the same direction as the
blood flow.
SUMMARY OF THE INVENTION
The invention is a miniature thermodilution catheter for use in the
internal jugular vein or a normal size thermodilution catheter for
measurement of left ventricular output. This catheter is used where it is
desired or required to introduce the catheter against the blood flow as,
for example, if it is desired to meausre internal jugular blood flow. The
upstream high frequency heating coil is distally located at the tip of the
catheter and the downstream platinum resistance thermometer is proximally
located on the catheter so as to measure blood temperature before any
tributaries enter the vein.
Fine lead wires are wound on the outside of the catheter and connected at
solder points to the heating coil, which consists of six constantan wires
for the internal jugular vein and 9 constantan wires for the left
ventricle, bifilarly wound onto the catheter. The downstream resistance
thermometer is also bifilarly wound. A thermocouple measures local
temperature rise at the heating coil and is connected in series with a
reference thermocouple, located between the heating coil and resistance
thermometer. In addition, electrodes are included on the catheter so that
electrocardiogram tracings may be made.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view showing the invention being used to measure
internal jugular flow.
FIG. 2 is a segmented view of the distal or tip portion of the catheter;
FIG. 3 is a segmented view of the base plug and the portion of the catheter
proximal to the heating coil;
FIG. 4 is a schematic representing the electronic aspects of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, catheter 11 is inserted into living organism 9 by
operator 10 through one of the tributaries of the antecubital veins at
elbow 8. Catheter 11 is then passed upwards to the neck of living organism
9 and guided manually under screening into the internal jugular vein. Left
ventricular catheters are introduced through one of the femoral arteries.
Referring to FIG. 2, the invention is built upon catheter body 11 which is
preferably 1.5 mm in diameter if used to measure venous blood flow, and
about 2.6-2.8 mm if used to measure left ventricular output. Heating coil
12 is composed of 2 windings each of six to nine strands of fine
constantan wires bifilarly wound onto the distal portion of catheter 11.
The two wire windings are joined on the distal portion at solder point 13.
Heating coil 12 terminates proximally in solder points 14 from which 3
lead wires 15 emanate.
Between heating coil windings 12 a copper constantan thermocouple 16 is
affixed to catheter 11. Lead wires 18 connect thermocouple 16 in series
with reference copper constantan thermocouple 17 located proximally of the
heating coil. These thermocouples provide a convenient measure of the
local temperature rise at the heating coil 12.
Referring to FIG. 3, lead wires 15 and 18 are wound on catheter 11 and
terminate in plug 19. Platinum resistance thermometer 21 is bifilarly
wound onto catheter 11. Thermometer 21 terminates in solder points 22 from
which lead wires 23 emanate. Lead wires 23 terminate at plug 19.
On opposite sides of resistance thermometer 21, bare platinum electrodes
24, 25 are provided for electrocardiogram tracings. Electrode 24 has lead
wire 26 terminating in plug 19. Electrode 25 has lead wire 27 terminating
in plug 19. Plug 19 connects the catheter assembly to the electronics
necessary to obtain the thermodilution measurements.
Catheter body 11, which is a flexible, woven body, along with the rest of
the above-described apparatus, is covered with inert plastic coating 28
which is applied in 3 layers after winding the wires. Multiple layers
ensure perfect electric insulation. The plastic is diluted with ethyl
acetate to make each layer thinner. A layer of inert plastic coating is
also placed on the inside of catheter 11. Silicone rubber may be used as
the inert coating.
Referring to FIG. 4, platinum resistance thermometer 21 is one arm of a
Wheatstone Bridge. Standard voltage source 31, which may be for example, a
1.35 V mercury battery, galvanometer 32, automatic balance and D.C.
recorder 33 and three variable resistors 29 complete the bridge. Two leads
23 are used to connect thermometer 21 to variable resistors 29.
Thermometer 21 thus forms one arm of the bridge. Temperature changes down
to 0.001.degree. C. can be detected using this configuration.
