US20040220484A1

US20040220484A1 - Method of measuring transcutaneous access blood flow

Info

Publication number: US20040220484A1
Application number: US10/855,384
Authority: US
Inventors: Robert Steuer; David Bell; David Miller
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-12-29
Filing date: 2004-05-28
Publication date: 2004-11-04
Also published as: JP2004524885A; WO2002053212B1; EP1345638A1; CA2433421A1; WO2002053212A1; KR20030081368A; US20020128545A1; US6746407B2

Abstract

Indicator dilution techniques are used to measure vascular access flow rates during routine hemodialysis. A bolus injection port is used to infuse a specific volume (V_i) of an indicator diluent into the patient cardiovascular circuit by one of the following:

1. Needle injection of a known volume of indicator diluent directly into the access site in the presence or absence of the hemodialysis circuit.

2. Infusion of an indicator diluent into the arterial, venous line upstream of the venous needle.

3. Turning the ultrafiltration of the dialysis delivery system from OFF to ON and OFF again over a predetermined time period.

4. In a hemodialysis circuit, turning on the hemodialysis pump and using the priming saline volume as a single saline bolus.

A transdermal sensor is used to measure the percent change in a blood parameter. The sensor is positioned directly over the vascular access site a prescribed distance downstream of the injection site and upstream of the access-vein connection. The sensor employs emitter and detector elements at multiple spacings (d₁, d₂) for the purpose of measuring the bulk absorptivity (α) of the area immediately surrounding and including the access site, and the absorptivity (α_o) of the tissue itself.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present patent application is a continuation of U.S. application Ser. No. 09/750,122, filed Dec. 29, 2000, which is incorporated herein by reference in its entirety.[0001]

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for transcutaneously measuring access blood flow. More specifically, the invention relates to a method for measuring access blood flow through the optical measurement of percentage change in a blood parameter and application of the Ficke dilution principle.

2. Related Art

Modern medical practice utilizes a number of procedures and indicators to assess a patient's condition especially in the dialysis setting. Hemodialysis is a process wherein an artificial kidney is required to function in the place of the patient's normal kidney in order to remove certain biologic waste products. When the human kidney no longer functions correctly removing waste products such as urea, potassium, and even excess water, blood must be removed from the patient via blood tubing lines and filtered through an artificial kidney or dialyzer. In this process blood is passed through the dialyzer, cleansed, then returned to the normal circulatory system of the patient. Access to the patient's circulatory system is achieved through the use of a surgically implanted shunt or fistula (access). This “access site” is typically located in the arm, leg, or neck of the patient. Typically needles are placed into the access in such a way as to facilitate the easy removal of blood on the “arterial” or upstream side of the dialyzer and typically return the purified blood downstream of the first needle placement on the “venous” side. Unfortunately, in many cases the access will clot or “stenos” over time. This results in decreased blood flow through the access site which ultimately necessitates either angioplasty or a surgical replacement of the shunt. As the access flow ceases or “clots off” part of the purified dialyzed blood is forced to flow back into the arterial withdrawal site and, hence, recirculates only to be dialyzed again; this is termed “access recirculation”.

Access Blood Flow (ABF, represented by the variable Q _a) is the rate at which blood passes through an arteriovenous (AV) graft or fistula. Poor or low Q_arates are generally indicative of hemo-dynamically significant access stenosis and/or thrombosis, which can reduce the adequacy of dialysis therapy and endanger the patient. In 1997 Dialysis Outcomes Quality Initiative (DOQI) Guidelines, the National Kidney Foundation (NKF) sets forth both the rationale and the procedural guidelines for the monitoring and maintenance of AV grafts and fistulas. These guidelines suggest that regular assessment of ABF may be predictive of access stenosis, which in turn may facilitate early intervention, thereby reducing the rate of thrombosis and loss.

NKF-DOQI Guidelines clearly identify access blood flow as a preferred method of monitoring AV grafts and fistulas: “Sequential, timely, repetitive measurement of access flow is the preferred method for monitoring AV grafts”, and “Flow measurements should be used when available to monitor for stenosis and thrombosis in AV fistulae.” NKF-DOQI Pocket Summary, Clinical Practice Guidelines for Vascular Access:

Guideline

10,11.

Lindsay and Leypoldt state, “Reductions in access blood flow rates if recognized may mandate reductions in QB and lead to difficulty in delivering adequate dialysis; if unrecognized these reductions can lead to the phenomenon of access recirculation, which will significantly decrease the efficiency of the hemodialysis treatment. Furthermore, such reductions may herald the problem of acute access thrombosis. It seems ideal, therefore, to monitor access blood flow.” Lindsay R, Leypoldt J: Monitoring Vascular Access Flow. Advances In Renal Replacement Therapy, Vol. 6, No. 3 (July), 1999: pp. 273-277.

Blood flow, Q, measured by the so-called Ficke dilutional techniques, has been described by A. C. Guyton, Textbook of Medical Physiology, Sixth Edition, pg. 287, 1981, wherein Q equals the volume of the injected diluent divided by the mean concentration of the diluent times the duration of the passage of the diluent through the vessel. A dilution curve is obtained by continuously monitoring changes in a given physical parameter of the blood over the time period of the injection. The change in the concentration of either the diluent (or the media) is measured over time.

Access Blood Flow (ABF) measurement is an area of concern in hemodialysis since it is a good indicator of access viability. Recent methods of determining ABF have included Doppler imaging, reversed line recirculation, and

\frac{Δ H}{H}

(the percentage change in hematocrit through the access site). The time, cost, and/or dialysis line reversal requirements of these methods have greatly limited their wide spread use and routine clinical applicability. With the exception of Doppler, ABF methods require the patient to be on dialysis and unencumbered by intradialytic activity such as blood pressure assessment or eating, further reducing flexibility in measurement. Conversely, Doppler measurements remain limited in accuracy due to uncertainty in measuring access size and cross-sectional area.

It is to the solution of these and other problems that the present invention is directed.

SUMMARY OF THE INVENTION

It is therefore a primary object of the present invention to provide a straightforward method of determining ABF, Q _a, using an optical sensor placed on the skin directly over the access site.

It is another object of the present invention to provide a transcutaneous method of ABF measurement that is not affected by size and/or depth of the access site, placement of the dialysis needles, pump speed variations, skin color, tissue composition, or access site location and type.

It is another object of the invention to provide a method of measuring a parameter transcutaneously downstream of a site where an indicator diluent is injected.

It is another object of the invention to provide a method of measuring a parameter transcutaneously in a perturbed system downstream of a site where the perturbation is introduced.

