IL97627A

IL97627A - Method and apparatus for measuring cardiac ouput

Info

Publication number: IL97627A
Application number: IL9762791A
Authority: IL
Inventors: Nitzan Meir
Original assignee: Jerusalem College Tech
Priority date: 1991-03-21
Filing date: 1991-03-21
Publication date: 1995-12-31
Also published as: IL97627A0

Description

METHOD AND APPARATUS FOR MEASURING CARDIAC OUTPUT s n n ^ob irmat> ipnni ηυ » - 2 - 97627/2 The present invention relates to a method for continuous measurement of human or animal cardiac output. It also relates to a device for carrying out this method.

Cardiac output, I.e., the volume of blood pumped by the heart per minute, is the principal parameter in the assessment of cardiac performance and its measurement 1s of importance for patients after heart surgery, or suffering from coronary diseases or cardiac insufficiency and other malfunctions.

These measurements are also of importance in animal experimentation undertaken, e.g., to examine the effect, on cardiac output, of various pharmacological substances and physiological procedures.

Because of the clinical importance of the assessment of cardiac output, many methods have been developed for Its measurement, including the method described in the U.S. Patent No. 4,901,734. The most widely used method today 1s the so-called thermal dilution method, based on the principle according to which a change of temperature of a flowing mass of liquid after being cooled (or heated) by the extraction (or addition) of a given amount of energy (calories) is a function, primarily, of the rate of flow of the liquid.

According to this method, 10 ml of a glucose solution at a temperature of 0°C are injected into the right atrium and the changes in blood temperature are measured by a thermal sensor located in the pulmonary artery. Blood temperature as a function of time (after injection of the cold solution) is plotted, and by a calculation involving the area below the plotted curve, the cardiac output can be found.

While this method is more reliable than others, it still has many drawbacks, the most important of which resides in the fact that it is not continuous. Another limitation of this method is due to its inferior accuracy with low cardiac outputs and the fact that the injection of liquids changes the liquid content of the blood, an aspect of importance mainly with children, preventing repeat measurement.

The relative success of the thermal dilution method has encouraged several researchers to develop a method which, while incorporating the basic principle, remedies its main deficiency: its noncontin-uousness.

With this relatively new method, the change of blood temperature is produced by an electric heating coil introduced into the right atrium, while temperature measurement in the pulmonary artery is taken over from the previous method.

Experience has, however, shown that when the temperature of the coil is raised by 2 - 4°C, (the maximum permissible temperature increase, since higher coil temperatures may cause damage to the blood coming into contact the coil), the rise of temperature at the sensor end of the dilution path would not exceed 0.05 - 0.1 C. A signal of this magnitude is not significantly larger than the "noise" which, in this case, is produced by spontaneous changes in blood temperature.

Trying to overcome the drawbacks and limitations of the above method, it was found that . by using an optical fiber introduced into the blood, much larger amounts of energy in the form of a suitably diffused light beam could be absorbed by the blood without causing local overheating, with a corresponding increase in the temperature at the sensor end of the flow path.

It is thus one of the objects of the present invention to overcome the disadvantages of prior-art methods of measuring cardiac output and to provide a method that is continuous and will give accurate and reproducible results while avoiding destructive local overheating.

According to the invention, this is achieved by a method for continuous measurement of human or animal cardiac output, comprising the steps of introducing into a first venous-blood-carrying member of the human or animal circulatory system a portion of an optional fiber having one end inside, and the other end outside of said member, the outside end of which fiber is optically coupled to a source of light, introducing into a second venous-blood-carrying member of said system temperature-sensing means, thus locating said means at a point up- stream of the location of the inside end of said fiber, measuring the blood temperature at the location of said temperature-sensing means before activation of said source of light, activating said source of light for a given time to raise, by irradiation, the temperature of the blood in direct contact with, or proximity to, said inside end of said optical fiber; measuring the change of temperature of the blood as produced by said irradiation as a function of time, and calculating therefrom said cardiac output.

