Method and apparatus for determining respiratory system resistance during assisted ventilation

• US 20030159695A1
• Filed: 02/24/2003
• Published: 08/28/2003
• Est. Priority Date: 04/26/2000
• Status: Active Grant

• Current Assignee: Province of Manitoba
• Original Assignee: Province of Manitoba
• Inventors: Younes, Magdy
• Granted Patent: US 6,837,242 B2
• Time in Patent Office: Days
• US Class Current: 128/204.18
• CPC Class Codes: A61M 16/00 Devices for influencing the respiratory system of patients by gas treatment, e.g. mouth-to-mouth respiration; Tracheal tubes stimulating the respiratory movement by mechanical, pneumatic or electrical means, iron lungs combined with gas breathing means A61H31/00
• CPC Class Codes: A61M 16/0051 with alarm devices
• CPC Class Codes: A61M 16/0063 Compressors
• CPC Class Codes: A61M 16/026 specially adapted for predicting, e.g. for determining an information representative of a flow limitation during a ventilation cycle by using a root square technique or a regression analysis
• CPC Class Codes: A61M 2016/0021 with a proportional output signal, e.g. from a thermistor
• CPC Class Codes: A61M 2016/0027 pressure meter
• CPC Class Codes: A61M 2016/0036 in the breathing tube and used in both inspiratory and expiratory phase
• CPC Class Codes: A61M 2230/46 Resistance or compliance of the lungs

Abstract

Method and apparatus are described for determining respiratory system resistance (R) in a patient receiving gas from a ventilator. A negative pulse in the pressure and/or flow output of the ventilator during selected inflation cycles is generated and Paw, V dot and V are measured at a point (T₀) near the beginning of the pulse, at a point (T₁) near the trough of the negative pulse and at a point (T₋₁) preceding T₀. The value of R is calculated from the difference between Paw. V dot and V at T_O and at T_I and where the change in patient generated pressure (Pmus) in the interval T_O−T_I is estimated by extrapolation from the different between Paw, V dot and V and T₀ and at T_−I, in accordance with Equation 8.

Claims

1. A method of determining respiratory system resistance (R) in a patient receiving gas from a ventilatory assist device (ventilator), comprising:

estimating the flow rate ({dot over (V)}) and volume (V) of gas received by the patient from the ventilator;

estimating pressure near the airway of the patient (Paw);

generating a signal that results in a step decrease (negative pulse) in the pressure and/or flow output of the ventilator during selected inflation cycles;

measuring Paw, {dot over (V)} and V at a point (T_O) near the beginning of the pulse (Paw_O, {dot over (V)}, V_O), at a point (T₁) near the trough of the negative pulse (Paw₁, {dot over (V)}₁, V₁), and at a point (T<sub>−

1</sub>) preceding T_O but after the onset of inspiratory effort (Paw<sub>−

1</sub>, {dot over (V)}<sub>−

1</sub>, V<sub>−

1</sub>); and

calculating the value of resistance (R) from the differences between Paw, {dot over (V)} and V at T_O and at T₁ and where the change in patient generated pressure (Pmus) in the interval T_O→

T_I (Δ

Pmus ( T_O→

T_I)) is estimated, by extrapolation, from the differences between Paw, {dot over (V)} and V at T_O and at T<sub>−

1</sub>, in accordance with equation (8).

2. The method of claim 1 wherein the estimating by extrapolation step is modified by estimating Δ

Pmus (T_O→

T_I) from the differences between Paw, {dot over (V)} and V values obtained at two time points preceding T_O, as opposed to T_O and a single preceding time point (T<sub>−

1</sub>).

3. The method of claim 1 wherein the estimating by extrapolation step is modified by estimating Δ

Pmus (T_O→

T₁) using regression coefficients obtained from regression analysis of Paw, {dot over (V)} and V values measured at multiple (>

2) points preceding the pulse.

4. The method of claim 1 wherein the estimating step to estimate Δ

Pmus (T_O→

T₁) is modified by estimating by interpolation, from the differences between Paw, {dot over (V)} and V values obtained at T_O and at a second point (T₂) beyond T_I, in addition to, or instead of, extrapolation of differences between values at T_O and T<sub>−

1</sub>.