Thermocouples 16 and 17 located on the catheter in the heating coil region
are connected to microvoltmeter 34 by lead wires 18. These temperature
transducers permit accurate measurement of temperature differences and
thus provide an indication of the local temperature rise at the heating
coil 12.
Heating coil 12 is connected to adjustable power supply 35 and external
dummy load 36. Double pole-double throw switch 37 selectively supplies
power to heating coil 12 or dummy load 36. Dummy load 36 acts as a warming
medium for the connecting wiring. The power supply is adjustable so that
the rate of heating of the coil may be determined prior to applying power
to the catheter. Electrodes 24 and 25 are connected to electrocardiograph
38 by leads 26 and 27 respectively.
Finally, electronic components 31, 34, 35 are electrically connected to
direct readout recorder 39.
MODE OF OPERATION
The present invention may be utilized when circumstances prevent insertion
of a catheter in the same direction as blood flow. Insertion of a catheter
against the blood flow requires that the heating element be upstream at
the tip or distal portion of the catheter and that the thermometer be
downstream at a more proximal portion of the catheter.
Referring to FIG. 1, to measure internal jugular blood flow, catheter 11 is
introduced first in the same direction as the blood stream through one of
the tributaries of the antecubital vein at elbow 8. Next, catheter 11 is
directed against the blood stream into the internal jugular vein under
x-ray screening and an image intensifier, and measurements are obtained
when the head of living organism 9 is below the level of the heart. This
prevents applying heat while the vein is empty.
Referring to FIG. 2, the heat applied by mean heating coil 12 at a
predetermined constant rate need not exceed 6-8 seconds in duration for
internal jugular measurements inasmuch as the temperasure rise measured by
downstream platinum resistance thermometer 21, reaches the "plateau" of
the asymoptote within four seconds. This relatively short heating time
reduces the recirculation problem which has affected prior devices and
methods. In addition, the capillary bed acts as a very large heat "sink"
which also minimizes recirculation problems. The modest mean temperature
rise ranges between 0.04.degree. C. and 0.15.degree. C. This rise affects
only the volume flow during the period of heating and not the entire
minute volume and is therefore not likely to affect heart function or
blood vessel intima during or after the measurements.
When the thermometer is located in the venous system and pulmonary
arteries, the heating time should be up to 12 seconds in normal
individuals. If there is heart failure, then the heating time should be
longer because in cases of Rt heart failure the stroke volume is small
relative to the residual fraction remaining in the right ventricle after
each systole. This is not the case for catheters designed for the internal
jugular vein because there is no residual fraction and the flow is
continuous, therefore the heating time need not exceed 6-8 seconds.
Referring to FIGS. 2 and 3, lead wires 15, which supply current to heating
coil 12, must pass under the proximal resistance thermometer 21 to reach
heating coil 12. Since lead wires 15 are heated while current passes
through them, a possible source of error is introduced because heat from
lead wires 15 will be conducted through catheter wall 11 and resistance
thermometer 21 will also be heated.
Referring to FIG. 4, this possible source of error is avoided by placing a
dummy load 36, an identical bifilarly wound heating coil, inside the
apparatus. Dummy load 36 has an impedance identical to that of heating
coil 12 on the distal segment of the catheter. Power is applied to dummy
load 36 at all times except during the measuring time. The circuit
supplying power to dummy load 36 includes lead wires 15 E and F which pass
under resistance thermometer 21 to reach heating coil 12, where they are
connected together without supplying any heat to heating coil 12. On
applying heat by means of double pole-double throw switch 37 to heating
coil 12, lead wire 15 E will be out of the circuit. Therefore either wires
15E and 15F, connected to dummy load 36, or 15F and 15G connected to main
heating coil 12, will heat thermometer 21 to an equal degree on applying
power either to heating coil 12 or to dummy load 36. The heating of wires
15E, F, and G will therefore not be detected by galvanometer 32 and
recorder 39 and will therefore not affect the measurement. Thus, dummy
load 36 and switch 37 constitute auxiliary heating means to heat the
electrical leads 15.
