Sensor for transcutaneous measurement of vascular access blood flow Number:6,987,993 from the United States Patent and Trademark Office (PTO) owispatent

Title: Sensor for transcutaneous measurement of vascular access blood flow

Abstract: An optical sensor includes photoemitter and photodetector elements at multiple spacings (d₁, d₂) for the purpose of measuring the bulk absorptivity (α) of an area immediately surrounding and including a hemodialysis access site, and the absorptivity (α_o) of the tissue itself. At least one photoemitter element and at least one photodetector element are provided, the total number of photoemitter and photodetector elements being at least three. The photoemitter and photodetector elements are collinear and alternatingly arranged, thereby allowing the direct transcutaneous determination of vascular access blood flow.

Patent Number: 6,987,993 Issued on 01/17/2006 to Steuer,   et al.

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Inventors:	Steuer; Robert R. (Pleasant View, UT); Bell; David A. (Farmington, UT); Miller; David R. (Morgan, UT)
Assignee:	Hema Metrics, Inc. (Kaysville, UT)
Appl. No.:	730102
Filed:	December 9, 2003

Current U.S. Class:	600/322; 600/310
Current Intern'l Class:	A61B 5/00     (20060101)
Field of Search:	600/309-310,322,504

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References Cited [Referenced By]

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Other References

J.P. Payne and J.W. Severinghaus, Eds., Pulse Oximetry, Chapters 1 and 2 (©1986). John D. Bower and Thomas G. Coleman, Circulatory Function During Chronic Hemodialysis, vol. XV Trans. Amer. Soc. Artif. Int. Organs, 1969, 373-377. Larry Reynolds, C. Johnson, A. Ishimaru, "Diffuse reflectance from a finite blood medium: applications to the modeling of fiber optic catheters," Sep. 1976, vol. 15, No. 9, Applied Optics pp. 2059-2067. R.N. Greenwood, C, Aldridge, L. Goldstein, L.R.I. Baker and W.R. Cattell, "Assessment of arteriovenous fistulae from pressure and thermal dilution studies: clinical experience in forearm fistulae," Clinical Nephrology, vol. 23, No. 4-1985, pp. 189-197. R.N. Greenwood, C. Aldridge and W.R. Cattell, "Serial blood water estimations and in-line blood viscometry: the continuous measurement of blood volume during dialysis procedures," Clinical Science (1984)66, pp. 575-583. C. Aldridge, R.N. Greenwood, W.R. Cattell and R.V. Barrett, "The assessment of arteriovenous fistulae created for haemodialysis from pressure and thermal dilution measurements," Journal of Medical Engineering & Technology, vol. 8, No. 3, (May/Jun.), pp. 118-124. L. Goldstein, L. Pavitt, R.N. Greenwood, C. Aldridge, L.R.I. Baker and W.R. Cattell, "The Assessment of Arteriovenous Fistulae From Pressure and Recirculation Studies," ProcEDTNA-ERCA (1985) vol. 14., pp. 207-215. R.N. Greenwood, C. Aldridge, L. Goldstein, L.R.I. Baker and W.R. Cattell, "Assessment of Arteriovenous Fistulas From Pressure Recirculation Studies: Clinical Experience In 215 Upper Limb Fistulas," ProcEDTA-ERA (1985), vol. 22, pp. 296-302. Joseph M. Schmitt, James D. Meindl and Frederick G. Mihm, "An Integrated Circuit-Based Optical Sensor for In Vivo Measurement of Blood Oxygenation," IEEE Transactions On Biomedical Engineering, vol. BME-33, No. 21, Feb. 1986, pp. 98-107. Joseph M. Schmitt, Fred G. Mihm and James Meindl, New Methods for Whole Blood Oximetry, Annals of Biomedical Engineering, vol., 14, pp. 35-52, 1986. Mark R. Arnfield, J. Tulip and Malcolm McPhee, "Optical Propagation in Tissue With Anisotropic Scattering," IEEE Transactions on Biomedical Engineering, vol. 35, No. 5, No. 5, May 1988, pp. 372-381. N.M. Krivitski, "Theory and validation of access flow measurements by dilution technique during hemodialysis," Kidney Int 48:244-250, 1995. N.M. Krivitski, "Novel method to measure access flow during hemodialysis by ultrasound velocity dilution technique," ASAIO J 41:M741-M745, 1995. T.A. Depner and N.M. Krivitski, "Clinical measurement of blood flow in hemodialysis access fistulae and grafts by ultrasound dilution," ASAIO J 41:M745-M749, 1995). D. Yarar et al., "Ultrafiltration method for measuring vascular access flow rates during hemodialysis," Kidney Int., 56: 1129-1135 (1999). N.M. Krivitski et al., "Saline Release Method to Measure Access Flow (AF) by Ultrasound Dilution during Hemodialysis," JASN Abstracts, 8:164A, 1997.