These and other objects of the invention are achieved by use of indicator dilution techniques to measure vascular access flow rates during routine hemodialysis, as well as in a clinic, before and/or after hemodialysis. A bolus injection port is used to infuse a specific volume (V _i) of an indicator diluent, such as saline or dye, into the patient cardiovascular circuit by one of the following:

1. Needle injection of a known volume (bolus) of indicator diluent directly into the access site in the presence or absence of the hemodialysis circuit.

2. Infusion of an indicator diluent into the arterial or venous needle or line upstream of the detector.

A transdermal optical sensor is used to measure the percent change in a blood parameter. The sensor is positioned directly over the vascular access site a prescribed distance downstream of the injection site and upstream of the access-vein connection in the case of grafts. The sensor employs complementary emitter and detector elements at multiple spacings (d ₁, d₂) for the purpose of measuring the bulk absorptivity (α) of the area 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 (a) 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 a 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 _ofor tissue regions not containing the access are then used to normalize the signal, thus providing a baseline from which can be assessed in access hematocrit in the access blood flowing directly under the skin.

In the case that hematocrit is the monitored parameter, these values are then related to the percentage change of the parameter by the relationship:

F (\frac{Δ H}{H}) = \frac{\frac{d i}{i}}{(d - \frac{1}{α}) (α^{2} - α_{o}^{2})},

where i is defined from a modified Beer's law as:

I≈I_oAe^−αd, where A≈α

More specifically, the diluent bolus is injected into the access site at an average flow rate of Q _i. Since the hematocrit of the indicator solution is zero, the red blood cell (RBC) mass does not change. The transcutaneous access blood flow (TQ_a) differential equation of state may be re-written in either a transient formulation,

Q_{a} = \frac{V_{i}}{\int F (\frac{Δ H}{H}) \partial t}, where \frac{Δ H}{H} = function of time

or time dependent or steady flow formulation. The steady flow form of the TQ _adifferential equation is obtained by assuming uniform and steady flow rates over the analysis time period, and is written as

Q_{a} = \frac{Q_{i}}{F (\frac{Δ H}{H})}

In both cases, the quantity

F (\frac{Δ H}{H})

is measured as the indicator bolus is injected into the system. If the bolus injection rate Q _iis uniform and constant, then the access flow Q_amay be determined from the steady flow formulation. Conversely, if the system remains dynamic and the bolus injection rate is uncertain or uncontrollable, then the transient solution must be used to determine Q_a.

The percentage change in blood parameters (both macroscopic and microscopic) passing through the access site can 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. In both cases, the measurement relies on the quantity

\frac{Δ H}{H}

when saline is injected. Thus, the method in accordance with the present invention can also be applied to the measurement of macroscopic parameters (percent change in density, temperature, conductivity, or energy) using ultrasonic, temperature, or conductivity sensors; and to the measurement of microscopic parameters like hematocrit using an optical sensor.

In the measurement of both macroscopic and microscopic blood parameters, it is necessary to differentiate the access site, and parameter changes therein, from the surrounding tissue structure. The method in accordance with the present invention utilizes a transdermal sensor incorporating photoemitters and photodetectors positioned directly over the access site itself and is based upon optical back-scattering of monochromatic light (λ=805 nm-880 nm) from the blood flow in the access site and the surrounding tissues, so that it is not limited to the extracorporeal circuit.

Light back-scattered from a turbid tissue sample follows the modified form of Beer's Law,

I≈I_oAe^−αd, A≈α

A transcutaneously measured a value is 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. The effects of absorption and scattering of the access site are separated from that of surrounding tissue structure by taking measurements in areas near but not including the access site. If the tissue is more or less homogeneous, it is only necessary to make a single, non-access site reference α _omeasurement. On the other hand, if a gradient in α_oexists in the area of interest, multiple measurements are made to establish the nature of the gradient and provide an averaged estimate of α_o.

The value of

\frac{d i}{i}

is defined as the time derivative of intensity i, normalized by i. To determine

\frac{d i}{i},

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. The quantity

\frac{d i}{i}

is then calculated as

\frac{d i}{i} = \frac{I_{baseline} - I (t)}{I_{baseline}}

The value

F (\frac{Δ H}{H})

consequently is:

F (\frac{Δ H}{H}) = \frac{\frac{d i}{i} α}{(d - \frac{1}{α}) (α^{2} - α_{o}^{2})}

Since d is fixed and known,

\frac{d i}{i},

α, and α _oare computed by the equations:

\frac{d i}{i} = \frac{I_{baseline} - I (t)}{I_{baseline}} and

α \approx \frac{- L n (\frac{I_{measured}}{I_{o}})}{d}

or from:

I≈I _oαe^−αd, where α is solved in polynomial form.

where access size and/or volume or depth dependence are not factors in either the transient or the steady state formulation of Q _a.

Although one embodiment of the invention uses saline as a diluent and measures the dilution of an existing endogenous material such as blood, the method in accordance with the present invention generally contemplates measuring a parameter transcutaneously in a perturbed system downstream of a site where the perturbation is introduced. The parameter can, for example, comprise a marker and the method comprises measuring the marker downstream using a sensor. Possible markers are proteins or red cells tagged with a radio nucleotide, which can be measured using a Geiger counter. Other parameters that can be measured in accordance with the present invention, and the devices for measuring them are: ultrasound, measured by an ultrasound detector; temperature, measured by a thermistor; impedance, measured by a bio-impedance measuring device; and albumen, glucose, and other blood constituents, measured using the optical sensor disclosed herein, but in which the LEDs emit different wavelengths suited to the specific constituent.

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.

BREF 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: [0051]
FIG. 1 is a diagrammatic view of a basic red blood cell mass balance model of an access site for a typical hemodialysis patient. [0052]
FIG. 2 is a diagrammatic view illustrating the illuminated volumes or “glowballs” produced by the emitters and seen by the detectors of a sensor used to measure TQ[0053] _awith the method of the invention.
FIG. 3 is a diagrammatic view of a patient circulatory system and associated in vivo dialysis circuit in which a TQ[0054] _asensor is placed to carry out the TQ_amethod of the invention.
FIG. 4 is a diagrammatic view of an in vitro cardiovascular-dialysis model constructed to test the TQ[0055] _amethod of the invention.
FIGS. 5-12 are graphical representations of a full test matrix performed using the model of FIG. 3 and the TQ[0056] _amethod of the invention.

