Division approximation for implantable medical devices

• US 20060094973A1
• Filed: 10/29/2004
• Published: 05/04/2006
• Est. Priority Date: 10/29/2004
• Status: Active Grant

• Current Assignee: Medtronic Incorporated (Medtronic Plc)
• Original Assignee: Medtronic Incorporated (Medtronic Plc)
• Inventors: Drew, Touby A.
• Granted Patent: US 7,526,340 B2
• Time in Patent Office: Days
• US Class Current: 600/544
• CPC Class Codes: G06F 7/4833 Logarithmic number system
• CPC Class Codes: G06F 7/535 Dividing only

Abstract

Methods and devices for performing division approximation in implantable and wearable self-powered medical devices. The present invention provides rapid methods for performing an approximation of division on fixed point numbers, where the methods are easily implemented in small, low power consumption devices as may be found in implantable medical devices. One example of use is in rapidly determining the approximate ratio between foreground and background activity in seizure detection algorithms. Some methods approximate the ratio of Numerator (N) to Denominator (D) by raising 2 to the power of the difference in the number of zeros to the left of the Most Significant Set Bit (MSSB) of D vs. N. Some methods may also pad bits to the right of the approximate ratio MSSB using bits from the right of the N MSSB, and/or pre-process the smaller of D or N by rounding the value upward. Methods may be implemented in firmware and/or in discrete logic.

Claims

1. A method for detecting a seizure in an implantable or wearable medical device (IMD), the method comprising:

executing an algorithm having a plurality of ratio determinations of foreground seizure energies divided by background seizure energies, in which the algorithm uses a ratio estimation in place of fixed point division to determine a majority of the ratios,

wherein the ratio estimation includes estimating a Numerator (N) divided by a Denominator (D); and

generating an indication of a detected seizure at least in part as a function of the ratio determinations.

2. The method of claim 1, in which the ratio estimation includes estimating the Numerator.

3. The method of claim 1, in which the ratio estimating includes estimating the Denominator.

4. The method of claim 1, in which the ratio estimating includes estimating the Numerator and the Denominator.

5. The method of claim 1, in which the estimating includes obtaining a result as a function of 2 raised to the power of the difference between the most significant set bit position (MSSB) of the D and the MSSB of the N.

6. The method of claim 5, in which the ratio result of claim 1 has a MSSB, further comprising setting a bit just to the right of the ratio MSSB to equal the value of a bit just to the right of the N MSSB.

7. The method of claim 5, in which the ratio result of claim 5 has a MSSB, further comprising setting a number of M bits to the right of the ratio MSSB to have the value as the same number of M bits just to the right of the N MSSB.

8. The method of claim 7, in which M has a value of at least 1.

9. The method of claim 7, in which M is equal to all bits to the right of the MSSB of the ratio result of claim 7.

10. The method of claim 5, in which the smaller of the N or the D is first processed by rounding up the smaller of the N or the D, and the rounded up number is used in the method of claim 5.

11. The method of claim 7, in which the smaller of the N or the D is first processed by rounding up the smaller of the N or the D, and the rounded up number is used in the method of claim 5.

12. A computer readable medium having computer executable instructions for executing the method of claim 1.

13. A self-powered implantable or wearable medical device having executable instructions or logic for executing the method of claim 1.

14. A method for detecting a seizure in an implantable or wearable medical device, the method comprising:

obtaining a first plurality of electrical signal samples indicative of seizure energy over a time first time window;

obtaining a second plurality of electrical signal samples indicative of seizure energy over a second time window, wherein the second time window is longer in duration and extends further into the past relative to the first window;

approximating a ratio of a first average of the first window values divided by a second average of the second window values; and

generating an indication of the seizure at least in part as a function of the ratio approximations.

15. The method of claim 14, in which the ratio approximating includes estimating the Numerator.

16. The method of claim 14, in which the ratio approximating includes estimating the Denominator.

17. The method of claim 14, in which the ratio approximating includes estimating the Numerator and the Denominator.

18. The method of claim 14, in which the ratio approximating includes raising 2 to the power of the difference in most significant set bit positions of the first signal and the second signal.

19. The method of claim 18, wherein the ratio result of claim 18 has a MSSB, further comprising setting a number of M bits to the right of the ratio MSSB to have the value as the same number of bits M just to the right of the N MSSB.

20. The method of claim 19, in which M has a value of at least 1.

21. The method of claim 18, in which the smaller of the N or the D is first processed by rounding up the smaller of the N or the D, and the rounded up number is used in the method of claim 18.

22. The method of claim 19, in which the smaller of the N or the D is first processed by rounding up the smaller of the N or the D, and the rounded up number is used in the method of claim 18.

23. A self-powered implantable or wearable medical device having executable instructions or logic for executing the method of claim 18.

24. A method for determining a physiological measurement ratio in a self powered Implantable Medical Device (IMD) or a self-powered wearable medical device the method comprising:

obtaining a first signal indicative of a first physiological measurement, wherein the first signal is represented in a first fixed point number;

obtaining a second signal indicative of a second physiological measurement, wherein the second signal is represented in a second fixed point number; and

determining a ratio approximation of a ratio of the first number divided by the second number.

