EP0530308A1

EP0530308A1 - Apparatus and method for monitoring cardiac output

Info

Publication number: EP0530308A1
Application number: EP19910910910
Authority: EP
Inventors: Samer Abdel-Malik Nashef
Original assignee: CPEC Corp
Current assignee: CPEC CORPORATION
Priority date: 1990-05-19
Filing date: 1991-05-17
Publication date: 1993-03-10
Also published as: BR9106463A; WO1991017703A1; GB9011259D0; CA2081453A1; AU8080591A; JPH05508567A

Abstract

Un appareil servant à mesurer la vitesse d'écoulement de fluides et le débit cardiaque comprend un élément de support (12, 32) pourvu d'une extrémité distale (14, 34), d'une extrémité proximale (16, 36) et d'une paroi externe (18, 38). Un premier élément de détection de température (20, 44) est situé sur ladite paroi externe (18, 38) à proximité d'un second élément de détection de température (22, 46) situé sur la même paroi externe (18, 38). Des éléments de chauffage (24, 26, 48), qui sont aussi placés sur ladite paroi externe (18, 38), sont juxtaposés audit premier élément de détection de température (20, 44). Ainsi, en excitant les éléments de chauffage (24, 26, 48), on permet d'établir un différentiel de température entre les premier et second éléments de détection de température (20, 22, 44, 46). L'énergie requise pour maintenir ce différentiel de température peut être réglée et surveillée, et mise en correspondance avec la vitesse d'écoulement de fluides et le débit cardiaque.An apparatus for measuring fluid flow speed and cardiac output includes a support member (12, 32) having a distal end (14, 34), a proximal end (16, 36) and d 'an external wall (18, 38). A first temperature sensing element (20, 44) is located on said outer wall (18, 38) near a second temperature sensing element (22, 46) located on the same outer wall (18, 38) . Heating elements (24, 26, 48), which are also placed on said outer wall (18, 38), are juxtaposed with said first temperature sensing element (20, 44). Thus, by exciting the heating elements (24, 26, 48), it is possible to establish a temperature differential between the first and second temperature detection elements (20, 22, 44, 46). The energy required to maintain this temperature differential can be adjusted and monitored, and mapped to the fluid flow velocity and the cardiac output.

Description

APPARATUS AND METHOD FOR MONITORING CARDIAC OUTPUT

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates in general to apparatus and methods for measuring fluid flow rates. More particularly, the present invention relates to apparatus and methods for continuously monitoring cardiac output by determining the rate of blood flow within a blood vessel. Description of Related Art

Cardiac output monitors are widely used within the healthcare field to monitor blood flow during the care of patients with a variety of medical indications. These cardiac monitors have found utility in such situations as, for example, during patient pre-operative diagnosis, the care of the critically ill, post-operative care, and during surgery itself, where an accurate determination of cardiac output in the form of blood flow rate can be of vital importance. This is true not only for providing early warning of possible deterioration in cardiac function, but in assisting in the assessment of the effectiveness of therapeutic intervention to aid or stimulate an ailing heart.

There are several methods currently utilized to monitor cardiac output, each of them having their attendant advantages and disadvantages. The Fick method involves injecting a known amount of dye into a patient's arm artery and subsequently measuring the dye concentration in the vein of the other arm. The degree of the rate of dilution in concentration of dye in the vein is assumed to be directly proportional to the patient's cardiac output. While the Fick method is attractive for its simplicity there are several problems associated with performing this method. Typically, the dye concentration measurements are performed outside the body and a single measurement can require multiple samples of blood. This is both burdensome and an annoyance to the patient. Additionally, many medical professionals prefer to avoid the presence of dye or significant amounts of dye in the blood. Thus, in limiting the amount of dye injected into the blood to keep the level of dye as low as possible, there is necessarily a limitation in the number of measurements which can be performed. Also inherent in this procedure are measurement errors owing to variations in the dye injection volume, the sampling volume, and timing the interval between samples.