Primary Examiner: Winakur; Eric F.
Assistant Examiner: Kremer; Matthew
Attorney, Agent or Firm: Jacobson Holman PLLC

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Parent Case Text

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CROSS-REFERENCE TO RELATED APPLICATIONS

The present patent application is a divisional of application Ser. No. 09/750,076, filed Dec. 29. 2000, now U.S. Pat. No. 6,725,072 which is incorporated herein by reference in its entirety.

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Claims

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What is claimed is:

1. A device for locating the flow of a fluid in a graft or vein in the body of a patient, the device comprising:

a sensor configured to measure the value of a parameter in an area of the skin of a patient including the graft or vein to be located and to measure the value of the parameter in the area of the skin adjacent to the graft or vein to be located;

means for placing the sensor on the skin of the patient near the flow of fluid to be found;

means based on the measurements detected by the sensor for calculating:

(a) the optical attenuation coefficient (α) in an area of the skin that includes the graft or vein to be located;

(b) the optical attenuation coefficient (α_o1) in a first area adjacent to the graft or vein to be located;

(c) the optical attenuation coefficient (α_o2) in a second area adjacent to the graft or vein to be located;

means for measuring a reference ratio between the optical attenuation coefficient (α_o1) and the optical attenuation coefficient (α_o2); and means for monitoring the reference ratio so that when the ratio reaches a certain value a signal indicates that the graft or vein is located.

2. The device of claim 1 wherein the reference ratio is determined by solving the equation (1-α_o1/α_o2) times 100.

3. The device of claim 1 wherein the parameter is Hematocrit.

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Description

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to apparatus for non-invasively measuring one or more blood parameters. More specifically, the invention relates to apparatus for the transcutaneous measurement of vascular access blood flow. The invention can also be used for precise access location, as a "flow finder," and also can be used to locate grafts and to localize veins in normal patients for more efficient canulatization.

2. Related Art

Routine determination of the rate of blood flow within the vascular access site during maintenance hemodialysis is currently considered an integral component of vascular access assessment. While the relative importance of vascular access flow rates and venous pressure measurements in detecting venous stenoses is still somewhat controversial, both the magnitude and the rate of decrease in vascular access flow rate have been previously shown to predict venous stenoses and access site failure. The traditional approach for determining the vascular access flow rate is by Doppler flow imaging; however, these procedures are expensive and cannot be performed during routine hemodialysis, and the results from this approach are dependent on the machine and operator.

