RENAL FUNCTION ANALYSIS METHOD AND APPARATUS

Abstract

A method for measuring a glomerular filtration rate in a mammalian kidney comprises a source of reporter and marker fluorescent molecules. The fluorescent molecules are introduced into the blood stream of a mammalian subject. Over a period of time, a measurement of the intensities of the reporter and marker fluorescent molecules is taken. A ratio is calculated to determine the health of the subject's kidney. This method measures volume of plasma distribution based on a fluorescence of a marker molecule relative to a fluorescence of a reporter molecule.

Claims

1-28. (canceled)

29. An apparatus for analyzing the condition of a kidney, the apparatus comprising: a source of an analysis composition, the analysis composition comprising a plurality of fluorescent reporter molecules having a first fluorescence and a plurality of fluorescent marker molecules having a second fluorescence, where the reporter molecules have a molecular weight that is less than the molecular weight of the marker molecules; a percutaneously introducible catheter configured for introducing the analysis composition into a blood steam of a patient and for in situ delivery of excitation energy from an optical fiber extensible from a distal end of the catheter into the bloodstream of the patient and for collection of emission energy from within the bloodstream of the patient; and a detector for monitoring a level of the analysis composition within the blood stream and reporting the operating condition of the kidney; wherein a volume of plasma distribution of the patient is calculated based on the fluorescence of the marker molecules, and wherein the fluorescence of the marker molecules relative to the fluorescence of the reporter molecules is calculated and reported as a measure of kidney function.

30. The apparatus of claim 29 wherein the reporter molecule is readily filtered by the kidney, and the marker molecule resists filtration by the kidney.

31. The apparatus of claim 30 wherein the first molecular weight is between about 3 kD and 70 kD.

32. The apparatus of claim 31 wherein the first molecular weight is between about 3 kD and 20 kD.

33. The apparatus of claim 29 wherein the reporter molecule and marker molecules are both dextrans.

34. The apparatus of claim 33 wherein the dextrans are conjugated with fluorescence.

35. The apparatus of claim 33 wherein the reporter molecule is fluorescein dextran.

36. The apparatus of claim 33 wherein the reporter molecule is a fluorescein isothiocyanate-inulin.

37. The apparatus of claim 33 wherein the marker molecule is a larger sulforhodamine 101 dextran that is not filtered by the kidney.

38. The apparatus of claim 29 wherein the fiber optic cable has a diameter of from about 0.5 mm to about 1.0 mm.

39. The apparatus of claim 29 wherein the detector is selected from the group consisting of photo multiplier tubes, photo detectors, solid state detectors, a charge-coupled device, and combinations thereof

40. The apparatus of claim 39 wherein the detector includes at least one LED source or laser diode, at least one optical filter, and at least one power supply.

41. The apparatus of claim 40 wherein the detector further comprises one of a dichroic and band pass filter.

42. The apparatus of claim 41 wherein the detector comprises a microcontroller.

43. The apparatus of claim 29 wherein the apparatus measures the glomerular filtration rate (GFR) of the kidney.

44. An apparatus for analyzing the condition of a kidney, said apparatus comprising: a source of a kidney function analysis composition, the kidney analysis composition comprising a plurality of fluorescent reporter molecules having a first fluorescence and a plurality of fluorescent marker molecules having a second fluorescence, where the reporter molecules have a molecular weight that is less than the molecular weight of the marker molecules; a percutaneously introducible catheter comprising: a tubular main member defining a passageway between a proximal end and a distal end thereof, and an optical fiber extensible from the distal end; and a detector, optically coupled to the optical fiber, for monitoring a level of the analysis composition within the blood stream and reporting vascular plasma volume and the operating condition of the kidney; wherein the volume of plasma distribution is calculated based on the fluorescence of the marker molecules and the fluorescence of the marker molecules relative to the fluorescence of the reporter molecules is calculated and reported as a measure of kidney function and condition.

45. The apparatus of claim 44 wherein said optical fiber is slidable within the passageway.

46. The apparatus of claim 44 wherein said optical fiber includes a bend proximate a distal end thereof

47. The apparatus of claim 44 wherein said optical fiber is configurable for in situ delivery of excitation energy and collection of emission energy within the bloodstream a patient.

48. The apparatus of claim 44 wherein said catheter is configured for introducing the analysis composition into the blood steam of a patient.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] To understand the present invention, it will now be described by way of example, with reference to the accompanying drawings in which:

[0048] FIG. 1 is an illustration of an apparatus of the present invention utilizing a method of the present invention;

[0049] FIG. 2 is a series of micrographs showing renal clearance of a small molecular weight dextran in a normal rat as visualized by intravital 2-photon microscopy. Micrographs taken from a time series reveal localization of both a small FITC-inulin (5.5 kD) (lower series of micrographs) and a large 500 kD Texas Red® dextran (upper series of micrographs) within the capillaries (CAP). The inulin is rapidly filtered into the proximal tubular lumen (PT lumen) resulting in a steady decrease in fluorescence signal over time (panels E, F, G, & H). In contrast, the 500 kD dextran is not cleared and its signal remains constant within the capillaries (panels A, B, C, & D). The fluorescence seen in the PT lumen in Panel A, B, & C is not clearance of the 500 kD dextran but bleed through emissions from the FITC-inulin. The current approach of sequential excitation and acquisition with separate LED's will minimize this bleed through phenomenon;

[0050] FIG. 3 is a series of micrographs showing the intensity ratio of FITC-inulin to the 500 kD Texas Red® dextran with the 500 kD Texas Red® dextran staying in the blood stream a longer time after dye infusion due to the larger molecular size;

