METHOD OF ESTIMATING PULSE WAVE VELOCITY

• US 20090030328A1
• Filed: 11/27/2006
• Published: 01/29/2009
• Est. Priority Date: 12/01/2005
• Status: Active Grant

• Current Assignee: Atcor Medical Pty Ltd (CardieX Ltd.)
• Original Assignee: Atcor Medical Pty Ltd (CardieX Ltd.)
• Inventors: Qasem, Ahmad M., Harpas, Peter Chris
• Granted Patent: US 8,273,030 B2
• Time in Patent Office: Days
• US Class Current: 600/485
• CPC Class Codes: A61B 5/0285 Measuring or recording phase velocity of blood waves
• CPC Class Codes: A61B 5/7257 using Fourier transforms

Abstract

A method of estimating blood pressure Pulse Wave Velocity (PWV) in the aorta from a recording of a pressure waveform at a single site. The method comprises the following steps: 1. measuring the patient'"'"'s arterial pressure relative to time in order to estimate a central pressure waveform (CPW); 2. estimating the patient'"'"'s aortic pressure pulse transit time (PPTT) from the CPW; 3. estimating the patient'"'"'s carotid to femoral arterial distance from the patient'"'"'s physical characteristics; and 4. dividing the patient'"'"'s estimated carotid to femoral arterial distance by the patient'"'"'s estimated PPTT to estimate the patient'"'"'s PWV.

Claims

1. A method of estimating blood pressure Pulse Wave Velocity (PWV) in the aorta from a recording of a pressure waveform at a single site, the method comprising the following steps:

1. measuring the patient'"'"'s arterial pressure relative to time in order to estimate a central pressure waveform (CPW);

2. estimating the patient'"'"'s aortic pressure pulse transit time (PPTT) from the CPW;

3. estimating the patient'"'"'s carotid to femoral arterial distance from the patient'"'"'s physical characteristics; and

4. dividing the patient'"'"'s estimated carotid to femoral arterial distance by the patient'"'"'s estimated PPTT to estimate the patient'"'"'s PWV.

2. The method as claimed in claim 1, wherein the patient'"'"'s single site pressure measurement is taken non-invasively.

3. The method as claimed in claim 2, wherein the patient'"'"'s single site pressure measurement is taken using the SphygmoCor Px (Trade Mark) Pulse Wave Analysis Q system.

4. The method as claimed in claim 1, wherein the patient'"'"'s radial artery pressure is measured, which is then used to derive the patient'"'"'s CPW.

5. The method as claimed in claim 1, wherein the patient'"'"'s carotid artery pressure is measured and used as an approximation of the patient'"'"'s CPW.

6. The method as claimed in claim 1, wherein the patient'"'"'s carotid to femoral arterial distance is estimated from the patient'"'"'s height, weight and body 0 mass index (BMI).

7. The method as claimed in claim 1, wherein the patient'"'"'s carotid to femoral arterial distance is estimated from a direct measurement of the distance between the carotid and femoral artery.

8. The method as claimed in claim 1, wherein the patient'"'"'s carotid to femoral arterial distance is estimated from superficial body surface measurements.

9. The method as claimed in claim 1, wherein the patient'"'"'s aortic PPTT is estimated from the patient'"'"'s CPW by estimating the incident pressure waveform (IPW) and reflected pressure waveform (RPW) components of the patient'"'"'s CPW, whereby the PPTT has a linear relationship with the time difference between the foot of the IPW and the foot of the RPW.

10. The method as claimed in claim 9, wherein the aortic PPTT, and both the IPW and RPW components, are calculated non-invasively from an ascending aortic CPW measurement that is derived using the SphygmoCor Px (Trade Mark) Pulse Wave Analysis system.

11. The method as claimed in claim 10, wherein, to decompose the CPW into the IPW and RPW components, a triangular flow waveform (FL) is estimated from zero at the time of the foot of the wave up to the first systolic peak (T1) in the CPW, which corresponds to the time of peak flow, and then back to zero at the time of the closure of the aortic valve, which is estimated as the ejection duration (ED) from the CPW.

12. The method as claimed in claim 11, wherein the Characteristic Impedance (Zc) is then calculated using the ratio of the Fast Fourier Transform (FFT) of both the CPW and the estimated triangular flow waveform (FL), the characteristic impedance (Zc) then being used derive the IPW and RPW components, based upon an arterial wave reflection model which utilises CPW, Zc and the estimated triangular flow waveform (FL) as input parameters.

