Device and method for the determination of dry weight by continuous measurement of resistance and calculation of circumference in a body segment using segmental bioimpedance analysis

• US 20060122540A1
• Filed: 10/07/2005
• Published: 06/08/2006
• Est. Priority Date: 08/14/2000
• Status: Active Grant

• Current Assignee: Fresenius Medical Care Holdings Incorporated (Fresenius SE & Co. KGaA)
• Original Assignee: Fresenius Medical Care Holdings Incorporated (Fresenius SE & Co. KGaA)
• Inventors: Levin, Nathan W., Zhu, Fansan
• Granted Patent: US 7,801,598 B2
• Time in Patent Office: Days
• US Class Current: 600/587
• CPC Class Codes: A61B 5/022 by applying pressure to close blood vessels, e.g. against the skin; Ophthalmodynamometers
• CPC Class Codes: A61B 5/0537 Measuring body composition by impedance, e.g. tissue hydration or fat content
• CPC Class Codes: A61B 5/6843 Monitoring or controlling sensor contact pressure

Abstract

A method and device for continuously calculating the circumference of a body segment is based on the continuous measurement of the extracellular resistance of the body segment. Also disclosed is a method and device for determining the dry weight of a dialysis patient wherein the time at which the dry weight of the patient has been achieved is identified as the time at which the patient'"'"'s normalized resistivity as a function of time (ρ_N(t)) is greater than or equal to a minimum level of normalized resistivity in healthy subjects (ρ_N,H).

Claims

1. A device for continuously calculating the circumference of a body segment, the device comprising:

a digital signal processor;

an electrical output means being in electrical communication with the digital signal processor and being attachable to the body segment, the electrical output means being adapted to apply electrical current to the body segment;

an electrical input means being in electrical communication with the digital signal processor and being attachable to the body segment, the electrical input means being adapted to receive the current transmitted through the body segment and transmit the same to the digital signal processor; and

a microprocessor being in electrical communication with the digital signal processor;

wherein the microprocessor is adapted to continuously calculate the circumference of the body segment based on the continuous measurement of the extracellular resistance of the body segment.

2. The device of claim 1, wherein the microprocessor continuously calculates the circumference of the body segment according to the following equation:

${\mathrm{\chi <br><br>}}_t=\sqrt{{\mathrm{\chi <br><br>}}_0^2-\frac{4<br><br>\mathrm{\pi <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\mathrm{\rho <br><br>}}_0<br><br>L}{R_0}<br><br>\left(1-\frac{R_0}{R_t}\right)}$

wherein χ

_t is the circumference of the body segment as a function of time;

χ

₀ is the initial circumference of the body segment;

ρ

₀ is a constant value of resistivity;

L is the length of the body segment;

R₀ is the initial extracellular resistance of the body segment; and

R_t is the extracellular resistance of the body segment as a function of time.

3. A method of continuously calculating the circumference of a body segment, the method comprising:

continuously measuring the extracellular resistance of the body segment;

continuously calculating the circumference of the body segment based on the continuous measurement of the extracellular resistance of the body segment.

4. The method of claim 3, wherein the circumference of the body segment is continuously calculated according to the following equation:

${\mathrm{\chi <br><br>}}_t=\sqrt{{\mathrm{\chi <br><br>}}_0^2-\frac{4<br><br>\mathrm{\pi <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\mathrm{\rho <br><br>}}_0<br><br>L}{R_0}<br><br>\left(1-\frac{R_0}{R_t}\right)}$

wherein χ

_t is the circumference of the body segment as a function of time;

χ

₀ is the initial circumference of the body segment;

ρ

₀ is a constant value of resistivity;

L is the length of the body segment;

R₀ is the initial extracellular resistance of the body segment; and

R_t is the extracellular resistance of the body segment as a function of time.

5. The method of claim 3, wherein the body segment is that of a dialysis patient, and a change in the circumference of the body segment is due to a change in fluid volume during a dialysis treatment.