The Fick method does not provide continuous monitoring and thus there is a possibility of that if a catastrophic event occurs the response time to this event can be significantly lengthened. Finally, because this method demands frequent invasive injections, there is a continuing possibility that an infection will result from the dye solutions or the equipment.

A more commonly performed method for measuring cardiac output is the thermodilution technique. This method consists of injecting a known amount of cold saline through a catheter lumen into the right atrium of the heart to cool the blood in the vicinity of the catheter tip. By measuring the temperature of the blood distal to the injectate and taking into consideration the distance the cooled blood has flowed and the time interval between measurement and injection, the cardiac output can be determined.

Other thermal techniques use methods similar to the thermodilution system described above. Instead of a cool injectate, however, the blood is heated with a bolus of heated fluid. Then the temperature of the blood is measured and the determination of cardiac output is similarly computed. This thermal dilution technique typically requires local blood o temperature increases on the order of 10 C to obtain adequate correlations. A significant disadvantage associated with these high temperature increases is the potential damage to blood proteins and other tissue. Additionally, these thermal dilution methods require lengthy measurement periods which result in long delays between measurement.

Though widely utilized, the thermodilution technique also has a number of disadvantages. It can be expensive, because it requires highly trained healthcare professionals and specialized equipment. By the nature of the measurements they are intermittent and do not lend themselves to continuous monitoring. Additionally, an active intervention is required in order to initiate the measurements, thus providing a certain level of risk. This risk is amplified by the risk of infection with each injection of cold saline. Moreover, there tends to be a wide variation in measurements which necessarily requires three or more cardiac output readings so that the results can be compared.

A third method is known as the mixed venous oxygen saturation (Sv02) method and consists of measuring the oxygen level in the venous blood. Then, assuming that the rate of oxygen consumed is directly proportional to cardiac output, the blood flow rate can be determined. A serious problem which frequently causes inaccurate blood flow determinations when the venous oxygen saturation method is utilized is that many factors affect the measurement of oxygen. Besides cardiac output these factors include catheter placement, tissue perfusion, and HCT. Thus, this indirect method merely implies a cardiac output, and depending upon other factors, its accuracy can be questionable.

Accordingly, there is a need to provide apparatus and methods for continuously and accurately providing information relating to cardiac output and blood temperature. There is also a need to provide apparatus and associated methods for monitoring cardiac output which results in a reduced likelihood of a catastrophic event occuring which goes undetected. Additionally, there is a need to provide a method for monitoring cardiac output in which the risk of infection is eliminated and significantly reduced.

SUMMARY OF THE INVENTION

The present invention provides apparatus and methods for the accurate and continuous determination of cardiac output in the form of blood flow rate. Because the apparatus of the present invention provides information on a continuous basis and there is no significant lapse of time between measurements, there is a highly reduced likelihood that a catastrophic event will go unnoticed. Accordingly, the response time for medical intervention in the event of a significant event is reduced. Moreover, there is a significant reduction or elimination of risk of infection because the practice of the present invention does not require the frequent injection of fluids to obtain cardiac output measurement.

In accordance with the present invention, an apparatus for measuring fluid flow rate is provided which includes a support member having a distal end, a proximal end, and an outer wall. A first temperature sensing means is positioned on the outer wall and a second temperature sensing means is positioned on the outer wall proximal to the first temperature sensing means. A heating means is positioned on the outer wall and juxtaposed to the first temperature sensing means.

The support member utilized in the apparatus of the present invention is preferably a catheter which can be positioned in a patient's blood vessel. The catheter preferably further includes an inflatable device positioned on the outside wall distal to the first temperature sensing means. Also in accordance with the present invention, the first and second temperature sensing means are preferably a thermistor and the heating means is preferably an electrical resistance wire in the form of a coil.

The apparatus for measuring fluid flow of the present invention further includes a power supply and a current control means in electrical communcation with the heating means. Additionally, in accordance with the present invention, a temperature monitoring means is in communication with the first and second thermistor.

