Methods for measuring blood pressure with automatic compensations

• US 7,479,111 B2
• Filed: 05/20/2005
• Issued: 01/20/2009
• Est. Priority Date: 05/20/2004
• Status: Active Grant

• Current Assignee: Chinese University of Hong Kong
• Original Assignee: Chinese University of Hong Kong
• Inventors: Poon, Chung Yan Carmen, Yip, Lung, Zhang, Yuan Ting, Teng, Xiao Fei, Lee, Chi Man
• Primary Examiner(s): Nasser; Robert L
• Assistant Examiner(s): Toth; Karen E
• Application Number:
  US11/134,637
• Publication Number:
  US 20050261593A1
• Time in Patent Office: 1,341 Days
• Field of Search: 600/481, 600/485
• US Class Current: 600/485
• CPC Class Codes: A61B 5/021 Measuring pressure in heart or blood vessels A61B5/0205 takes precedence
• CPC Class Codes: A61B 5/02125 of pulse wave propagation time
• CPC Class Codes: A61B 5/0285 Measuring or recording phase velocity of blood waves
• CPC Class Codes: A61B 5/352 Detecting R peaks, e.g. for synchronising diagnostic apparatus; Estimating R-R interval
• CPC Class Codes: A61B 7/04 Electric stethoscopes microphones, acoustic transducers therefor H04R

Abstract

Disclosed is a method for measuring an arterial blood pressure of a subject comprising detecting a pulse-wave-related signal of the subject; extracting a feature from the signals; determining a factor that can affect the arterial blood pressure of the subject; and determining the arterial blood pressure based on the feature with an automatic compensation for the factors. The disclosed method reduces the error in measuring the blood pressure based on characteristics of the pulse-wave-related signals.

Claims

1. A method for measuring an arterial blood pressure of a subject, comprising:

a) detecting pulse-wave-related signals of the subject with a sensor;

b) extracting a feature from the signals;

c) determining factors that can affect the feature; and

d) determining the arterial blood pressure based on the feature with an automatic compensation for the factors,

wherein the feature is a pulse transit time and the factors comprise a contacting force to the sensor, nervous activities and/or cardiac outputs of the subject, and ambient temperatures,

and wherein the step d) further comprises;

1) obtaining coefficients from a calibration process;

2) counterbalancing effects of the factors on the pulse transit time with a first compensation term;

3) determining an estimated value of the arterial blood pressure according to the result of the counterbalancing;

4) counterbalancing the effects of the nervous activities and/or cardiac outputs on the estimated value with a second compensation term; and

5) counterbalancing the effects of the ambient temperatures on the estimated value of the arterial blood pressure with a third compensation term.

2. The method of claim 1, wherein the sensor is positioned on the subject.

3. The method of claim 2, wherein the pulse-wave-related signals comprise an electrocardiographic (ECG) signal and a photoplethysmographic (PPG) signal.

4. The method of claim 3, wherein the pulse transit time is a time interval between a first characteristic point at the electrocardiographic signal, and a second characteristic point at the photoplethysmographic signal of the subject.

5. The method of claim 4, wherein the second characteristic point is at a peak or a foot of the photoplethysmographic signal.

6. The method of claim 4, wherein the pulse transit time is a time interval between a first characteristic point at an R wave of the electrocardiographic signal and a second characteristic point at the photoplethysmographic signal of the subject.

7. The method of claim 6, wherein the first characteristic point is at the peak of the R wave.

8. The method of claim 4, wherein the pulse transit time is a time interval between a first characteristic point at the electrocardiographic signal and a second characteristic point at photoplethysmographic signal within a cardiac cycle.

9. The method of claim 1, wherein the step d4) is performed in accordance with a non-linear function.

10. The method of claim 9, wherein the blood pressure is determined by one group of following equations:

Psys=As/(Ts+Tcs)ⁿ¹+Bs+Cs<sup>α

1</sup>+Ds<sup>β

1</sup>

Pdia=Ad/(Td+Tcd)ⁿ²+Bd+Cd<sup>α

2</sup>+Dd<sup>β

2</sup>;

Psys=As′

×

ln(Ts+Tcs)ⁿ¹+Bs′

+Cs<sup>α

1</sup>+Ds<sup>β

1</sup>

Pdia=Ad′

×

ln(Td+Tcd)ⁿ²+Bd′

+Cd<sup>α

2</sup>+Dd<sup>β

2</sup>; and

Psys=As″

×

Cs<sup>α

1</sup>×

Ds<sup>β

1</sup>/(Ts×

Tcs)ⁿ¹+Bs″

Pdia=Ad″

×

Cd<sup>α

2</sup>×

Dd<sup>β

2</sup>/(Td×

Tcd)ⁿ²+Bd″

,

wherein Psys and Pdia are an estimated systolic blood pressure and an estimated diastolic blood pressure, respectively;

Ts and Td are the pulse transit time used to measure the systolic blood pressure and the diastolic blood pressure, respectively;

