Methods and systems for determining physiological information using reference waveforms

• US 9,119,598 B2
• Filed: 09/11/2012
• Issued: 09/01/2015
• Est. Priority Date: 09/11/2012
• Status: Active Grant

• Current Assignee: Covidien LP (Medtronic Plc)
• Original Assignee: Covidien LP (Medtronic Plc)
• Inventors: Engelbrecht, Pirow, Rodriguez-Llorente, Fernando
• Primary Examiner(s): Le, Long V
• Assistant Examiner(s): Negarestan, Farshad
• Application Number:
  US13/609,447
• Publication Number:
  US 20140073954A1
• Time in Patent Office: 1,085 Days
• US Class Current: 1/1
• CPC Class Codes: A61B 5/02416 using photoplethysmograph signals, e.g. generated by infrared radiation A61B5/14552 takes precedence
• CPC Class Codes: A61B 5/02427 Details of sensor
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: A61B 5/7203 for noise prevention, reduction or removal
• CPC Class Codes: A61B 5/7228 Signal modulation applied to the input signal sent to patient or subject; demodulation to recover the physiological signal modulation H03C
• CPC Class Codes: A61B 5/7246 using correlation, e.g. template matching or determination of similarity

Abstract

A physiological monitoring system may process a physiological signal such a photoplethysmograph signal from a subject. The system may determine physiological information, such as a physiological rate, from the physiological signal. The system may use search techniques and qualification techniques to determine one or more initialization parameters. The initialization parameters may be used to calculate and qualify a physiological rate. The system may use signal conditioning to reduce noise in the physiological signal and to improve the determination of physiological information. The system may use qualification techniques to confirm determined physiological parameters. The system may also use autocorrelation techniques, cross-correlation techniques, fast start techniques, and/or reference waveforms when processing the physiological signal.

Claims

1. A subject monitoring system for determining physiological information of a subject, comprising:

a sensor configured to generate a photoplethysmograph signal, wherein the sensor detects light attenuated by the subject; and

processing equipment coupled to the sensor, wherein the processing equipment is configured to;

calculate a correlation sequence based on a portion of the photoplethysmograph signal and a stored reference waveform;

determine physiological rate information based on the correlation sequence;

select a new reference waveform that is derived from the photoplethysmograph signal, wherein a size of the new reference waveform is selected based on a period associated with the physiological rate information;

determine whether to qualify the new reference waveform; and

when the new reference waveform is qualified,

store the new reference waveform;

calculate a subsequent correlation sequence based on a more recent portion of the photoplethysmograph signal and the new reference waveform; and

determine subsequent physiological rate information based on the subsequent correlation sequence.

2. The system of claim 1, wherein when the processing equipment does not qualify the new reference waveform, the processing equipment is further configured to:

discard the new reference waveform;

calculate a subsequent correlation sequence based on a more recent portion of the photoplethysmograph signal and the stored reference waveform; and

determine subsequent physiological rate information based on the subsequent correlation sequence.

3. The system of claim 1, wherein the processing equipment is further configured to analyze a shape of the new reference waveform.

4. The system of claim 1, wherein the processing equipment is further configured to:

generate a threshold based on the correlation sequence;

identify threshold crossings; and

determine a difference associated with at least two threshold crossings.

5. The system of claim 1, wherein the physiological rate is a pulse rate.

6. A processing module for determining physiological information of a subject, wherein the processing module is configured to:

receive a photoplethysmograph signal derived from a detector output, wherein the detector detects light attenuated by the subject;

calculate a correlation sequence based on a portion of the photoplethysmograph signal and a stored reference waveform;

determine physiological rate information based on the correlation sequence;

select a new reference waveform that is derived from the photoplethysmograph signal, wherein a size of the new reference waveform is selected based on a period associated with the physiological rate information;

determine whether to qualify the new reference waveform; and

when the new reference waveform is qualified,

store the new reference waveform;

calculate a subsequent correlation sequence based on a more recent portion of the photoplethysmograph signal and the new reference waveform; and

determine subsequent physiological rate information based on the subsequent correlation sequence.

7. The processing module of claim 6, wherein when the processing module does not qualify the new reference waveform, the processing module is further configured to:

discard the new reference waveform;

calculate a subsequent correlation sequence based on a more recent portion of the photoplethysmograph signal and the stored reference waveform; and

determine subsequent physiological rate information based on the subsequent correlation sequence.

8. The processing module of claim 6, further configured to analyze a shape of the new reference waveform.

9. The processing module of claim 6, further configured to:

generate a threshold based on the correlation sequence;

identify threshold crossings; and

determine a difference associated with at least two threshold crossings.

10. The processing module of claim 6, wherein the physiological rate is a pulse rate.

11. A method for determining physiological information of a subject, comprising:

receiving a photoplethysmograph signal from a detector, which detects light attenuated by the subject;

calculating a correlation sequence based on a portion of the photoplethysmograph signal and a stored reference waveform;

determining physiological rate information based on the correlation sequence;

selecting a new reference waveform that is derived from the photoplethysmograph signal, wherein a size of the new reference waveform is selected based on a period associated with the physiological rate information;

determining whether to qualify the new reference waveform; and

when the new reference waveform is qualified,

storing the new reference waveform;

calculating a subsequent correlation sequence based on a more recent portion of the photoplethysmograph signal and the new reference waveform; and

determining subsequent physiological rate information based on the subsequent correlation sequence.

12. The method of claim 11, further comprising, when the new reference waveform is not qualified,

discarding the new reference waveform;

calculating a subsequent correlation sequence based on a more recent portion of the photoplethysmograph signal and the stored reference waveform; and

determining subsequent physiological rate information based on the subsequent correlation sequence.

13. The method of claim 11, wherein determining whether to qualify the new reference waveform comprises analyzing a shape of the new reference waveform.

14. The method of claim 11, wherein determining the physiological rate information comprises:

generating a threshold based on the correlation sequence;

identifying threshold crossings; and

determining a difference associated with at least two threshold crossings.

15. The method of claim 11, wherein the physiological rate is a pulse rate.

16. A non-transitory computer-readable medium for use in determining physiological information of a subject, the computer-readable medium having computer program instructions recorded thereon for:

receiving a photoplethysmograph signal from a detector, which detects light attenuated by the subject;

calculating a correlation sequence based on a portion of the photoplethysmograph signal and a stored reference waveform;

determining physiological rate information based on the correlation sequence;

selecting a new reference waveform that is derived from the photoplethysmograph signal, wherein a size of the new reference waveform is selected based on a period associated with the physiological rate information;

determining whether to qualify the new reference waveform; and

when the new reference waveform is qualified,

storing the new reference waveform;

calculating a subsequent correlation sequence based on a more recent portion of the photoplethysmograph signal and the new reference waveform; and

determining subsequent physiological rate information based on the subsequent correlation sequence.

17. The non-transitory computer-readable medium of claim 16, having further computer program instructions recorded thereon for, when the new reference waveform is not qualified,

discarding the new reference waveform;

calculating a subsequent correlation sequence based on a more recent portion of the photoplethysmograph signal and the stored reference waveform; and

determining subsequent physiological rate information based on the subsequent correlation sequence.

18. The non-transitory computer-readable medium of claim 16, having further computer program instructions recorded thereon for analyzing a shape of the new reference waveform.

19. The non-transitory computer-readable medium of claim 16, having further computer program instructions recorded thereon for:

generating a threshold based on the correlation sequence;

identifying threshold crossings; and

determining a difference associated with at least two threshold crossings.

20. The non-transitory computer-readable medium of claim 16, wherein the physiological rate is a pulse rate.