Techniques for detecting heart pulses and reducing power consumption in sensors

• US 20050187446A1
• Filed: 02/25/2004
• Published: 08/25/2005
• Est. Priority Date: 02/25/2004
• Status: Active Grant

• Current Assignee: Covidien LP (Medtronic Plc)
• Original Assignee: NELLCOR PURITAN BENNETT
• Inventors: Petersen, Ethan, Shea, William, Nordstrom, Brad
• Granted Patent: US 7,162,288 B2
• Time in Patent Office: Days
• US Class Current: 600/323
• CPC Class Codes: A61B 2560/0209 adapted for power saving
• 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 5/7207 of noise induced by motion artifacts
• CPC Class Codes: A61B 5/7239 using differentiation including higher order derivatives

Abstract

Low power techniques for sensing cardiac pulses in a signal from a sensor are provided. A pulse detection block senses the sensor signal and determines its signal-to-noise ratio. After comparing the signal-to-noise ratio to a threshold, the drive current of light emitting elements in the sensor is dynamically adjusted to reduce power consumption while maintaining the signal-to-noise ratio at an adequate level. The signal component of the sensor signal can be measured by identifying systolic transitions. The systolic transitions are detected using a maximum and minimum derivative averaging scheme. The moving minimum and the moving maximum are compared to the scaled sum of the moving minimum and moving maximum to identify the systolic transitions. Once the signal component has been identified, the signal component is compared to a noise component to calculate the signal-to-noise ratio.

Claims

1. A pulse oximeter system comprising:

a drive interface that controls drive current of light emitting elements in a pulse oximeter sensor; and

a feedback loop coupled around the pulse oximeter sensor and the drive interface that dynamically adjusts the drive current of the light emitting elements based on results of a comparison between a signal-to-noise ratio of a pulse oximeter signal and a threshold,

wherein the pulse oximeter signal is generated by a photodetector in the pulse oximeter sensor.

2. The pulse oximeter system as defined in claim 1 wherein the feedback loop causes the drive current of the light emitting elements to decrease if the signal-to-noise ratio of the pulse oximeter signal is greater than a maximum threshold, and the feedback loop causes the drive current of the light emitting elements to increase of the signal-to-noise ratio of the pulse oximeter signal is less than a minimum threshold.

3. The pulse oximeter system as defined in claim 1 wherein the feedback loop further comprises:

a pulse detection block that calculates the signal-to-noise ratio of the pulse oximeter signal; and

a comparator that performs the comparison of the signal-to-noise ratio of the pulse oximeter signal to the threshold.

4. The pulse oximeter system as defined in claim 3 wherein the pulse detection block calculates a moving average of a derivate of the pulse oximeter signal to generate a first output, calculates a moving average of the first output to generate a second output, calculates a moving average of the second output to generate a third output, and identifies a moving minimum and a moving maximum of the third output.

5. The pulse oximeter system as defined in claim 4 wherein the pulse detection block compares the moving minimum and the moving maximum of the third output to a scaled sum of the moving minimum and the moving maximum of the third output to generate a fourth output that identifies a systolic period.

6. The pulse oximeter system as defined in claim 5 wherein the pulse oximeter system filters out false positives from the fourth output using pulse qualification tests to generate a signal component of the pulse oximeter signal.

7. The pulse oximeter system as defined in claim 6 wherein the pulse oximeter system compares systolic area, width, and number of sub-peaks in the fourth output to first thresholds;

compares diastolic area, width, and number of sub-peaks in the fourth output to second thresholds;

compares systolic area and width to diastolic area and width;

compares pulse area and width to third thresholds.

8. The pulse oximeter system as defined in claim 6 wherein the pulse oximeter system compares systolic area, width, and number of sub-peaks in the fourth output;

diastolic area, width, and number of sub-peaks in the fourth output; and

pulse area and width to N detected heart pulses.

9. The pulse oximeter system as defined in claim 6 wherein the pulse oximeter system performs additional qualification tests to generate the signal component by comparing the lag/lead time between infrared pulse detection and red pulse detection, comparing pulse size to N qualified pulses, comparing a statistically significant coefficient of a best-fit line plot of a moving average between the infrared and the red signals to thresholds, and comparing a saturation rate-of-change to thresholds.

