Noninvasive method of measuring blood density and hematocrit

• US 20040249292A1
• Filed: 01/20/2004
• Published: 12/09/2004
• Est. Priority Date: 01/21/2003
• Status: Active Grant

• Current Assignee: Hemonix, Inc.
• Original Assignee: Hemonix, Inc.
• Inventors: Davis, Charles L., Bronson, John E., Harrison, Patrick D.
• Granted Patent: US 7,011,631 B2
• Time in Patent Office: Days
• US Class Current: 600/481
• CPC Class Codes: A61B 5/02125 of pulse wave propagation time
• CPC Class Codes: A61B 5/0295 using plethysmography, i.e. measuring the variations in the volume of a body part as modified by the circulation of blood therethrough, e.g. impedance plethysmography
• CPC Class Codes: A61B 5/0535 Impedance plethysmography for measuring blood flow A61B5/0295
• CPC Class Codes: A61B 5/14535 for measuring haematocrit

Abstract

Apparatus and methods for noninvasively measuring blood density and hematocrit in a human subject can include an appliance for applying pressure around a segment of a subject'"'"'s limb or appendage that includes a pressure sensing means and encloses an array of impedance sensing devices applied to the subject'"'"'s skin for independent fluid volume measurements. While applying pressure to the limb segment by the pressure appliance, measurements of blood pressure, blood pulse wave velocity, and limb segment impedance are recorded. The measurements are subsequently processed to yield changes of arterial blood volume during pulse wave passage. Then pressure, wave velocity, and derived volume data are combined to yield the density of the blood. Blood density is converted to hematocrit by means of a linear relationship between the two. Independent of the blood hematocrit, such apparatus and means may also be used to measure the density of enclosed fluid in other pulsed flow non-rigid wall vessel systems.

Claims

1. A method of noninvasively measuring fluid density of a pulsed flow system with non-rigid vessel walls, comprising:

a) noninvasively measuring a local relative fluid volume ratio V_b/Δ

V_b, where V_b is a basal volume of fluid in a vessel and Δ

V_b is a volume change of the fluid in the vessel due to the pulse;

b) noninvasively measuring a fluid pulse propagation velocity ν

;

c) noninvasively measuring a basal fluid pressure P and a local change in fluid pressure Δ

P;

d) calculating fluid density ρ

_b as a function of the local relative fluid volume ratio, the fluid pulse propagation velocity, the basal fluid pressure, and local change in fluid pressure according to the generalized functional relationship

$F<br><br>\left(\frac{{\mathrm{\rho <br><br>}}_b<br><br>v^2}{\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>P}\right)=G<br><br>\left(\frac{V_b}{\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>V_b},\frac{P}{\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>P}\right)$

or its derivatives.

2. The method of noninvasively measuring fluid density of claim 1 wherein the fluid is blood.

3. The method of noninvasively measuring fluid density according to claim 2, further comprising deriving a hematocrit value from the blood density.

4. The method of noninvasively deriving a hematocrit value from blood density of claim 3 further comprising calculating the equation

5. The method of noninvasively deriving a hematocrit value from blood density of claim 4 further comprising using an average of mean reported values for ρ

_plasma and ρ

_bc.

6. The method of noninvasively measuring fluid density according to claim 1, further comprising:

a) placing a coextensive pressure applicator and electrical impedance monitoring apparatus on a measurement area of a body;

b) applying a constant amplitude current to the measurement area;

c) applying pressure to the measurement area to remove blood therefrom;

d) measuring a change in impedance values of the measurement area while applying pressure;

e) measuring an arterial pulse pressure of the measurement area;

f) measuring a velocity of a pulse wave of the measurement area; and

g) deriving a blood density value from measurements of arterial pulse pressure, impedance, and velocity of pulse wave.

7. An apparatus for noninvasively measuring fluid density of a pulsed flow system with non-rigid vessel walls at a local measurement area, comprising:

a) means for noninvasively measuring a local relative fluid volume ratio V_b/Δ

V_b, where V_b is a basal volume of fluid in a vessel and Δ

V_b is a volume change of the fluid in the vessel due to the pulse;

b) means for noninvasively measuring a fluid pulse propagation velocity ν

;

c) means for noninvasively measuring a basal fluid pressure P and a local change in fluid pressure Δ

P; and

d) a calculator for determining fluid density ρ

_b as a function of the local relative fluid volume ratio, the fluid pulse propagation velocity, the basal fluid pressure, and local change in fluid pressure according to the generalized functional relationship

$F<br><br>\left(\frac{{\mathrm{\rho <br><br>}}_b<br><br>v^2}{\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>P}\right)=G<br><br>\left(\frac{V_b}{\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>V_b},\frac{P}{\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>P}\right)$

or its derivatives.