13. The method as claimed in claim 12, wherein the estimated triangular flow waveform (FL) is improved by imposing three conditions:

the RPW is constant until at least T1;

there is no reflection in the IPW after T1; and

the beginning of RPW is smoothed using spline interpolation.

14. The method as claimed in claim 13, wherein the patient'"'"'s estimated PPTT is then calculated using the time between the foot of the IPW and the RPW.

15. The method as claimed in claim 13, wherein the patient'"'"'s estimated PPTT is then s calculated using a cross correlation algorithm.

16. The method as claimed in claim 14, wherein a regression equation is used to convert the patient'"'"'s estimated PPTT into representing the carotid-femoral transit time, based on analysis of a database containing SphygmoCor Pulse Wave Velocity and G SphygmoCor Pulse Wave Analysis measurements for the same patients at the same time.

17. The method as claimed in claim 16, wherein the patient'"'"'s carotid to femoral PWV is then calculated by dividing the patient'"'"'s carotid to femoral arterial distance by the carotid to femoral transit time.

18. The method as claimed in claim 9, wherein the carotid to femoral PWV is calculated from the IPW and RPW components of the patient'"'"'s CPW using the following steps:

1. measuring the time from the foot of the CPW to the first systolic peak (T1);

2. measuring the time from the foot of the CPW to the closure of the aortic valve, which represents the ejection duration (ED);

3. estimating a triangular flow waveform (FL) assuming zero flow at the foot of IPW, peak flow at T1 and zero flow again at ED;

4. calculating a Fast Fourier Transform (FFT) of both the CPW and the estimated triangular flow waveform (FL) to determine the impendence magnitude (Z) using the following formula;

Z=FFT(CPW)/FFT(FL);

5. calculating the characteristic impedance Zc as the average of the 4^th to 7^th harmonics;

6. calculating the IPW (forward) pressure waveform function (P_f(t)) using the following equation;

P_f(t)=[CPW(t)+ZcFl(t)]/2;

7. calculating the RPW (backward) pressure waveform function (P_b(t)) using the following equation;

P_b(t)=[CPW(t)−

ZcFl(t)]/2;

8. calculating the max dp/dt of the P_b and locating the intersection of the upstroke slope with the baseline to initially estimate the start (T_i) of the reflected wave;

9. making pressure values before T_i equal to the baseline pressure in P_b and calculating a new estimate of P_b and P_f (P_b1 and P_f1) using the following formulas;

P_b1(0 . . . T_i)=P_b(0)

P_b1(T_i+T_s . . . T)=P_b(T_i+T_s . . . T)

P_f1(0 . . . T_i)=P_f(0 . . . T_i)+(P_b(0 . . . T_i)−

P_b1(0 . . . T_i))

P_f1(T_i+T_s . . . T)=P_f(T_i+T_s . . . T)

where T_s=sampling period and T=cardiac period;

10. calculating a new estimate of the forward wave (P_f2) by smoothing the peak of the wave to ED using spline interpolation;

11. calculating a new estimate of P_b (P_b2) using the following equation;

P_b2(T_peak . . . ED)=P_b1(T_peak . . . ED . . . +(P_f1(T_peak . . . ED)−

P_f2(T_peak . . . ED))

where T_peak=Time at the peak of the forward wave;

12. calculating an improved estimated flow waveform (FL_imp) from P_b2 and P_f2 using the following equation;

FL_imp=(2×

P_f2)−

CPW=(2×

P_b2)+CPW

13. if any pressure value of P_b2 before ED is less than the baseline pressure then reduce T₁ by one sampling point and repeat steps 3 to 12. If not continue to the next step;

14. re-calibrating P_b2 and P₁₂ so both have the same height and calculating a cross correlation, wherein the time of the highest correlation represents the aortic PPTT;

15. converting the estimated PPTT to carotid femoral PPTT using a linear regression equation;

16. calculating the carotid to femoral distance from direct external measurement, or automatically from height and weight or BMI;

17. determining the carotid-femoral PWV by dividing the carotid to femoral distance by the carotid to femoral PPTT.

19. (canceled)