Division approximation for implantable medical devices

• US 7,526,340 B2
• Filed: 10/29/2004
• Issued: 04/28/2009
• Est. Priority Date: 10/29/2004
• Status: Expired due to Fees

• Current Assignee: Medtronic Incorporated (Medtronic Plc)
• Original Assignee: Medtronic Incorporated (Medtronic Plc)
• Inventors: Drew, Touby A.
• Primary Examiner(s): EVANISKO, GEORGE ROBERT
• Application Number:
  US10/976,474
• Publication Number:
  US 20060094973A1
• Time in Patent Office: 1,642 Days
• Field of Search: 600/300, 600/301, 600508-528, 607 1- 46
• US Class Current: 607/45
• 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 determining a physiological measurement ratio for incorporation in a neurological event detection calculation used to analyze a neurological event status of a patient and carried out by 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;

determining a ratio approximation of a ratio of the first fixed point number divided by the second fixed point number, the ratio approximation determining including raising 2 to the power of the difference between a most significant set bit position of the second fixed point number and a most significant set bit position of the first fixed point number; and

detecting a neurological event based on the ratio approximation with the self-powered IMD or the self-powered wearable medical device.

2. The method of claim 1, in which the ratio approximation determining includes estimating the first fixed point number.

3. The method of claim 1, in which the ratio approximation determining includes estimating the second fixed point number.

4. The method of claim 1, in which the ratio approximation determining includes estimating the first and second fixed point numbers.

5. The method of claim 1, in which the ratio approximation determining comprises raising 2 to the power of the difference between the number of zeros to the left of the most significant set bit in the second fixed point number and the number of zeros to the left of the most significant set bit in the first fixed point number.

6. The method of claim 1, in which the first fixed point number is an average over a first time period and in which the second fixed point number is an average over a second time period, where the first time period is shorter than the second time period, and wherein the first and second signals are both derived from a common signal source.

7. The method of claim 6, in which the first time period is more recent than the second time period.

8. The method of claim 6, in which the first fixed point number is a function of brain neurological activity occurring during the first time period, and in which the second fixed point number is a function of average neurological activity occurring during the second time period.

9. The method of claim 1, in which the first fixed point number is indicative of a physiological signal and in which the second fixed point number is indicative of physiological noise.

10. The method of claim 1, further comprising setting a number of M bits to the right of a most significant set bit position of the ratio approximation to have the value as the same number of M bits just to the right of the most significant set bit position of the first fixed point number.

11. The method of claim 10, wherein M has a value of at least 1.

12. The method of claim 11, wherein M has a value equal to the absolute value of the difference between the most significant set bit position of the second fixed point number and the most significant set bit position of the first fixed point number.

13. The method of claim 1, further comprising rounding up the smaller of the first fixed point number and second fixed point number prior to determining the ratio approximation.

14. The method of claim 13, further comprising setting a number of M bits to the right of a most significant set bit position of the ratio approximation to have the value as the same number of M bits just to the right of the most significant set bit position of the first fixed point number.

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

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

17. A method for determining a ratio approximation for incorporation in a neurological event detection calculation used to analyze a neurological event status of a patient and carried out by a self-powered implantable or wearable medical device, the method comprising:

obtaining a first fixed point number correlated to a first physiological parameter measurement;

obtaining a second fixed point number correlated to a second physiological parameter measurement;

calculating a ratio approximation of the first fixed point number over the second fixed point number by using a method consisting essentially of raising 2 to the power of the difference between a most significant set bit position of the second fixed point number and a most significant set bit position of the first fixed point number; and

detecting a neurological event based on the ratio approximation with the self-powered implantable or wearable medical device.

18. The method of claim 17, in which the first and second physiological parameter measurements are both derived from the same physiological source, but where the second physiological parameter measurement is a background level measurement and the first physiological parameter measurement is a more recent measurement than the second physiological parameter measurement.

19. The method of claim 17, in which the first and second physiological parameter measurements are both derived from the same source but over different time periods.

20. A computer readable medium having computer executable instructions for executing the method of claim 17.

21. A self-powered implantable or wearable medical device having executable instructions or logic for executing the method of claim 17.

22. A method for characterizing neurological energy for incorporation in a neurological event detection calculation carried out by a self-powered implantable or wearable medical device to analyze a neurological event status of a patient, the method comprising:

obtaining a first fixed point number correlated to a foreground neurological energy measured by the self-powered implantable or wearable medical device;

obtaining a second fixed point number correlated to a background neurological energy measured by the self powered implantable or wearable medical device; and

determining with the self-powered implantable or wearable medical device an approximate ratio of the foreground neurological energy divided by the background neurological energy by calculating an approximate ratio of the first fixed point number over the second fixed point number by using a method consisting essentially of raising 2 to the power of the difference between a most significant set bit position of the second fixed point number and a most significant set bit position of the first fixed point number.

23. The method of claim 22, further comprising setting a number of M bits to the right of a most significant set bit position of the approximate ratio to have the value as the same number of M bits just to the right of the most significant set bit position of the first fixed point number.

24. The method of claim 23, wherein M has a value of at least 1.

25. The method of claim 24, wherein M has a value equal to the absolute value of the difference between the most significant set bit position of the second fixed point number and the most significant set bit position of the first fixed point number.

26. The method of claim 22, further comprising rounding up the smaller of the first fixed point number and second fixed point number prior to determining the ratio approximation.

27. The method of claim 26, further comprising setting a number of M bits to the right of a most significant set bit position of the approximate ratio to have the value as the same number of M bits just to the right of the most significant set bit position of the first fixed point number.

28. An implantable medical device having a processor programmed with instructions for executing a method for analyzing neurological energy for incorporation in a neurological event determination, the method comprising

obtaining a first fixed point number correlated to a foreground neurological energy;

obtaining a second fixed point number correlated to a background neurological energy; and

determining an approximate ratio of the foreground neurological energy divided by the background neurological energy by calculating an approximate ratio of the first fixed point number over the second fixed point number by using a method consisting essentially of raising 2 to the power of the difference between a most significant set bit position of the second fixed point number and a most significant set bit position of the first fixed point number.

29. A wearable medical device having a processor programmed with instructions for executing a method for analyzing neurological energy for incorporation in a neurological event determination, the method comprising

obtaining a first fixed point number correlated to the foreground neurological energy;

obtaining a second fixed point number correlated to the background neurological energy; and

determining an approximate ratio of a foreground neurological energy divided by a background neurological energy by calculating an approximate ratio of the first fixed point number over the second fixed point number by using a method consisting essentially of raising 2 to the power of the difference between a most significant set bit position of the second fixed point number and a most significant set bit position of the first fixed point number.

30. A computer readable medium having computer executable instructions for causing a processor to execute a method for analyzing neurological energy for incorporation in determining the occurrence of a neurological event in a patient, the method comprising

obtaining a first fixed point number correlated to a foreground neurological energy of the patient;

obtaining a second fixed point number correlated to a background neurological energy of the patient; and

determining an approximate ratio of the foreground neurological energy divided by the background neurological energy by calculating an approximate ratio of the first fixed point number over the second fixed point number by using a method consisting essentially of raising 2 to the power of the difference between a most significant set bit position of the second fixed point number and a most significant set bit position of the first fixed point number.