Determination of the vascular access flow rate can also be accurately determined using indicator dilution methods. Early indicator dilution studies determined the vascular access flow rate by injecting cardiogreen or radiolabeled substances at a constant rate into the arterial end of the access site and calculated the vascular access flow rate from the steady state downstream concentration of the injected substance. These early attempts to use indicator dilution methods were limited to research applications since this approach could not be routinely performed during clinical hemodialysis. It has long been known that in order to determine the vascular access flow (ABF) rate during the hemodialysis procedure, the dialysis blood lines can be reversed (by switching the arterial and venous connections) to direct the blood flow within the hemodialysis circuit in order to facilitate the injection of an indicator in the arterial end of the access site and detect its concentration downstream (N. M. Krivitski, "Theory and validation of access flow measurements by dilution technique during hemodialysis," Kidney Int 48:244-250, 1995; N. M. Krivitski, "Novel method to measure access flow during hemodialysis by ultrasound velocity dilution technique," ASAIO J 41:M741-M745, 1995; and T. A. Depner and N. M. Krivitski, "Clinical measurement of blood flow in hemodialysis access fistulae and grafts by ultrasound dilution," ASAIO J 41:M745-M749, 1995)). D. Yarar et al., Kidney Int., 65:1129-1135 (1999), developed a similar method using change in hematocrit to determine ABF. Various modifications of this approach have been subsequently developed. While these latter indicator dilution methods permit determination of the vascular access flow rate during routine hemodialysis, reversal of the dialysis blood lines from their normal configuration is inconvenient and time-consuming since it requires that the dialyzer blood pump be stopped and the dialysis procedure is relatively inefficient during the evaluation of the flow rate which can take up to twenty minutes. Furthermore, some of these indicator dilution methods also require accurate determination of the blood flow rate.

Clinical usefulness and ease of use are major developmental criteria. From a routine clinical point of view the need to design a simple sensor, easily attached to the patient, requiring no line reversals, no knowledge of the dialysis blood flow rate, Q_b, and transcutaneously applied to skin, thereby accomplishing the measurement within a total of 1-2 minutes, is crucial to have repeated, routine meaningful ABF trend information, whereby access health is easily tracked.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide apparatus for non-invasively measuring one or more blood parameters.

It is another object of the present invention to provide an optical hematocrit sensor that can detect changes in hematocrit transcutaneously.

It is still another object of the invention to provide an optical hematocrit sensor that can be used to determine the vascular access flow rate within 2 minutes and without reversal of the dialysis blood lines or knowledge of Q_b, all transcutaneously.

These and other objects of the invention are achieved by the provision of an optical sensor including complementary emitter and detector elements at multiple spacings (d₁, d₂) for the purpose of measuring the bulk absorptivity (α) of the volume immediately surrounding and including the access site, and the absorptivity (α_o) of the tissue itself.

In one aspect of the invention, the optical sensor system comprises an LED of specific wavelength and a complementary photodetector. A wavelength of 805 nm-880 nm, which is near the known isobestic wavelength for hemoglobin, is used.

When the sensor is placed on the surface of the skin, the LED illuminates a volume of tissue, and a small fraction of the light absorbed and back-scattered by the media is detected by the photodetector. The illuminated volume as seen by the photodetector can be visualized as an isointensity ellipsoid, as individual photons of light are continuously scattered and absorbed by the media. Because a wavelength of 805 nm-880 nm is used, hemoglobin of the blood within the tissue volume is the principal absorbing substance. The scattering and absorbing characteristics are mathematically expressed in terms of a bulk attenuation coefficient (α) that is specific to the illuminated media. The amount of light detected by the photodetector is proportional via a modified Beer's law formula to the instantaneous net α value of the media.

When the volume of tissue illuminated includes all or even part of the access, the resultant α value includes information about both the surrounding tissue and the access itself. In order to resolve the signal due to blood flowing within the access from that due to the surrounding tissues, the sensor system illuminates adjacent tissue regions on either side of the access. Values of α_o for tissue regions not containing the access are then used to normalize the signal, thus providing a baseline from which relative changes in access hematocrit can be assessed.

Other objects, features and advantages of the present invention will be apparent to those skilled in the art upon a reading of this specification including the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is better understood by reading the following Detailed Description of the Preferred Embodiments with reference to the accompanying drawing figures, in which like reference numerals refer to like elements throughout, and in which:

FIG. 1 is a diagrammatic view of a dialysis circuit in which a TQ_a hematocrit sensor in accordance with the present invention is placed at the hemodialysis vascular access site.