6. The method of claim 4, wherein the body segment is that of a dialysis patient, and a change in the circumference of the body segment is due to a change in fluid volume during a dialysis treatment.

7. The method of claim 3, wherein a change in the circumference of the body segment is due to an internal bleeding of the body segment.

8. The method of claim 4, wherein a change in the circumference of the body segment is due to an internal bleeding of the body segment.

9. The method of claim 3, wherein the body segment is that of an animal'"'"'s body.

10. The method of claim 4, wherein the body segment is that of an animal'"'"'s body.

11. A method for determining a dialysis patient'"'"'s dry weight comprising the steps of:

continuously measuring the extracellular resistance of a body segment during dialysis and graphing the ratio of R₀/R_t as a function of time, wherein R₀ is the extracellular resistance at the start of dialysis and R_t is the extracellular resistance as a function of time;

flattening the graph of the ratio of R₀/R_t as a function of time;

comparing the patient'"'"'s normalized resistivity as a function of time (ρ

_N(t)) to a minimum level of normalized resistivity in healthy subjects (ρ

_N,H);

identifying the time at which the dry weight of the patient has been achieved when

ρ

_N(t)≧

ρ

_N,H; and

calculating the patient'"'"'s dry weight.

12. The method of claim 11, wherein the patient'"'"'s normalized resistivity as a function of time (ρ

N(t)) is calculated according to the following equation;

${\mathrm{\rho <br><br>}}_N<br><br>\left(t\right)=\frac{\mathrm{\rho <br><br>}<br><br>\left(t\right)}{\mathrm{BMI}}⨯<br><br>{10}^{-2}<br><br>\mathrm{\Omega <br><br>}·<br><br>m^3/\mathrm{kg}$

wherein BMI is the patient'"'"'s body mass index defined as body weight divided by body height squared; and

ρ

(t) is the resistivity of the body segment as a function of time, calculated according to the following equation;

$\mathrm{\rho <br><br>}<br><br>\left(t\right)=\frac{{\mathrm{\chi <br><br>}}_t^2·<br><br>R_t}{4<br><br>\mathrm{\pi <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>L}<br><br>\mathrm{\Omega <br><br>}·<br><br>\mathrm{cm}$

wherein L is the length of the body segment; and

χ

_t is the circumference of the body segment as a function of time which is continuously calculated according to the following equation;

${\mathrm{\chi <br><br>}}_t=\sqrt{{\mathrm{\chi <br><br>}}_0^2-\frac{4<br><br>\mathrm{\pi <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\mathrm{\rho <br><br>}}_0<br><br>L}{R_0}<br><br>\left(1-\frac{R_0}{R_t}\right)}$

wherein χ

₀ is the initial circumference of the body segment; and

ρ

₀ is a constant value of resistivity.

13. The method of claim 12, wherein the patient'"'"'s dry weight (DW) is calculated according to the following equation:

$\mathrm{DW}=W_{\mathrm{pre}}-{\int <br><br>}_{t_0}^{t_{\mathrm{DW}}}<br><br>\mathrm{UFR}<br><br>\text{\hspace{1em}<br><br>}<br><br>\left(t\right)·<br><br>d<br><br>t$

where W_pre is the patient'"'"'s body weight prior to dialysis;

UFR(t) is the ultrafiltration rate (UFR) as a function of time during dialysis, with t₀ to t_DW being the time interval of a functioning UFR in which t₀ is the start time of dialysis and t_DW is the time at which the dry weight of the patient is achieved.

14. The method of claim 13, wherein the flattening of the graph of the ratio of R₀/R_t as a function of time is achieved during a time interval Δ

t in accordance with at least one of the following equations;

δ

(Δ

t)<

C₁; and


k_s=δ

(Δ

t)/Δ

t<

C₂;

wherein χ

(t)=R₀/R_t;

δ

(Δ

t)=λ

(t)−

λ

(t+Δ

t); and

C₁ and C₂ are constants; and

wherein the flattening of the graph of the ratio of R₀/R_t as a function of time is confirmed during the time interval Δ

t by an algorithm of least squares.

