Plethysmographic signal processing method and system

• US 20030073890A1
• Filed: 10/10/2001
• Published: 04/17/2003
• Est. Priority Date: 10/10/2001
• Status: Abandoned Application

• Current Assignee: Datex-Ohmeda
• Original Assignee: Datex-Ohmeda
• Inventors: Hanna, D. Alan
• Application Number:
  US09/975,289
• Publication Number:
  US 20030073890A1
• Time in Patent Office: Days
• US Class Current: 600/323
• 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

Abstract

The present invention provides a plethysmographic signal processing method and system that achieves improved S/N ratios leading to improved patient heart rate estimates and improved plethysmographic waveform displays. The plethysmographic signal processing method and system of the present invention may be implemented using analog and/or digital components within a pulse oximeter. In one embodiment, first and second plethysmographic signals S₁, S₂ associated with first and second wavelengths, respectively (e.g., infrared and red), are received on first and second channels 210, 212. First and second multipliers 214, 216 multiply the first and second plethysmographic signals S₁, S₂ by first and second multiplication factors T₁, T₂. A summer 218 sums the products from the first and second multipliers 214, 216 to output a composite plethysmographic signal C on an output channel 220. The composite plethysmographic signal C may then be displayed and/or utilized to make heart rate determinations and the like.

Claims

1. A signal processing method for use in plethysmography, said method comprising the steps of:

receiving at least two plethysmographic signals, each plethysmographic signal being associated with a particular wavelength;

multiplying each plethysmographic signal by an associated multiplication factor; and

generating a composite plethysmographic signal comprising a linear combination of the plethysmographic signals by adding the results of the multiplications.

2. The method of claim 1 wherein the multiplication factors are chosen to provide an improved S/N ratio of the composite signal as compared with S/N ratios of the plethysmographic signals over a specified range of SpO₂ levels.

3. The method of claim 2 wherein the specified range of SpO₂ levels is from 40% to 100%.

4. The method of claim 2 wherein the multiplication factors are dependent upon an R value that varies in accordance with an SpO₂ level in arterial blood circulated through a patient tissue site.

5. The method of claim 4 further comprising the step of:

obtaining the multiplication factors from a look-up table comprising sets of multiplication factors cross-referenced with corresponding incremental R values.

6. The method of claim 5 wherein the look-up table includes multiplication factors corresponding with incremental R values ranging from 0.4 to 1.4.

7. The method of claim 6 wherein the R values in the look-up table are incremented in equal increments, the increments being between 0.001 and 0.1.

8. The method of claim 4 wherein there are first and second plethysmographic signals and the wavelength associated with the first plethysmographic signal is between 800 and 950 nm and the wavelength associated with the second plethysmographic signal is between 600 and 700 nm.

9. The method of claim 8 wherein a first multiplication factor designated T₁ associated with the first plethysmographic signal is given by the following formula:

10. The method of claim 9 wherein R is given by the following formula:

11. The method of claim 1 wherein the plethysmographic signals comprise digital signals including pluralities of signal sample values taken at sequential temporal instances and said steps of multiplying and generating are performed for each temporally corresponding signal sample value.

12. A signal processing method for use in plethysmography, said method comprising the steps of:

receiving first and a second plethysmographic signals S₁ and S₂, the first and second plethysmographic signals S₁ and S₂ being associated with first and second wavelengths, respectively;

forming a complex signal vector S, wherein S is given by S=S₁+iS₂;

forming a complex transformation vector T from first and second scalar multiplication factors T₁ and T₂, wherein T is given by T=T₁+iTd₂; and

multiplying the complex signal vector S by the complex transformation vector T to generate a composite plethysmographic signal C.

13. The method of claim 12 wherein the scalar multiplication factors T₁ and T₂ are dependent upon an R value, wherein the R value varies in accordance with an SpO₂ level in arterial blood circulated through a patient tissue site.

14. The method of claim 13 further comprising the step of:

obtaining the scalar multiplication factors T₁ and T₂ from a look-up table comprising a plurality of pairs of scalar multiplication factors T₁ and T₂ cross-referenced with corresponding incremental R values.

15. The method of claim 14 wherein the look-up table includes multiplication factors T₁ and T₂ corresponding with incremental R values ranging from 0.4 to 1.4.

16. The method of claim 15 wherein the R values in the look-up table are incremented in equal increments, the increments being between 0.0001 and 0.1.

17. The method of claim 13 wherein the first scalar multiplication factor T₁ is given by the following formula:

18. The method of claim 17 wherein R is given by the following formula:

19. The method of claim 12 wherein the first and second plethysmographic signals S₁ and S₂ are digital signals including pluralities of signal sample values taken at sequantial temporal instances and said steps of multiplying and generating are performed for each temporally corresponding signal sample value.

20. The method of claim 12 wherein the first and second plethysmographic signals S₁ and S₂ are associated with infrared and red wavelengths, respectively.

21. A plethysmographic signal processing system comprising:

a first input channel for receiving a first plethysmographic signal thereon, said first plethysmographic signal being associated with a first wavelength;

a second input channel for receiving a second plethysmographic signal thereon, said second plethysmographic signal being associated with a second wavelength;

a first multiplier operable to receive the first plethysmographic signal and a first scalar multiplication factor as inputs and output a first product comprising the first plethysmographic signal multiplied by the first scalar multiplication factor;

a second multiplier operable to receive the second plethysmographic signal and a second scalar multiplication factor as inputs and output a second product comprising the second plethysmographic signal multiplied by the second scalar multiplication factor;

a summer operable to receive the first and second products as inputs and add the first and second products to output a composite signal comprising the sum of the first and second products.

22. The system of claim 21 wherein said first channel, said second channel, said first multiplier, said second multiplier, and said summer are implemented in software executable by a digital processor.

23. The system of claim 21 wherein said first channel, said second channel, said first multiplier, said second multiplier, and said summer comprise analog components.

24. The system of claim 21 wherein the first and second scalar multiplication factors are dependent upon an R value, wherein the R value varies in accordance with an SpO₂ level in arterial blood circulated through a patient tissue site.

25. The system of claim 24 further comprising:

a look-up table including a plurality of pairs of first and second scalar multiplication factors cross-referenced with corresponding incremental R values.

26. The system of claim 25 wherein said look-up table includes pairs of first and second multiplication factors corresponding with incremental R values ranging from 0.4 to 1.4.

27. The system of claim 26 wherein the R values in said look-up table are incremented in equal increments, said increments being between 0.001 and 0.1.

28. The system of claim 25 wherein the first scalar multiplication factor designated T₁ is given by the following formula:

29. The system of claim 28 wherein R is given by the following formula:

30. The system of claim 21 wherein the first and second plethysmographic signals are associated with infrared and red wavelengths, respectively.

31. The system of claim 21 further comprising:

at least one additional input channel for receiving at least one additional plethysmographic signal thereon, said at least one additional plethysmographic signal being associated with at least one additional wavelength; and

at least one additional multiplier operable to receive said at least one additional plethysmographic signal and at least one additional scalar multiplication factor as inputs and output at least one additional product comprising said at least one additional plethysmographic signal multiplied by said at least one additional scalar multiplication factor;

said summer being operable to receive the first, second, and at least one additional products as inputs and add the first, second, and at least one additional products together to output a composite signal comprising the sum of the first, second, and at least one additional products.