Apparatus of the present invention can be utilized to determine cardiac output by positioning the support member or catheter at a location within a blood vessel having blood at ambient blood temperature. Creating a temperature differential between a heating means temperature and the ambient blood temperature by activating the heating means, monitoring blood temperature at the second temperature sensing means, monitoring the heating means temperature at the first temperature sensing means and controlling the heating means in order to maintain the temperature differential, results in the determination of the cardiac output in terms of blood flow rate.

Advantageously, the apparatus of the present invention provides accurate and continuous information relating to cardiac output and blood temperature without subjecting the patient to exposure to dyes or unnecessarily high blood temperature. Additionally, the present invention can be practiced without the need for high cost equipment and the constant attention to highly trained personnel.

The apparatus for monitoring fluid flow of the present invention can be formed according to well known methods within the art. These methods include extruding the support means and mounting the first and second temperature sensing means and the heating means on the outer walls using conventional techniques such as adhesive and heat bonding.

Further objects and advantages of the apparatus for monitoring fluid flow rates of the present invention, as well as a better understanding thereof, will be afforded to those skilled in the art from a consideration of the following detailed explanation of preferred exemplary embodiments and the drawings.

A BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic representation of an apparatus for monitoring cardiac output in accordance with the present invention.

Fig. 2 is a schematic representation of an apparatus for monitoring cardiac output including display, monitoring and controlling attributes.

DESCRIPTION OF PREFERRED EMBODIMENTS The teachings of the present invention provide apparatus which can be minaturized and inserted in small areas. Accordingly, the practice of the present invention is particularly suitable for applications in which small volumes of fluid flow are measured. However, those skilled in the art will appreciate that the present invention can be practiced in any situation requiring the measurement of fluid flow rates, and thus is not limited to a specific application. For purposes of explanation only, the apparatus and methods described herein are considered in the context of monitoring cardiac output in terms of blood flow rate.

Turning now to Figure 1 there is shown a schematic representation of an exemplary apparatus 10 for measuring cardiac output in accordance with the present invention. The apparatus 10 includes a support member 12 having a distal end 14, a proximal end 16, and an outer wall 18. A first temperature sensing means 20 is positioned on the outer wall

18 and a second temperature sensing means 22 is also positioned on the outer wall 18 proximal to the first temperature sensing means 20. At least one heating means 24,

26 is positioned on the outer wall 18 juxtaposed to the first temperature sensing means 20.

In accordance with the present invention, the first and second temperature sensing means 20, 22 can be a thermistor, a thermocouple, or any means known in the art for detecting temperature or temperature changes. Preferably, the temperature sensing means is a thermistor of the type which detects small changes in temperature, on the order of o less that 10 C. The choice of temperature sensing means is dependent upon design configurations and particularly the range of temperature and temperature differentials which are under consideration. It is also contemplated to be within the scope of the present invention to utilize combinations of different temperature sensing means.

The heating means 24, 26 utilized in the apparatus of the present invention can be an electrical resistance wire in the form of an insulated heating coil shown at 24 or a heat transfer device such as a plate shown at 26. The choice of heating means 24, 26 is dependent upon the range of fluid flow rates, the heat capacity of the fluid and the ambient temperature as well as degree of temperature elevation which is required. For purposes of measuring blood flow rate in a blood vessel, the heating means can be a combination of a nickel chromium heating coil 24 and a heat transfer device 26. As illustrated in Figure 1 when utilizing the heat transfer device heating means 26, it is positioned on-_..the outer wall so that the first temperature sensing means is located between the heat transfer device heating means 26 and the electrical resistance coil heating means 24. Combining the two types of heating means 24, 26 in this manner provides a more uniform heating of the blood which results in higher precision and accuracy in the measurement. However, using only a coil as described above provides accurate measurements for the purpose of determining cardiac output.

