Methods and apparatus for improving time domain relationships between signals obtained from respiration

• US 20060253038A1
• Filed: 07/10/2006
• Published: 11/09/2006
• Est. Priority Date: 02/22/2000
• Status: Active Grant

• Current Assignee: Joseph A. Orr, Kai Kuck, Lara Brewer
• Original Assignee: Joseph A. Orr, Kai Kuck, Lara Brewer
• Inventors: Kuck, Kai, Orr, Joseph A., Brewer, Lara
• Granted Patent: US 8,398,559 B2
• Time in Patent Office: Days
• US Class Current: 600/484
• CPC Class Codes: A61B 5/029 Measuring or recording blood output from the heart, e.g. minute volume A61B8/065 takes precedence
• CPC Class Codes: A61B 5/0836 Measuring rate of CO2 production
• CPC Class Codes: G01N 2001/2244 Exhaled gas, e.g. alcohol detecting
• CPC Class Codes: G01N 33/497 of gaseous biological material, e.g. breath

Abstract

Methods for estimating the volume of the carbon dioxide stores of an individual'"'"'s respiratory tract include determining a carbon dioxide store volume at which a correlation between corresponding signals of carbon dioxide elimination and an indicator of the content of carbon dioxide in blood of the individual is optimized. The estimate of the volume of carbon dioxide stores, which comprises a model of the respiratory tract, or lungs, of the individual, may be used as a transformation to improve the accuracy of one or both of the carbon dioxide elimination and carbon dioxide content signals. Transformation, or filtering, algorithms are also disclosed, as are systems in which the methods and algorithms may be used. The methods, algorithms, and systems may be used to accurately and noninvasively determine one or both of the pulmonary capillary blood flow and cardiac output of the individual.

Claims

1. A method for improving a time domain relationship between two signals obtain from respiration of a subject, comprising:

measuring a first respiratory signal corresponding to a respiratory system;

substantially concurrently measuring a second respiratory signal corresponding to a circulatory system;

generating a filter that improves a correlation between the first respiratory signal and the second respiratory signal in the time domain.

2. The method of claim 1, wherein measuring the first respiratory signal comprises measuring a signal that corresponds to carbon dioxide elimination.

3. The method of claim 2, wherein substantially concurrently measuring the second respiratory signal comprises measuring a signal that corresponds to an amount of end tidal carbon dioxide.

4. The method of claim 1, wherein generating the filter comprises generating a filter comprising an algorithm that employs a transformation coefficient based on carbon dioxide stores of the subject.

5. The method of claim 4, wherein generating the filter comprises generating a filter comprising an algorithm that employs a transformation coefficient based on at least a functional residual capacity of the subject.

6. The method of claim 5, wherein generating the filter comprises generating a filter comprising an algorithm that employs a transformation coefficient also based on carbon dioxide within respiratory tissues of the subject.

7. The method of claim 6, further comprising:

estimating carbon dioxide excretion for the at least another breath.

8. The method of claim 7, wherein estimating carbon dioxide excretion for the at least another breath includes accounting for carbon dioxide stores in the respiratory tract of the subject during the at least another breath.

9. The method of claim 8, wherein accounting for carbon dioxide stores comprises accounting for functional residual capacity during the at least another breath.

10. The method of claim 8, wherein accounting for carbon dioxide stores comprises accounting for functional residual capacity and carbon dioxide within tissues of the respiratory tract during the at least another breath.

11. The method of claim 7, wherein estimating carbon dioxide excretion of the at least another breath includes employing the formula:

{dot over (V)}_MCO₂(n)+{circumflex over (V)}_A*(n)[f_ACO₂(n)−

f_ACO₂(n−

1)]RR,

where {dot over (V)}_MCO₂(n) is the carbon dioxide elimination measurement for the at least another breath, {circumflex over (V)}_A*(n) is an estimate of a volume of the carbon dioxide stores of the respiratory tract of the subject during the at least another breath, [f_ACO₂(n)−

f_ACO₂(n−

1)] is a difference between a fraction of carbon dioxide in alveoli of the respiratory tract of the subject for the at least another breath and an immediately preceding breath, and RR is a respiratory rate of the subject.