The invention further provides a device for continuous · measurement of human or animal cardiac output, comprising an optical fiber, one end of which is optically coupled to a source of light and the other end of which is adapted to be introduced into a venous-blood-carrying member of the human or animal circulatory system, temperature-sensing means locatable in a similar member of said system at a point upstream of said other end when thus introduced and including means for relaying its signals to points outside of the human or animal body, a tip attached to, or being an integral part of, said other end of said optical fiber, said tip being adapted to diffuse the light emerging from said optical fiber, wherein said diffused light emerging from said tip introduces heat into the blood in direct contact with, or in proximity to, said tip, said heat, as diluted on the way to said temperature-sensing means and detected thereby, causes a rise in the initial temperature of said blood, said rise in temperature being a function of cardiac output.

The invention will now be described in connection with certain preferred embodiments with reference to the following illustrative figures so that it may be more fully understood.

With specific reference now to the figures in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.

In the drawings: Fig. 1 is a schematic illustration of the device according to the invention in situ; Fig. 2 is an enlarged cross-sectional view of a first embodiment of the tip of the optical fiber, and Fig. 3 is a similar view of a second embodiment of this tip.

Referring now to the drawings, there is seen in Fig. 1, a schematic drawing showing the right half of the heart only, an optical fiber 2 of a diameter of about 1 mm, which has been introduced into the inferior vena cava IVC. At the end of the fiber 2 there is seen a cylindrical tip 4 the nature and purpose of which will be explained further below. Also seen is a temperature-sensing element such as, e.g., a thermistor 6 located in the pulmonary artery PA. The thermistor and its catheter-encased leads 8 have been introduced into the pulmonary artery via the inferior vena cava IVC, the valve of the IVC, the right atrium RA, the right atrioventricular aperture and its valve AV, the right ventricle RV and the semilunar valve SLV of the pulmonary artery PA.

The lower end 10 of the optical fiber is optically coupled to a light source. This source can be a laser or a high-intensity incandescent bulb. Advantageously used is a laser emitting in the red and near-intrared region (e.g., an Nd YAG laser emitting at 1.06 m). This laser has the advantage that its light is absorbed to a greater depth, thus reducing local heating. However, a small proportion of the light escapes via the blood vessel. A CO^ laser, emitting in the far infrared region (10.6 vim), while much less expensive, has a much lower absorption depth. The same is also true for an argon laser, emitting green light. The advantage of these latter two lasers is that their light is wholly absorbed by the blood. When an incandescent source 12 is used, an optical system 14 is required to concentrate the light into the fiber 2. The system 14 (shown in dashed lines) is seen to consist of a concave mirror 16 behind the source 12, a pair of condenser lenses 18 and an imaging lens 20.

The function of the tip 4 is to diffuse the light emerging from the end of the fiber 2 so as to spread the energy over a relatively large volume of blood, thus keeping local temperature rise at a minimum. There is seen in Fig. 2 the fiber 2 comprising the light-conducting core 22 (in this case, a multi-strand fiber) and the sheath 24 surrounding the core. The tip 4 is fixedly attached to the sheath 24. Its length is about 10 mm and its surface is rendered matt (e.g., by etching, sandblasting or with the aid of emery paper), so as to frustrate total internal reflection. The material the tip 4 is made of is one which is transmissive of the wavelengths produced by the light source employed.

Another embodiment is shown in Fig. 3. In this single-fiber type optical fiber the end has been stripped of its sheath 24 for the required length and the bared core 22 is then etched to remove the cladding and roughen the core surface for the above-stated purpose.

While Fig. 1 shows the tip 4 to be located in the inferior vena cava IVC, it is also possible to advance the fiber 2 so that the tip 4 is introduced into the right atrium RA. The superior vena cava is also a suitable location for the tip 4.

The measurement procedure is quite simple. The optical fiber 2 and its tip 4, and the thermistor 6 having been positioned in their respective locations with the aid of the per se known Swan-Ganz catheter, the initial blood temperature in the pulmonary artery, T , is measured before the light source is switched on. The source is then activated for a period of time long enough for the now rising temperature to stabilize at its new level T after which the cardiac output CO can be calculated from the expression CO = p/cAT where p = output of light at the fiber tip 4, c = specific heat of blood, and The method according to the invention being continuous, measurements can be repeated at shorter or longer intervals as desired by switching off the light source and restarting the above procedure. The shortest interval between measurements is that required for the raised blood temperature to drop again to its initial level, which is no more than a few seconds.