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. [0057]

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



AV =	arteriovenous
α =	access region optical attenuation coefficient
α_o=	non-access bearing region optical attenuation coefficient
B_o=	composite of all the non-access region S, K coefficients
C =	proportionality scalar
d =	distance between the emitter and the detector
H =	hematocrit, generally
H_a=	access site hematocrit
H_ao=	hematocrit beneath the sensor (outside the dialyzer)
H_i=	hematocrit of normal saline
ΔH =	change in hematocrit (H_a− H_ao)
i =	intensity of light, generally
I_baseline=	baseline measured intensity (taken in the absence of a bolus)
I_measured=	light back-scattered from a turbid tissue sample
I_o=	emitted radiation intensity
K =	bulk absorption coefficient
K_b=	access blood absorption coefficient
Q =	blood flow
Q_a=	access blood flow
Q_ao=	output blood flow of the access site between the venous
	portion of the access site and into the vascular system
Q_b=	dialyzer pump flow rate
Q_i=	average flow rate at which diluent bolus is injected into
	the access site
Q_h=	flow rate of the heart (cardiac output)
RBC_in=	red blood cell mass flowing into the access site
RBC_out=	red blood cell mass flowing out from the access site
S =	bulk scattering coefficient
SNR =	signal-to-noise ratio
V_a=	net volume of blood in the access site during the
	time interval, t
V_ao=	volume of access blood plus indicator volume
V_i=	specific volume of an indicator diluent infused into the
	access site
X_b=	percentage of the access volume to the total volume
	illuminated
	(access blood proration value)