FIG. 2 is a perspective view of a first embodiment of a TQ_a hematocrit sensor in accordance with the present invention.

FIG. 3 is a bottom plan view of the TQ_a hematocrit sensor of FIG. 2.

FIG. 4 is a side elevational view of the TQ_a hematocrit sensor of FIG. 2.

FIG. 5 is a top plan view of the TQ_a hematocrit sensor of FIG. 2.

FIG. 6 is a cross-sectional view taken along line 6—6 of FIG. 2.

FIG. 7 is a diagrammatic view illustrating the TQ_a sensor of FIG. 2 and the illuminated volumes or "glowballs" produced by the emitters and seen by the detectors thereof.

FIG. 8 is a perspective view of a second embodiment of a TQ_a hematocrit sensor in accordance with the present invention.

FIG. 9 is a bottom plan view of the TQ_a, hematocrit sensor of FIG. 8.

FIG. 10 is a side elevational view of the TQ_a hematocrit sensor of FIG. 8.

FIG. 11 is a top plan view of the TQ_a hematocrit sensor of FIG. 8.

FIG. 12 is a cross-sectional view taken along line 12—12 of FIG. 9.

FIG. 13 is a diagrammatic view illustrating the TQ_a hematocrit sensor of FIG. 8 and the illuminated volumes or "glowballs" produced by the emitters and seen by the detector thereof.

FIG. 14 is a perspective view of a third embodiment of a TQ_a hematocrit sensor in accordance with the present invention.

FIG. 15 is a bottom plan view of the TQ_a hematocrit sensor of FIG. 14.

FIG. 16 is a side elevational view of the TQ_a hematocrit sensor of FIG. 14.

FIG. 17 is a top plan view of the TQ_a hematocrit sensor of FIG. 14.

FIG. 18 is a cross-sectional view taken along line 18—18 of FIG. 15.

FIG. 19 is a diagrammatic view illustrating the TQ_a hematocrit sensor of FIG. 14 and the illuminated volumes or "glowballs" produced by the emitter and seen by the detectors thereof.

FIG. 20 is a perspective view of a fourth embodiment of a TQ_a hematocrit sensor in accordance with the present invention.

FIG. 21 is a partial cross-sectional view of the TQ_a hematocrit sensor of FIG. 20.

FIG. 22 is a diagrammatic view of the TQ_a hematocrit sensor of FIG. 20 showing the placement of the emitters and detectors relative to the access site.

FIGS. 23-26 are diagrammatic views illustrating the TQ_a hematocrit sensor of FIG. 20 and the illuminated volumes or "glowballs" produced by the emitters and seen by the detectors thereof.

FIG. 27 is a perspective view of a fifth embodiment of a TQ_a hematocrit sensor in accordance with the present invention.

FIG. 28 is a partial cross-sectional view of the TQ_a hematocrit sensor of FIG. 27.

FIG. 29 is a diagrammatic view of the TQ_a hematocrit sensor of FIG. 27 showing the placement of the emitters and detectors relative to the access site.

FIGS. 30-33 are diagrammatic views illustrating the TQ_a hematocrit sensor of FIG. 27 and the illuminated volumes or "glowballs" produced by the emitters and seen by the detectors thereof.

FIG. 34 is a cross-sectional view of a TQ_a hematocrit sensor in accordance with the present invention in the form of a disposable adhesive patch.

FIG. 35 is a graphical representation of a signal proportional to the hematocrit in the vascular access as recorded by a sensor and associated monitoring system in accordance with the invention.

FIG. 36 is a graphical representation of plotted values of the vascular access flow rate determined using a TQ_a sensor in accordance with the present invention versus that determined by a conventional HD01 monitor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In describing preferred embodiments of the present invention illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner to accomplish a similar purpose.