25. The method of claim 24, in which the ratio approximation determining includes estimating the first fixed point number.

26. The method of claim 24, in which the ratio approximation determining includes estimating the second fixed point number.

27. The method of claim 24, in which the ratio approximation determining includes estimating the first and second fixed point numbers.

28. The method of claim 24, in which the ratio approximation determining includes raising 2 to the power of the difference in most significant set bit positions between the first number and the second number.

32. The method of claim 28, in which the first number is indicative of a physiological signal and in which the second number is indicative of physiological noise, and in which the ratio is a signal to noise ratio.

33. A computer readable medium having computer executable instructions for executing the method of claim 28.

34. A self-powered implantable or wearable medical device having executable instructions or logic for executing the method of claim 28.

29. The method of claim 29, in which the first number is an average over a first time period and in which the second number is an average over a second time period, where the fist time period is shorter than the second time period, wherein the first and second signals are both derived from a common signal source.

30. The method of claim 29, in which the first time period is more recent than the second time period.

31. The method of claim 29, in which the first number is indicative of brain neurological activity extending back a first period, and in which the second number is indicative of average neurological activity extending back a second time period that is longer than the first time period.

35. A method for determining at least an approximate ratio between a numerator (N) and a denominator (D), both having most significant set bits (MSSBs), as least in part as a function of a method, the method comprising:

estimating the ratio as 2 raised to the power of the difference between the number of zeros to the left of the MSSB in the D and the number of zeros to the left of the MSSB in the N.

36. The method of claim 35, wherein the ratio result of claim 35 has a MSSB, further comprising setting a bit just to the right of the ratio MSSB to equal the value of a bit just to the right of the N MSSB.

37. The method of claim 35, wherein the ratio result of claim 35 has a MSSB, further comprising setting a number of M bits to the right of the ratio MSSB to have the value of the same number of bits M just to the right of the N MSSB.

38. The method of claim 37, in which M has a value of at least 1.

39. The method of claim 37, in which M is equal to the number of all bits to right of the MSSB of the ratio result of claim 37.

40. The method of claim 35, in which the smaller of the N or the D is processed by rounding up the smaller of the N or the D, and the rounded up number is used in the method of claim 35.

41. The method of claim 37, in which the smaller of the N or the D is first processed by rounding up the smaller of the N or the D, and the rounded up number is used in the method of claim 35.

42. A computer readable medium having computer executable instructions for executing the method of claim 35.

43. A self-powered implantable or wearable medical device having executable instructions or logic for executing the method of claim 35.

44. A method for performing a division approximation of a denominator by a numerator to obtain a result, the method comprising:

determining the result as a function of a first number of zero bits more significant than the Most Significant Set Bit (MSSB) of the denominator and a second number of zero bits more significant than the numerator MSSB, including raising the difference between the first number and the second number to the second power.

45. The method of claim 44, further comprising filling a least one bit to the right of the MSSB of the claim 44 result with at least one corresponding bit to the right of the MSSB of the denominator.

46. The method of claim 45, in which the determining consists essentially of the steps of claim 44.

47. A computer readable medium having computer executable instructions for executing the method of claim 44.

48. A method for determining a ratio approximation in a self-powered implantable or wearable medical device, the method comprising:

obtaining a first fixed point number (N) indicative of a first physiological parameter measurement;

obtaining a second fixed point number (D) indicative of a second physiological parameter measurement; and

calculating the ratio approximation of N/D by using a method consisting essentially of raising 2 to the power of the difference in most significant set bit positions between the first number and the second number.

49. The method of claim 48, in which the first and second physiological parameter measurements are both derived from the same physiological source, but where the first physiological parameter is indicative of a background level and the second physiological parameter is indicative of a more recent measurement than the first.

50. The method of claim 48, in which the first and second physiological parameters are both derived from the same source but over different time periods.

51. A computer readable medium having computer executable instructions for executing the method of claim 48.

52. A self-powered implantable or wearable medical device having executable instructions or logic for executing the method of claim 48.

53. A method for determining an approximate ratio of a foreground seizure energy divided by a background seizure energy in a seizure detection calculation in a self powered implantable or wearable medical device, the method comprising:

obtaining a first fixed point number (N) indicative of the foreground seizure energy;

obtaining a second fixed point number (D) indicative of the background seizure energy; and

calculating an approximate ratio of N/D by using a method consisting essentially of raising 2 to the power of the difference in most significant set bit positions between the first number and the second number.

54. The method of claim 53, in which the method consists essentially of the steps of claim 53.

55. An implantable medical device having a processor executing the method of claim 53.

56. A wearable medical device having a processor executing the method of claim 53.

57. A computer readable medium having computer executable instructions for executing the method of claim 53.

58. A computer readable medium having computer executable instructions for executing the method of claim 54.