Method and apparatus for improving the accuracy of noninvasive hematocrit measurements

• US 20020165439A1
• Filed: 03/16/2001
• Published: 11/07/2002
• Est. Priority Date: 03/16/2001
• Status: Active Grant

• Current Assignee: Covidien LP (Medtronic Plc)
• Original Assignee: Nellcor Puritan Bennett Incorporated (Medtronic Plc)
• Inventors: Schmitt, Joseph M.
• Granted Patent: US 6,606,509 B2
• Time in Patent Office: Days
• US Class Current: 600/309
• CPC Class Codes: A61B 5/0059 using light, e.g. diagnosis by transillumination, diascopy, fluorescence A61B5/0093, A61B5/1455, A61B5/02416 and A61B5/0261 take precedence
• CPC Class Codes: A61B 5/022 by applying pressure to close blood vessels, e.g. against the skin; Ophthalmodynamometers
• CPC Class Codes: A61B 5/14535 for measuring haematocrit
• CPC Class Codes: A61B 5/14546 for measuring analytes not otherwise provided for, e.g. ions, cytochromes
• CPC Class Codes: A61B 5/6826 Finger
• CPC Class Codes: A61B 5/6838 Clamps or clips
• CPC Class Codes: G01N 21/359 using near infrared light
• CPC Class Codes: G01N 33/721 Haemoglobin

Abstract

A device and a method to provide a more reliable and accurate measurement of hematocrit (Hct) by noninvasive means. The changes in the intensities of light of multiple wavelengths transmitted through or reflected light from the tissue location are recorded immediately before and after occluding the flow of venous blood from the tissue location with an occlusion device positioned near the tissue location. As the venous return stops and the incoming arterial blood expands the blood vessels, the light intensities measured within a particular band of near-infrared wavelengths decrease in proportion to the volume of hemoglobin in the tissue location; those intensities measured within a separate band of wavelengths in which water absorbs respond to the difference between the water fractions within the blood and the displaced tissue volume. A mathematical algorithm applied to the time-varying intensities yields a quantitative estimate of the absolute concentration of hemoglobin in the blood. To compensate for the effect of the unknown fraction of water in the extravascular tissue on the Hct measurement, the tissue water fraction is determined before the occlusion cycle begins by measuring the diffuse transmittance or reflectance spectra of the tissue at selected wavelengths.

Claims

1. A device for measuring hematocrit values using optical spectrophotometry comprising:

a probe housing configured to be placed proximal to a tissue location which is being monitored;

an occlusion device connected to said housing and configured to magnify the fractional change in the vascular blood volume to a value greater than said fractional change produced by normal arterial pulsations;

light emission optics connected to said housing and configured to direct radiation at said tissue location;

light detection optics connected to said housing and configured to receive radiation from said tissue location;

a processing device connected to said housing and configured to process radiation from said light emission optics and said light detection optics to compute said hematocrit values.

2. The device of claim 1, further comprising a display device connected to said probe housing and configured to display said hematocrit values.

3. The device of claim 1, further comprising means for measuring hemoglobin.

4. The device of claim 1, further comprising a pressure transducer connected to said housing and positioned to be placed adjacent to said tissue location, and configured to interface with said processing device and said occlusion device to provide an input to said processing device to control the timing of the data acquisition by said processing device.

5. The device of claim 1, wherein said light emission optics are tuned to emit radiation at at least a first and a second wavelength, where said at least first wavelength is within a band of wavelengths where hemoglobin is the dominant absorber and where said at least second wavelength is within a band of wavelengths where water is the dominant absorber.

6. The device of claim 1, wherein said light emission optics are tuned to emit radiation at at least a first and a second wavelength, where said at least first wavelength is within a band of wavelengths in the range of approximately 800-1000 nm and where said at least second wavelength is within a band in the range of approximately 1250-1600 nm.

7. The device of claim 1, wherein said light emission optics are tuned to emit radiation at at least a first and a second wavelength, where said first and said second wavelengths are related to each other such that 0.34Hμ

_a^Hb>

>

μ

_a^w at said first wavelengths and μ

_a^w>

>

0.34Hμ

_a^Hb at said second wavelength, where

H is the hematocrit value,

μ

_a^Hb is the sum of the absorption coefficient of the two forms of hemoglobin, and

μ

_a^w is the absorption coefficient of water.

8. The device of claim 1, wherein said light emission optics are tuned to emit radiation at at least a first and a second wavelength, where said at least first wavelength is in a range approximately between and including 805 to 850 nm and said at least second wavelength is in a range approximately between and including 1310 to 1370 nm.

9. The device of claim 1, wherein said light emission optics and said light detection optics are mounted within said probe housing and positioned with appropriate alignment to enable detection in a transmissive mode.

