Volumetric Flow Rate Determination in Conduits Not Directly Accessible

This invention generally relates to methods for fluid flow determinations and, more particularly, to such methods that determine the volumetric flow rate of blood within, and the effective internal cross-sectional area of, portions of a biological circulatory system.

Determination of the volumetric flow rate of fluid within a conduit that is not directly accessible is typified by the determination of cardiac output, i.e., the volumetric flow rate of blood in the pulmonary artery. Various methods have been commonly implemented in cardiac output determination and various other methods and apparatus have been proposed. The hallmark method is that expounded by Adolf Fick in 1870, and subsequently developed coincident with modern cardiac catheterization techniques. Blood samples are obtained from the pulmonary artery and from a systemic artery, and their oxygen content is measured. The consumption of oxygen per unit time, under steady state conditions, is either assumed or measured. Cardiac output is then calculated by dividing the oxygen consumption by the difference between the measured arterial and mixed-venous oxygen content. Unfortunately, the method is complicated, cumbersome to use, and yields poorly reproducible results.

Dye-dilution methods also have been used widely but are not considered to be significantly more accurate or precise. Such methods employ an indicator dye which is injected as a bolus into a blood vessel. Thereafter, the changing concentration of indicator dye is measured at a downstream site and plotted versus time. Cardiac output is then computed by integrating a portion under the resultant curve. Dye-dilution methods are hampered by problems of cumbersome apparatus, loss of indicator dye, recirculation of indicator dye, and anatomic circulatory shunts.

The problems of indicator dye loss and recirculation essentially were solved by the thermal dilution modification introduced by Fegler in 1953. In this method, a bolus of cold solution or "dye" is injected into the blood vessel through a proximal port of a multiple lumen catheter, and the subsequent "dilution" or changing blood temperature with time is measured downstream through the use of a thermistor located on the distal end of the catheter. The resultant concentration-time variation is electronically integrated and cardiac output computed therefrom. A representative catheter for use in thermal dilution methods can be seen in U.S. Pat. No. 3,726,269, Webster, Jr. In its most widely used form, this type of catheter also has an inflatable segment or balloon which is used as a flotation device to facilitate positioning of the catheter in the pulmonary artery. Reference, for example, U.S. Pat. Nos. 3,995,623, Blake et al., 4,024,873, Antoshkiw et al., 4,105,022, Antoshkiw et al., and 4,329,993, Lieber et al. Thermal dilution methods have proven, however, to be no more accurate than the Fick method, require cumbersome apparatus, and yield information intermittently.

A plethora of additional methods and apparatus, all employing intravascular catheters or probes, have been proposed for cardiac output measurements. These include apparatus that determine cardiac output by measurement of the differential temperature that results from localized heating of the blood (U.S. Pat. Nos. 3,359,974, Khalil; 3,798,967, Gieles et al.); that use electromagnetic energy (U.S. Pat. No. 3,347,224, Adams); or that measure the conductivity of the blood (U.S. Pat. No. 3,896,373, Zelby). Yet another alternative technique determines cardiac output from balloon and arterial pressure signals obtained from a balloon catheter (U.S. Pat. No. 3,985,123, Herzlinger et al.).

Perhaps the most promising methods for cardiac output determinations are those utilizing a catheter bearing an ultrasonic transducer. Reference, for example, U.S. Pat. No. 3,430,625, McLeod, Jr., which discloses an intravascular catheter bearing a pair of ultrasonic transducers at its tip. The transducers are coupled with a Doppler circuit which applies a high-frequency electrical signal (typically in the mHz range) to one of the transducers so as to cause the transmission of ultrasonic energy therefrom. An electrical output signal from the other transducer, resulting from returns of the transmitted ultrasonic energy from the blood cells, is compared with the high-frequency electrical signal applied to the transmitting transducer to develop a Doppler signal that is representative of any frequency shift caused by relative movement between the blood cells and the catheter. The Doppler signal is therefore directly related to blood velocity. Improved apparatus of this type, using a single transducer for transmission and reception, can be seen in U.S. Pat. No. 3,443,433, Liston et al.