For purposes of measuring cardiac output the support member 12 is preferably a catheter designed to be easily inserted within a blood vessel such as the pulmonary artery. Catheters which are suitable for such uses are well known in the art and are normally fabricated from biologically compatible materials such as plasticized polyvinylchloride, polyurethanes, polyethylenes, polypropylenes, or nylons. Preferably, the material can be easily formed into elongated flexible tubular configurations.

It is further desirable that the apparatus of the present invention have an inflatable device 26 positioned on the outer wall 18 distally from the first temperature sensing means 20. In this case, there is also included a means for inflating (not shown) the inflatable device in fluid communication with the inflatable device. Preferably, the inflatable device is a balloon, as shown, however, a number of different devices can be utilized including blisters. As described below, this allows for the proper flotation of the catheter into the desired position which is typically the pulmonary artery.

In order to provide a means for monitoring temperature and controlling the heating, the cardiac output monitoring apparatus for the present invention further includes instrumentation for performing these functions. Figure 2 is a schematic representation which illustrates an apparatus for monitoring cardiac output according to the present invention including associated instrumentation for controlling and monitoring. The illustrated apparatus 30 includes a catheter support member 32 having a distal end 34, a proximal end 36, an outer wall 38, and at least one lumen 40 integral therewith. An inflatable balloon 42 is positioned at the distal end 34 in fluid communication with the at least one lumen 40. A first themistor 44 is positioned on the outer wall 38 in communication with the lumen 40. First thermistor 44 is further positioned proximally with the inflatable balloon 42. A second thermistor 46 is positioned on the outer wall 38 proximally with the first thermistor 44 and the second thermistor 46 is also in communication with the at least one lumen 40.

An insulated electrical resistance heater coil 48 is axially positioned on the outer wall 38 and it is further juxtaposed with the first thermistor 44 and in electrical communication with the at least one lumen 40.

A means for inflating (not shown) the balloon 42 is included and is in fluid communication with the at lea stone lumen 40. This communication can be with the at least one lumen 40 directly or via a separate fluid line 50 within the at least one lumen 40 as shown.

In accordance with the teachings of the present invention there is further included a power supply 52 in electrical communication with the electrical resistance heater coil 48 for supplying current to the electrical resistance coil 48. There is also included a temperature monitoring means 54 in communication with the first thermistor 44 and the second thermistor 46 and an electric current controlling means 56 in electrical communication with the power supply 52 and the temperature monitoring means 54.

As further illustrated in Figure 2 the apparatus of the present invention preferably includes an inflation port 58 and an electrical wire port 60 at the proximal end 36 of the catheter support member 32. The inflation port 58 is in fluid communication with the at least one lumen 40, either directly or through a separate fluid line within said at least one lumen 40 as shown. Inflation port 58 is utilized functionally as a port for the application of fluid to the inflatable balloon 42. The electrical wire port 60 is also in communication with the at least one lumen 40 and provides an entry point for connecting wires between the first and second thermistor 44, 46 and the temperature monitoring means 54. The electrical wire port 60 further provides an entry point for connecting wires from the power supply 52 to the electrical resistance coil.

As illustrated in Figure 2, the catheter 32 can have one lumen 40 in communication with the first and second thermistor, heating means, and balloon. Those skilled in the art, however, will recognize that the catheter can also have a plurality of lumens, each lumen communicating with one of the first and second thermistor, heating means, or balloon. For example, one lumen can communicate with the first thermistor and the electrical wire port and contain live and return wires for the first thermistor connecting the temperature monitor means. Similarly, a second lumen can communicate with the second thermistor and electrical wire port and contain live and return wires for the second thermistor connecting with the temperature monitoring means. A third lumen can communicate with the electrical resistance coil and the electrical wire port and thereby connecting the power supply with the electrical resistance coil.

It is also contemplated to be within the scope of the present invention to provide visual and or audible display devices or signal monitoring devices which can be activated by the temperature monitoring means and the electrical current control means. It is also preferred that the temperature monitoring means, the electrical current controlling means and the display device are integral with a microprocessor control system which is appropriately designed to provide continual and instantaneous indications of cardiac output and or blood ambient temperature to the professional user. Such a display system and microprocessor are represented in combination as numeral 62 in Figure 2.