10. The device of claim 1, wherein said light emission optics and said light detection optics are mounted within said probe housing and positioned with appropriate alignment to enable detection in a reflective mode.

11. The device of claim 1, wherein said light emission optics and said light detection optics are placed within a remote unit and which deliver light to and receive light from said probe housing via optical fibers.

12. The device of claim 1, wherein said light emission optics comprise at least one of a (a) incandescent light source, (b) narrowband light source, wherein a narrowband light source comprises one of a light emitting diode (“

LED”

) and a filtered white light source.

13. The device of claim 1, wherein said processing device receives at least two sets of optical measurements, where the at least first set of optical measurements corresponds to the detection of light whose absorption is primarily due to hemoglobin, and where the at least second set of optical measurements corresponds to the detection of light whose absorption is primarily due to water, and where a combination of said at least two sets of optical measurements provides a measure of said hematocrit value.

14. The device of claim 1, wherein said processing device:

receives at least two sets of optical measurements at an at least a first and a second wavelength, where for each wavelength two optical measurements are obtained corresponding to measurements before and after a venous occlusion conducted by said occlusion device to obtain before and after occlusion measurements at each wavelength;

combines said before and after occlusion measurements at each wavelength to determine a blood pulse spectrum at each wavelength;

combines said blood pulse spectra at each wavelength to obtain a ratio of said blood pulse spectra; and

combines said ratio with measurements of tissue water fractions to determines the concentration of hemoglobin in the blood.

15. The device of claim 1, wherein said processing device determines hematocrit based on optical measurements such that

17. The device of claim 16, further comprising a pressure transducer connected to said housing and positioned to be placed adjacent to said tissue location, and configured to interface with said processing device and said occlusion device to provide an input to said processing device to control the timing of the data acquisition by said processing device.

18. The device of claim 16, wherein said light emission optics are tuned to emit radiation at at least a first and a second wavelength, where said at least first wavelength is within a band of wavelengths in the range of approximately 800-1000 nm and where said at least second wavelength is within a band in the range of approximately 1250-1600 nm.

20. The method of claim 19, wherein said first wavelength is chosen from within a band of wavelengths in the range of approximately 800-1000 nm and where said second wavelength is chosen from within a band of wavelengths in the range of approximately 1350-1600 nm.

21. The method of claim 19, wherein said first and said second wavelengths are related to each other such that 0.34Hμ

_a^Hb>

>

μ

_a^w at said first wavelengths and μ

_a^w>

>

0.34Hμ

_a^Hb at said second wavelength, where

H is the hematocrit value,

μ

_a^Hb is the sum of the absorption coefficient of the two forms of hemoglobin, and

μ

_a^w is the absorption coefficient of water.

22. The method of claim 19, wherein said first and second wavelengths are approximately between and including 805 to 850 nm and approximately between and including 1310 to 1370 nm respectively.

24. The method of claim 23, wherein said first wavelength is chosen from within a band of wavelengths in the range of approximately 800-1000 nm and where said second wavelength is chosen from within a band of wavelengths in the range of approximately 1350-1600 nm.

25. The method of claim 23, wherein said first and said second wavelengths are related to each other such that 0.34Hμ

_a^Hb>

>

μ

_a^w at said first wavelengths and μ

_a^w>

>

0.34Hμ

_a^Hb at said second wavelength, where

H is the hematocrit value,

μ

_a^Hb is the sum of the absorption coefficient of the two forms of hemoglobin, and

μ

_a^w is the absorption coefficient of water.

26. The method of claim 23, wherein said first and second wavelengths are approximately between and including 805 to 850 nm and approximately between and including 1310 to 1370 nm respectively.

16. A device for measuring hematocrit values using optical spectrophotometry comprising:

a probe housing configured to be placed proximal to a tissue location which is being monitored;

an occlusion device connected to said housing and configured to magnify the fractional change in the vascular blood volume to a value greater than said fractional change produced by normal arterial pulsations;

light emission optics connected to said housing and configured to direct radiation at said tissue location, wherein said light emission optics comprise at least one of a (a) incandescent light source, (b) white light source and (c) light emitting diodes (“

LEDs”

) which are tuned to emit radiation at at least a first and a second wavelength, where said at least first wavelength is within a band of wavelengths where hemoglobin is the dominant absorber and where said at least second wavelength is within a band where water is the dominant absorber;

a photodiode connected to said housing and configured to receive radiation from said tissue location;

a processing device connected to said housing and configured to process radiation from said light emission optics and said light detection optics to compute said hematocrit values, wherein said processing device;

receives at least two sets of optical measurements at an at least a first and a second wavelength, where for each wavelength two optical measurements are obtained corresponding to measurements before and after a venous occlusion conducted by said occlusion device, to obtain before and after occlusion measurements at each wavelength;

combines said before and after measurements at each wavelength to determine a blood pulse spectrum at each wavelength;

combines said blood pulse spectra at each wavelength to obtain a ratio of said blood pulse spectra;

combines said ratio with measurements of tissue water fractions to determines the blood hematocrit value, such that