Blood velocity by itself is not sufficient to determine cardiac output because the concurrent effective internal cross-sectional area of the blood vessel must also be known. In the past, such determination of effective internal cross-sectional area has been accomplished by the use of transducer arrays and range-gating or signal-power ratio processing methods, all of which require intricate and meticulously constructed transducers as well as complex electronic processing units. Reference, in this regard, U.S. Pat. Nos. 3,542,014, Peronneau, 4,142,412, McLeod et al., 4,237,729, McLeod et al., 4,259,870, McLeod et al., and 4,232,373, Jackson et al. Although these methods are capable of providing superior accuracy and precision in the determination of cardiac output, the complexity and expense of the catheter and of the related signal processing apparatus has significantly limited their widespread commercial application.

It is therefore the principal object of the invention to provide a method for volumetric flow rate determination, such as cardiac output determination, which employs a simply and inexpensively constructed catheter bearing an ultrasonic transducer that can be discarded after a single use and which requires minimal signal processing in the necessary concurrent determination of conduit effective internal cross-sectional area.

Briefly, the invention resides in a method for determining the volumetric flow rate of a fluid in a conduit whose effective internal cross-sectional area is unknown. The method comprises the steps of:

measuring, by the use of ultrasonic energy, the unrestricted velocity of the fluid at points where it passes through an imaginary plane transverse to the conduit;

introducing a flow restriction into the conduit, the flow restriction having a predetermined cross-sectional area in the imaginary plane;

measuring, by the use of ultrasonic energy, the restricted velocity of the fluid at points where it passes through the imaginary plane;

determining the effective internal cross-sectional area from the measured unrestricted and restricted velocities and from the predetermined cross-sectional area of the flow restriction; and

determining the volumetric flow rate from the measured unrestricted velocity and the determined effective internal cross-sectional area.

The invention can best be understood by reference to the following portion of the specification, taken in conjunction with the accompanying drawings in which:

FIG. 1 is a pictorial view of a first embodiment of a catheter constructed for use with the invention, particularly illustrating the distal portion thereof;

FIG. 2 is a cross-sectional view taken along the line 2--2 in FIG. 1;

FIG. 3 is a cross-sectional view taken along the line 3--3 in FIG. 1;

FIG. 4 is a pictorial view illustrating use of the catheter;

FIGS. 5 and 6 are elevational views of the catheter distal portion, as situated within a blood vessel during use;

FIG. 7 is a pictorial view of another embodiment of the catheter, in combination with a conventional balloon catheter; and,

FIG. 8 is a cross-sectional view taken along the line 8--8 in FIG. 7.

With reference now to FIGS. 1-3, the distal portion of one embodiment of the catheter is formed by a hollow, substantially cylindrical member 10 that is preferably fabricated from a rigid plastic material such as an acrylic and that is fitted over and secured in an appropriate manner to an elongated flexible tubing 20 having defined therein a plurality of lumens 12, 14, 16 and 18. The proximal portion of the catheter is provided with a plurality of smaller-diameter, elongated flexible tubings 22, 24 and 26. A preferred material for tubings 20, 22, 24 and 26 is a standard medical grade of a flexible polyurethane or polypropylene. Tubings 22, 24 and 26 are fitted into or otherwise brought into sealed fluid communication with lumens 12, 14 and 18, respectively, and an insulated electrical cable 28 passes into and along lumen 16. Lumen 12 terminates in a port 30 disposed in the exterior surface of tubing 20 that is located approximately 30-35 cm. from the distal end of the catheter, lumen 14 terminates in a port 32 in the substantially cylindrical exterior surface of member 10 intermediate the proximal and distal ends thereof, the lumen 18 terminates in a port 34 disposed in the distal end of the catheter.