12. The method of claim 7, wherein generating the filter comprises employing the following formula:

$\left(1-\mathrm{\alpha <br><br>}\right)<br><br>{\stackrel{.}{V}}_M<br><br>{\mathrm{CO}}_2<br><br>\left(n\right)+\mathrm{\alpha <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\widehat{\stackrel{.}{V}}}_B<br><br>{\mathrm{CO}}_2<br><br>\left(n-1\right),$

where {dot over (V)}_MCO₂(n) is a measure of an indicator of carbon dioxide elimination, {circumflex over ({dot over (V)})}_BCO₂(n−

1) is the estimate of carbon dioxide excretion of another breath, and α

is the transformation coefficient based on an estimate of the carbon dioxide stores of the respiratory tract of the subject.

13. The method of claim 1, further comprising:

optimizing the filter.

14. The method of claim 13, wherein optimizing comprises determining an optimal value for the transformation coefficient.

15. The method of claim 14, wherein determining the optimal value comprises at least one of iterative searching, rote searching, gradient searching, use of a set of predetermined equations, and adaptive filtering.

16. The method of claim 1, wherein generating the filter comprises generating a filter that effectively delays the first respiratory signal.

17. The method of claim 1, wherein generating the filter comprises generating a filter that effectively advances the second respiratory signal.

18. A respiratory monitoring device configured to improve a time domain relationship between a first respiratory signal that corresponds to a respiratory system and substantially concurrently obtained second respiratory signal that corresponds to a circulatory system, comprising:

at least one processing element programmed to generate a filter comprising an algorithm that employs a transformation coefficient based on carbon dioxide stores of a respiratory tract of the subject to at least one of the first and second respiratory signals.

19. The respiratory monitoring device of claim 18, wherein generating the filter comprises generating a filter that effectively delays the first respiratory signal.

20. The respiratory monitoring device of claim 18, wherein generating the filter comprises generating a filter that effectively advances the second respiratory signal.

21. The respiratory monitoring device of claim 18, wherein the first respiratory signal comprises a signal that corresponds to carbon dioxide elimination.

22. The respiratory monitoring device of claim 21, wherein the second respiratory signal comprises a signal that corresponds to an amount of end tidal carbon dioxide.

23. The respiratory monitoring device of claim 18, wherein the filter employs a transformation coefficient based on carbon dioxide stores of the subject.

24. The respiratory monitoring device of claim 23, wherein the filter comprises an algorithm that employs a transformation coefficient based on at least a functional residual capacity of the subject.

25. The respiratory monitoring device of claim 24, wherein the filter comprises an algorithm that employs a transformation coefficient also based on carbon dioxide within respiratory tissues of the subject.

26. The respiratory monitoring device of claim 18, wherein the filter comprises an algorithm that considers an estimate of carbon dioxide excretion for a different breath than a breath from which the first and second signals are obtained.

27. The respiratory monitoring device of claim 26, wherein the at least one processing element is programmed to estimate carbon dioxide excretion for the different breath by employing the formula:

{dot over (V)}_MCO₂(n)+{circumflex over (V)}_A*(n)[f_ACO₂(n)−

f_ACO₂(n−

1)]RR,

where {dot over (V)}_MCO₂(n) is carbon dioxide elimination measurement for the different breath, {circumflex over (V)}_A*(n) is an estimate of a volume of the carbon dioxide stores of the respiratory tract of a subject during the different breath, [f_ACO₂(n)−

f_ACO₂(n−

1)] is a difference between a fraction of carbon dioxide in alveoli of the respiratory tract of the subject for the different breath and an immediately preceding breath, and RR is a respiratory rate of the subject.

28. The respiratory monitoring device of claim 26, wherein the at least one processing element is programmed to apply the following formula in generating the filter:

$\left(1-\mathrm{\alpha <br><br>}\right)<br><br>{\stackrel{.}{V}}_M<br><br>{\mathrm{CO}}_2<br><br>\left(n\right)+\mathrm{\alpha <br><br>}<br><br>{\widehat{\stackrel{.}{V}}}_B<br><br>{\mathrm{CO}}_2<br><br>\left(n-1\right),$

where {dot over (V)}_MCO₂(n) is a measure of an indicator of carbon dioxide elimination,

${\widehat{\stackrel{.}{V}}}_B<br><br>{\mathrm{CO}}_2<br><br>\left(n-1\right)$

is the estimate of carbon dioxide excretion of the different breath, and α

is the transformation coefficient based on an estimate of the carbon dioxide stores of the respiratory tract of the subject.

29. The respiratory monitoring device of claim 18, wherein the at least one processing element is further programmed to optimize the filter.

30. The respiratory monitoring device of claim 29, wherein the at least one processing element is programmed to determine an optimal value for the transformation coefficient.