$\frac{1}{H}\cong <br><br>\frac{0.34}{1-f_w-f_{p<br><br>\text{\hspace{1em}<br><br>}<br><br>p}}<br><br>\left(1+R<br><br>\frac{{\mathrm{\mu <br><br>}}_a^{H<br><br>\text{\hspace{1em}<br><br>}<br><br>b}<br><br>\left({\mathrm{\lambda <br><br>}}_1\right)+\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\mathrm{\mu <br><br>}}_s<br><br>\left({\mathrm{\lambda <br><br>}}_1\right)}{{\mathrm{\mu <br><br>}}_a^w<br><br>\left({\mathrm{\lambda <br><br>}}_2\right)+\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\mathrm{\mu <br><br>}}_s<br><br>\left({\mathrm{\lambda <br><br>}}_2\right)}\right),\mathrm{where};<br><br>$

H is the hematocrit value;

ƒ

_w is the tissue water fraction;

ƒ

_pp is the plasma protein fraction;

R is the ratio of magnitudes of the blood pulse spectrum;

μ

_a^Hb(λ

₁) is the sum of the absorption coefficient of the two forms of hemoglobin at a first wavelength;

μ

_a^w(λ

₂) is the absorption coefficient of water at a second wavelength;

Δ

μ

_s(λ

₁) is the difference between the scattering coefficients of the blood and surrounding tissue at a first wavelength;

Δ

μ

_s(λ

₂) is the difference between the scattering coefficients of the blood and surrounding tissue at a second wavelength; and

0.34 is the fraction of the red cell volume occupied by hemoglobin, which is assumed to be constant.

19. A method of measuring a percent hematocrit near a tissue location using optical spectrophotometry comprising:

placing a probe housing proximal to said tissue location;

occluding the venous blood flow adjacent to said tissue location;

emitting radiation at at least two wavelengths using light emission optics configured to direct radiation at said tissue location;

detecting radiation using light detection optics configured to receive radiation from said tissue location;

processing said radiation from said light emission and said light detection optics using a processing device;

computing said percent hematocrit, where said percent hematocrit is determined by;

receiving at least two sets of optical measurements at an at least a first and a second wavelength, where for each wavelength two optical measurements are obtained corresponding to measurements before and after a venous occlusion, to obtain before and after occlusion measurements at each wavelength;

combining said before and after measurements at each wavelength;

determining a blood pulse spectrum at each wavelength;

obtaining a ratio of said blood pulse spectra;

combining said ratio with measurements of tissue water fractions; and

determining said percent hematocrit value such that

$\frac{1}{H}\cong <br><br>\frac{0.34}{1-f_w-f_{p<br><br>\text{\hspace{1em}<br><br>}<br><br>p}}<br><br>\left(1+R<br><br>\frac{{\mathrm{\mu <br><br>}}_a^{H<br><br>\text{\hspace{1em}<br><br>}<br><br>b}<br><br>\left({\mathrm{\lambda <br><br>}}_1\right)+\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\mathrm{\mu <br><br>}}_s<br><br>\left({\mathrm{\lambda <br><br>}}_1\right)}{{\mathrm{\mu <br><br>}}_a^w<br><br>\left({\mathrm{\lambda <br><br>}}_2\right)+\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\mathrm{\mu <br><br>}}_s<br><br>\left({\mathrm{\lambda <br><br>}}_2\right)}\right),\mathrm{where};<br><br>$

H is the hematocrit value;

ƒ

_w is the tissue water fraction;

ƒ

_pp is the plasma protein fraction;

R is the ratio of magnitudes of the blood pulse spectrum;

μ

_a^Hb(λ

₁) is the sum of the absorption coefficient of the two forms of hemoglobin at a first wavelength;

μ

_a^w(λ

₂) is the absorption coefficient of water at a second wavelength;

Δ

μ

_s(λ

₁) is the difference between the scattering coefficients of the blood and surrounding tissue at a first wavelength;

Δ

μ

_s(λ

₂) is the difference between the scattering coefficients of the blood and surrounding tissue at a second wavelength; and

0.34 is the fraction of the red cell volume occupied by hemoglobin, which is assumed to be constant.

23. A method of computing percent hematocrit based on optical measurements, wherein said percent hematocrit is determined such that

27. A method of measuring a percent hematocrit near a tissue location using optical spectrophotometry comprising:

irradiating said tissue location and processing received signals from said tissue location to measure tissue water;

occluding the venous blood flow adjacent to said tissue location;

repeat irradiating said tissue location;

detecting radiation from said tissue following said repeat irradiating; and

calculating hematocrit values using tissue water measurements.