In an alternative calculation procedure, there is no need to wait for the blood temperature, raised from T , to stabilize. Irradiation is allowed for a period of time t, during which the temperature is monitored as a function of t. The cardiac output CO can then be calculated using the expression: CO = !0 ; °°ATcdt o where m = quantity of heat (cal) injection into blood stream, ΔΤ = change of blood temperature over time t, and c = specific heat of blood. m is, of course, derived from p, the effective output of the light at the tip 4.

For extended monitoring of cardiac output, the above-described routine can also be pre-programmed and carried out automatically, with curves describing cardiac output vs. clock time provided by computer printout.

It is also possible to use a thermocouple instead of the thermistor 6, with the cold junction advantageously located in the vena cava downstream of the fiber tip 4.

It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims

WHAT IS CLAIMED IS:

1. A method for continuous measurement of human or animal cardiac output, comprising the steps of: introducing into a first venous-blood-carrying member of the human or animal circulatory system a portion of an optional fiber having one end inside, and the other end outside of said member, outside end of which fiber is optically coupled to a source of light; introducing into a second venous-blood-carrying member of said system temperature-sensing means, thus locating said means at a point upstream of the location of the inside end of said fiber; measuring the blood temperature at the location of said temperature-sensing means before activation of said source of light; activating said source of light to raise, by irradiation, the temperature of the blood in direct contact with, or proximity to, said inside end of said optical fiber; monitoring the change of temperature of the blood as produced by said irradiation as a function of time, and calculating therefrom said cardiac output.

2. The method as claimed in claim 1, comprising: •continuing with said ir^idiation for a period of time long enough for said blood temperature, raising from the initial temperature T , to stabilize at a new level T and calculating cardiac output CO using the expression: CO = p/cAT where p = output of light at the fiber tip, c = specific heat of blood, and

3. The method as claimed in claim 1, comprising: continuing with said irradition for a period of time t; monitoring the course of temperature change of the blood during said time t, and calculating cardiac output CO using the expression: CO ÷ m /o°°ATcdt where m = quantity of heat (cal) injected into blood stream, ΔΤ = change of blood temperature over time t, and c = specific heat of blood.

4. The method as claimed in claim 1, wherein said first member of said circulatory system is a vena cava.

5. The method as claimed in claim 1, wherein said first member of said circulatory system is the right atrium.

6. The method as claimed in claim 1, wherein said second member of said circulatory system is the pulmonary artery.

7. A device for continuous measurement of human or animal cardiac output, comprising: an optical fiber, one end of which is optically coupled to a source of light and the other end of which is adapted to be introduced into a venous-blood-carrying member of the human or animal circulatory system; temperature-sensing means locatable in a similar member of said system at a point upstream of said other end when thus introduced and including means for relaying its signals to points outside of the human or animal body; a tip attached to, or being an integral part of, said other end of said optical fiber, said tip being adapted to diffuse the light emerging from said optical fiber, wherein said diffused light emerging from said tip causes a rise in the initial temperature of the blood in direct contact with, or in proximity to, said tip, said rise in temperature, as diluted on the way to said temperature-sensing means and detected thereby, being a function of cardiac output.

8. The device as claimed in claim 7, wherein said optical fiber is of the single-fiber type.

9. The device as claimed in claim 7, wherein' said optical fiber is of the multi-strand type.

10. The device as claimed in claim 7, wherein said tip is in the form of an elongated body made of a material transmissive of the light from said source, fixedly attached to said other end and extending there-beyond in substantial coaxial ity therewith, a substantial portion of the surface of which tip is roughened to frustrate total internal reflection.

11. The device as claimed in claim 7, wherein said tip is part of said optical fiber, an end portion of which is stripped of its sheath as well as of its cladding and has been roughened by etching..

12. The device as claimed in claim 7, wherein said temperature-measuring means is a thermistor.

13. The device as claimed in claim 7, wherein said temperature-measuring means is a thermocouple.

14. The device as claimed in claim 7, wherein said source of light is a laser.

15. The device as claimed in claim 7, wherein said source of light is an incandescent bulb.

16. The device as claimed in claim 14, further comprising an optical system for focusing the light from said incandescent bulb into said optical fiber.

17. A method for continuous measurement of human or animal cardiac output, substantially as hereinbefore described and with reference to the accompanying drawings.

18. A device for continuous measurement of human or animal cardiac output, substantially as hereinbefore described and with reference to the accompanying drawings. FOR THE APPLICANT WOLFF, BREGMAN AND GOLLER