[0051] FIG. 4 is a plot of the intensity time-series of the 500 kD FITC dextran measured from a blood vessel following a bolus infusion up to 60 minutes;

[0052] FIG. 5 is a plot of the intensity ratio of a bolus injection of 20 kD FITC dextran to 500 kD Texas Red® dextran as a function of time along with the result of a least square fit;

[0053] FIG. 6 is a comparison of plots between using a ratio technique and directly using intensity for measuring plasma clearance;

[0054] FIG. 7 is a plot showing the kidney vascular plasma intensity ratios resulting over time from two rats after bolus infusion, one with a mixture of 3 kD FITC dextran and 500 kD Texas-Red dextran and the other with 20 kD FITC dextran and 500 kD Texas Red® dextran;

[0055] FIG. 8 is a plot showing liver vascular plasma intensity ratios over time from two anephric rats, one injected with a mixture of 10 kD FITC dextran and 500 kD Texas Red® dextran, the other with a mixture of 20 kD FITC dextran and 500 kD Texas Red® dextran;

[0056] FIG. 9 is a block diagram a diagnostic apparatus of the present invention;

[0057] FIG. 10 is a model of the apparatus of FIG. 9;

[0058] FIG. 11 is a plot of excitation and emission spectra of FITC, rhodamine and Texas Red® dyes;

[0059] FIG. 12 is a block diagram of a single channel optical system;

[0060] FIG. 13 is a block diagram of a two channel optical system;

[0061] FIG. 14 is an exploded view of the two channel optical system of FIG. 13;

[0062] FIG. 15 is a block diagram of a two compartment model;

[0063] FIG. 16 is an illustration of a dissembled catheter having an optical fiber; and

[0064] FIG. 17 is an illustration of the catheter of FIG. 16 assembled and inserted within a patient's vein.

DETAILED DESCRIPTION

[0065] While this invention is susceptible of embodiments in many different forms, there are shown in the drawings and will herein be described in detail preferred embodiments of the invention with the understanding that the present disclosures are to be considered as exemplifications of the principles of the invention and are not intended to limit the broad aspect of the invention to the embodiments illustrated.

[0066] The inventors have found that a combination of both structural and functional markers of AKI presents a high level of clinical utility in diagnosing kidney function and kidney-related diseases. Thus, one objective of the present invention is to provide tests for analyzing and quantifying organ function and physiological parameters that have been difficult or impossible to measure in the past. The present invention focuses on a method and device for rapid detection of acute kidney injury and chronic kidney-related diseases. This development utilizes technology developed by and licensed from the Indiana Center for Biological Microscopy. Such technology is described in U.S. Provisional Patent Application No. 60/672,708, PCT Application No. US2006/014576, published as WO/2006/113724, and U.S. application Ser. No. 11/911,895, which are hereby incorporated by reference as if fully set forth herein. Specifically, figures of the apparatuses shown in FIGS. 6-9 of WO/2006/113724 and the descriptions of same at the paragraphs numbered 96 to 104 are directed to the technology utilized.

[0067] In early animal studies, this technology has proven efficacious in providing accurate and rapid measurement of the true Glomerular Filtration Rate (GFR)—the rate by which the kidney is able to filter waste products from the blood stream. While the need for disease diagnostics varies according to the specific disease, in kidney disease, GFR is the primary clinical indicator of injury, disease progression, or recovery.

[0068] GFR measures the amount of plasma filtered through glomeruli within a given period of time. It is clinically the most widely used indicator of kidney function. Physicians routinely use it for both diagnostic and therapeutic decisions. In fact, the National Kidney Foundation has now divided chronic kidney disease patients into five groups (I-V) based upon their estimated GFR (eGFR). This has assisted clinicians in recognizing and understanding the severity of the kidney disease in patients. It has also allowed for the initiation of appropriate therapies based on the patient's baseline GFR.

[0069] A variety of techniques such as radioactive and non-radioactive contrast agents, as well as radiographic renal imaging, can measure GFR rapidly. Plasma clearance techniques are based on measuring the plasma clearance of GFR marker molecules. By using radioactive markers, such as [51]Cr-EDTA or [99]m Tc-DTPA ([99]m Technetium diethylene triamine pentaacetic acid), it has been reported that plasma clearance and GFR could both be determined independently using a radiation detector. Using radioactive GFR markers, such as [51]Cr-EDTA and [99]mTc-DTPA ([99]m-Technetium diethylene triamine pentaacetate), in conjunction with a radiation detector, one can monitor GFR in patients with acute kidney injury at rates close to real-time. The measured plasma clearance shows excellent correlations with GFRs simultaneously measured using the standard method with urine collection. However, the use of radioactive GFR markers and the clinical difficulties in administering this test make this method unattractive. By using a fluorescent GFR marker, such as FITC-inulin, with a bolus intravenous infusion followed with drawing blood samples at multiple time points, one can accurately determine GFR. Potentially, with the development of a suitable contrast agent, magnetic resonance imaging (MRI) techniques can be very useful for providing kidney functional diagnostics. The downside of using such technologies is the low accessibility, associated high cost, difficulty repeating the study and the need to move the patient for the study.

[0070] Similarly, the plasma concentration of non-radioactive markers, e.g. iothalamate, determined by standard methods, such as high-performance liquid chromatography (HPLC), has also been used to evaluate renal function in critically ill patients. Such plasma clearance based GFR measurement techniques have been reported to have good time resolution in detecting changes of renal function in patients with severely impaired renal function. By using bolus infusion of a single fluorescent GFR marker, FITC-inulin, GFR has been determined by sequentially measuring the fluorescence signals in the blood samples drawn as a function of time after infusion. The inventors have expanded upon and enhanced this approach offering improved accuracy, rate of determination, and reduced exposure to potentially toxic radioactive molecules.