In the present invention automatic balance and D.C. recorder 33 for
balancing the bridge is provided in the design of the electronic apparatus
necessary to obtain thermodilution measurements. A predetermined rate of
heating can be introduced only after automatic balancing of the bridge is
reached. During the balancing time, a push button on the face of direct
readout recorder 39 is lit with a sign to "wait". When automatic balancing
of the bridge is reached the "wait" light is turned off and another push
button with a sign "exp." (exposure) is lit indicating that the electronic
circuit is ready to introduce heat. When "exp." button is pushed, the
predetermined rate of heating starts and remains on for the predetermined
number of seconds and automatic suppression and D.C. recorder 33 is
disconnected. On starting "exposure" and throughout the exposure time, the
automatic balancing device is cancelled so that the required deflection is
obtained. The automatic balancing of the bridge is reactivated after
heating time is over.
Referring to FIG. 2, heating coil 12 is made of constantan wires 0.04
millimeters O.D. Referring to FIG. 3, resistance thermometer 21 is made of
99.99% pure platinum wires 0.02-0.03 millimeters O.D. All of the wires in
heating coil 12 and resistance thermometer 21 are electrically insulated
with a double polyurethane coating which melts during the soldering
process. These fine wires cannot stand even extra fine sandpaper to remove
the conventional enamel insulation.
Platinum wires were used for thermometer winding 15 in order to obtain high
resistance. Copper may also be used in a resistance thermometer but the
resistance is too low for short windings. In a left ventricular catheter
with long resistance thermometers in the aorta either platinum or copper
wires may be used, each with its own temperature coefficient of
resistivity, since the thermometer does not move with each heart beat.
Platinum has a high strain coefficient of resistivity but strain errors
can be eliminated. Two types of errors are caused by strain phenomena in
thermometer material.
The first type of error is strain caused by winding and coating of the
thermometer during the making of the catheter. This source of error is
corrected by measuring the new temperature coefficient of the resistance
thermometer, that is, finding the resistance of the thermometer at the
0.degree. C. point and at 100.degree. C.
The second type of strain error is caused by the rhythmic movements of the
thermometer with heart beats and the change in O.D. of the catheter caused
by changing pressures of systole and diastole during the measuring time.
These rhythmic movements of the catheter and changes in its O.D. affect
the strain coefficient of resistivity of the wound thermometer. At rest
these movements are repetitive and cause almost identical waves which may
be filtered. However, during exercise the body movements and deep
respiratory movements are reflected on the base line with irregular waves
and affect the accuracy of each measurement. Use of a double spiral
thermometer winding overcomes this problem.
The length and number of strands of heating coil winding 12 and platinum
resistance thermometer winding 21 will vary with the size of the catheter.
The length of heating coil 12 on the internal jugular catheter varies from
4 to 5 centimeters (2-3 centimeters for pediatrics) and the range of the
length of resistance thermometer winding 21 is 5 to 7 centimeters (4
centimeters for pediatrics). The heating coil 12 for the left ventricular
catheter should be around 7 cms. The resistance thermometer 21 should be
15-20 cms. long since it will be located inside the thoracic and abdominal
aorta. These windings on the catheter are made with a weaving machine as
it is known in the art.
The length and number of strands of fine wires used in winding of heating
coil 12 and resistance thermometer 21 should be selected according to: (1)
the site from which it is introduced to reach the regional blood flow to
be measured, and (2) size of the patient or experimental animal. The
length of heating coil 12, resistance thermometer 21 and distance between
them depends on the expected range of the minute volume of blood flow at
the site. Thus, catheters of several sizes and lengths, each for its own
expected range of blood flow measurement may be needed.
When the catheter is introduced through one of the tributaries of the
antecubital vein to reach the internal jugular vein, the overall length of
catheter should be 110 cms. Catheters designed to measure left ventricular
blood flow are introduced through one of the femoral arteries and
therefore should be 125 cms long. These two lengths, 110 & 125, are
standard lengths of conventional cardiac catheters.