The following abbreviations and variables are used throughout the present disclosure in connection with the present invention:

α=access site optical attenuation coefficient

α_o=non-access site optical attenuation coefficient

B_o=composite of all the non-access region S, K coefficients

C=proportionality scalar

CPR=cardio-pulmonary recirculation

d=distance between the emitter and the detector

H=hematocrit, generally

H_a=hematocrit within the vascular access site

H_ao=hematocrit beneath the sensor (outside the dialyzer)

ΔH=change in hematocrit (H_a-H_ao)

i=intensity of light, generally

I_baseline=baseline intensity (taken in the absence of a bolus)

I_measured=light back-scattered from a turbid tissue sample

I_o=emitter radiation intensity

K=bulk absorption coefficient

K_b=access site blood coefficient

Q_a=vascular access blood flow rate

Q_b=dialyzer blood flow rate

Q_f=dialyzer ultrafiltration rate

Q_i=average injection inflow rate

S=bulk scattering coefficient

SD=standard deviation

SNR=signal-to-noise ratio

t=time (measured from time of injection)

TQ_a=transcutaneous access blood flow

V=known volume of saline injected into dialysis venous line

X_b=percentage of the access volume to the total volume illuminated (access blood proration value)

X_o=percentage of the non-access area to the total volume

The optical hematocrit sensor in accordance with the present invention comprises a light emitting source (emitter) (preferably an LED of specific wavelength) and a complementary photodetector that can be placed directly on the skin over a vascular access site. The LED preferably emits light at a wavelength of 805 nm-880 nm, because it is near the known isobestic wavelength for hemoglobin, is commercially available, and has been shown to be effective in the optical determination of whole blood parameters such as hematocrit and oxygen saturation.

When the sensor is placed on the surface of the skin, the LED illuminates a volume of tissue, and a small fraction of the light absorbed and back-scattered by the media is detected by the photodetector. While light travels in a straight line, the illuminated volume as seen by the photodetector can be visualized as an isointensity ellipsoid, as individual photons of light are continuously scattered and absorbed by the media. Because a wavelength of 805 nm-880 nm is used, hemoglobin of the blood within the tissue volume is the principal absorbing substance. The scattering and absorbing characteristics are mathematically expressed in terms of a bulk attenuation coefficient (α) that is specific to the illuminated media. The amount of light detected by the photodetector is proportional via a modified Beer's law formula to the instantaneous net α value of the media.

When the volume of tissue illuminated includes all or even part of the access, the resultant α value includes information about both the surrounding tissue and the access itself. In order to resolve the signal due to blood flowing within the access from that due to the surrounding tissues, the sensor system illuminates adjacent tissue regions on either side of the access. Values of α_o for tissue regions not containing the access are then used to normalize the signal, thus providing a baseline from which relative changes can be assessed in access hematocrit in the access blood flowing directly under the skin.

FIG. 1 illustrates a dialysis circuit in which a TQ_a hematocrit sensor 12 in accordance with the present invention is placed over the hemodialysis vascular access site 14, with the dialysis arterial and venous blood lines 16a and 16b in the normal configuration, for measuring TQ_a. A dialyzer 20 downstream of the vascular access site 14 and a syringe 22 for injecting a reference diluent (for example, saline) downstream of the dialyzer 20 are indicated. The hematocrits and flow rates under steady state conditions are also indicated, where Q_a is the access flow rate, Q_b is the dialyzer blood flow rate, Q_i is the injection flow rate, H_a is the hematocrit in the access flow, and H_o is the hematocrit at the sensor 12. The hematocrit sensor 12 is placed directly on the skin over the vascular access site 14 downstream of the venous dialysis needle 24.

As shown in FIG. 35, the sensor 12 and an associated monitoring system 30 records a signal proportional to the hematocrit in the vascular access site 14 (H_a). The monitoring system 30 can be a computer including a computer processor and memory, and output means such as a video monitor and printer (not shown). After a stable H_a value is obtained, a known volume (V) of normal saline is injected via the syringe 22 into the dialysis venous line 16b, which reduces the hematocrit beneath the sensor 12 to a time-dependent hematocrit H_o during the injection.