[0071] Inulin, a small fructose polymer that is filtered, and cleared from the body only by glomerular filtration, is a reference standard GFR marker. Other non-radioactive markers (such as iothalamate, iohexol, polyfructosan) and radioactive ones (such as [125]I-iothalamate and [51]Cr-EDTA) are also commonly used.

[0072] In clinical practice, endogenous markers such as serum creatinine and cystatin C are routinely used to estimate GFR, since the production and tubular reabsorption rates of these molecules vary significantly from different individuals. Cystatin C has received recent attention as a superior endogenous serum marker of GFR, compared to serum creatinine, as it is elevated up to a day earlier than creatinine in an ICU population with AKI.

[0073] The inventors have developed a minimally invasive device for direct measurement of GFR in mammalian subjects, such as humans, using a multi-photon microscopy method, preferably a two photon microscopy method. The method relies on reading two fluorescent molecules attached to different size dextran molecules. Dextran is a complex, branched polysaccharide made of many glucose molecules joined into chains of varying lengths (from 3 to 2,000 kD). Thus, another objective of the present invention is to provide both a method and apparatus using a catheter based fiber optic probe to read the fluorescent markers. This catheter can be placed into a vascular system, e.g., an arm vein of a mammalian patient, to allow the concentration of fluorescent markers to be monitored in real time, providing a direct measurement of GFR.

[0074] A rapid and accurate measurement of GFR in an early stage of acute kidney injury is important for diagnosis, stratification of extent of injury and therapeutic purposes. An advantage of the present invention is that it will rapidly identify and determine the extent of injury allowing for early treatment, including dialysis initiation, as well as enrollment and stratification for clinical studies. It could also be used to determine the effect of a clinical maneuver on GFR, such as volume resuscitation. Therefore, this technical advance is of major clinical importance, especially in high risk patients where intense surveillance is necessary for early diagnosis, injury stratification and determination of therapeutic potential.

[0075] The inadequacies of methods currently clinically used for estimating GFR are established both in literature and in practice. While progress is being made to identify biomarkers for detecting presence of injury, little progress has been made in finding a functional marker that is practical enough for broad acceptance. The inventors' method represents a true advancement in the ability to accurately quantify and track the degree of kidney function with near real-time efficiency. The inventors have also developed a device that is easy to operate in a busy medical environment—a critical adoption barrier in medical technology.

[0076] The optical technique developed by the inventors is based on plasma clearance measurements of a fluorescent bioreporter molecule and allows for the rapid, frequent, and safe evaluation of GFR. To further validate the values, other standard GFR tests, including but not limited to inulin clearance, may be performed. Upon comparison of these values, a correction factor may be applied to the data obtained using this novel method if needed.

[0077] Referring to FIG. 1, an apparatus 10 which incorporates a method of the present invention is illustrated. The apparatus 10 comprises a source of a GFR measurement composition 100, a kidney fluorescent detector 200, and a catheter 300. The GFR measurement composition 100, which comprises a plurality of reporter molecules and a plurality of marker molecules, is introduced into the blood stream of a human subject 20 via the catheter 300. The fluorescent detector 200 monitors the level of the GFR measurement composition within the blood stream and reports an operating condition of the human subject's kidney in at least substantially real time. This apparatus measures volume of plasma distribution based on a fluorescence of a marker molecule relative to the fluorescence of a reporter molecule. “Substantially real time” is intended to encompass the duration elapsed between measurement of the levels of the reporter and the marker within the blood stream, calculation of the operating condition of the kidney, and reporting of that condition. It is contemplated by the inventors that this elapsed time will be very near real time as to be negligible in relation to the prior techniques discussed above.

The GFR Measurement Composition

[0078] By utilizing intravital multi-photon microscopic imaging of the kidney, the inventors have quantified glomerular filtration and tubular reabsorption processes independent of each other. The inventors have developed ratiometric imaging techniques permitting quantitative analysis of fluorescence signals within local regions of the kidney using multi-fluorescent probe experiments. To measure GFR by plasma clearance, the inventors use a fluorescent GFR reporter molecule, e.g. FITC dextran, together with a large different fluorescent marker molecule that does not pass through the glomerular filtration barrier. This large fluorescent marker serves to quantify the plasma volume of distribution in the vascular space and allow for the ratiometric technique.

[0079] The inventors have been able to quantify plasma clearance of the fluorescent GFR marker by examining the ratio of fluorescence intensities of the two molecules from within the blood vessel regions of the image. GFR can be rapidly determined using this ratio technique. This method has been tested in a number of animal models. Since the fluorescent signals are being measured from within the blood vessels to quantify the kinetics of plasma clearance, the ratio signal of the two fluorescent molecules is independent of the body location where the measurement is performed.

[0080] To measure GFR accurately, the inventors have determined that the ideal GFR marker molecule should be stable within the vascular compartment during the study and have a glomerular sieving coefficient (GSC) of 0.0, be retained within the vasculature, and it should not be, or should substantially not be, secreted, reabsorbed, or filtered within the kidney and may have a molecular weight greater than 100 kD. “Substantially” as used here is limited to ±5%. Satisfying these conditions, the GFR would be equal to the urinary clearance of the reporter after its intravenous infusion. In theory, one could use a GFR reporter with any known GSC that is greater than 0. The preferable marker molecule is a sulphorhodamine 101 having a molecular weight greater than 100 kD.

