Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
2000-12-29
2004-06-08
Nasser, Robert L. (Department: 3736)
Surgery
Diagnostic testing
Cardiovascular
C604S004010
Reexamination Certificate
active
06746407
ABSTRACT:
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
a
rates 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
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.
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 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
, d
2
) for the purpose of measuring the bulk absorptivity (&agr;) of the area immediately surrounding and including the access site, and the absorptivity (&agr;
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 illumin
Bell David A.
Miller David R.
Steuer Robert R.
Hema Metrics, Inc.
Jacobson & Holman PLLC
Nasser Robert L.
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