I. Introduction [0059]
The novel transcutaneous ABF (TQ[0060] _a) measurement technique that is the subject of the present invention provides a straightforward method of determining ABF without the limitations of previous methods. Much like other transcutaneous measurements, such as oxygen saturation, hematocrit, and glucose, it is based upon optical techniques. The TQ_aapproach continuously measures the percentage change in hematocrit or another blood parameter (either macroscopic or microscopic) resulting from a bolus of saline (or other diluent) injected into the patient's access site, which typically is in the form of an arteriovenous graft or fistula. In turn, using the Ficke dilution principle, and more specifically, the Henriques, Hamilton, Bergner principle, an access blood flow rate value is determined.
II. Model and Theory [0061]
FIG. 1 illustrates a basic red blood cell mass balance model of a typical hemodialysis patient access site. Blood flows into the access site from an arterial source at a flow rate Q[0062] _aand constant hematocrit value H_a. The magnitude of this inflow is primarily determined by the hemodynamics of the patient and the status of the access site itself. The hematocrit of the blood flowing into the shunt portion of the access site is typical of the bulk blood flow of the cardiovascular circuit, and for short time durations (less than 1 minute) this hematocrit may be considered as a constant. The red blood cell (RBC) mass flowing into the access site is, $\begin{matrix} {RBC}_{i n} = Q_{a} H_{a} = \frac{\partial V_{a}}{\partial t} H_{a} & (1) \end{matrix}$
A bolus injection port is used to infuse a specific volume (V[0063] _i) of an indicator diluent into the access site. Normal saline whose red cell content or hematocrit is zero (H_i=0) is typically used. However, the indicator diluent does not have to be saline, but can also, for example, be a dye or another liquid having a hematocrit of zero.
The diluent bolus is injected into the access site at an average flow rate of Q[0064] _i, $\begin{matrix} Q_{i} = \frac{\partial V_{i}}{\partial t} (bolus injection flow rate) & (2) \end{matrix}$
The output blood flow from the shunt portion of the access site into the venous portion of the access site and into the vascular system, Q[0065] _ao, is the nodal sum of the two inflow rates, Q_aand Q_i, where $\begin{matrix} Q_{ao} = \frac{\partial V_{ao}}{\partial t} = \frac{\partial V_{a}}{\partial t} + \frac{\partial V_{i}}{\partial t} (flow balance equation) & (3) \end{matrix}$
and the outgoing red blood cell mass flow from the access site is [0066] $\begin{matrix} {RBC}_{out} = Q_{ao} H_{ao} = \frac{\partial V_{ao}}{\partial t} H_{ao} & (4) \end{matrix}$
Since the hematocrit of the indicator solution is zero, the RBC mass does not change and therefore the RBC mass balance becomes: [0067] $\begin{matrix} {RBC}_{i n} - {RBC}_{out} = \frac{\partial (VH)}{\partial t} = 0 or, & (5) \\ \frac{\partial V_{a}}{\partial t} H_{a} = \frac{\partial V_{ao}}{\partial t} H_{ao} (mass balance equation) & (6) \end{matrix}$
Equations (3) and (6) may be combined to yield [0068] $\begin{matrix} \frac{\partial V_{a}}{\partial t} H_{a} = (\frac{\partial V_{a}}{\partial t} + \frac{\partial V_{i}}{\partial t}) H_{ao} or, & (7) \\ \frac{\partial V_{a}}{\partial t} (H_{a} - H_{ao}) = \frac{\partial V_{i}}{\partial t} H_{ao} and finally, \frac{\partial V_{a}}{\partial t} (\frac{Δ H}{H}) = \frac{\partial V_{i}}{\partial t} (T Q_{a} differential equation of state) & (8) \end{matrix}$
The TQ[0069] _adifferential equation of state may be re-written in a transient (that is, a time dependent) formulation or a steady flow formulation.
The transient formulation is derived by substituting [0070] $Q_{a} = \frac{\partial V_{a}}{\partial t}$
into equation (8) and integrating, which yields: [0071] $\begin{matrix} Q_{a} = \frac{V_{i}}{\int F (\frac{Δ H}{H}) \partial t}, where \frac{Δ H}{H} = a function of time & (9) \end{matrix}$
The steady flow form of the TQ[0072] _adifferential equation is obtained by assuming uniform and steady flow rates over the analysis time period. In this case, the net access blood flow rate Q_ais assumed to be $\frac{\partial V_{a}}{\partial t}$
and the injection inflow rate [0073] $\frac{\partial V_{i}}{\partial t} = Q_{i}$
is assumed to be uniform and steady. Therefore, [0074] $\begin{matrix} Q_{a} = \frac{Q_{i}}{\frac{Δ H}{H}} (the steady flow formulation) & (10) \end{matrix}$
In both cases, the quantity [0075] $\frac{Δ H}{H}$
(the percentage change in the access hematocrit) is measured as the indicator bolus is injected into the system. If the injection rate Q[0076] _iis uniform and constant, then the access flow Q_amay be determined from the steady flow formulation (10). Conversely, if the system remains dynamic and the bolus injection rate is uncertain or uncontrollable, then the transient solution (9) must be used to determine Q_a. In a practical sense, it may be easier to control the bolus volume as opposed to the rate of injection. In vitro tests have been conducted that show the two methods to be equivalent, as discussed below.
Determination of [0077] $F (\frac{Δ H}{H})$
The percentage change in blood parameters (both macroscopic and microscopic) passing through the access site can 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 [0078] $F (\frac{Δ H}{H}) .$
Inherent in all of these is the need to differentiate the access site, and parameter changes therein, from the surrounding tissue structure. Because of the complicating factors associated with the tissue structure of the access site, most previous methods (with exception of color Doppler) have generally been limited to extracorporeal evaluation within the dialysis delivery circuit which is attached to the access site during hemodialysis. The method in accordance with the present invention is not limited to the extracorporeal circuit but rather utilizes a transdermal [0079] optical sensor 10 positioned directly over the access site 12 itself and is based upon optical back-scattering of monochromatic light (λ=805 nm-880 nm) from the blood flow in the access site 12 and the surrounding tissues 14. As shown in FIG. 2, the sensor 10 incorporates complementary photoemitters (such as LEDs) 10 a and photodetectors 10 b, and may be of the type described in co-pending application Ser. No. 09/750,076, entitled “Sensor For Transcutaneous Measurement Of Access Blood Flow,” filed on even date herewith (Dec. 29, 2000), which is incorporated herein in its entirety.
Preferably, the [0080] optical sensor 10 comprises an LED of specific wavelength and a complementary photodetector. A wavelength of 805 nm-880 nm is used 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. As shown in FIG. 2, 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 a value of the media. [0081]
When the volume of tissue illuminated includes all or even part of the access, the resultant a 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[0082] _ofor 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.
The present technique is related to that used in transcutaneous oxygen saturation and hematocrit measurements and requires the use of optical physics and laws associated with optical determination of physiologic elements including hematocrit. [0083]
Modified Beer's Law [0084]
Numerous studies have shown that light back-scattered from a turbid tissue sample follows a modified form of Beer's Law, [0085]
I_measured≈I_oAe^−αd, where A≈α (11)
where I[0086] _ois the radiation intensity emitted from the LED, A is a complex function of d and a, d is the distance between the LED and detector, and a is the bulk optical attenuation coefficient. The a term is a function of the absorption and scattering nature of the tissue and has a strong dependence on hematocrit, and can be computed as: $\begin{matrix} α \approx \frac{- Ln (\frac{I_{measured}}{I_{o}})}{d} & (12) \end{matrix}$
or from equation (11), α in a polynomial form. [0087]
Compartmentalization of α[0088]
A transcutaneously measured a value is actually a prorated composite measure of all the absorption and scattering elements contained within the illuminated volume or “glowball” [0089] 16 of the emitter source 10 a (see FIG. 2), and typically includes the effects of tissue, water, bone, blood, and in the case of hemodialysis patients, the access site 12. In the determination of a, clearly only the blood flowing through the access site 12 is of interest. The task therefore becomes one of separating the effects of absorption and scattering of the access site 12 from that of surrounding tissue structure 14. Starting with the well known definition,
α=⇄{square root over (3K(K+S))} (13)
where K is the bulk absorption coefficient and S is the bulk scattering coefficient, and separating the access blood coefficients from non-access blood coefficients and rearranging terms, [0090]
X _b K _b≈α² −B _o (14)
where X[0091] _b=percentage of the access volume to the total volume illuminated
K[0092] _b=access blood absorption coefficient
B[0093] _o=composite of all the non-access region S, K coefficients
Now, letting B[0094] _o=α_o ², we have
X _b K _b=α²−α_o ² (15)
In equation (14), the access blood coefficient, K[0095] _b, is directly proportional to hematocrit (H), K_b=H·C. Therefore,
X _b ·H·C=X _b K _b=α²−α_o ² (16)
To determine α[0096] _o, measurements are made in areas near but not including the access site 12, as depicted in FIG. 2. If the tissue 14 is more or less homogenous, it is only necessary to make a single reference α_omeasurement. On the other hand, if a gradient in α_oexists in the area of interest (and this is typically the case in vivo) multiple measurements are made to establish the nature of the gradient and provide an averaged estimate of α_o.
Determination of [0097] $\frac{di}{i}$
The value of [0098] $\frac{di}{i}$
is defined as the time derivative of intensity I, normalized by I. This is expressed from equation (11) as: [0099] $\frac{di}{i} = X_{b} \cdot Δ K_{b} (d - \frac{1}{α}), where A \approx α$ $or, X_{b} \cdot Δ K_{b} = \frac{\frac{di}{i} α}{(d - \frac{1}{α})}$
which is proportional to ΔH. Hence, [0100] $\begin{matrix} X_{b} \cdot Δ H \cdot C = X_{b} \cdot Δ K_{b} = \frac{\frac{di}{i} α}{(d - \frac{1}{α})} & (17) \end{matrix}$
To determine [0101] $\frac{di}{i},$
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, I(t). The quantity [0102] $\frac{di}{i}$
is then calculated as [0103] $\begin{matrix} \frac{di}{i} = \frac{I_{baseline} - I (t)}{I_{baseline}} & (18) \end{matrix}$
Final Determination of [0104] $F (\frac{Δ H}{H})$
The value [0105] $F (\frac{Δ H}{H})$
is the ratio of equations (17) and (16), [0106] $\begin{matrix} F (\frac{Δ H}{H}) = \frac{\frac{di}{i}}{(d - \frac{1}{α}) (α^{2} - α_{o}^{2})} & (19) \end{matrix}$
Since d is fixed and known, [0107] $\frac{di}{i},$
α, and α[0108] _oare computed by equations (17) and (12). It is important to note that in the final ratio of $F (\frac{Δ H}{H}),$
the access blood proration value, X[0109] _b, cancels out. This removes access size and/or volume and depth dependence from the final result. Likewise, the $\frac{di}{i} and \frac{α}{α^{2} - α_{o}^{}}$
common mode ratios eliminate skin color variations, as known in pulse oximetry. [0110]