[0081] FIG. 2 contains several fluorescence intensity images of the kidney from a live and healthy male rat. These images were taken as function of time after a bolus intravenous infusion of a dye mixture containing a FITC-inulin (5.5 kD) and 500 kD dextran labeled with a sulforhodamine 101, i.e. Texas Red®. The fluorescence intensity signal from FITC-inulin is shown in the lower series of micrographs, and the 500 kD Texas Red® dextran intensity is shown in the upper series of micrographs. At about 12 seconds after dye infusion, the fluorescence intensity was seen in both the capillaries of the kidney and in the proximal tubule (PT) lumen. The variations in the blood vessel over time indicate that both the FITC-inulin and 500 kD Texas Red® dextran were in these blood vessels. At 50 seconds, the FITC-inulin was already decreasing in intensity in the capillary and in the PT lumen as a result of immediate plasma clearance (glomerular filtration) of this molecule. This was not true for the red 500 kD dextran where the capillary intensity was similar to the 12 second value. A red blood cell (RBC) appears as a dark object as it excludes dye. At 100 and 200 seconds the FITC-inulin intensity continued to decrease in the capillary and in the PT lumen as filtration continued to remove it from the body. This again was not true for the 500 kD Texas Red® dextran which did not change in intensity during this time interval as it was not filtered. Consequently, the relative strength of the intensity from the blood vessels increases indicating a relative increase in the 500 kD Texas Red® dextran to FITC-inulin concentration ratio due to plasma clearance of the FITC-inulin. This type of time-series image collection contains dynamic information about a given molecule passing through the glomerular filtration barrier of the kidney, and becoming part of the filtrate. This provides the basis for the inventors' measurement of plasma clearance rates and GFR.

[0082] To quantify molecular filtration dynamics, the inventors used the intensity ratio of the FITC-inulin and the 500 kD Texas Red® dextran (See FIG. 3). The intensity ratio from the blood vessels in FIG. 3 changes over time with a change of relative concentrations of the two dyes. Since the 500 kD dextran molecules are minimally cleared from the vascular culture, not by the kidneys, due to its large size, it remains stable in the plasma for a long time after infusion. Typically, there was no noticeable intensity drop from the 500 kD dextran within the time period following a dye infusion (anywhere between 5-30 minutes). This resulted in a decreasing intensity ratio visualized over time. It is this type of ratio that greatly minimizes the problems with using fluorescence intensity as a read out for biological studies.

[0083] The inventors have also used a 500 kD fluorescent dextran for similar studies in order to further minimize filtration and extend the dye's plasma survival time. FIG. 4 is an example of the intensity time-series of the 500 kD FITC dextran measured from a blood vessel following a bolus infusion up to 60 minutes. The initial intensity spike (see inlet) was due to dye injection and fast distribution of the dye molecules into the whole plasma volume. It did not show significant intensity drop for the rest of the curve. Effectively, the decrease of the fluorescence intensity ratio of labeled inulin to labeled 500 kD dextran correlates with the concentration decrease of the labeled inulin.

[0084] FIG. 5 is a plot of the intensity ratio of a bolus injection of 20 kD FITC dextran to 500 kD Texas Red® dextran as a function of time along with the result of a least square fit. Each data point in FIG. 5 was the average ratio value of the same region from a blood vessel extracted from an image time-series (such as the images shown in FIG. 3). The data points were plotted every 0.5 seconds up to 200 seconds. The decay occurred in two phases, the initial phase and the clearance phase (or the filtration phase/elimination phase). The gradual increase of the initial phase was due to relative dye distributions and accumulations in the kidney following IV injection. The highest point (around 12 seconds) of the curve marks the starting point of the clearance phase and correlates with the beginning of the appearance of FITC-inulin in the proximal tubule.

[0085] The data points of the clearance phase fit well with a single exponential. The inventors obtained a 20 kD FITC dextran plasma clearance rate constant, k, of 0.00458 (s.sup.−1) (using 95% confidence limits).

[0086] Following a bolus infusion of GFR reporter molecules, the plasma concentration of the GFR reporter molecules decreases as a function of time due to renal clearance. By acquiring plasma samples at different time points, one can either directly calculate or perform least square fit of the time trace to retrieve the plasma clearance rate constant (k). GFR can then be determined according to the equation:

GFR=kV.sub.d   (1)

where k is the plasma clearance rate and V.sub.D is the volume of distribution into which the GFR marker is diluted. GFR measured using this technique has been validated in patients with stable renal function as well as in rodents and proven to be accurate and correlated well with what was measured using other methods.

[0087] A comparison between using the intensity ratio and directly using the intensity value of a 3 kD FITC conjugated dextran (3 kD FITC dextran) for measuring the clearance rate is shown in FIG. 6. The chief differences are significantly less noise and better identification of distribution phase.

[0088] The intensity fluctuations of the 3 kD FITC dextran alone were quite significant (FIG. 6-B). Consequently, the fitting result of the clearance rate constant k contained larger errors and was less defined. In contrast, the intensity ratio (between 3 kD FITC dextran and a 500 kD Texas Red® dextran) had significantly less noise, and the measured clearance rate constant k was much better defined with only 3% error. This was partially because fluorescence intensity is typically very sensitive to even a slight change in microscope focus and movement of the sample. The intensity ratio, on the other hand, is insensitive to minor changes in imaging depth and motion. The inventors are focusing on the fluorescence signals from the blood. Furthermore, the intensity signal of a dye from the blood can change when the blood flow rate changes. However, the relative intensity ratio between two molecules does not change even when the blood flow rate or blood volume changes (assuming there is no clearance). This is because both dye molecules are present in the blood and move together. The method developed by the inventors limits this problem.