The ratio of the surface area of the length and number of strands used to
volume flow must be considered in establishing the heating coil and
resistance thermometer lengths. For example, in the present invention for
the study of blood flow in the internal jugular vein, the flow is about
400 ml/min. This compares to 4000-12000 ml/min. for cardiac output. Using
the equation for the area of a cylinder, 2.pi.rh, where, .pi. equals
3,1416 . . . , r is the radius of the catheter body 11 in nm, and h is the
length of the windings in mm, and the above given lengths for heating coil
12 and resistance thermometer 21 along with an outside diameter of 1.5 mm,
the surface area to volume flow ratio may be computed.
The resistance (and impedance) of heating coil 12 is always kept at a
predetermined level by reducing or increasing the number and length of the
wires used in parallel, to suit the range of blood flow to be measured. An
impedance of 50 ohms was selected for use with the present invention
because high frequency power and 50 ohms impedance coaxial cables were
employed.
The volume of blood flow, which is directly proportional to the rate of
heating and inversely proportional to temperature rise is obtained from
the following formula
##EQU1##
Where: W is the predetermined rate of high frequency power in Watts, which
is supplied to heater 12 or dummy load 36.
0.239 is the conversion factor from watts to calories,
60 is the conversion factor to obtain the minute volume,
0.92 is the product of specific heat (0.87) & density (1.056) of the blood,
T is the rise of the mean temperature of blood flowing through the vein or
artery.
An advantage in having a constant predtermined heating coil resistance is
that it allows for a direct milliliter per minute reading in direct
readout recorder 39. This is accomplished by taking the above blood flow
equation and substituting V.sup.2 /R for W, where V.sup.2 is the square of
the voltage applied to heating coil 12 and R is the heating coil
resistance. Because R is now 50 ohms the equation simplifies to:
blood flow milliliters per minute=V.sup.2 /T.times.0.311
Where:
V.sup.2 =Square of the voltage applied to heater 12; and
T=rise of mean temperature of blood flowing through the blood vessel.
Since the applied voltage and temperature rise are readily available, the
blood flow can be directly computed inside the apparatus and a direct
readout in milliliters per minute made available. This is done by
including analog multipliers inside recorder 39 to square the applied
voltage, divide by the observed temperature rise, and multiply by the
0.311 constant. Direct readout provides a significant advantage over
prior devices in that it makes data available more quickly and avoids
computation errors.
Continual measurements of blood flow within a few seconds using this
invention offers a number of other advantages. For example, each
measurement can be carried out without the injection of any substance into
the blood stream. Other thermodilution measurements made in man and in
experimental animals have utilized a saline injection during each
measurement. These injections are likely to increase cardiac output during
the measuring period, and the error is further enlarged when the minute
volume is calculated. Furthermore, the flexible connections to the subject
allow the application of heat and measurement of temperature rise with
ease and without sacrifice of accuracy. It is also possible to apply heat
by remote control and obtain the signal indicative of temperature rise by
telemetry.
Many modifications and variations of the present invention are possible in
light of the above teachings. For example, this invention may also be used
to measure the left ventricular output when catheter 11 is introduced
through the femoral artery so that heating coil 12 is in the left
ventricle while resistance thermometer 21 is in the aorta (FIG. 5). The
base line indicating the temperature in the arterial system and left
ventricle is far more stable than in the venous system, right ventricle or
pulmonary arteries. This is because, in the venous syytem, right
ventricle, and pulmonary arteries, the blood is not thoroughly mixed
before reaching the lungs and its temperature is affected by respiration.
Once the blood passes through the lungs however, the process of mixing is
complete and the temperature of blood in the left atrium, left ventricle
and arteries is constant, giving a stable baseline. With a stable
temperature baseline from the aorta, a rate of heating ranging between 5-8
watts is all that is required to measure the left ventricular output.
It is therefore understood that within the scope of the disclosed inventive
concept, the invention may be practiced otherwise than specifically
described.
* * * * *
|
|
 |
|
 |
Description |
|