Derivation of the equation used to calculate the vascular access flow rate when using the bolus injection indicator dilution approach is complex. However, the constant infusion and bolus injection indicator dilution approaches yield identical results; therefore, the governing equation was derived from steady state constant infusion principles. Consider the dialysis circuit in FIG. 1 where a steady infusion of saline occurs in the dialysis venous blood line 16b (ultrafiltration at the dialyzer 20 is neglected). Red cell balance where the dialysis venous blood flow enters the access site 14 requires

H_a(Q_a-Q_b)+H_aQ_b=H_o(Q_a+Q_i)  (1)

Solving for Q_a, the vascular access flow rate, yields
##EQU1##

where ΔH denotes H_a-H_ao. This equation describes the changes in hematocrit at the sensor 12 during a constant infusion of normal saline in the dialysis venous blood line 16b. (If ultrafiltration at the dialyzer 20 occurs at a rate of Q_f, then the numerator in this equation becomes Q_i-Q_f).

Noting that Q_i is equivalent to the volume of saline injected in a specified time interval, equation (2) is therefore equivalent to:
##EQU2##

to yield the vascular access flow rate (Q_a), where ΔH denotes H_a-H_ao and the integral (area under the curve) in the above equation is from the time of injection (t=0) to where the signal has returned to the baseline value (t=∞). This equation is valid independent of the rate of saline injection or the dialyzer blood flow rate. The signals detected by the TQ_a sensor 12 can be used to calculate
##EQU3##

Determination of
##EQU4##


The percentage change in blood parameters (both macroscopic and microscopic) passing through the access site 14 may be measured in a variety of ways. Macroscopic parameters such as bulk density or flow energy can be measured by ultrasonic, temperature, or conductivity means. Microscopic parameters (sometimes called "physiologic or intrinsic" parameters) such as hematocrit or red cell oxygen content are measured by optical means. Each technique has its respective advantages and disadvantages, both rely on the quantity ΔH/H. Inherent in all of these is the need to differentiate the access site 14, and parameter changes therein, from the surrounding tissue structure. The TQ_a sensor 12 in accordance with the present invention is positioned directly over the access site region 14 itself approximately 25 mm downstream of the venous needle 24, and is based upon optical back-scattering of monochromatic light (λ=805 nm-880 nm) from the blood flow in the access site 14 and the surrounding tissues. The theory on which the construction of the TQ_a sensor 12 is based requires the use of optical physics and laws associated with optical determination of physiologic elements including hematocrit.

Modified Beer's Law

Numerous studies have shown that light back-scattered from a turbid tissue sample follows a modified form of Beer's Law,

I_measured=I_oAe^-αd  (4)

where I_o is the radiation intensity emitted from the LED, A is a complex function of d and α of the various layers of tissue (epidermis, dermis, and subcutaneous tissue), d is the distance between the LED and detector, and α is the bulk optical attenuation coefficient. The α term is a function of the absorption and scattering nature of the tissue and has a strong dependence on hematocrit.
##EQU5##

Compartmentalization of α

A transcutaneously measured α value is actually a prorated composite measure of all the absorption and scattering elements contained within the illuminated volume or "glowball" of the emitter source, and typically includes the effects of tissue, water, bone, blood, and in the case of hemodialysis patients, the access site 14. In the determination of α, clearly only the blood flowing through the access site 14 is of interest. The task therefore becomes one of separating the effects of absorption and scattering of the access site 14 from that of surrounding tissue structure. Starting with the well known definition,

α=√{square root over (3K(K+S))}  (6)

where K is the bulk absorption coefficient and S is the bulk scattering coefficient, and separating the access site 14 from non-access blood coefficients and rearranging terms,

X_bK_b≈α²-B_o  (7)


where X_b=ratio of the access volume to the total volume illuminated

• K_b=access blood coefficient
• B_o=composite of all the non-access region S and K coefficients
  Now, letting the non-access components become α_o²=B_o, we have
  
  X_bK_b=α²-α_o²  (8)
  
  In equation (6), the access blood coefficient, K_b, is directly proportional to hematocrit (H), K_b=H·C. Therefore,
  
  X_bK_b=X_b·H·C=α²-α_o²  (9)
  
• where C is a proportionality scalar known from the literature or empirically derived.

