FALL DETECTION ALGORITHM UTILIZING A THREE-AXIS ACCELEROMETER

• US 20090292227A1
• Filed: 05/23/2008
• Published: 11/26/2009
• Est. Priority Date: 05/23/2008
• Status: Active Grant

• Current Assignee: Medtronic Incorporated (Medtronic Plc)
• Original Assignee: Medtronic Incorporated (Medtronic Plc)
• Inventors: Kerver, Harry Bernardus Antonius, Scholten, Patrick
• Granted Patent: US 8,814,811 B2
• Time in Patent Office: Days
• US Class Current: 600/595
• CPC Class Codes: A61B 5/0031 Implanted circuitry
• CPC Class Codes: A61B 5/1117 Fall detection

Abstract

A three-axis accelerometer signal is used in a medical device for detecting that a patient has fallen. The accelerometer signal is sampled at a low sampling rate for determining a patient position. The accelerometer signal is sampled at a high sampling rate for determining an acceleration magnitude in response to determining the patient position. The patient position and acceleration magnitude are used to detect that a patient has fallen.

Claims

1. A method for use in a medical device including a three-axis accelerometer, the method comprising:

sensing a signal from the three-axis accelerometer;

sampling the sensed signal at a low sampling rate;

determining a patient position using the low sampling rate signal;

sampling the sensed signal at a high sampling rate greater than the low sampling rate;

determining an acceleration magnitude using the high sampling rate signal in response to determining the patient position; and

detecting that the patient has fallen in response to the determined patient position and acceleration magnitude.

2. The method of claim 1, wherein determining the patient position comprises:

determining a three-dimensional reference vector from the sensed signal corresponding to an upright patient position,

determining a current position vector from the low sampling rate sampled signal; and

computing a current angle between the reference vector and the current position vector.

3. The method of claim 2 further comprising comparing the computed current angle to a first threshold angle and determining the acceleration magnitude in response to the comparison.

4. The method of claim 3 wherein detecting that the patient has fallen further comprises:

computing a previous angle between the reference vector and a previous position vector determined from the low sampling rate sampled signal, the previous position vector occurring prior to the current position vector; and

comparing the previous angle to a second threshold angle, the second threshold angle being less than the first threshold angle.

5. The method of claim 4 wherein detecting that the patient has fallen further comprises:

computing a next angle between the reference vector and a next position vector determined from the low sampling rate sampled signal, the next position vector occurring after the current position vector; and

comparing the next angle to a third threshold angle, the third threshold angle being greater than the first threshold angle.

6. The method of claim 5 wherein the second threshold angle corresponding to an upright position and the third threshold angle corresponding to a non-upright position.

7. The method of claim 2 wherein the low sampling rate being up to approximately 1 sample per second.

8. The method of claim 2 further comprising setting an analysis window in response to detecting the current angle exceeding the first threshold, and wherein detecting the patient fall comprises detecting the magnitude crossing a predetermined magnitude threshold within the analysis window.

9. The method of claim 8 wherein the computed magnitude being a Euclidean norm of the high rate sampled signal.

10. The method of claim 8 wherein the analysis window beginning at least approximately two low sampling rate signal points prior to the current angle exceeding the first threshold and ending at least approximately one low sampling rate signal point after the current angle exceeding the first threshold.

11. The method of claim 8 wherein the magnitude is computed in response to the current angle exceeding the first threshold.

12. The method of claim 11 wherein the magnitude threshold corresponds to a high acceleration occurring upon patient impact with a surface.

13. The method of claim 11 wherein the magnitude threshold corresponds to an acceleration less than gravity occurring during a patient fall, prior to patient impact with a surface.

14. The method of claim 6 wherein detecting that the patient has fallen further comprises determining a difference between two of the previous angle, the current angle and the next angle, comparing the difference between the two angles to a difference between the second threshold angle and the third threshold angle, and detecting that the patient has fallen if the difference between the two angles exceeds the difference between the second and third thresholds when the two angles occur within a predetermined maximum time interval.

15. A medical device, comprising;

a physiological sensor for sensing a signal in a patient;

a three-axis accelerometer for generating a 3D accelerometer signal;

means for sampling the accelerometer signal at a low sampling rate for providing a low sample rate signal;

means for sampling the accelerometer signal at a high sampling rate greater than the low sampling rate for providing a high sample rate signal; and

a processor receiving the low sample rate signal and configured to determine a patient position using the low sample rate signal, determine an acceleration magnitude using the high sample rate signal in response to determining the patient position, and detect that the patient has fallen in response to the determined patient position and acceleration magnitude.

16. The device of claim 15 further comprising:

means for computing a three-dimensional reference vector corresponding to an upright patient position using the 3D accelerometer signal;

means for determining a current position vector from the low sampling rate signal; and

means for computing a current angle between the reference vector and the current position vector.

17. The device of claim 16 wherein the processor is further configured to compare the computed current angle to a first threshold angle and determines the acceleration magnitude in response to the comparison.

18. The device of claim 17 wherein the processor further includes:

means for determining a previous position vector from the low sample rate signal, the previous position vector occurring prior to the current position vector;

means for computing a previous angle between the reference vector and the previous position vector;

means for comparing the previous angle to a second threshold angle less than the first threshold angle; and

means for detecting that the patient has fallen in response to the previous angle being less than the second threshold angle and the current angle being greater than the first threshold angle.

19. The device of claim 18 wherein the processor is further being configured to:

determine a next position vector from the low sample rate signal, the next position vector occurring prior to the current position vector;

compute a next angle between the reference vector and the next position vector;

compare the next angle to a third threshold angle greater than the first threshold angle; and

detect the patient has fallen in response to the next angle being greater than the third threshold angle.

20. The device of claim 19 wherein the second threshold angle corresponding to an upright position and the third threshold angle corresponding to a non-upright position.

21. The device of claim 15 wherein the low sampling rate being up to approximately 1 sample per second.

22. The device of claim 17 wherein the processor further configured to set an analysis window in response to the detecting the current angle exceeding the first threshold, and wherein detecting the patient fall comprises detecting the magnitude crossing a predetermined magnitude threshold within the analysis window.

23. The device of claim 22 wherein the computed magnitude being a Euclidean norm of the sampled three-dimensional position vector.

24. The device of claim 22 wherein the analysis window beginning at least two low sample rate signal points prior to the current angle exceeding the first threshold and ending at least one low sample rate signal point after the current angle exceeding the first threshold.

25. The device of claim 18 wherein the magnitude threshold corresponds to a high acceleration occurring upon patient impact with a surface.

26. The device of claim 18 wherein the magnitude threshold corresponds to an acceleration magnitude less than gravity occurring during a patient fall, prior to patient impact with a surface.

27. The device of claim 20 wherein detecting that the patient has fallen further comprises determining a difference between two of the previous angle, the current angle and the next angle, comparing the difference between the two angles to a difference between the second threshold angle and the third threshold angle, and detecting that the patient has fallen if the difference between the two angles exceeds the difference between the second and third thresholds when the two angles occur within a predetermined maximum time interval.

28. A computer readable medium for storing a set of instructions which when implemented in a system cause the system to:

sense a signal from the three-axis accelerometer;

sample the sensed signal at a low sampling rate;

determine a patient position using the low sampling rate sampled signal;

sample the sensed signal at a high sampling rate;

compute an acceleration magnitude using the high sampling rate signal in response to the determined patient position; and

detect that a patient has fallen in response to the patient position and the computed magnitude