Surgery – Diagnostic testing – Measuring or detecting nonradioactive constituent of body...
Reexamination Certificate
2002-03-19
2004-10-12
Winakur, Eric F. (Department: 3736)
Surgery
Diagnostic testing
Measuring or detecting nonradioactive constituent of body...
C600S322000
Reexamination Certificate
active
06804543
ABSTRACT:
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 (“TQA”) that is capable of generating accurate TQA measurements, even when the volume of access being measured is extremely small in size or extremely deep or when the access is of varying nature, such as a synthetic or native fistula. Further, it is possible to infer additional information about the access area, such as collateral veins or competing vessels.
BACKGROUND OF THE INVENTION
Access blood flow for hemodialysis patients can now be measured non-invasively through a novel photo-optic transcutaneous technique as described in co-pending application Ser. No. 09/750,122, filed Dec. 29, 2000 (which is incorporated herein by reference in its entirety), using a transcutaneous TQA sensor as disclosed in application Ser. No. 09/750,076, filed Dec. 29, 2000 (which is also incorporated herein by reference in its entirety), and more particularly, the transcutaneous TQA sensor described in connection with
FIGS. 2-6
thereof (hereinafter, “the prior art linear sensor”).
With reference to
FIGS. 1
,
2
, and
2
A, the prior art linear sensor
10
includes two light emitting sources (emitters)
12
a
and
12
b
, preferably light emitting diodes (LEDs) of specific wavelengths, and two complementary silicon photodiode detectors
14
a
and
14
b
alternatingly arranged in a straight line at identical intervals to form three LED/detector pairs with identical separations between the members of each pair, for the purpose of measuring the bulk absorptivity (&agr;) of the volume immediately surrounding and including the access site A, and the absorptivity (&agr;
o
) of the tissue itself. The LEDs preferably emit 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.
The technique is accomplished by directly placing the prior art linear sensor
10
on the skin of a patient with the aligned emitters
12
a
and
12
b
and detectors
14
a
and
14
b
perpendicular to the vascular access site A, and measuring the back-scattered light from a turbid tissue sample to determine the percentage change in hematocrit &Dgr;H as a bolus of saline passes through the access vessel.
When the prior art linear sensor
10
is placed on the surface of the skin, each LED
12
a
and
12
b
illuminates a volume of tissue T, and a small fraction of the light absorbed and back-scattered by the tissue and red blood cells is detected by its adjacent photodetector
14
a
or
14
b
, which generates a detection signal. When the volume of tissue illuminated includes all or even part of the access A, the resultant &agr; value includes information about both the surrounding tissue T and the access itself. In order to resolve the signal due to blood flowing within the access A from that due to the surrounding tissues T, the prior art linear sensor
10
illuminates adjacent tissue regions T on either side of the access A. Values of &agr;
o
for tissue regions T not containing the access A 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 intensity of the signal produced by each photodetector
14
A or
14
B is proportional to the total absorption and scattering within a given volume of tissue between each detector
14
a
or
14
B and its adjacent LED
12
a
or
12
b
. During saline dilution, only the hematocrit inside the access A varies, and the detected signal changes are solely dependent upon the optical property changes within the small volume of access viewed by the sensor
10
.
By correcting the signal in the volume containing the access A with the average reference signal in the volumes without access, the sensor
10
provides a signal solely dependent on the hematocrit flowing in the access. Then, traditional Ficke principle mathematics can be used to calculate the blood flow rate using the following equation:
Q
a
=
V
∫
Δ
H
(
t
)
H
a
ⅆ
t
For a given separation between LED and photodiode in the sensor
10
, the volume of tissue illuminated and viewed by the prior art linear sensor
10
is relatively constant and the signal-to-noise ratio of this technique depends on the volume of access included inside the tissue volume. When the volume of access included inside the tissue volume is small enough due to extremely small size or excessive depth, the signal-to-noise ratio falls to a level that would not generate accurate measurement results. It would accordingly be desirable to improve the signal-to-noise ratio so that accurate measurements can be taken even when the access is extremely small or very deep.
According to W. Cui (“Photon Diffusion Theory and Noninvasive Tissue Optical Property Measurement,” PhD. Thesis, Biomedical Engineering Department, Rensselaer Polytechnic Institute (1990)), the principle path of diffused photons in a turbid medium is in the gradient direction of the photon density distribution, which is perpendicular to the contour surfaces. Along this direction, photons consistently travel all the way from the LED to the detector in a curved path. In a later study, W. Cui et al. (“Experimental Study of Migration Depth for the Photons Measured at Sample Surface,”
SPIE
, Vol. 1431, pp 180-191 (1991)) further showed that the photon flux path from LED to detector has a “banana” shape that reaches deepest into the tissue at the mid-portion of the “banana.” More significantly, in this “banana”-shaped photon path, there is a region in the middle between LED and detector near the tissue surface that is totally outside the detected photon flux path. This means that anything in this region will not interact with the photons that reach the detector and will never be “seen” by the detector. This finding was verified by S. Feng et al. (“Monte Carlo Simulations of Photon Migration Path Distributions in Multiple Scattering Media,”
SPIE
, Vol. 1888, pp 78-89 (1993)), using both analytical perturbative diffusion theory and Monte Carlo simulations. This phenomenon also explains the clinical observations that with a visually observable shallow graft, no significant difference in &agr; is detected with the injection of a saline bolus.
The configuration of the prior art linear sensor
10
allows it (or more precisely, the aligned LEDs
12
a
and
12
b
and the detectors
14
a
and
14
b
) to be perpendicular to the access A and the photon flux F to travel across the access to generate an illuminated volume of access within the illuminated tissue volume, as shown in
FIGS. 1 and 2
. For a graft in the center of the photon flux path F, the volume of the access viewed by the prior art linear sensor
10
is limited to the cross-section of the graft and the photon flux path F as indicated by
FIGS. 1 and 2
. For a graft that is nearly out of the photon flux path F (because it is too shallow, as shown in
FIG. 2A
, or too deep) the volume of access “seen” by the prior art linear sensor
10
is so small that the signal-to-noise ratio is too low to give accurate measurements.
It is to the solution of this and other problems that the present invention is directed.
BRIEF SUMMARY OF THE INVENTION
It is therefore a primary object of the present invention to provide apparatus for non-invasively measuring one or more blood parameters associated with a vascular access, even when the volume of access being measured is extremely small in size or extremely deep.
It is another object of the present invention to provide a sensor for transcutaneous TQA measurement that is capable of generating accurate TQA measurements, even when the volume of access being measured is extremely small in size or extremely deep.
This and
Bell David A.
Cox Douglas L.
Miller David R.
Zhang Songbiao
Hema Metrics, Inc.
Jacobson & Holman PLLC
McCrosky David J.
Winakur Eric F.
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