DESCRIPTION OF THE METHOD

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. Reversing the dialysis blood lines during the hemodialysis treatment permits application of indicator dilution by direct injection of the indicator into the venous dialysis tubing. The TQ[0111] _amethod that is the subject of the present invention permits a unique application of indicator dilution principles since the sensor 10 can detect a dilution signal downstream of the venous needle through the skin. This geometry permits determination of the vascular access flow rate without reversal of the dialysis blood lines.
The method in accordance with the present invention will now be described in connection with the measurement of hematocrit using a transcutaneous [0112] optical sensor 10. The general environment of hemodialysis and typical components are described in detail in U.S. Pat. No. 5,351,686, which is incorporated herein by reference in its entirety. Referring to FIG. 3, there is shown a diagrammatic view of a patient cardiovascular circuit 20 and associated in vivo dialysis circuit 30 in which a TQ_asensor 10 is placed to carry out the TQ_amethod of the invention.
The heart volume is denoted as [0113] 20 a, the access/artery connection is denoted as 20 b, the access/vein connection is denoted as 20 c, and the capillary bed venous pool is denoted as 20 d. As is conventional, the dialysis circuit 30 incorporates a dialyzer 30 a and a dialyzer (Q_b) pump 30 b upstream of the dialyzer 30 a; a saline drip bag 30 c is connected to the dialysis tubing circuit 30 through a first or arterial needle port 30 d inserted in the dialysis tubing circuit 30 on the arterial side between the arterial needle site 12 a and the dialyzer pump 30 b. Blood is taken out of a patient by a needle inserted into the hemodialysis access site 12 (a surgically implanted shunt or fistula) at the arterial needle site 12 a on the “arterial” or upstream side of the dialyzer circuit 30. The arterial needle is connected to an intake catheter, so that unclean blood flows from an artery in the patient through the intake catheter to the dialyzer pump 30 b. From the dialyzer pump 30 b, the blood flows to the input port of the dialyzer 30 a, where it is cleaned. The clean blood is returned to the patient by a return catheter connected to a needle inserted into the access site 12 at the venous needle site 12 b on the “venous” or downstream side of the dialyzer 30 a. The sensor 10 is positioned downstream of the diluent injection point and upstream of the fistula/vein connection 20 c.
As shown in FIG. 2, the [0114] sensor 10 comprises a light source (photoemitter, e.g., an LED) 10 a and a detector 10 b and is placed directly on the skin over the vascular access site 12 downstream of the venous dialysis needle. The sensor 10 emits light at a wavelength of 805 nm-880 nm, near the isobestic wavelength for hemoglobin, and can accurately determine the relative changes in hematocrit in the access blood flowing directly under the skin. The fraction of light absorbed by the blood flowing through the vascular access site 12, which has been reflected by the blood and underlying tissue 14 and can be detected by the sensor 10, is proportional to the hematocrit in the vascular access site 12 (and more particularly, to the ratio $\frac{Δ H}{H})$
as discussed above. [0115]
The [0116] sensor 10 outputs a signal proportional to the hematocrit in the vascular access site 12 (H_ao), from which the relative change in hematocrit in the vascular access site 12 can accurately be determined. This signal is recorded by a monitoring system 40 associated with the sensor 10. The monitoring system 40 can be a computer including a computer processor and memory, and output means such as a video monitor and printer (not shown).
In a first embodiment of the method, a stable (baseline) value Ha proportional to the hematocrit in the access is first obtained. Then, a known volume (V) of a reference diluent (for example, normal saline) is injected into the dialysis venous line upstream of the venous needle. The diluent reduces the hematocrit in the [0117] vascular access site 12 beneath the sensor 10 to a time-dependent hematocrit H(t) during the injection. Using the signals produced from the time the diluent is injected to the time the signal returns to the baseline value, Q_acan be calculated by the monitoring system using either the transient formulation (equation (9)) or the steady state formulation (equation (10)).
A method in accordance with a second embodiment of the present invention is a method of measuring TQ[0118] _abased on Q_iΔT (the steady flow formulation, a transit time). The sensor may be of the type shown in FIGS. 2, 8, or 14 of the copending application.
The TQ[0119] _adetector/emitter-set array is applied parallel to the flow and atop the access site 12 at H_aobetween the venous needle site 12 b and the access/vein connection 20 c. The TQ_aarray emitters can be spaced at 8, 16, 20, and 24 mm for averaging Q_aand better SNR. Saline injection is achieved via the drip bag 30 c and the dialyzer pump 30 b or via a syringe inserted into the arterial needle port 12 a. Before dialysis begins and while the AV circuit is primed with saline, the arterial line in the hemodialysis circuit is clamped off, the saline drip bag 30 c is opened, and two priming flushes of two seconds duration each are infused into the shunt flow at differing blood pump flow rates of Q_i1and Q_i2, such that the combined flow of diluted blood at H_aois:
Q=Q _a +Q _i (20)
For example, Q[0120] _i1=100 ml/min and Q_i2=400 ml/min.
Alternately, two infusions of Q[0121] _i1and Q_i2are pushed at either the arterial needle port 12 a or the venous needle port 12 b.
As each bolus combines with the shunt flow and passes the TQ[0122] _asensor at H_ao, the transit time ΔT of the combined diluted flow (Q=Q_a+Q_i) is measured at H_aoby the TQ_aarray.
For bolus 1: [0123] $\begin{matrix} Q_{a} + Q_{i1} = \frac{V}{Δ T_{1}} & (21) \end{matrix}$
and for bolus 2: [0124] $\begin{matrix} Q_{a} + Q_{i2} = \frac{V}{Δ T_{2}} & (22) \end{matrix}$
where V is the blood volume in the shunt between emitters spaced at d[0125] ₁and d₂.
Combining the two results, equations (21) and (22), and canceling V: [0126] $\begin{matrix} Q_{a} = \frac{(Q_{i2} Δ T_{2} - Q_{i1} Δ T_{1})}{(Δ T_{1} - Δ T_{2})} & (23) \end{matrix}$
Also, if V or the shunt size (diameter) is known (for example, if a new GORTEX shunt is used), a single Q[0127] _iinjection gives Q_a.
A method in accordance with a third embodiment of the present invention is a method of measuring TQ[0128] _ain a hemodialysis circuit based on prime TQ_a. In this embodiment, a hemodialysis circuit, the priming saline volume is used as a single 10 second saline bolus when Q_b=300 ml/min. Preferably, the TQ_asensor is as shown in FIG. 20 of the copending application, is 24 mm square, and employs an outboard sensor array capable of making both parallel and perpendicular measurements. The TQ_asensor is applied over the H_aoshunt area, preferably between the venous needle site 12 b and the access/vein connection 20 c, as shown in FIG. 3 in connection with the sensor 10.
Prior to the priming dilution, the TQ[0129] _asensor makes a perpendicular measurement of α of normal shunt flow. The (arterial) dialyzer Q_bpump 30 b is then run for 10 seconds to clear the saline. As the saline enters and mixes with the shunt flow, the second TQ_asensor 10 makes perpendicular measurements of the diluted shunt flow to determine $\frac{d i}{i}$
and α terms. [0130]
Outboard detectors and emitters determine α[0131] _oin the non-shunt, tissue area parallel to the shunt. Solving, $Q_{a} = \frac{\frac{k}{α \frac{di}{i}}}{(α - α_{o}) (d - \frac{1}{α})} = \frac{k}{F (\frac{Δ H}{H})}, where$
k is a gain factor due to the electronics and is a linear function of Q[0132] _band
d=the emitter-detector separation distance [0133]
In the method in accordance with the third embodiment, the measurement of α[0134] _ois straightforward, but its validity is dependent upon the degree of local tissue homogeneity. Also, the depth of the shunt generally requires at least 20 mm spacing between the emitter and the detector to enclose the shunt cross-section within the illuminated volume or “glowball” of the emitter and detector.
The method in accordance with the third embodiment can be used to corroborate the Q[0135] _imethod with Q_iΔT.
A method in accordance with a fourth embodiment of the present invention is a method of measuring TQ[0136] _abased on any of the embodiments; wherein a bolus is introduced via a direct shunt injection upstream of the measurement site.
A method in accordance with a fifth embodiment of the present invention is a method of measuring TQ[0137] _abased on AT, transit time. In this embodiment, a 24 mm square TQ_asensor array employs both outboard and parallel arrays, as shown in FIG. 27 of the copending application. A bolus is introduced either via a direct shunt injection at the arterial needle site 12 a or via a short arterial/venous circuit bolus from a drip bag 30 c. The perpendicular arrays measure the transit time of the bolus at H_aoperpendicular to the shunt or fistula. The outboard sensor arrays “size” the shunt or fistula diameter via a glowball interaction perpendicular to the shunt or fistula. Q_ais then directly calculated as from transit time (velocity) and cross-sectional area.
Referring now to FIG. 4, there is shown an in vitro cardiovascular-dialysis model that was constructed to test the TQ[0138] _amethod. In the in vitro model, a patient cardiovascular circuit 20′ was simulated using a 1 L central blood volume 20 a′, 4 L venous pool 20 d′, and a cardiac pump 20 e′ placed downstream of the central blood volume 20 a′. A PT FE access site 12 was placed in a shallow tunnel cut in a piece of chicken breast muscle and covered with 15 mm of chicken skin. To complete the simulation, a Q_apump 20 f′ was connected to the access site 12. A typical arterial-venous dialyzer circuit 30 with a dialyzer 30 a, a dialyzer (Q_b) pump 30 b, and a drip bag 30 c was connected to the access site 12 via arterial and venous needles at arterial and venous needle sites 12 a and 12 b, simulating hemodialysis treatment conditions. A saline injection pump 30 f was also provided.
The test protocol for the in vitro model comprised the following steps: [0139]
1. Attach the [0140] sensor 10 to a reference material with known α and determine all reference I values, i_o.
2. Attach the [0141] sensor 10 on the chicken skin in line with the access site 12.
3. Set the [0142] cardiac pump 20 e′ at 3000 ml/min.
4. Stop the Q[0143] _bpump 30 b, clamp both ends of the dialyzer and connect the venous needle to the saline injection pump 30 f. Set the saline injection pump 30 f at 400 ml/min.