In accordance with the present invention a generalized process for monitoring cardiac output is provided which includes the steps of providing a catheter having a proximal end, a distal end, and at least one lumen. Then by positioning the catheter at a predetermined position within a blood vessel having blood at an ambient blood temperaure; creating a temperature differential between the ambient blood temperature and a heating means temperature by activating a heating means which is located near the distal end of the catheter; monitoring heating means temperature at a first temperature sensing means juxtaposed with the heating means; monitoring ambient blood temperature at a second temperature sensing means; and controlling the heating means to maintain the temperature differential, the cardiac output can be determined from the current or power required to maintain the temperature differential.

Preferably the catheter includess an inflatable device such as a balloon positioned near the distal tip and positioning the cather further includes inflating the balloon with a fluid to a slidable fit within the inner wall of the blood vessel. The fluid is preferably air which is injected through the inflation port and into the balloon. When the catheter can be floated into position as the blood flow behind the inflated balloon causes the transport of the catheter to a selected distance within the vessel. Because air is a low density medium, balloons which are air inflated are easily floated in blood. However, other fluid mediums such as saline may also be used to inflate the balloon.

The blood vessel in which the catheter is positioned has flowing blood with a flow direction and an ambient blood temperature. Positioning the catheter with the blood vessel is such that the blood flow direction is from the proximal end to the distal end of the catheter.

In accordance with the teachings of the present invention, once the catheter is in place, the actual operation of the cardiac monitoring apparatus is as follows. The first and second temperature sensing means provide signals to the temperature monitor which in turn initially indicates ambient blood temperature at both the first and second temperature sensing means. Activating the heating means is performed to create a temperature differential between the heating means temperature in the vicinity of the first temperature sensing means and the ambient blood temperature as sensed by the second temperature sensing means. By activating the heating means, the heating means temperature is elevated to a o o temperature which is preferably about 1 C to about 3 C above the ambient blood temperature. Meanwhile, the ambient blood temperature continues to be sensed by the second temperature sensing means and the heating means temperature is sensed by the first temperature sensing means. As blood flows over the heating means, it is cooled and the rate of cooling as well as the amount of cooling is directly proportional to the rate of blood flow. Thus the higher the blood flow rate, the higher the cooling effect and the lower the blood flow rate, the lower the cooling effect.

The electrical current supplied by the power supply to the heating means is controlled by the electric current control means in such a manner as to maintain the small o o temperature differential of about 1 C to about 3 C between the first temperature sensing means and the second temperature sensing means. By determining the amount of current which is required to maintain this temperature differential a direct reading of changes in blood flow rate which is accurate and continuous can be determined. Determining the actual rate of blood flow can be accomplished by first providing a calibration of known rates of blood flow vs electrical current and then directly correlating the measured values with the calibration.

Those skilled in the art will appreciate that the power supply can be monitored with relative ease in all of its aspects. Thus for example instead of current, other electrical indicators such as resistance and voltage can also be monitored to give the same accurate and continuous cardiac output determinations. In addition to monitoring the amperage supplied to the heater coil, and calibrating the monitored amperage to convert the readings into blood flow rate, voltage or resistance can also be measured. In all cases, compared to existing technologies, an exceedingly more accurate blood flow rate determination is provided with the added considerable advantage that the signal can be instantaneously monitored at any time interval or it can be continuously monitored.

As mentioned above, by providing microprocessor control of the process of the present invention very small changes in temperature between the first and second temperature sensing means will immediately signal the electric current control means to change the rate of current flow to the heating means in order to maintain the desired temperature o o differential of from about 1 C to about 3 C. The microprocessor can them immediately translate this required decrease or increase in current to a quantitative expression of blood flow rate from a previously determined calibration.

