Continuous, non-invasive technique for determining blood pressure using a transmission line model and transcutaneous ultrasound measurements

• US 20060211942A1
• Filed: 03/17/2005
• Published: 09/21/2006
• Est. Priority Date: 03/17/2005
• Status: Active Grant

• Current Assignee: GE Medical Systems Information Technologies Incorporated (General Electric Company)
• Original Assignee: GE Medical Systems Information Technologies Incorporated (General Electric Company)
• Inventors: Hersh, Lawrence T., Friedman, Bruce A., Hoctor, Ralph T.
• Granted Patent: US 7,621,876 B2
• Time in Patent Office: Days
• US Class Current: 600/438
• CPC Class Codes: A61B 5/02125 of pulse wave propagation time
• CPC Class Codes: A61B 5/022 by applying pressure to close blood vessels, e.g. against the skin; Ophthalmodynamometers
• CPC Class Codes: A61B 8/04 Measuring blood pressure
• CPC Class Codes: A61B 8/06 Measuring blood flow measuring volume flow in general G01F, e.g. G01F1/66, G01F1/72; measuring speed of fluids in general G01P5/00
• CPC Class Codes: A61B 8/13 Tomography A61B8/10, A61B8/12 take precedence; tomography for radiation diagnosis A61B6/02
• CPC Class Codes: A61B 8/4236 characterised by adhesive patches
• CPC Class Codes: A61B 8/4472 Wireless probes

Abstract

A method and technique for the continuous, non-invasive measurement of blood pressure. The blood pressure measurement technique of the present invention utilizes ultrasound measurements to determine the diameter of the blood vessel in which the blood pressure is being measured as well as the flow rate of blood at both an input point and an output point along the blood vessel. The system utilizes a transmission line model to relate various blood vessel measurements with electrical components. The transmission line model, in combination with data management techniques including state variable representations and Kalman filtering, is used to develop a blood pressure measurement in real time.

Claims

1. A method for continuously and non-invasively monitoring blood pressure of a patient, the method comprising the steps of:

positioning at least two non-invasive ultrasound sensors aligned with a blood vessel of the patient, the ultrasound sensors being operable to determine the diameter of the blood vessel and the flow rate of blood through the blood vessel;

utilizing a transmission line model to represent the blood vessel, where the voltage at each node of the transmission line model corresponds to the blood pressure; and

using a transmission-line-based computation to estimate the blood pressure.

2. The method of claim 1 wherein the two ultrasound sensors are each aligned with the same blood vessel, the ultrasound sensors being spaced by a distance.

3. The method of claim 2 wherein the transmission line model includes an input current source and an output current source positioned between an RLC network, wherein the input current source is a first blood flow rate determined by the first ultrasound sensor and the output current source is a second blood flow rate determined by the second ultrasound sensor.

4. The method of claim 3 wherein the RLC ladder network includes an inductance (L), a capacitance (C), and a series resistance (R), wherein the capacitance is analogous to the compliance of the blood vessel, the inductance is analogous to the density of the blood divided by the cross-sectional area of the blood vessel and the series resistance is analogous to eight times the viscosity of blood divided by π

times the radius of the lumen of the blood vessel to the fourth power.

5. The method of claim 4 wherein the RLC ladder network further includes a transverse resistance corresponding to loss of blood mass due to branching from the blood vessel.

6. The method of claim 1 further comprising the step of initializing the measurement system by determining a measured blood pressure of the patient by utilizing a blood pressure cuff, the measured blood pressure being utilized to determine the pulse wave velocity.

7. The method of claim 3 wherein the step of using the transmission-line based computation includes the steps of:

developing a series of state variable equations based on the transmission line network;

inputting the measured first blood flow rate, the measured second blood flow rate and a measured blood vessel radius into a Kalman filter; and

utilizing the Kalman filter to estimate the state variables of the transmission line model, wherein the state variables include a capacitor voltage for each node in the transmission line model.

8. The method of claim 7 further comprising the steps of:

determining the pulse wave velocity for the blood flowing through the blood vessel; and

determining the capacitance (C) of the RLC circuit by

$C=\frac{\mathrm{\pi <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>r^2}{\mathrm{\rho <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>v_p^2}$

where r is the measured radius of the blood vessel, ρ

is the density of the blood and v_p is the pulse wave velocity of the blood.

9. The method of claim 7 further comprising the step of utilizing the initialization measurement of the blood pressure to define an initial state vector used in the Kalman filter.

10. The method of claim 7 further comprising the steps of:

comparing a current value of the state variables with an initialized value of the state variables; and

re-initializing the measurement system when the current value of the state variables differs from the initialized value of the state variables by more than a pre-determined threshold value.

11. The method of claim 7 further comprising the steps of:

continuously determining the radius of the artery of the patient;

continuously determining the first blood flow rate and the second blood flow rate; and

continuously updating the state variable equation with the blood vessel radius, first blood flow rate and second blood flow rate, such that the Kalman filter provides a continuous, real time blood pressure measurement.

12. The method of claim 7 wherein the state variable equations include at least one undetermined parameter, the method further comprising the steps of:

providing multiple, separate estimates for the value of the undetermined parameter;

calculating a prediction error for each of the estimates, wherein the prediction error is the difference between the response of the Kalman filter for the estimate and a measured blood pressure of the patient;

selecting the estimate of the undetermined parameter that results in the smallest prediction error; and

utilizing the selected estimate with the series of state variable equations and the Kalman filter to determine the continuous blood pressure.