A balloon 36 is carried by member 10 so as to cover port 32, and the respective ends of balloon 36 are secured in any appropriate manner to the exterior surface of member 10 so as to provide a fluid-tight seal. Preferably, balloon 10 is composed of a flexible, expandable material such as a standard medical grade of Silastic.TM. silicon rubber. At least one and preferably a pair of ultrasonic transducers 38 are disposed at the distal end of member 10 and secured thereto by appropriate adhesive material 40. Preferably, each ultrasonic transducer 38 includes a piezoelectric crystal, although various ceramic polymers can also be used. Suitable electrical connections (not illustrated) are made between the ultrasonic transducers 38 and the leads and shield of cable 28 resident within lumen 16. The transducers (and their connections) are covered with a layer of appropriate sealing material (not illustrated) such as an epoxy resin to provide an electrical and fluid seal. Finally, each of the lumens 12, 14 and 16 is filled with an appropriate sealing material, from a point along its length beyond which the lumen is no longer required to the distal end of the catheter, as has been illustrated by sealing material 14a in lumen 14 (FIG. 3).

As best illustrated in FIGS. 5 and 6, transducers 38 are mounted in diametrically opposing positions on member 10, and each transducer 38 is disposed so that its beam is directed toward the proximal end of member 10 and thus toward balloon 36. More specifically, the beam axis of each ultrasonic transducer is arranged at an angle .theta. with respect to the longitudinal axis of the catheter so as to intersect a plane PL transversely bisecting balloon 36 at the approximate midpoint thereof. Each beam axis must be unobstructed by balloon 36 not only when the balloon is unexpanded (FIG. 5) but also when the balloon is expanded (FIG. 6).

With additional reference now to FIG. 4, the distal end of the catheter is inserted into an appropriate vein and then advanced until it passes through the superior vena cava SVC, the right atrium RA, the tricuspid valve TV, the right ventricle RV, and into the main pulmonary artery PA. Tubing 22 going to the "proximal" port 30 is coupled to an appropriate pressure measuring apparatus by a connector 42, and tubing 26 going to the "distal" port 34 is connected to an appropriate pressure measuring apparatus by a connector 44. During the advancement of the catheter, pressure at either or both of these ports may be measured to assist in positioning of the catheter within the pulmonary artery PA. Cable 28 going to ultrasonic transducers 38 is coupled to a Doppler circuit and processor by an electrical connector 46. When the catheter has been appropriately positioned, the Doppler circuit applies a pulsed high-frequency electrical signal to the transducers which causes them to transmit ultrasonic energy. Reflected returns of the transmitted ultrasonic energy from objects within the beam of each transducer (e.g., blood cells moving within the main pulmonary artery PA) are detected by the ultrasonic transducers and result in a corresponding return signal that is used by the Doppler circuit to develop a Doppler signal reepresentative of the velocity of the objects within the beams of the ultrasonic transducers. By appropriate signal processing techniques as are well known, the Doppler signal can be enhanced by the processor to eliminate artifact and noise so that the Doppler signal represents solely blood velocity within the main pulmonary artery PA.

Referring again to FIGS. 5 and 6, the blood velocity thus determined by the Doppler circuit is measured for the two conditions of balloon 36 illustrated. In FIG. 5, the balloon is substantially unexpanded and the cross-sectional area of the catheter within transverse plane PL is such so as to have no appreciable effect upon the blood velocity within the main pulmonary artery PA in the vicinity of plane PL. In FIG. 6, the balloon is substantially expanded so that the balloon has a predetermined and known cross-sectional area within transverse plane PL. One way to achieve such expansion is illustrated in FIG. 4, in which a syringe 50 containing a predetermined volume of fluid has been inserted into a connector 48 coupled to tubing 24, whereupon the fluid within syringe 50 may be injected into balloon 36 via tubing 24, lumen 14, and port 32 (FIGS. 1-3). Whatever the mechanism used to expand the balloon, the resultant cross-sectional area within plane PL must be known and must be such so as to significantly increase the blood velocity through the remaining, unrestricted portions of the pulmonary artery in the vicinity of plane PL.