The microprocessor then provides an output signal for visually or audibly indicating accurate cardiac output in terms of blood flow rate.

The more accurate and continuous cardiac output monitoring provided by the process and apparatus of the present invention advantageously provides instantaneous indication of cardiac function, both trending and absolute. Because any resposne to catastrophic events which may occur can be more accurately and timely detected, the use of the apparatus of the present invention additionally increases the effectiveness of any medical intervention which may be required. Moreover, the likelihood of infection is substantially reduced because the process of the present invention does not required injectates such as cold and hot solutions or dyes.

The just described apparatus and process for monitoring cardiac output can be incorporated within a catheter or other supporting medium which is designed to provide medical indications in addition to cardiac output. For example, a 6 lumen Swanz-Ganz type catheter can be designed to incorporate the features of the present invention as well as perform certain pressure monitoring functions for which this type of catheter is known. Thus, the central lumen of a Swanz-Ganz catheter can incorporate a transducer for monitoring pulmonary artery pressure. Since the preferred blood vessel location for positioning the catheter for cardiac output monitoring is the pulmonary artery, the pulmonary artery pressure monitor provides an indication as to whether the distal end of the catheter is in the ventricle or the artery. Accordingly, in addition to providing continual pressure monitoring, this information allows the medical professional to effectively position the catheter within the pulmonary artery for monitoring cardiac output.

Another catheter lumen of the six lumens of a Swanz- Ganz type catheter can provide the communication for an injectate for central venous pressure monitoring in the same manner as currently provided. The remaining four lumens can each separately provide the access for the fluid and electrical communications described above the for cardiac output monitor of the present invention. Accordingly, two of the four lumens contain the live and return wires for the first and second temperature sensing means. These wires are accessed through the electrical wire port as described above. A third lumen similarly contains the wires for communication between the power supply and the heating means and the fourth provides the route for inflating the inflation device and is accessed from the means for inflating through the inflation port.

The apparatus of the present invention can be fabricated using common assembling techniques available to those skilled in the art of designing and forming catheters for medical use. For example, single and multi-lumen catheters are both readily available commercially and they can be formed using common polymeric extrusion techniques known in the industry. Thermistors are conveniently and advantageouly mounted on the outer wall of the catheter using well known methods such as adhesive bonding. Similarly, lengths of electrical resistance wire can be coiled round a given length of catheter and then spot bonded to the outer wall of the catheter using adhesive or mechanical methods to secure the wire to the catheter outer wall.

The following non-limiting example illustrates the feasibility of providing a cardiac output monitor in accordance with the teachings of the present invention. Example 1

A Swanz-Ganz catheter was obtained from Baxter Healthcare. The catheter was additionally fitted with a thermistor and a 38 gauge nickel chrome resistance wire having a resistance of about 26 kilo ohms per foot. Both the resistance wire and the thermistor were insulated with a coat of shellac. The thermistor was adhered to the outer wall of the catheter near the distal end, and the resistance wire was wound axially around the catheter on top of the thermistor. About a one foot length of resistance wire was used. The two ends of the wire were connected to copper lead wires which were further connected to the terminals of a multimeter. The distal end including the thermistor and the electrical resistance wire was placed in a temperature controlled water bath. Then the wire resistance was measured as a function of the temperature of a heated water bath to verify response to temperature changes. The following results were obtained: o Temperature ( C) Resistance (ohms)

26.7 20.30

29.7 18.30

The catheter was then placed in a silastic tube within a controlled fluid flow system. The copper wire portions of nickel chrome resistance wire were connected to a constant power supply of 5 volts. The thermistor wire terminals were connected to a multimeter. Then the fluid flow rate through the silastic tubing was varied and measured. Additionally, the resistance of the nickel chrome wire was measured as the flow rate varied. Graph I illustrates the results of this experiment. The time needed for obtaining a resistance reading was less than 10 seconds.

The following non-limiting example illustrates the in vivo operation of an apparatus for monitoring cardiac output according to the present invention.