5. Set the Q[0144] _apump 20 d′ to a pre-calibrated flow rate at scale “2”.
6. Start the data capturing process for 20 seconds and save the data into a file of the [0145] computer 40 associated with the sensor 10.
7. Five seconds after data capturing begins, inject saline from the [0146] saline drip bag 30 c into the access site 12 for 5 seconds through the injection pump 30 c.
8. Process the data with two different algorithms (the transient and the steady flow formulations) and calculate the Q[0147] _a.
9. Record the calculated Q[0148] _aresults from the computer 40.
10. Repeat step 5-9 with different pre-calibrated flow rate settings on the Q[0149] _apump 20 d′.
While the [0150] cardiac output pump 20 e′ was running at 4-6 L/min, the Q_apump 20 d′ was varied from 300 to 2200 ml/min. A small 25×30 mm TQ_asensor 10 was placed on top of the chicken skin directly over the access site 12 to measure the hematocrit approximately 25 mm downstream of the venous needle and a single 5 second bolus of saline was infused at 400 ml/min (Q_i) directly into the access site 12.
A full test matrix was performed using the model and methodology described in the preceding paragraph. The matrix consisted of independently varying Q[0151] _b, Q_a, Q_i, Q_h, saline bolus volumes, skin and tissue thickness, access materials, access sizes and depth, skin color (melanin), and sensor geometries (distance from venous needle). A graphical representation of the stated results is shown in FIGS. 5-12, which represent a composite of 34 individual runs conducted to validate the accuracy versus a known Q_apump as reference. FIG. 5 shows the results of varying skin thickness and access depth. FIG. 6 shows the results of varying access size. FIG. 7 shows the results of varying access materials. FIG. 8 shows the results of varying the distance of the sensor from the needle. FIG. 9 shows the results of varying the blood pump rates. FIG. 10 shows the results of varying the cardiac pump rate. FIG. 11 shows the results of varying the bolus volume. FIG. 12 shows the results of varying the melanin content.
The average TQ[0152] _ameasurement error over the Q_arange of 360 to 2200 ml/min was ±1.4%, n=34, R=0.99, p<0.001 and y=0.98+51. TQ_ameasurements were independent of Q_b, Q_haccess size or material, or chicken skin thickness and melanin content (which were varied during the tests).
The TQ[0153] _amethod in accordance with the present invention yielded very good in vitro results under a variety of conditions. The robustness of the optical approach to measure the $F (\frac{Δ H}{H})$
continuously from the saline dilution mitigated or virtually eliminated all of the traditional difficulties associated with present ABF measurements. Size and depth of the access site, placement of the dialysis needles, pump speed variations, skin color, tissue composition, and access site location and type, all had negligible or no effect on the TQ[0154] _areading, as discussed below.
A. Size and Depth of Access [0155]
Access measurements such as Doppler imaging accurately measure the velocity of the blood through the access site but are limited by volumetric uncertainties. The volume of the access site area under measure is either estimated or inferred from the image. In either case this becomes a major source of error. Additionally, geometric concerns associated with the uncertainty of the depth of the access site below the skin tend to exacerbate the volumetric estimate. However, the TQ[0156] _aapproach of the present invention eliminates this problem by using a percentage change of hematocrit in the access site during the dilution. As shown above in the discussion of the model and theory of the present invention, the size and depth-prorating variable, X_b, is ratiometrically eliminated. This cancellation makes the TQ_ameasurement impervious to variations in access site, size, and location. It is only necessary that a portion of the access site 12 be contained within the field of view of the optical sensor 10, as shown in FIG. 2.
B. Placement of Dialysis Needles [0157]
In reverse line ABF measurements, needle placement can become problematic. As fluid is drawn in from and returned to the access site it is susceptible to potential streaming under the laws of classical fluid dynamics, i.e. laminar flow. This streaming effect causes a certain portion of an injected bolus to pass through the system undetected and greatly bias the ABF measurement. Typically, this problem is addressed by assuring adequate needle separation and placement. The problem is in determining what is adequate and then in having sufficient latitude in needle placement under the constraints of patient physiology (that is, sufficient access length). Also needle orientation relative to the direction of blood flow can greatly affect the streaming and measurement error. [0158]
As indicated in the discussion of the model and theory of the invention, transcutaneous optical measurements indicate a net effect of all the tissue contained within the optical view. In essence, the various absorption and scattering effects of the tissue constituents are optically integrated over the entire illuminated volume, α or α[0159] _o. Therefore, this optical integration eliminates the effect of streamlines and poor mixing within the shunt, since the entire region is integrated. Again it is only necessary that a portion of the access site 12 be contained within the field of view of the optical sensor 10, as shown in FIG. 2.
C. Pump Speed Variations [0160]
Measurements that rely on the interaction of the dialysis circuit with the patient's access site such as reversed line recirculation are dependent upon pump speed variations. The forward and reverse line AH method is dependent upon two ultrafiltration rate variations. In addition, all of these methods require that the patient be on dialysis while the ABF measurement is made. [0161]
However, because the measurement is made directly over the access site, the TQ[0162] _amethod does not even require the dialysis circuit. When present, the arterial or venous line may be used to provide an access port for the saline bolus injection, but a direct injection into the access site works equally well. This independence from the dialysis machine (and Q_b) allows greater flexibility in making ABF measurements during the interdialytic periods, in a physician's office, or in emergency situations.
D. Skin Color, Tissue Composition and Synthetic Grafts [0163]
Many studies have been conducted on the optical properties of human tissues. The result of these studies is an emerging spectral picture of how monochromatic light interacts with the various biological constituents. The key to success in making optical transcutaneous measurements is the appropriate selection of the wavelength for the desired biological constituent. For example, 660 nm and 805 nm wavelengths are desirable for pulse oximetery because of their oxygen saturation dependence and isobestic properties, respectively. [0164]
An 880 nm wavelength of light has shown a strong hematocrit dependence and it also affords good tissue penetration depth. 880 nm light is also able to penetrate hemodialysis synthetic PTFE grafts such as GORTEX. Skin color and skin blemishes, such as scars, have been shown to have little effect at 880 nm, especially when [0165] $\frac{di}{i} \cdot α$
a is the major mathematical operator, skin color is eliminated. Additionally, as explained above, alpha common mode separation, α[0166] ²−α_o ², eliminates any effects of skin color, tissue composition, and synthetic graft materials.
E. Access Location and Type [0167]
Generally dialysis patient access sites are grouped into two categories, native fistula and synthetic grafts. These access sites are typically located in the upper or lower arm but occasionally access sites are in legs as well. Sub-clavian catheter access sites [?] are not considered in TQ[0168] _adiscussions. The type and location of the access site does not effect the TQ_ameasurement to the extent that the access site location is discernable or palpable to the person placing the sensor 10 over the access site.
Finally, the results demonstrate that the TQ[0169] _amethod in accordance with the invention has potential for highly accurate and reproductive Q_ameasurements without line reversals. Because the saline bolus can be injected directly into the access site, it is also possible to routinely measure Q_ainterdialytically.
Modifications and variations of the above-described embodiments of the present invention are possible, as appreciated by those skilled in the art in light of the above teachings. For example, the method in accordance with the present invention generally contemplates measuring a parameter transcutaneously in a perturbed system downstream of a site where the perturbation is introduced. The parameter can, for example, comprise a marker and the method comprises measuring the marker downstream using a sensor. Possible markers are proteins or red cells tagged with a radio nucleotide, which can be measured using a Geiger counter. Other parameters that can be measured in accordance with the present invention, and the devices for measuring them are ultrasound, measured by an ultrasound detector; temperature, measured by a thermistor; impedance, measured by a bio-impedance measuring device; and albumen, glucose, and other blood constituents, measured using the optical sensor disclosed herein, but in which the LEDs emit different wavelengths suited to the specific constituent. [0170]
Further, the detector-emitter arrangement of the [0171] sensor 10 shown in FIG. 2 allows 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. In this connection, the sensor 10 is placed directly on the skin over the approximate area of the access, graft, or vein, and values of α, α_o1, and α_o2are calculated as described above. A reference ratio, RR, is developed, where: $RR = (1 - \frac{α_{o1}}{α_{02}}) \times 100$
When RR<±15, then the access or graft or vein is “centered” correctly or found between the [0172] inboard LED 10 a and the inboard detector 10 b. Also, a signal strength (SS) indicator advises the user whether a sufficient signal is present for an accurate measurement, where $SS = [(α - (\frac{α_{o1} + α_{o2}}{2})] \times 100$
When SS>40, then a sufficient amount of the access or graft or vein is within the illuminated volume of tissue. If RR is not <±15 (that is, if RR≧2±15), or if SS is not >40 (that is, if SS is ≦40), then the [0173] sensor 10 is moved right or left (+ or −) to find the appropriate spot or location.
It is therefore to be understood that, within the scope of the appended claims and their equivalents, the invention may be practiced otherwise than as specifically described. [0174]

