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 7,801,598 B2
• Filed: 10/07/2005
• Issued: 09/21/2010
• Est. Priority Date: 08/14/2000
• Status: Expired due to Fees

• 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
• Primary Examiner(s): Hindenburg; Max
• Assistant Examiner(s): EISEMAN, ADAM JARED
• Application Number:
  US11/246,635
• Publication Number:
  US 20060122540A1
• Time in Patent Office: 1,810 Days
• Field of Search: 600/547, 600/595
• US Class Current: 600/547
• 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 method for determining a dialysis patient'"'"'s dry weight comprising the steps of:

continuously measuring the extracellular resistance of a body segment using a plurality of electrodes, while applying a pressure to the 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;

using a microprocessor to perform each of the following steps;

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.

2. The method of claim 1, wherein the patient'"'"'s normalized resistivity as a function of time (ρ

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

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

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;

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

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;

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

wherein χ

₀ is the initial circumference of the body segment; and

ρ

₀ is a constant value of resistivity.

3. The method of claim 2, wherein the patient'"'"'s dry weight (DW) is calculated according to the following equation:

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

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.

4. The method of claim 3, 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

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.

5. 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;

a compression means being in electrical communication with the digital signal processor, said compression means adapted to apply pressure to the body segment; and

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

wherein the microprocessor is configured to determine a dialysis patient'"'"'s dry weight by;

continuously measuring 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;

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); and

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

ρ

_N(t)≧

ρ

_N,H.

6. The device of claim 5, 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;

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

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;

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

wherein χ

₀ is the initial circumference of the body segment; and

ρ

₀ is a constant value of resistivity.

7. The device of claim 6, wherein the patient'"'"'s dry weight (DW) is calculated according to the following equation:

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

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.

8. The device of claim 7, 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

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.

9. The device of claim 5, wherein said compression means is a pressure cuff.