Example 2

A Swanz-Ganz catheter was obtained from Baxter

Healthcare and further modified with thermistors and electrical resistance coils as follows. Two thermistors were adhered to the outer wall of the catheter about 5 cm apart and near the distal end. One and one half feet of a polyurethane coated 40 gauge nickel chromium wire was wound around the distal thermistor. At ambient conditions the resistance of this length of wire is about 100 ohms. Two ends of the nickel chromium wire were connected to copper wire placed within the length of one lumen. The copper wires were in turn connected to a power supply. The two thermistors were connected to a multimeter to monitor their resistance. The connecting leads between the thermistors and the multimeter were contained within a catheter lumen. The catheter was then placed in a controlled rate circulating system of normal o saline at 34.5 C. The following results were obtained.

RESISTANCE

FLOW CURRENT (Killo Ohms)

Graph II illustrates these results in terms of the log of the current vs flow rate where it is shown the good correlation between flow rate and the current supplied to the nickel o chromium wire necessary to maintain a 1 - 3 C temperature differential at the more distal thermistor.

Also contemplated to be within the scope of the present invention is a cardiac monitoring apparatus which includes a catheter support member having one temperature sensing means and one or more heating means. The temperature sensing means is juxtaposed with the heating means. It is readily recognized by one skilled in the art that such a cardiac output monitor differs from the embodiments described above in that a second heat sensing means is not included. This apparatus can contain all other aspects described above including an inflatable device, electrical and inflation ports, and the accompanying instrumentation for monitoring and controlling the operating of the apparatus. Additionally, typical heating means, temperature sensing means and catheter materials as described above are applicable.

In operation, the process according to the present invention for utilizing this exemplary embodiment includes measuring an ambient blood temperature at the temperature sensing means. After the measurement, the heating means is activated to create a temperature differential between the just sensed ambient blood temperature and the heating means temperature. The energy required to achieve this temperature differential is measured by the associated controlling instrumentation. This energy is proportional to the blood flow rate and is an indication of the blood flow rate. Following this measurement, the heating means is deactivated and the heating means temperature return to ambient blood temperature. The ambient blood temperature is again monitored and the cycle is repeated. In this manner, cardiac output determinations can be made on a semi-continuous basis.

Having thus described exemplary embodiments of the present invention, it should be noted by those skilled in the art that the disclosures are exemplary only and that various other alternatives, adaptations and modifications may be made within the scope of the present invention. Accordingly, the present invention is not limited to the specific embodiments as illustrated herein, but is limited only by the following claims.

Claims

1. An apparatus for measuring fluid flow rate, said apparatus comprising, a support member having a distal end, a proximal end, and an outer wall, and characterised by a first temperature sensing means positioned on said outer wall, a second temperature sensing means positioned on said outer wall and proximal to said first temperature sensing means, and a heating means positioned on said outer wall, said heating means being juxtaposed to said first temperature sensing means.

2. The apparatus of claim 1 characterised by a power supply in electrical communication with said heating means.

3. The apparatus of claim 1 or claim 2 characterised by temperature monitoring means in communication with said first and second temperature sensing means.

4. The apparatus of any of claims 1 to 3, characterised by electric current control means in electrical communication with said power supply and said temperature monitoring means.

5. The apparatus of any of claims 1 to 4 characterised in that said temperature sensing means are a thermistor, a thermocouple, or combinations thereof.

6. The apparatus of any of claims 1 to 5, characterised in that said heating means is an electrical resistance heater wire, a heat transfer device, or combinations thereof.

7. The apparatus of any of claims 1 to 6, characterised in that said support means is a flexible elongated tube.

8. The apparatus of claim 7 characterised in that an inflatable device is positioned on said outer wall distal to said first temperature sensing means, and means are provided for inflating said inflatable device, in communication with said inflatable device.