Claims

What is claimed is:

1. A method of measuring a blood parameter transcutaneously in the vascular system of a patient, the vascular system having a vascular access site and a needle site in the vascular access site, comprising the steps of:

perturbing a region of the vascular system;

using a sensor placed on the skin of a patient over the access site downstream of the needle site to transcutaneously measure the perturbation over a predetermined period of time at a measurement site downstream of the perturbation region in the vascular system; and

calculating the blood parameter based on the measured perturbation.

2. The method of claim 1, wherein the region that is perturbed is a vascular access site, and the perturbation is accomplished by injecting a marker into an upstream end of the vascular access site.

3. The method of claim 2, wherein the marker is a saline solution.

4. The method of claim 2, wherein the marker is tagged red blood cells.

5. The method of claim 1, wherein the region that is perturbed is a vascular access site, and the perturbation is accomplished by changing a parameter of the blood.

6. The method of claim 1, wherein the blood parameter is macroscopic.

7. The method of claim 1, wherein the blood parameter is microscopic.

8. A method of transcutaneously measuring access blood flow comprising the steps of:

infusing a specific volume (Vi) of an indicator diluent into a patient cardiovascular circuit at an access site in the presence of a hemodialysis circuit to effect a change in a blood parameter; and

using a transdermal sensor to measure the percent change in the parameter at a measurement site downstream of where the indicator is infused.

9. The method of claim 8, wherein the blood parameter is selected from the group consisting of bulk density, flow energy, hematocrit, and red cell oxygen content.