Given that the blood velocity V for the unexpanded balloon condition illustrated in FIG. 5 and the blood velocity v for the expanded balloon condition illustrated in FIG. 6 have both been determined and recorded, and considering that the cross-sectional area a of the balloon when expanded is known, the effective internal cross-sectional area A of the main pulmonary artery PA can be easily determined by considering that the proportional relation: ##EQU1## can be rearranged as ##EQU2##

Having thus determined the effective internal cross-sectional area A, the volumetric flow rate (or cardiac output) is easily determined by multiplication of the effective internal cross-sectional area A by the blood velocity V.

As can be readily appreciated, the catheter described can be simply and inexpensively constructed so as to permit one-time sterilization, use and disposal. If desired, automated apparatus can be provided for filling and emptying the balloon when measurements are to be made. In any event, the Doppler signal measurements and related determinations can be accomplished by a conventional Doppler circuit and a very simply programmed microprocessor.

Preferably, balloon 36 is hydrodynamically shaped so as to maintain the distal portion of the catheter centered within the blood stream and away from the walls of the pulmonary artery. To this end, the ends of balloon 36 are not rolled over on themselves during construction of the catheter, as is conventionally the case in the manufacture of other types of balloon catheters. There is still a distinct possibility, however, that the distal portion of the catheter may lodge against one of the pulmonary artery walls, thereby substantially blocking blood flow on that side of the catheter. To avoid the resultant loss of velocity information that would result in such an event if a single ultrasonic transducer were used, the preferred embodiment contemplates as described the use of a pair of diametrically opposed transducers so that the beam of at least one of the transducers remains unobstructed at all times. Other transducer constructions are contemplated to avoid this problem, such as a ring-type transducer emitting a conical beam at the angle .theta. relative to the catheter longitudinal axis.

The proximal and distal ports 30, 34 may be used for purposes other than pressure measurements, such as the infusion and withdrawal of appropriate fluids. In fact, the principles of the invention can be applied to numerous catheter constructions such as the alternative embodiment illustrated in FIGS. 7 and 8, which is substantially identical to that previously described with the exception that tubing 20' includes an additional, central lumen 60 extending from its proximal end to its distal end. Lumen 60, which is larger in diameter than the remaining lumens in tubing 20', is used for the passage of a conventional balloon catheter 61 consisting of an elongated flexible tubing 62 that carries an inflatable balloon 64 on its distal portion. As best seen in FIG. 8, tubing 60 includes a pair of lumens 66 and 68, with lumen 66 terminating in a port 70 that is surrounded by balloon 64 and with lumen 68 terminating in the distal end of tubing 62.

In use, the catheter assembly in FIG. 7 is inserted into the venous system until the distal portion thereof is resident within the main pulmonary artery PA as illustrated in FIG. 3. Thereafter, the conventional balloon catheter 61 is further advanced an appropriate distance. When catheter 61 has been properly positioned, balloon 64 thereon is inflated (by injection of fluid into the balloon via lumen 66 and port 70) so as to create a wedge within the peripheral artery branch. As will be appreciated by those skilled in the art, measurements can be made concurrently of not only pulmonary artery pressure and cardiac output (as previously described) but also of pulmonary artery wedge pressure (by the use of an appropiate pressure measuring apparatus coupled to lumen 68 within catheter 61).

While the invention has been described with reference to a preferred embodiment, it is to be clearly understood by those skilled in the art that the invention is not limited thereto. For example, the rigidification of the catheter's distal portion afforded by member 10 is desirable to insure that the beam of each transducer 38 remains at a substantially constant angle relative to the catheter's longitudinal axis (and thus relative to balloon 36) during measurements. An alternate embodiment of the catheter would eliminate member 10 (wherein the transducers 38 and balloon 36 would be directly affixed to tubing 20) and would maintain the desired transducer/balloon relationship by appropriate design of those elements and choice of their materials or by the insertion of a relatively stiff yet flexible wire into one of the tubing lumens after the catheter's distal portion had been propriately positioned in-situ. Accordingly, the scope of the invention is to be interpreted only in conjunction with the appended claims.