[0089] The separation between the initial dye distribution and the clearance phase is well-defined using the intensity ratio. When using the intensity of a single dye alone, it is more difficult to determine at what time point the clearance phase begins. The highest data point in the intensity curve typically does not correlate in time with the appearance of the smaller molecule in the proximal tubule lumen. Therefore, the dye distribution and the filtration phases are convoluted in the intensity only curve. Using multi-photon microscopy approaches allow such correlations and is highly beneficial.

[0090] It is believed that purity, in terms of size distribution or molecular weight, of the dextrans is vitally important. In addition, the distribution of molecular weight plays an important role in how well GFR can be measured. Even though dextrans are widely used in medical applications, these previous applications did not require the more stringent size control needed for use in the present invention.

[0091] Referring to FIG. 7, a plot of intensity values obtained from two rats after bolus infusion is illustrated. One rat was infused with a mixture of 3 kD FITC dextran and 500 kD Texas Red® dextran. The second rat was infused with 20 kD FITC dextran and 500 kD Texas Red® dextran. The plot shows a rapid decay with the 3 kD/500 kD fluorescence ratio curve. This indicates a fast clearance with movement into the interstitial space. However, a substantial part of it is due to non-renal plasma clearance as seen from 10 kD dextran data of liver imaging illustrated in FIG. 8.

[0092] FIG. 8 was generated from a pair of anephric rats (with both kidneys removed). One of the rats was injected with a mixture of 10 kD FITC dextran and 500 kD Texas Red® dextran. The other rat was injected with a mixture of 20 kD FITC dextran and 500 kD Texas Red® dextran. The 10 kD/500 kD ratio curve shows that there is clear evidence that non-renal plasma clearance of 10 kD dextran is still substantial. Meanwhile, the 20 kD dextran, which can be filtered by glomeruli, shows minimal non-renal plasma clearance. Therefore, it can be used to determine GFR.

[0093] Additionally, smaller molecules of 3kD to 5 kD as reporter molecules in conjunction with a two compartment kinetic model can be used to measure organ function. Thus, the inventors have determined that the preferred molecular weight of the filtered molecule to be within the range of 3 kD to 500 kD, more preferably 3 kD to 150 kD, still more preferably 3 kD to 150 kD, still more preferably 3 kD to 70 kD, still more preferably 3 kD to 5 kD, and most preferably on the order of 5 kD, or any range or combination of ranges therein. The method also contemplates the use of known common sizes such as 10 kD and 500 kD dextrans as well less common sizes 20 kD, 70 kD and 150 kD. An amino fluorescein dextran is preferred.

Fluorescent Detector

[0094] The fluorescent detector 200 includes software for reading and reporting data, a user interface 202 to control the apparatus 10 and review results, and an apparatus for sending and receiving fluorescent signals 204 (see FIGS. 9, 10, and 12-14). This unit 200 is designed to be compatible with a standard IV pump stand, or it can be operated on a table top. It incorporates a battery backup system that is capable of running for 2 hours without connection to AC power.

[0095] The user interface 202 is capable of being used by any clinician. It includes touch screen technology for most of the software user interface. This provides flexibility in how the data is shown to the clinicians.

[0096] Based on the body of work done to perfect the ratio technique using multi-photon microscopy, the inventors determined that a fiber optic catheter placed in the blood stream of a subject would be capable of measuring the fluorescent molecules. The current method of using multi-photon microscopy is responsible for generating much of the variation due to the drop off in fluorescence intensity as the tissue is penetrated more deeply. Using a fiber optic catheter as disclosed herein will eliminate these variations since the measurements will be taken in real time, or substantially real time, directly in the blood. The fiber optic catheter is explained in more detail below.

[0097] FIGS. 9 and 10 illustrate a two channel apparatus 204 using a single multi-colored LED 206 (light emitting diode) as a light source. An objective of this apparatus 204 is to determine how much fluorescent signal would be returned from a fiber optic element 210. FIGS. 9 and 10 show both a diagram and computer model of the optical system used in this device. The apparatus includes photo multiplier tubes (PMT) 208a,b as detectors, since these devices have well-known characteristics. Alternatively, the detector may be a photo detector, a solid state detector, a charge-coupled device, or any other equivalent device without departing from the spirit of the invention. This apparatus may have one or more power supplies 207,209, and/or controllers, for providing power to the LED 206 and PMTs 208a,b.

[0098] An optical path 212 focuses the light from an LED source 206 through a selection of band pass 216 and dichroic filters 220, then onto the fiber optic element 210. An excitation light is then passed down the fiber optic element 210 into a test solution chosen to simulate the approximate level of fluorescent dextrans in a blood stream. The fiber optic element 210 is generally a fiber optic cable in the range of 0.5 to 1 mm in diameter or even smaller.

[0099] Once excited, a small portion of the fluorescence signal then passes back through the fiber optic element 210. The signal then passes through a focusing lens 224, dichroic beam splitter 228 and band pass filter 232 before landing on the cathode of the PMT 208a.

[0100] An easily detectable fluorescent signal is measured from the PMT 208a,b for fluorescein dye. This dye has an excitation peak of about 494 nm and emits light in a broad band of wavelengths centered on 519 nm. Fluorescein dye is only one example of a marker dye. A rhodamine dye may also be used; however, the LED source 206 must have sufficient intensity to excite the rhodamine dye. The spectral response of fluorescein, rhodamine and Texas Red®, can be seen in FIG. 11.

