Systems and methods for high-pass filtering a photoplethysmograph signal

• US 9,649,071 B2
• Filed: 06/10/2015
• Issued: 05/16/2017
• Est. Priority Date: 09/29/2009
• Status: Active Grant

• Current Assignee: Nellcor Puritan Bennett Ireland
• Original Assignee: Nellcor Puritan Bennett Ireland
• Inventors: Watson, James N., Addison, Paul Stanley
• Primary Examiner(s): Natnithithadha, Navin
• Assistant Examiner(s): MESSERSMITH, ERIC J
• Application Number:
  US14/736,178
• Publication Number:
  US 20150272507A1
• Time in Patent Office: 706 Days
• CPC Class Codes: A61B 2562/0238 Optical sensor arrangements for performing transmission measurements on body tissue
• CPC Class Codes: A61B 5/0205 Simultaneously evaluating both cardiovascular conditions and different types of body conditions, e.g. heart and respiratory condition
• 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 5/02416 using photoplethysmograph signals, e.g. generated by infrared radiation A61B5/14552 takes precedence
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: A61B 5/14552 Details of sensors specially adapted therefor
• CPC Class Codes: A61B 5/1495 Calibrating or testing of in-vivo probes
• CPC Class Codes: A61B 5/7225 Details of analog processing, e.g. isolation amplifier, gain or sensitivity adjustment, filtering, baseline or drift compensation A61B5/30, A61B5/316 take precedence; transformers for specific applications H01F38/00; amplifiers H03F
• CPC Class Codes: A61B 5/7239 using differentiation including higher order derivatives
• CPC Class Codes: A61B 5/725 using specific filters therefor, e.g. Kalman or adaptive filters A61B5/316 take precedence
• CPC Class Codes: A61B 5/7278 Artificial waveform generation or derivation, e.g. synthesising signals from measured signals A61B5/327 takes precedence

Abstract

According to embodiments, systems and methods for high-pass filtering a plethysmograph or photoplethysmograph (PPG) signal are disclosed. A sensor or probe may be used to obtain a plethysmograph or PPG signal from a subject. The sensor may be placed at any suitable location on the body, e.g., the forehead, finger, or toe. The PPG signal generated by the sensor may be high-pass filtered to disambiguate certain features of the PPG signal, including one or more characteristic points. The cut-off frequency for the high-pass filter may be greater than 0.75 Hz and less than 15 Hz. The cut-off frequency for the high-pass filter may be selected to be greater than the subject'"'"'s computed pulse rate. These characteristic points on the filtered PPG signal may be used to compute non-invasive blood pressure measurements continuously or on a periodic basis. For example, the time difference between two or more characteristic points in a high-pass filtered version of the generated PPG signal may be computed. The time difference may be used to compute non-invasive blood pressure measurements continuously or on a periodic basis.

Claims

1. A system for high-pass filtering a photoplethysmograph (PPG) signal used for determining a blood pressure value of a subject comprising:

a sensor configured to generate the PPG signal; and

a pulse oximetry monitor configured to;

store a predetermined frequency value, wherein the predetermined frequency value is equal to one of;

0.5 Hz, 1 Hz, 2 Hz, 3 Hz, 4 Hz, 5 Hz, 10 Hz, and 15 Hz;

receive the PPG signal from the sensor;

determine a pulse frequency value indicative of a pulse rate of the subject;

add the predetermined frequency value to the determined pulse frequency value that is indicative of the pulse rate of the subject;

compute a cut-off frequency for a high-pass filter such that the cut-off frequency is approximately equal to the result of the addition;

generate a filtered PPG signal by applying the high-pass filter to the PPG signal; and

determine, based at least in part on the filtered PPG signal, the blood pressure value of the subject.

2. The system of claim 1, wherein the sensor comprises a pulse oximetry sensor configured to detect light attenuated by blood perfused tissue of the subject.

3. The system of claim 1, wherein determining the blood pressure value of the subject comprises identifying at least one characteristic point in the filtered PPG signal.

4. The system of claim 3, wherein the pulse oximetry monitor is further configured to identify at least one stationary point or inflection point of the filtered PPG signal.