9. An apparatus for measuring cardiac output, said apparatus, comprising a flexible catheter having a distal end, a proximal end, an outer wall therewith, and an inflatable balloon positioned at said distal end on said outer wall, characterised in that a first thermistor is positioned on said outer surface, said first thermistor being in communication with at least one lumen, and said first thermistor being positioned proximal to said inflatable balloon, a second thermistor being positioned on said outer wall proximal to said first thermistor, said second thermistor being in communication with said at least one lumen, an insulated electrical resistance heater coil being axially positioned on said outer wall and juxtaposed with said first thermistor, said electrical resistance heater wire being in communication with said at least one lumen, there being means for inflating said inflatable balloon in communication with said at least one lumen, and a power supply in electrical communication with said electrical resistance coil for supplying electric current to said electrical resistance coil, together with temperature monitoring means in communication with said first and second thermistor, and an electric current control means in electrical communication with said electrical resistance heating coil.

10. The apparatus of claim 9 characterised in that said catheter further includes an inflation port at said proximal end, said inflation port being in fluid communication with said at least one lumen.

11. The apparatus of claim 9 or Claim 10, characterised in that said catheter further includes an electrical wire port at said proximal end, said electrical wire port being in communication with said at least one lumen.

12. The apparatus of any of claims 9 to 11, characterised in that said catheter is formed from a polymer selected from the group consisting of polyvinylchloride, polyethylene, polyurethane, polypropylene, and nylon.

13. The apparatus of any of claims 9 to 12, characterised in that said electrical resistance heater wire is nickel chrome.

14. The apparatus of any of claims 9 to 13, characterised in that a heat transfer device is positioned at said first thermistor and said insulated electrical resistance heater wire.

15. The apparatus of any of claims 9 to 14, characterised in that said at least one lumen is a plurality of lumens and wherein said first thermistor, said second thermistor, said electrical resistance heating coil, said inflatable balloon are each in communication with one lumen of said pluraity of lumens.

16. A process for monitoring cardiac output, said process characterised by the steps of, providing a catheter, said catheter having a proximal end, a distal end, and at least one lumen, positioning said catheter at a predetermined location within a blood vessel, said blood vessel having flowing blood with a blood flow direction and an ambient blood temperature, said blood flow direction being from said proximal end to said distal end, activating a heating means to provide a heating means temperature, said heating means positioned near said distal end, creating a temperature differential between said ambient blood temperature and said heating means temperature, monitoring said heating means temperature at a first temperature sensing means and monitoring said ambient blood temperature at a second temperature sensing means, said first temperature sensing means juxtaposed with said heating means, and said second temperature means positioned proximal to said first temperature means, controlling said heating means to maintain said temperature differential, and determining said cardiac output by correlating energy required to maintain said temperature differential with blood flow.

17. The process of claim 16 characterised in that said catheter further includes an inflatable device positioned near said distal tip and wherein said process further includes the steps of, inflating said inflatable device to a slidable fit within said blood vessel subsequent to providing said catheter, floating said catheter to said predetermined position within said blood vessel prior to positioning said catheter, and deflating said inflatable device.

18. The process of claim 16 or claim 17, characterised in that said temperature differential is from o o about 1 C to about 3 C above said ambient blood temperature.

19. The process of any of claims 16 to 18, characterised in that said blood vessel is the pulmonary artery.

20. The process of any of claims 17 to 19, characterised in that inflating said inflatable device is accomplished by injecting air through said at least one lumen to said inflatable device.

21. An apparatus for monitoring cardiac output, said apparatus comprising, a catheter support member having a distal end, a proximal end, and an outer wall, and characterised by a temperature sensing means positioned on said outer wall, and a heating means juxtaposed to said temperature sensing means.

22. The apparatus of claim 21 characterised in that a power supply is in electrical communication with said heating means.

23. The apparatus of claim 22 characterised in that a temperature monitoring means is in communication with said temperature sensing means.

24. The apparatus of claim 23 characterised in that an electrical current control means is in electrical communication with said power supply and said temperature monitoring means.