Techniques for detecting heart pulses and reducing power consumption in sensors

• US 20070208240A1
• Filed: 01/08/2007
• Published: 09/06/2007
• Est. Priority Date: 02/25/2004
• Status: Active Grant

• Current Assignee: Covidien LP (Medtronic Plc)
• Original Assignee: Nellcor Puritan Bennett Incorporated (Medtronic Plc)
• Inventors: Petersen, Ethan, Shea, William, Nordstrom, Brad
• Granted Patent: US 7,499,740 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.

4-13. -13. (canceled)

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.

16-21. -21. (canceled)

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.

25. A method for controlling drive current of light emitting elements in a pulse oximeter sensor, comprising:

measuring a noise component of a pulse oximeter signal;

identifying a systolic period of the pulse oximeter signal;

performing pulse qualification tests to qualify the systolic period for oxygen saturation calculations;

determining a strength of the systolic period if the systolic period is qualified for oxygen saturation calculations;

identifying a signal-to-noise ratio by comparing the strength of the systolic period to the noise component; and

controlling the drive current based on a comparison of the signal-to-noise ratio to a threshold.

26. The method of claim 25, comprising measuring the noise component before determining the strength of the systolic period and storing the measured noise component in memory for comparison with the strength of the systolic period.

27. The method of claim 25, comprising measuring the noise in the pulse oximeter signal at various gain values.

28. The method of claim 25, wherein the threshold comprises a maximum signal-to-noise ratio value of 128:

1.

29. The method of claim 25, wherein the threshold comprises a minimum signal-to-noise ratio value of 8:

1.

30. The method of claim 25, comprising calculating a series of moving averages based on a derivative of the pulse oximeter signal to identify the systolic period.

31. The method of claim 30, comprising identifying a moving minimum and moving maximum of a last moving average of the series of moving averages to identify the systolic period.

32. The method of claim 25, 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, and identifying a moving minimum and a moving maximum of the third output to identify the systolic period.

33. The method of claim 32, comprising comparing the moving minimum and the moving maximum to a scaled sum of the moving minimum and the moving maximum to determine the systolic period.

34. A monitor for controlling drive current of light emitting elements in a pulse oximeter sensor, comprising:

an identification module configured to identify a systolic period of a pulse oximeter signal;

a qualification module configured to perform pulse qualification tests to qualify the systolic period for oxygen saturation calculations;

a strength determination module configured to determine a strength of the systolic period if the systolic period is qualified for oxygen saturation calculations;

a ratio module configured to identify a signal-to-noise ratio by comparing the strength of the systolic period to a measured value of a noise component of the pulse oximeter signal stored in a memory; and

a controller configured to control the drive current based on a comparison of the signal-to-noise ratio to a threshold.

35. The monitor of claim 34, comprising a measurement module configured to measure the noise component.

36. The monitor of claim 35, wherein the measurement module is configured to measure the noise component at various gain values.

37. The monitor of claim 34, wherein the threshold comprises a maximum signal-to-noise ratio value of 128:

1.

38. The monitor of claim 34, wherein the threshold comprises a minimum signal-to-noise ratio value of 8:

1.

39. The monitor of claim 34, comprising a calculation module configured to calculate a series of moving averages based on a derivative of the pulse oximeter signal to identify the systolic period.

40. The monitor of claim 39, wherein the calculation module is configured to identify a moving minimum and moving maximum of a last moving average of the series of moving averages to identify the systolic period.

41. The monitor of claim 34, comprising a calculation module configured to calculate a moving average of a derivative of the pulse oximeter signal to generate a first output, calculate a moving average of the first output to generate a second output, calculate a moving average of the second output to generate a third output, and identify a moving minimum and a moving maximum of the third output to identify the systolic period.

42. The monitor of claim 41, comprising a comparison module configured to compare the moving minimum and the moving maximum to a scaled sum of the moving minimum and the moving maximum to determine the systolic period.

43. A tangible computer-readable medium, comprising:

code configured to identify a systolic period of a pulse oximeter signal;

code configured to perform pulse qualification tests to qualify the systolic period for oxygen saturation calculations;

code configured to determine a strength of the systolic period if the systolic period is qualified for oxygen saturation calculations;

code configured to identify a signal-to-noise ratio by comparing the strength of the systolic period to a measured value of a noise component of the pulse oximeter signal; and

code configured to control the drive current based on a comparison of the signal-to-noise ratio to a threshold.

44. The tangible computer-readable medium of claim 43, comprising code configured to measure the noise component of the pulse oximeter signal.

45. The tangible computer-readable medium of claim 43, comprising code configured to calculate a moving average of a derivative of the pulse oximeter signal to generate a first output, calculate a moving average of the first output to generate a second output, calculate a moving average of the second output to generate a third output, and identify a moving minimum and a moving maximum of the third output to identify the systolic period.