Systems and methods for high-pass filtering a photoplethysmograph signal

• US 9,066,660 B2
• Filed: 09/29/2009
• Issued: 06/30/2015
• 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): Mallari, Patricia
• Assistant Examiner(s): MESSERSMITH, ERIC J
• Application Number:
  US12/568,946
• Publication Number:
  US 20110077531A1
• Time in Patent Office: 2,100 Days
• Field of Search: 600485-507
• US Class Current: 1/1
• 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 method for high-pass filtering a photoplethysmograph (PPG) signal used for determining a blood pressure measurement of a subject comprising:

detecting the PPG signal;

computing a pulse frequency of the subject;

selecting a cut-off frequency for a high-pass filter based at least in part on the computed pulse frequency, such that the cut-off frequency is approximately equal to the computed pulse frequency;

filtering the detected PPG signal with the high-pass filter;

identifying at least one characteristic point in the filtered PPG signal; and

determining, based at least in part on the at least one identified characteristic point, the blood pressure measurement.

2. The method of claim 1 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.

3. The method of claim 1 wherein identifying at least one characteristic point in the filtered PPG signal comprises identifying a local turning point in the filtered PPG signal.

4. The method of claim 1 wherein identifying at least one characteristic point in the filtered PPG signal comprises identifying two peaks in the filtered PPG signal.

5. The method of claim 1 wherein determining, based at least in part on the at least one identified characteristic point, a blood pressure measurement comprises taking a natural logarithm of a time difference determined from the at least one identified characteristic point.

6. The method of claim 1 wherein determining, based at least in part on the on the at least one identified characteristic point, a blood pressure measurement comprises solving a multi-parameter equation, wherein the multi-parameter equation is

p=a+b·

ln(T)

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

7. The method of claim 1 further comprising performing at least one calibration of the blood pressure measurement, the calibration based at least in part on a known reference blood pressure measurement.

8. The method of claim 1 wherein the cut-off frequency is less than 15 Hz.

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

a sensor capable of generating the PPG signal; and

a processor configured for;

detecting the PPG signal;

computing a pulse frequency of the subject;

selecting a cut-off frequency for a high-pass filter based at least in part on the computed pulse frequency, such that the cut-off frequency is approximately equal to the computed pulse frequency;

filtering the detected PPG signal with the high-pass filter;

identifying at least one characteristic point in the filtered PPG signal; and

determining, based at least in part on the at least one identified characteristic point, the blood pressure measurement.

10. The system of claim 9 wherein the sensor comprises a pulse oximeter.

11. The system of claim 9 wherein the processor is further configured for identifying at least one stationary point or inflection point of the filtered PPG signal.

12. The system of claim 9 wherein the processor is further configured for identifying a local turning point of the filtered PPG signal.

13. The system of claim 9 wherein the processor is further configured for identifying two peaks of the filtered PPG signal.

14. The system of claim 9 wherein the processor is further configured for determining, based at least in part on the at least one identified characteristic point, a blood pressure measurement by solving a multi-parameter equation, wherein the multi-parameter equation is

p=a+b·

ln(T)

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

15. The system of claim 9 wherein the processor is further configured for performing at least one calibration of the blood pressure measurement, the calibration based at least in part on a known reference blood pressure measurement.

16. The system of claim 9 wherein the cut-off frequency is less than 15 Hz.

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

detecting the PPG signal;

computing a pulse frequency of the subject;

selecting a cut-off frequency for a high-pass filter based at least in part on the computed pulse frequency, such that the cut-off frequency is a predetermined amount higher than the computed pulse frequency, wherein the predetermined amount is less than 0.5 Hz.;

filtering the detected PPG signal with the high-pass filter;

identifying at least one characteristic point in the filtered PPG signal; and

determining, based at least in part on the at least one identified characteristic point, the blood pressure measurement.