15. A device for determining the dry weight of a dialysis patient comprising:

a digital signal processor;

an electrical output means being in electrical communication with the digital signal processor and being attachable to a body segment, the electrical output means being adapted to apply electrical current to the body segment;

an electrical input means being in electrical communication with the digital signal processor and being attachable to a body segment, the electrical input means being adapted to receive the current transmitted through the body segment and transmit the same to the digital signal processor; and

a microprocessor being in electrical communication with the digital signal processor;

wherein the microprocessor is adapted to;

continuously measure an extracellular resistance of the body segment during dialysis and graph the ratio of R₀/R_t as a function of time, wherein R₀ is the extracellular resistance at the start of dialysis and R_t is the extracellular resistance as a function of time;

flatten the graph of the ratio of R₀/R_t as a function of time;

compare the patient'"'"'s normalized resistivity as a function of time (ρ

_N(t)) to a minimum level of normalized resistivity in healthy subjects (ρ

_N,H);

identify the time at which the dry weight of the patient has been achieved when

ρ

_N(t)≧

ρ

_N,H; and

calculate the patient'"'"'s dry weight.

16. The device of claim 15, wherein the patient'"'"'s normalized resistivity as a function of time (ρ

_N(t)) is calculated according to the following equation;

${\mathrm{\rho <br><br>}}_N<br><br>\left(t\right)=\frac{\mathrm{\rho <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>\left(t\right)}{\mathrm{BMI}}\times <br><br>{10}^{-2}<br><br>\mathrm{\Omega <br><br>}·<br><br>m^3<br><br>\text{/}<br><br>\mathrm{kg}$

wherein BMI is the patient'"'"'s body mass index defined as body weight divided by body height squared; and

ρ

(t) is the resistivity of the body segment as a function of time, calculated according to the following equation;

$\mathrm{\rho <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>\left(t\right)=\frac{{\mathrm{\chi <br><br>}}_t^2·<br><br>R_t}{4<br><br>\mathrm{\pi <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>L}<br><br>\mathrm{\Omega <br><br>}·<br><br>\mathrm{cm}$

wherein L is the length of the body segment; and

χ

_t is the circumference of the body segment as a function of time which is continuously calculated according to the following equation;

${\mathrm{\chi <br><br>}}_t=\sqrt{{\mathrm{\chi <br><br>}}_0^2-\frac{4<br><br>{\mathrm{\pi <br><br>\rho <br><br>}}_0<br><br>L}{R_0}<br><br>\left(1-\frac{R_0}{R_t}\right)}$

wherein χ

₀ is the initial circumference of the body segment; and

ρ

₀ is a constant value of resistivity.

17. The device of claim 16, wherein the patient'"'"'s dry weight (DW) is calculated according to the following equation:

$\mathrm{DW}=W_{\mathrm{pre}}-{\int <br><br>}_{t_0}^{t_{\mathrm{DW}}}<br><br>\mathrm{UFR}<br><br>\text{\hspace{1em}<br><br>}<br><br>\left(t\right)·<br><br>d<br><br>t$

where W_pre is the patient'"'"'s body weight prior to dialysis;

UFR(t) is the ultrafiltration rate (UFR) as a function of time during dialysis, with t₀ to t_DW being the time interval of a functioning UFR in which t₀ is the start time of dialysis and t_DW is the time at which the dry weight of the patient is achieved.

18. The device of claim 17, wherein the flattening of the graph of the ratio of R₀/R_t as a function of time is achieved during a time interval Δ

t in accordance with at least one of the following equations;

δ

(Δ

t)<

C₁; and


k_s=δ

(Δ

t)/Δ

t<

C₂;

wherein λ

(t)=R₀/R_t;

δ

(Δ

t)=λ

(t)−

λ

(t+Δ

t); and

C_{1 and C2} are constants; and

wherein the flattening of the graph of the ratio of R₀/R_t as a function of time is confirmed during the time interval Δ

t by an algorithm of least squares.