10. The pulse oximeter system as defined in claim 6 wherein the pulse oximeter system compares the signal component to a determined noise component to calculate the signal-to-noise ratio.

11. The pulse oximeter system as defined in claim 6 wherein the pulse oximeter system compares the signal component to a noise component, the noise component being obtained by a continuously updated measurement of noise in the pulse oximeter signal.

12. The pulse oximeter system as defined in claim 1 wherein a reduced amount of processor resources are required to perform oximetry calculations on the pulse oximeter signal.

13. The pulse oximeter system as defined in claim 6 wherein the pulse detection block detects and qualifies pulses using CPU, RAM, and ROM efficient algorithms.

14. A method for reducing power consumption in a pulse oximeter sensor, the method comprising:

providing drive current to light emitting elements in the pulse oximeter sensor; and

determining a signal-to-noise ratio of a pulse oximeter signal generated by a photodetector in the pulse oximeter sensor; and

dynamically adjusting the drive current of the light emitting elements based on results of a comparison between the signal-to-noise ratio of the pulse oximeter signal and a threshold.

15. The method as defined in claim 14 wherein dynamically adjusting the drive current of the light emitting elements further comprises:

increasing the drive current provided to the light emitting elements if the signal-to-noise ratio of the pulse oximeter signal is less than a minimum threshold; and

decreasing the drive current provided to the light emitting elements if the signal-to-noise ratio of the pulse oximeter signal is greater than a maximum threshold.

16. The method as defined in claim 15 wherein determining the signal-to-noise ratio of the pulse oximeter signal further comprises:

calculating a moving average of a derivate of the pulse oximeter signal to generate a first output;

calculating a moving average of the first output to generate a second output;

calculating a moving average of the second output to generate a third output; and

identifying a moving minimum and a moving maximum of the third output.

17. The method as defined in claim 16 wherein determining the signal-to-noise ratio of the pulse oximeter signal further comprises:

comparing the moving minimum and the moving maximum of the third output to a scaled sum of the moving minimum and the moving maximum of the third output to generate a fourth output that identifies a systolic period.

18. The method as defined in claim 17 wherein determining the signal-to-noise ratio of the pulse oximeter signal further comprises:

filtering out false positives from the fourth output using pulse qualification tests to generate a signal component of the pulse oximeter signal.

19. The method as defined in claim 18 wherein determining the signal-to-noise ratio of the pulse oximeter signal further comprises:

comparing the signal component to a determined, noise component to calculate the signal-to-noise ratio.

20. The method as defined in claim 18 wherein determining the signal-to-noise ratio of the pulse oximeter signal further comprises:

comparing the signal component to a noise component, wherein the noise component is obtained by a continuously updated measurement of noise in the pulse oximeter signal.

21. A method for identifying systolic transitions in a pulse oximeter signal generated by a pulse oximeter sensor, the method comprising:

calculating a moving average of a derivative of the pulse oximeter signal to generate a first output;

calculating a moving average of the first output to generate a second output;

calculating a moving average of the second output to generate a third output;

identifying a moving minimum and a moving maximum of the third output;

comparing the moving minimum and the moving maximum of the third output to a scaled sum of the moving minimum and the moving maximum of the third output to generate a fourth output; and

filtering out false positives in the fourth output using pulse qualification routines to generate a fifth output corresponding to a systolic transition in the pulse oximeter signal.

22. A system coupled to a sensor, the system comprising:

a transimpedance amplifier that receives a current signal from the sensor and converts the current signal to a voltage signal based on a transimpedance gain;

an analog-to-digital converter that converts the voltage signal into a digital signal; and

a feedback loop that provides a feedback signal indicating a magnitude of the voltage signal from the transimpedance amplifier when light emitting elements in the sensor are on or off,

wherein the transimpedance gain is adjusted in response to the feedback signal to accommodate the environmental DC bias on the voltage signal.

23. The system as defined in claim 22 wherein the sensor is a pulse oximeter sensor containing a photodetector.

24. The system as defined in claim 22 further comprising:

a pulse detector that calculates the signal-to-noise ratio of the signal from the sensor;

a comparator that compares the signal-to-noise ratio to a threshold; and

a drive interface that controls drive current of the light emitting elements.