13. A method for continuously and non-invasively monitoring the blood pressure of a patient, the method comprising the steps of:

utilizing an ultrasound measurement to determine the diameter of a blood vessel of a patient;

utilizing an ultrasound measurement to determine a first blood flow rate at a first location along the blood vessel;

utilizing an ultrasound measurement to determine a second blood flow rate at a second location along the blood vessel, the second location being spaced from the first location;

estimating the pulse wave velocity along the blood vessel;

creating a transmission line model for the blood vessel, the transmission line model having multiple RLC ladder stages extending between a first current source and a second current source, where the first current source corresponds to the first blood flow rate and the second current source corresponds to the second blood flow rate; and

using a transmission-line-based computation to estimate a voltage at each node of the transmission line model, wherein the voltage corresponds to the blood pressure.

14. The method of claim 13 wherein each RLC ladder stage of the transmission line model includes a capacitance (C) analogous to the compliance of the blood vessel, an inductance (L) analogous to the density of the blood divided by the area of the blood vessel and a series resistance (R) analogous to eight times the viscosity of blood divided by π

times the radius of the lumen of the blood vessel to the fourth power.

15. The method of claim 13 wherein the diameter of the blood vessel, the first blood flow rate and the second blood flow rate are determined by at least two ultrasound sensors positioned on the skin of the patient and aligned with the blood vessel.

16. The method of claim 13 further comprising the step of:

initializing the measurement system by determining an initialized value of the blood pressure of the patient by utilizing a blood pressure cuff, the initialized value of the blood pressure being used to solve the transmission line model.

17. The method of claim 13 wherein the step of using the transmission-line-based computation to estimate the voltage at each node includes the steps of:

developing a series of state variable equations based on the transmission line network;

inputting the measured first blood flow rate, the measured second blood flow rate and the measured blood vessel diameter into a Kalman filter; and

utilizing the Kalman filter to estimate the state variables of the transmission line model, wherein the state variables include a capacitor voltage for each node in the transmission line model.

18. The method of claim 17 further comprising the steps of:

determining the pulse wave velocity for the blood flowing through the blood vessel; and

determining the capacitance of the RLC circuit by

$C=\frac{\mathrm{\pi <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>r^2}{\mathrm{\rho <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>v_p^2}$

where r is the measured radius of the blood vessel, ρ

is the density of the blood and v_p is the pulse wave velocity of the blood.

19. The method of claim 17 further comprising the step of utilizing the initialization measurement of the blood pressure to define an initial state vector used in the Kalman filter.

20. The method of claim 17 further comprising the steps of:

comparing a current value of the state variables with an initialized value of the state variables; and

re-initializing the measurement system when the current value of the state variables differs from the initialized value of the state variables by more than a pre-determined threshold value.

21. The method of claim 13 further comprising the steps of:

continuously determining the radius of the blood vessel of the patient;

continuously determining the first blood flow rate and the second blood flow rate; and

continuously updating the state variable equation with the blood vessel radius, first blood flow rate and second blood flow rate, such that the Kalman filter provides a continuous, real time blood pressure measurement.

22. The method of claim 21 wherein the real time blood pressure measurement is presented as a systolic and a diastolic pressure on a beat by beat basis.

23. The method of claim 17 wherein the state variable equations include at least one undetermined parameter, the method further comprising the steps of:

providing multiple, separate estimates for the value of the undetermined parameter;

calculating a prediction error for each of the estimates, wherein the prediction error is the difference between the response of the Kalman filter for the estimate and a measured blood pressure of the patient;

selecting the estimate of the undetermined parameter that results in the smallest prediction error; and

utilizing the selected estimate with the series of state variable equations and the Kalman filter to determine the continuous blood pressure.

24. A continuous, non-invasive blood pressure monitoring system comprising:

at least a pair of non-invasive ultrasound sensors each being operable to determine the diameter of the blood vessel and the flow rate of blood through the blood vessel;

a blood pressure cuff operable to determine an initialization value of the blood pressure of the patient; and

a processor connected to both the ultrasound sensors and the blood pressure cuff for determining the blood pressure of the patient continuously and non-invasively, wherein the processor utilizes a transmission line model to estimate the blood pressure.

25. The apparatus of claim 24 wherein the transmission line model utilized by the processor includes multiple RLC ladder stages extending between a first current source and a second current source, where the first current source corresponds to the first blood flow rate measured by the ultrasound sensor and the second current source corresponds to the second blood flow rate, where the processor develops a series of state variable equations based on the transmission line network and utilizes Kalman filtering to estimate the state variables.

26. The apparatus of claim 24 wherein the capacitance (C) in the transmission line model is analogous to the compliance of the blood vessel, the inductance (L) is analogous to the density of the blood divided by the cross-sectional area of the blood vessel and the resistance (R) is analogous to eight times the viscosity of blood divided by π

times the radius of the lumen of the blood vessel to the fourth power.

27. The apparatus of claim 24 wherein the processor compares the current value of the state variables with an initializing value of the state variables and re-initializes the measurement system when the current value of the state variables differs from the initialized value of the state variables by more than a pre-determined threshold value.