[0101] The emergence of white LEDs based on adding a phosphor to the LED die may be used in the present device 200, but the narrow spectral bandwidth associated with standard LEDs is superior for reducing background light. The intensity of the light source and how efficiently energy can be delivered to the fiber optic 210 is critical.

[0102] Laser diodes may be used as a substitute for LEDs. The laser diode provides additional light energy which may allow a reduction in the concentration of dye markers in the blood stream. However, most of the wavelengths available are not ideal for the preferred fluorescent molecules of fluorescein and sulforhodamine 101.

[0103] LEDs from several vendors have been evaluated. Several LEDs meet the needs of the apparatus. These LEDs provide the best flux density per unit area and work well with the filters providing excellent elimination of off wavelength background.

[0104] For fluorescein, a LED490-03U made by ROITHNER LASERTECHNIK GmbH of Austria may be chosen. This LED has a peak wavelength of 490 nm for fluorescein excitation. This LED is rated at 1.2 mw. Alternatively, for fluorescein, an XREBLU-L1-0000-00K01 LED made by Cree Inc. of Durham, N.C. is preferable. This part is a high power surface mount LED with good thermal characteristics. The peak wavelength for this application is 485 nm with a minimum flux output of 30.6 lumens. A surface mount part that can be sorted to have similar characteristics may be substituted for this part.

[0105] For sulforhodamine 101 excitation, an 828-OVTL01LGAAS from OPTEK, having distribution in North America and throughout the world, with a peak wavelength of 595 nm, may be used. This surface mount LED has a higher flux density, so it can be run at lower power settings to minimize wavelength thermal drift. The target output power for the 1 mm fiber optic will be about 50 microwatts. For sulphorhodamine 101 excitation, an XRCAMB-L1-0000-00K01 LED from Cree Inc. of Durham, N.C. is preferable. LEDs of this type can be sorted for peak wavelength over the range of 585 nm to 595 nm. A peak output of 590 nm has been chosen for the application. These are high power surface mount LEDs with good thermal characteristics. The luminus flux output of this LED is also 30.6 lumens.

[0106] Filter selection is critical to performance of this system. Since the fluorescence signal returning through the fiber optic 210 will be many orders of magnitude below the excitation energy, filter blocking and bandpass characteristics are critical to proper performance. The fluorescent markers which have been used in microscopy and other applications for many years are well known in the art. Thus, excellent filter sets are available from a variety of manufacturers such as SEMROCK of the United States. These filters are ideal for this application.

[0107] Two additional apparatuses for sending and receiving fluorescent signals 204 have been contemplated by the inventors. These apparatuses are illustrated in FIGS. 12 and 13 and are aimed at improved optical geometry. A single channel apparatus is illustrated in FIG. 12. One objective of the single channel device is to improve the signal to background ratio and determine the target signal strengths for the fluorescently tagged dextrans in whole blood. An improved optical geometry has significantly reduced the background levels over an order of magnitude. This new optical geometry and fiber coupling has provided us with a 30 to 1 signal to background ratio.

[0108] FIG. 12 is a block diagram of a single channel optical system. The single channel device shown in FIG. 12 uses a simple optical design. Light from a 490 nm LED 206 is relayed through a band pass dichroic filter 220, then focused onto the fiber optic surface mount adaptor (SMA) connector 244. A simple condenser lens element 240 is used to minimize spherical aberrations that would limit ability to focus onto the small 0.5 to 1 mm fiber optic target 210. The fiber lens 224 works as both a final focusing element for the source light and the initial collimator for the fluorescent emission. The fluorescent emission light is relayed back through the dichroic filter 220 and refocused onto the PMT 208. Simple bi-convex lenses 248 are used for this since the target size on the PMT 208 is not critical, and a FITC emission filter is provided as a band pass filter. Close attention is given to stray light and reflections in this system by utilizing good light absorbing coating materials in the component construction.

[0109] Referring to FIGS. 13 and 14, a two channel optical system is illustrated. Similar components to those chosen in the single channel design are used in the two channel design. The main differences are an additional dichroic filter within holders 254a,b and spaced from a main block 255 by spacers 256a,b used to combine light from the 490 nm LED 206a and 595 nm LED 206b together. Each source utilizes its own condenser lens assembly 240a,b and band-pass filter 216a,b, a 595 nm filter and a 490 nm filter respectively, within holders 242a,b. The light beams from the LEDs 206a,b are then relayed through a special dual band dichroic filter 252 before being focused by the lens 224 onto the fiber coupler 244, specifically the fiber optic target 210. This dual band filter is readily available from SEMROCK. The emission from both fluorescent molecules then travels back through the fiber optic cable 210. The fiber lens 224 is attached to main block 255 within holder 258 and ring 262 and is used to collimate this light for relay back through the dual band dichroic filter 252 and then split to the appropriate PMT 208a,b using a final emission dichroic filter 228. Each PMT assembly 208a,b has a final focusing lens 248a,b and an emission filter 232a,b, preferably a FITC emission filter and a sulphorhodamine 101 emission filter respectively, within holders 260a,b and PMT adaptors 262a,b. Main block 255 is closed by sealing plates 264a,b and gaskets 266a,b with fasteners 268

[0110] An electrical circuitry contains a microcontroller to control both the pulse rate to the LEDs 206a,b and synchronize the readings from the PMTs 208a,b. The LEDs 206a,b are energized for a short time at a frequency of 100 Hz. At no time are both LEDs 206a,b illuminated, eliminating some of the bleed through of the two fluorescent markers. A high speed 16 bit analogue/digitalconverter is used to read the PMTs 208a,b and average the data. A laptop computer may be used for the software component of this system, or the electrical circuitry, microcontroller, and software may be housed within the fluorescent detector 200.