As, Bs, Ad, Bd, As′

, Bs′

, Ad′

, Bd′

, As″

, Bs″

, Ad″

, and Bd″

are coefficients used in step d1), which are predefined or obtained from the calibration process;

Tcs and Tcd are the first compensation terms for counterbalancing effects of the contacting force on the pulse transit times Ts and Td, respectively;

n1 and n2 are a power of Ts and a power of Td, respectively, where 0 <

n1,n2≦

3;

Cs and Cd are the second compensation terms for counterbalancing the effects of nervous activities and/or cardiac outputs on the estimation of the systolic and diastolic blood pressure, respectively;

α

1 and α

2 are a power of Cs and a power of Cd, respectively, where 0≦

α

1,α

2≦

2;

Ds and Dd are the third compensation terms for counterbalancing the effects of the ambient temperatures on the estimation of the systolic and/or diastolic blood pressure, respectively; and

β

1 and β

2 are a power of Ds and a power of Dd, respectively, where 0≦

β

1,β

2≦

2.

11. The method of claim 10, wherein the pulse-wave-related signals comprise photoplethysmographic signals.

12. The method of claim 11, wherein each of the compensation terms further comprises at least one photoplethysmographic signal characteristic, and at least one weighting factor, and optionally at least one calibration constant.

13. The method of claim 12, wherein the photoplethysmographic signal characteristic comprises one of a magnitude, a characteristic time interval, a pulse area, and a higher order moment.

14. The method of claim 13, wherein Tcs and Tcd are determined by the magnitude, a first calibration constant and a first weighting factor.

15. The method of claim 14, wherein the first weighting factor is determined by a ratio of a change of the pulse transit time to a change of the blood pressure.

16. The method of claim 13, wherein Cs and Cd are determined by a characteristic time interval of the photoplethysmographic signal, a second calibration constant and a second weighting factor.

17. The method of claim 16, wherein the characteristic time interval includes a time interval from the bottom of the photoplethysmographic signal to a characteristic point of a peak of the photoplethysmographic signal within a cardiac cycle, or a time interval from the peak of the photoplethysmographic signal to a characteristic point of a bottom of the photoplethysmographic signal at the next cardiac cycle.

18. The method of claim 16, wherein the second weighting factor is determined by the characteristic time interval of the photoplethysmographic signal and a percentage difference in the characteristic time interval obtained during the measurement of the blood pressure as compared to that obtained during the calibration.

19. The method of claim 18, wherein the percentage difference is:

$\%<br><br><br><br>\mathrm{Diff}.=\frac{\mathrm{CTI}<br><br>{❘<br><br>}_{\mathrm{during}<br><br><br><br>\mathrm{measurement}}<br><br>-\mathrm{CTI}<br><br>{❘<br><br>}_{\mathrm{during}<br><br><br><br>\mathrm{calibration}}}{\mathrm{CTI}<br><br>{❘<br><br>}_{\mathrm{during}<br><br><br><br>\mathrm{calibration}}},\mathrm{or},<br><br>\%<br><br><br><br>\mathrm{Diff}.=\frac{\mathrm{CTI}<br><br>{❘<br><br>}_{\mathrm{during}<br><br><br><br>\mathrm{calibration}}}{\mathrm{CTI}<br><br>{❘<br><br>}_{\mathrm{during}<br><br><br><br>\mathrm{measurement}}<br><br>-\mathrm{CTI}<br><br>{❘<br><br>}_{\mathrm{during}<br><br><br><br>\mathrm{calibration}}}$

where CTI |_{during measurement} is the characteristic time interval obtained during the measurement, and

CTI |_{during calibration} is the characteristic time interval obtained during the calibration.

20. The method of claim 18, wherein the second calibration constant is obtained from a ratio of As to Bs for the estimation of the systolic blood pressure, and that of Ad to Bd for the estimation of the diastolic blood pressure.

21. The method of claim 13, wherein Ds and Dd are determined by a pulse wave area of the photoplethysmographic signal, a third calibration constant and a third weighting factor.

22. The method of claim 21, wherein the third weighting factor is a constant ranging from 10 to 100.

23. The method of claim 10, wherein Ts and Td are respectively obtained by the following equations:

$\mathrm{Ts}=\frac{1}{P}<br><br>\underset{i=1}{\overset{P}{\sum <br><br>}}<br><br>{\mathrm{Ts}}_i,\mathrm{and},\mathrm{Td}=\frac{1}{P}<br><br>\underset{i=1}{\overset{P}{\sum <br><br>}}<br><br>{\mathrm{Td}}_i$

where i is an index of a series of pulse transit times and 1≦

i≦

P.

24. The method of claim 23, wherein the difference between Ts and Ts_i or that between Td and Td_i, is within a boundary defined below:

Th_Errlow<

Ts−

{Ts_i}<

Th_Errhigh; and

Th_Errlow<

Td−

{Td_i}<

Th_Errhigh

where Th_Errhigh and Th_Errlow are a pre-defined boundary.