8. The apparatus for noninvasively measuring fluid density of claim 7 wherein the fluid is blood.

9. The apparatus for noninvasively measuring fluid density according to claim 8, further comprising means for deriving a hematocrit value from the blood density.

10. The apparatus for noninvasively deriving a hematocrit value from blood density of claim 9 further comprising means for calculating the equation

11. The apparatus for noninvasively measuring fluid density according to claim 7, wherein the means for noninvasively measuring a local relative fluid volume ratio, means for noninvasively measuring a fluid pulse propagation velocity, and means for noninvasively measuring a basal fluid pressure and a local change in fluid pressure, further comprise:

a) a coextensive pressure applicator and electrical impedance monitoring apparatus;

b) means for applying a constant amplitude current to the measurement area; and

c) means for measuring a change in impedance values of the measurement area while applying pressure.

12. The apparatus for noninvasively measuring fluid density according to claim 7, wherein the means for noninvasively measuring a local relative fluid volume ratio, means for noninvasively measuring a fluid pulse propagation velocity, and means for noninvasively measuring a basal fluid pressure and a local change in fluid pressure, further comprise:

a) an impedance volume sensor;

b) an inflatable cuff pressure generator;

c) a pressure sensor, and

d) a monitor for determining volume states and pressure states and for controlling the application of pressure to the measurement area.

13. An apparatus for noninvasively measuring fluid density of a pulsed flow system with non-rigid vessel walls at a local measurement area, comprising:

a) a pressure applicator for applying external pressure to the local measurement area,

b) an impedance measurer coextensive with the pressure applicator; and

c) a calculator operably connected to the pressure applicator and the impedance measurer for calculating a fluid density value of the local measurement area.

14. The apparatus for noninvasively measuring fluid density of claim 13 wherein the fluid is blood.

15. The apparatus for noninvasively measuring blood density according to claim 14, further comprising a calculator for calculating a hematocrit value from the blood density value.

16. A method of noninvasively measuring fluid density of a pulsed flow system with non-rigid vessel walls, comprising:

a) noninvasively measuring a local relative fluid volume ratio V_b/Δ

V_b, where V_b is a basal volume of fluid in a vessel and Δ

V_b is a volume change of the fluid in the vessel due to the pulse;

b) noninvasively measuring a fluid pulse propagation velocity ν

;

c) noninvasively measuring a local change in fluid pressure Δ

P;

d) calculating fluid density ρ

_b as a function of the local relative fluid volume ratio, the fluid pulse propagation velocity, and local change in fluid pressure according to the relationship

$\frac{{\mathrm{\rho <br><br>}}_b<br><br>v^2}{\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>P}=\frac{V_b}{\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>V_b}$

or its derivatives.

17. The method of noninvasively measuring fluid density of a pulsed flow system with non-rigid vessel walls of claim 16 wherein the local relative fluid volume ratio V_b/Δ

V_b is determined by;

a) measuring the electrical admittance (Y_b) of the basal fluid volume;

b) measuring the electrical admittance change (Δ

Y_b) due to increased local volume from pulsed flow; and

c) equating

$\frac{Y_b}{\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>Y_b}=\frac{V_b}{\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>V_b},$

whereby the need to know the resistivity of the fluid in determining the local fluid volume ratio is eliminated.

18. A method of noninvasively measuring a local relative fluid volume ratio V_b/Δ

V_b in a pulsed flow system with non-rigid vessel walls comprising;

a) measuring the electrical admittance (Y_b) of the local basal fluid volume;

b) measuring the electrical admittance change (Δ

Y_b) due to increased local volume from pulsed flow; and

c) equating

$\frac{Y_b}{\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>Y_b}=\frac{V_b}{\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>V_b},$

whereby the need to know the resistivity of the fluid in determining the local fluid volume ratio is eliminated.

19. A method of determining a hematocrit value of blood comprising:

a) measuring density of the blood (ρ

_b); and

b) determining the hematocrit value according to the relationship

$\mathrm{Hct}=\frac{{\mathrm{\rho <br><br>}}_b-{\mathrm{\rho <br><br>}}_{\mathrm{plasma}}}{{\mathrm{\rho <br><br>}}_{b<br><br>\text{\hspace{1em}<br><br>}<br><br>c}-{\mathrm{\rho <br><br>}}_{\mathrm{plasma}}}\times <br><br>100.$