Mathematical Model

[0111] A two compartment mathematical model may be used to calculate GFR from the intensity ratio of the two tagged dextran molecules. This model may be included in software which may be stored on an external computer or within the fluorescent detector 200. Alternatively, the mathematical model may be hard wired circuitry either internal or external to the apparatus.

[0112] GFR and apparent volume of distribution can be measured by monitoring the plasma disappearance of the fluorescently labeled dextran molecule intravenously administered by a single dose bolus injection. FIG. 15 illustrates a widely used two-compartment model, also known as three-component model. The two compartments in question are vascular space and interstitial space. The basic assumption for this model is that the infused reporter molecule will distribute from the vascular space to interstitial space after the bolus injection, but the marker molecule will be retained in the vascular space. The plasma removal of the reporter molecule only occurs from the vascular space.

[0113] The plasma clearance rate and the inter-compartment clearance rate are denoted as G and k, respectively. The virtual volume for the vascular space and interstitial space are V.sub.1 and V.sub.2, respectively. As demonstrated by Sapirstein et al. (Sapirstein, L. A., D. G. Vidt, et al. (1955). “Volumes of distribution and clearances of intravenously injected creatinine in the dog.” American Journal of Physiology 181(2): 330-6.) the amount change per unit time in V.sub.1 is given by the following equation:

[00001]$\begin{array}{cc}{V}_1\frac{{\mathrm{dC}}_1}{\mathrm{dt}}=-{\mathrm{GC}}_1-k\left(C_1-C_2\right)& \left(2\right)\end{array}$

[0114] Total injected amount D can be expressed as the following:

D=C.sub.1V.sub.1+C.sub.2V.sub.2+G∫.sub.0.sup.tC.sub.1dt   (3)

where C.sub.1 and C.sub.2 denote the concentrations of the reporter molecule in the vascular and interstitial space, respectively.

[0115] Combining the two equations above yields the following second order linear differential equation (Sapirstein, Vidt et al. 1955):

[00002]$\begin{array}{cc}{V}_1\frac{d^2{C}_1}{{\mathrm{dt}}^2}+\left(\frac{G+k}{V_1}+\frac{k}{V_2}\right)\frac{d{C}_1}{\mathrm{dt}}+\frac{kG{C}_1}{V_1{V}_2}=0& \left(4\right)\end{array}$

[0116] The general solution to equation (4) is a bi-exponential function expressed in equation (16) below:

C.sub.1(t)=Ae.sup.−αt+Be.sup.−βt   (5)

where the decay constants α and β can be expressed in k, G, V.sub.1 and V.sub.2 (Sapirstein, L. A., D. G. Vidt, et al. (1955). “Volumes of distribution and clearances of intravenously injected creatinine in the dog.” American Journal of Physiology 181(2): 330-6.).

[0117] Assuming the inter-compartment movement is negligible before the intra-compartment mixing in V.sub.1 is completed, then the following two boundary conditions at t=0 become valid: C.sub.0=D/V.sub.1 and C.sub.2=0.

[0118] From equations (2), (3), (5), and the two boundary conditions we can derive the following (Sapirstein, L. A., D. G. Vidt, et al. (1955). “Volumes of distribution and clearances of intravenously injected creatinine in the dog.” American Journal of Physiology 181(2): 330-6.):

[00003]$\begin{array}{cc}\mathrm{GFR}=\frac{D}{A/\alpha +B/\beta }& \left(6\right)\end{array}$$\begin{array}{cc}{V}_1=\frac{D}{A+B}& \left(7\right)\end{array}$$\begin{array}{cc}{V}_d=\frac{D\left(\frac{A}{{\alpha }^2}+\frac{B}{{\beta }^2}\right)}{{\left(\frac{A}{\alpha }+\frac{B}{\beta }\right)}^2}& \left(8\right)\end{array}$

where the total extracellular volume of distribution V.sub.d, is the sum of V.sub.1 and V.sub.2.

[0119] Parameters A, B, α, and β can be obtained by fitting the experimental data to equation (5).

[0120] In practice we may obtain V.sub.1 using the marker molecule. If the linear relationship between the concentration and fluorescence intensity holds for the reporter molecule, equation (5) can then be rewritten as:

F.sub.1(t)=A.sub.1e.sup.−αt+B.sub.1e.sup.−βt   (9)

where F.sub.1 is the fluorescence intensity of the reporter molecule as a function of time. A.sub.1 and B.sub.1 are constants.

[0121] Thus, equations (6) and (8) can be rewritten as follows:

[00004]$\begin{array}{cc}\mathrm{GFR}=\frac{V_1\left(A_1+B_1\right)}{A_1/\alpha +B_1/\beta }& \left(10\right)\end{array}$$\begin{array}{cc}{V}_d=\frac{V_1\left(A_1+B_1\right)\left(\frac{A_1}{{\alpha }^2}+\frac{B_1}{{\beta }^2}\right)}{{\left(\frac{A_1}{\alpha }+\frac{B_1}{\beta }\right)}^2}& \left(11\right)\end{array}$

where equation (10) represents GFR from intensity of a single, freely filterable reporter molecule type, and equation (11) represents the volume distribution associated with a single, freely filterable reporter molecule type.