To determine α_o, measurements are made in areas 130b and 130c near but not including the access site 14, as depicted, for example, in FIG. 7. If the tissue is more or less homogenous, it is only necessary to make a single reference α_o measurement, using either two emitters 202a and 202b and one detector 204 (as shown in FIG. 13) or one emitter 302 and two detectors 304a and 304b (as shown in FIG. 19), as discussed in greater detail hereinafter. On the other hand, if a gradient in α_o exists in the area of interest (and this is often the case in vivo) multiple measurements are made to establish the nature of the gradient and provide an averaged estimate of α_o, using two emitters 102a and 102b and two detectors 104a and 104b, as discussed in greater detail hereinafter in connection with FIGS. 2-6.

Determination of
##EQU6##


The value of
##EQU7##

is defined as the time derivative of intensity i, normalized by i. This is expressed as
##EQU8##

wherein ΔK_b is proportional to ΔH. Hence,
##EQU9##

To determine
##EQU10##

a baseline intensity (taken in the absence of a bolus) is first measured to establish a reference. The intensity is then measured as a time varying signal as the saline bolus is injected, I(t). The quantity
##EQU11##

is then calculated as
##EQU12##


Final Determination of
##EQU13##


The value
##EQU14##

is the ratio of equations (11) and (8),
##EQU15##

Since d is fixed and known,
##EQU16##

α, and α_o are computed by equations (10) and (5). It is important to note that in the final ratio of
##EQU17##

the access blood proration value, X_b, cancels out. This removes vascular access size, volume, or depth dependence from the final result. Likewise, the
##EQU18##

and
##EQU19##

ratios eliminate skin color variations.

In order to use indicator dilution techniques to measure vascular access flow rates during routine hemodialysis, the indicator must be injected upstream and its concentration detected downstream in the blood flowing through the vascular access site 14. Reversing the dialysis blood lines 16a and 16b during the hemodialysis treatment permits application of indicator dilution by direct injection of the indicator into the dialysis venous tubing 16b. Because the TQ_a sensor 12 can detect a dilution signal downstream of the venous needle 24 through the skin, a unique application of indicator dilution principles permits determination of the vascular access flow rate without reversal of the dialysis blood lines 16a and 16b. Various methods of measuring vascular access blood flow rate, as well as a method for locating accesses and grafts and localizing veins in normal patients, using the TQ_a sensor 12 are described in co-pending U.S. application Ser. No. 09/750,122 (published U.S. application No. US-2002-0128545-A1) entitled "Method of Measuring Transcutaneous Access Blood Flow," filed Dec. 29, 2000, which is incorporated herein in its entirety.