5. The system of claim 3, wherein the pulse oximetry monitor is further configured to identify a local turning point of the filtered PPG signal.

6. The system of claim 3, wherein the pulse oximetry monitor is further configured to identify two peaks of the filtered PPG signal.

7. The system of claim 3, wherein the pulse oximetry monitor is further configured to take a natural logarithm of a time difference determined from the at least one identified characteristic point.

8. The system of claim 3, wherein the pulse oximetry monitor is further configured to determine the blood pressure value of the subject by solving a multi-parameter equation, wherein the multi-parameter equation is

p=a+b·

ln(T)

where p is the blood pressure value of the subject, T is a time difference determined from the at least one identified characteristic point, and a and b are constants.

9. The system of claim 1, wherein the pulse oximetry monitor is further configured to perform at least one calibration of the blood pressure value of the subject, wherein the calibration is based at least in part on a known reference blood pressure value.

10. The system of claim 1, wherein the pulse oximetry monitor is further configured to update the cut-off frequency when the pulse frequency value of the subject changes, wherein the updating comprises:

adding the predetermined frequency value to an updated pulse frequency value that is indicative of a changed pulse rate of the subject; and

computing an updated cut-off frequency for the high-pass filter such that the cut-off frequency is approximately equal to the result of the addition of the predetermined frequency value and the updated pulse frequency value.

11. A method for high-pass filtering a photoplethysmograph (PPG) signal used for determining a blood pressure value of a subject comprising:

generating, using a sensor, the PPG signal;

storing, using a pulse oximetry monitor, a predetermined frequency value, wherein the predetermined frequency value is equal to one of;

0.5 Hz, 1 Hz, 2 Hz, 3 Hz, 4 Hz, 5 Hz, 10 Hz, and 15 Hz;

determining, using the pulse oximetry monitor, a pulse frequency value indicative of a pulse rate of the subject;

adding, using the pulse oximetry monitor, the predetermined frequency value to the determined pulse frequency value that is indicative of the pulse rate of the subject;

computing, using the pulse oximetry monitor, a cut-off frequency for a high-pass filter such that the cut-off frequency is approximately equal to the result of the addition;

generating, using the pulse oximetry monitor, a filtered PPG signal by applying the high-pass filter to the PPG signal; and

determining, using the pulse oximetry monitor, based at least in part on the filtered PPG signal, the blood pressure value of the subject.

12. The method of claim 11, wherein the sensor comprises a pulse oximetry sensor configured to detect light attenuated by blood perfused tissue of the subject.

13. The method of claim 11, wherein determining the blood pressure value of the subject comprises identifying at least one characteristic point in the filtered PPG signal.

14. The method of claim 13 wherein identifying at least one characteristic point in the filtered PPG signal comprises identifying at least one stationary point or inflection point of the filtered PPG signal.

15. The method of claim 13 wherein identifying at least one characteristic point in the filtered PPG signal comprises identifying a local turning point in the filtered PPG signal.

16. The method of claim 13 wherein identifying at least one characteristic point in the filtered PPG signal comprises identifying two peaks in the filtered PPG signal.

17. The method of claim 13 wherein determining the blood pressure value of the subject comprises taking a natural logarithm of a time difference determined from the at least one identified characteristic point.

18. The method of claim 13 wherein determining the blood pressure value comprises solving a multi-parameter equation, wherein the multi-parameter equation is

p=a+b·

ln(T)

where p is the determined blood pressure value, T is a time difference determined from the at least one identified characteristic point, and a and b are constants.

19. The method of claim 11 further comprising performing, using the pulse oximetry monitor, at least one calibration of the blood pressure of the subject, wherein the calibration is based at least in part on a known reference blood pressure value.

20. The method of claim 11, further comprising updating, using the pulse oximetry monitor, the cut-off frequency when the pulse frequency value of the subject changes, wherein the updating comprises:

adding, using the pulse oximetry monitor the predetermine frequency value to an updated pulse frequency value that is indicative of a changed pulse rate of the subject; and

computing, using the pulse oximetry monitor, an updated cut-off frequency for the high-pass filter such that the cut-off frequency is approximately equal to the result of the addition of the predetermined frequency value and the updated pulse frequency value.