10. The method of claim 8, wherein the needle site comprises an arterial needle site and a venous needle site downstream of the arterial needle site, and wherein the transdermal sensor is placed between the venous needle site and the access site/vein connection.

11. The method of claim 8, wherein the blood parameter is macroscopic.

12. The method of claim 8, wherein the blood parameter is microscopic.

13. The method of claim 8, wherein the blood parameter is macroscopic.

14. The method of claim 8, wherein the blood parameter is microscopic.

15. A method of transcutaneously measuring access blood flow in a patient cardiovascular circuit having a vascular access site and a needle site in the vascular access site, comprising the steps of:

infusing a specific volume (V_i) of an indicator diluent into a patient cardiovascular circuit at an access site in the absence of a hemodialysis circuit to effect a change in a blood parameter; and

using a transdermal sensor placed on the skin of the patient over the access site downstream of the needle site to measure the percent change in the parameter.

16. The method of claim 15, wherein the blood parameter is selected from the group consisting of bulk density, flow energy, hematocrit, and red cell oxygen content.

17. The method of claim 15, wherein the transdermal sensor is placed over the access site.

18. A method of transcutaneously measuring access blood flow through the measurement of percentage change in a blood parameter, in a hemodialysis circuit including a vascular access site having an arterial needle site and a venous needle site downstream of the arterial needle site, and a dialyzer having an inlet and an outlet, a dialysis arterial line connecting the dialyzer inlet to the arterial needle site, and a dialysis venous line connecting the dialyzer outlet to the venous needle site, the method comprising the steps of:

placing over the vascular access site downstream of the venous needle site a transdermal sensor capable of accurately determining the relative changes in a blood parameter in the access blood flowing directly under the skin;

infusing a specific volume (V_i) of an indicator diluent into the patient cardiovascular circuit at one of the arterial line and the venous line upstream of the sensor to effect a change in the blood parameter; and

using the transdermal sensor to measure the percent change in the parameter.

19. The method of claim 18, wherein the blood parameter is selected from the group consisting of bulk density, flow energy, hematocrit, and red cell oxygen content.

20. The method of claim 18, wherein the transdermal sensor is placed directly on the skin between the venous line and the access site/vein connection.

21. The method of claim 18, wherein the blood parameter is macroscopic.

22. The method of claim 18, wherein the blood parameter is microscopic.

23. A method of transcutaneously measuring access blood flow in a hemodialysis circuit through the measurement of percentage change in a blood parameter, the method comprising the steps of:

infusing a specific volume (V_i) of an indicator diluent into the patient cardiovascular circuit through an arterial or venous line by turning the ultrafiltration of the dialysis delivery system from OFF to ON and OFF again over a predetermined time period to effect a change in a blood parameter; and

using a transdermal sensor placed on the skin of the patient over the vascular access downstream of the arterial and venous lines to measure the percent change in the parameter.

24. The method of claim 23, wherein the blood parameter is selected from the group consisting of bulk density, flow energy, hematocrit, and red cell oxygen content.

25. The method of claim 23, wherein the transdermal sensor is placed over a vascular access site of the hemodialysis circuit.

26. The method of claim 23, wherein the blood parameter is macroscopic.

27. The method of claim 23, wherein the blood parameter is microscopic.

28. A method of transcutaneously measuring access blood flow in a hemodialysis circuit including a vascular access site having a needle site, the hemodialysis circuit including a hemodialysis pump and employing a priming saline volume, the method comprising the steps of:

infusing the priming saline volume into the hemodialysis circuit by turning on the hemodialysis pump; and

29. The method of claim 28, wherein the blood parameter is selected from the group consisting of bulk density, flow energy, hematocrit, and red cell oxygen content.

30. The method of claim 28, wherein the vascular access site comprises an arterial needle site and a venous needle site downstream of the arterial needle site and wherein the transdermal sensor is placed between the venous needle site and the access site/vein connection.

31. The method of claim 28, wherein the blood parameter is macroscopic.

32. The method of claim 28, wherein the blood parameter is microscopic.

33. A method of transcutaneously measuring access blood flow through the measurement of percentage change in a blood parameter, in a hemodialysis circuit having a vascular access site that includes an arterial needle site and a venous needle site downstream of the arterial needle site, a dialyzer having an inlet and an outlet, a dialysis arterial line connecting the dialyzer inlet to the arterial needle site, and a dialyzer venous line connecting the dialyzer outlet to the venous needle site, the method comprising the steps of:

placing on the skin of the patient downstream of the venous needle site a sensor having a field of view so that at least a portion of the access site is within the field of view of the sensor and that a portion of the tissue surrounding the access site occupies the remainder of the field of view;

using the sensor to output a first signal containing information about both the surrounding tissue and the portion of the access site;

using the sensor to output a second signal containing information about the surrounding tissue;

mathematically manipulating the first and second signals to provide a stable baseline value of the blood parameter in the vascular access site;

after obtaining the baseline value of the blood parameter in the vascular access site, injecting a diluent into the access site upstream of the sensor at an average flow rate;

monitoring the first and second signals from the time the diluent is injected until the time the signal returns to the baseline value; and

calculating access blood flow based on the monitored first and second signals.

34. The method of claim 33, wherein the blood parameter chosen to calculate access blood flow is Hematocrit.

35. The method of claim 33, wherein the reference diluent is injected at a predetermined average flow rate.

36. The method of claim 33, wherein the reference diluent is injected at a uniform and constant flow rate.

37. The method of claim 33, wherein the reference diluent is introduced into dialysis venous line.

38. The method of claim 33, wherein the reference diluent is introduced through the dialysis arterial line.

39. A method of transcutaneously measuring access blood flow in a hemodialysis circuit, the hemodialysis circuit including a dialyzer, a dialyzer pump upstream of the dialyzer, a hemodialysis access site previously established in a patient for the purpose of carrying out dialysis and having a vein connection and an artery connection, an arterial needle inserted into the hemodialysis access site at an arterial needle site on the arterial, upstream side of the hemodialysis circuit, an intake catheter connecting the arterial needle to the dialyzer pump, a venous needle inserted into the hemodialysis access site at a venous needle site on the venous, downstream side of the dialyzer, the hemodialysis circuit employing a priming saline volume, the method comprising the steps of:

using a transdermal sensor placed over the access site between the venous needle site and the access/vein connection to measure the percent change in the parameter.

40. The method of claim 39, wherein the blood parameter is selected from the group consisting of bulk density, flow energy, hematocrit, and red cell oxygen content.

41. The method of claim 39, wherein the priming saline volume is used as a single 10 second saline bolus when the dialyzer pump flow rate Qb=300 ml/min.