The accuracy of the measurements taken using the TQ_a sensor 12 depends critically on at least two factors. As can be seen in equation (3) above, the calculated access flow rate depends directly on the volume of saline injected; therefore, care must be taken to inject a given amount of saline over a specified time interval. The latter does not need to be known precisely; however, it is important that it be less than approximately 10 seconds to avoid significant interference due to cardiopulmonary recirculation (CPR) of the injected saline. The second factor that is important to consider in the accuracy of the TQ_a measurements is the placement of the TQ_a sensor 12 to accurately determine changes in hematocrit through the skin. The sensor 12 must be placed directly over the vascular access site 14 approximately 25 mm downstream of the venous needle 24 in the specified orientation to accurately determine the relative changes in hematocrit. Additional variability due to sensor placement does not appear, however, to be significant, in that small variations in sensor placement do not significantly influence the measured vascular access flow rate. An additional concern is whether variations in accuracy of measurements taken using the TQ_a sensor 12 may occur with access sites that are not superficial or if the access diameter is very large; however, varying the spacing of sensor elements eliminates difficulties associated with very large accesses or with deeper access sites such as those typically found in the upper arm or thigh. Less accurate results would also be obtained if the sensor 12 does not accurately detect changes in hematocrit due to significant variation in skin pigmentation. The TQ_a sensor in accordance with the invention has been specifically designed to account for the individual absorption and scattering properties of patient tissues, through the use of 805 nm-880 nm LED optical technology, and the normalized nature of the measurements (di/i) suggests that the sensitivity of the calculated vascular access flow rate to skin melanin content is minimal.

Referring now to FIGS. 2-6, there is shown a first embodiment of the TQ_a sensor 100 in accordance with the present invention for the transcutaneous measurement of vascular access blood flow in a hemodialysis shunt or fistula 14. In this embodiment two emitters 102a and 102b and two detectors 104a and 104b are arranged in alignment along an axis A1 on a substrate 110. As mentioned above, this embodiment is employed if a gradient in α_o exists in the area of interest (as is often the case in vivo), as multiple measurements must be made to establish the nature of the gradient and provide an averaged estimate of α_o.

The sensor 100 has an access placement line L1 perpendicular to the axis A1. For proper operation, the sensor 100 must be placed with the access placement line L1 over the venous access site (shunt) 14. One of the emitters (the "inboard emitter") 102a and one of the detectors (the "inboard detector") 104a are placed at inboard positions on either side of and equidistant from the access placement line L1. The second emitter (the "outboard emitter") 102b is placed at a position outboard of the inboard detector 104a, while the second detector (the "outboard detector") 104b is placed at a position outboard of the inboard emitter 102a, so that the emitters 102a and 102b and detectors 104a and 104b alternate. The spacing between the emitters 102a and 102b and the detectors 104a and 104b is uniform.

The substrate 110 is provided with apertures 116 in its lower surface (the surface which in use faces the access site 20) for receiving the emitters 102a and 102b and the detectors 104a and 104b. The apertures 116 are sized so that the emitters 102a and 102b and the detectors 104a and 104b lie flush with the lower surface of the substrate 110.

Preferably, the upper surface of the substrate 110 is marked with the access placement line L1. The upper surface of the substrate 110 may also be provided with small projections 120 or other markings above the apertures 116 indicating the locations of the emitters 102a and 102b and the detectors 104a and 104b.

The circuitry (not shown) associated with the emitters 102a and 102b and the detectors 104a and 104b can be provided as a printed circuit on the lower surface of the substrate 110. The substrate 110 is made of a material that is flexible enough to conform to the contours of the underlying tissue but rigid enough to have body durability.

As shown in FIG. 7, there are three illuminated volumes or "glowballs" 130a, 130b, and 130c in the tissue, T, seen by the two detectors 104a and 104b: a first glowball 130a representing the reflective penetration volume (α) of the inboard emitter 102a through the access site tissue as seen by the inboard detector 104a in the process of determination of the access Hematocrit; a second glowball 130b representing the reflective penetration (α_o1) of the inboard emitter 102a through the non-access site tissue that surrounds the access site 14 as seen by the outboard detector 104b; and a third glowball 130c representing the reflective penetration (α_o2) of the outboard emitter 102b through the non-access site tissue that surrounds the access site 14 as seen by the inboard detector 104a. An estimate of α_o is made by averaging α_o1 and α_o2. That is,
##EQU20##


Due to the depth of the access site 14, in order for the cross-section of the access site 14 to be enclosed by the glowball 130a of the inboard emitter 102a seen by the inboard detector 104a, the spacing between the inboard emitter 102a and the inboard detector 104a is typically 24 mm.