[0122] In addition, since the fluorescence of the marker is a constant over time, equation (9) can be also expressed in terms of fluorescence ratio of the reporter molecule over the marker molecule. Thus, the bi-exponential equation becomes:

R(t)=A.sub.2e.sup.−αt+B.sub.2e.sup.−βt   (12)

where R(t) is the fluorescence ratio of the reporter molecule over the marker molecule.

[0123] Constants A.sub.2, B.sub.2, α, and β can be obtained by fitting the experiment data to the above equation. Thus, the clearance GFR and the total volume of distribution can be expressed as:

[00005]$\begin{array}{cc}\mathrm{GFR}=\frac{V_1\left(A_2+B_2\right)}{A_2/\alpha +B_2/\beta }& \left(13\right)\end{array}$$\begin{array}{cc}{V}_d=\frac{V_1\left(A_2+B_2\right)\left(\frac{A_2}{{\alpha }^2}+\frac{B_2}{{\beta }^2}\right)}{{\left(\frac{A_2}{\alpha }+\frac{B_2}{\beta }\right)}^2}& \left(14\right)\end{array}$

where equation (13) represents GFR from the intensity ratio between a freely filterable reporter molecule type and a larger marker molecule type, and equation (11) represents the volume distribution associated with from a freely filterable reporter molecule type and a larger marker molecule type.

[0124] Evidently, when the inter-compartment volume exchange rate approaches zero, this model collapses to a single compartment model. However, it has been shown that as the plasma clearance level increases this mono-exponential approximation will lead to an overestimation of the GFR (Schwartz, G. J., S. Furth, et al. (2006). “Glomerular filtration rate via plasma iohexol disappearance: pilot study for chronic kidney disease in children.” Kidney International 69(11): 2070-7; Yu, W., R. M. Sandoval, et al. (2007). “Rapid determination of renal filtration function using an optical ratiometric imaging approach.” American Journal of Physiology—Renal Physiology 292(6): F1873-80.)

Optical Catheter

[0125] Referring to FIGS. 16 and 17, the catheter 300 for use with the present invention is illustrated. The catheter 300 includes a fiber optic insertion tool 304, a dual port luman 308, and a French 5 to 8 size introducer 312. A standard 1 mm plastic fiber optical cable 316 may be inserted through a passageway 320 defined by a combination of the insertion tool 304 joined with the luman 308 joined with the introducer 312. Accordingly, each of these components is of a tubular configuration. Preferably, a 0.75 mm fiber optic cable is inserted through the passageway 320. The 0.75 mm diameter was only chosen to allow use of a standard 18 gauge introducer.

[0126] The insertion tool 304 includes a first tubular member 324 slidable within a second tubular member 328. Fluid-tight seals are provided on opposing ends of the second tubular member 328 by o-rings 332 about the first tubular member 324 and the fiber optic cable 316, respectively. The fiber optic cable 316 is securely held or fixed within the insertion tool 304 by a seal 336 at an opposite end of the insertion tool 304.

[0127] The insertion tool 304 is joined to one of the ports 340a on the luman 308. Homostatic seals 344a,b are located on the ports 340a,b. The other port 340b is to provide for bolus injection or a continuous infusion of the fluorescent molecule. A luer connector 348 at an opposite end of the luman 308 joins the subject with the introducer 312.

[0128] The fiber optic cable 316 may comprise either single or multiple single fibers for light delivery and collection of the emission and excitation. The fiber optic cable 316 is inserted within a subject's vein 352 by pressing the first tubular member 324 and the captive optical cable 316 through the second tubular member 328 wherein the fiber optic cable 316 is extensible from the catheter 300. The optical cable 316 traverses through the subject by the luer connector 348 through the introducer 312 and into the subject's vein 352. The fiber optic cable 316 may have a small permanent bend on an end inserted into the subject's vein 352. This bend helps penetrate the tissue and minimizes interference of the fiber optic cable 316 within the vein.

[0129] In use, the fiber optic cable 316 is an extension of, or placed in communication with, the fiber optic cable 210 of the fluorescent detector 200 to transmit a signal or signals generated at the subject's vein to the fluorescent detector 200 for evaluation.

[0130] The present invention discloses a unique and novel method and device for quantifying kidney function, but it also presents a unique method of quantitatively determining liver function. For example, a dye composition of a larger molecular weight marker and smaller molecular weight reporter molecules is injected into a subject, and the ratio of the decrease of the reporter molecule to the marker molecule is used to detect kidney function. The smaller reporter molecules are filtered by the kidney while the marker molecules are remained in the vascular system. For a reasonable ratio of marker molecules to signal molecules to be detected, the marker molecule must remain in the blood at relatively consistent levels during the diagnostic test. Eventually on a much longer time scale (typically 12 to 24 hours) the marker molecule will typically be absorbed and processed from the vascular system by the liver instead of the kidney. Here a novel method and device are described, where relative liver function and health may be quantitatively determined by measuring the absolute decrease of the marker molecule in the blood over time. This method will have advantages over other methods by providing a quantitative value on an arbitrary scale that correlates to liver health. As a result, medical care professionals will be provided with a new tool allowing them to better treat their patients and predict proper dosing of certain drugs. This method would use the same device as described previously, and would also utilize the same dye composition as described previously; however, it would provide a method of analyzing the results to provide additional function and utility using the following equation:

[00006]$\begin{array}{cc}\mathrm{LiverFunction}=\frac{Emi\mathrm{ssion}\mathrm{from}\mathrm{Marker}\mathrm{Molecule}}{time}& \left(15\right)\end{array}$

[0131] While the specific embodiments have been illustrated and described, numerous modifications come to mind without significantly departing from the spirit of the invention, and the scope of protection is only limited by the scope of the accompanying Claims.