5. The method of claim 1 or 3 wherein Δ

Pmus (T_O→

T_I) is estimated by back extrapolation of values obtained beyond T_I.

6. The method of any one of claim 1 to 5 wherein the single R value in equation (8) is replaced by mathematical functions that allow for non-linear pressure-flow relations.

7. The method of claim 6, wherein said mathematical function is given by R=K₁+K₂ {dot over (V)}, wherein K1 and K2 are the coefficients defining the non-linear pressure-flow relation.

8. The method of claim 7, wherein K₂ is replaced by a known or assumed K₂ value of the endotracheal tube of the patient.

9. The method of claim 6 or 7 wherein the coefficients defining the non-linear pressure-flow relation (K₁, K₂) are obtained by regression analysis performed on the results of two or more pulses applied in separate breaths.

10. The method of any one of claim 1 to 8, wherein a default elastance value (E) is used in lieu of an actually measured elastance value for the sake of computing differences in elastic recoil pressure between T₀, T_I and T₋

I in equation (8).

11. The method of any one of claims 1 to 10, wherein positive pulses are delivered instead of, or in addition to, negative pulses and the T_I values of Paw, {dot over (V)} and V are measured at or near peak Paw or flow of the positive pulse.

12. The method of any one of claims 1 to 11, including automatically adjusting the amplitude of the pulse depending on the response to previous pulses.

13. The method of any one of claims 1 to 12, including automatically adjusting the timing of pulse application during the inflation phase.

14. The method of any one of claims 1 to 13, including automatically adjusting the shape of the pulse to insure the presence of a flat segment in the Paw/flow signal during the pulse for use in measuring T_I values.

15. The method of any one of claims 1 to 14, wherein the pulses are delivered at random intervals.

16. The method of any one of claims 1 to 15 including user selecting one or more pulse characteristics.

17. The method of any one of claims 1 to 16, wherein the resistance results (R) are reported as averages of the results of several pulses.

18. The method of any one of claims 1 to 17, wherein resistance results (R) are stored over time to permit the display of time dependent trends.

19. The method of any one of claims 1 to 18 including identifying pulses with results that fall outside the normal variability of the data as determined from several data sampling and excluding the results of these pulses from analysis and determination of R.

20. The method of any one of claims 1 to 19 including deleting early data as new data are acquired and reporting the results of the determination of R for a specified number of pulses.

21. The method of claim 20 wherein the specified number of pulses is selected, either by a user or, in the absence of user input, as a default value (e.g. 20).

22. The method as claimed in any one of claims 1 to 21, wherein the step decrease or increase in Paw or {dot over (V)} is produced by an electromechanical system attached to the external tubing of the ventilator as opposed to directly interfacing with the ventilator control system.

23. The method of any one of claims 1 to 22, wherein the results of the resistance values are used in closed loop control of an assist level provided by the ventilator.

24. An apparatus which interfaces with ventilatory assist devices (ventilators) determining respiratory system resistance (R), comprising:

a flowmeter, with associated electronic circuitry, that estimates the flow rate ({dot over (V)}) and volume (V) of gas received by a patient;

a pressure sensor that estimates pressure near the airway of the patient (Paw); and

electronic circuitry which receives the Paw, {dot over (V)} and V signals from above mentioned circuitry and which is also connected to the control system of the ventilator, comprising;

circuitry that generates an output that results in a step decrease (negative pulse) in the pressure and/or flow output of the ventilator during selected inflation cycles;

circuitry that measures Paw, {dot over (V)} and V at a point (T_O) near the beginning of the pulse (Paw_o, {dot over (V)}o, V_O), at a point (T_I) near the trough of the negative pulse (Paw₁, {dot over (V)}₁, V₁), and at a point (T₁) preceding T_O but after the onset of inspiratory effort (Paw<sub>−

1</sub>, {dot over (V)}<sub>−

1</sub>, V<sub>−

1</sub>);

circuitry to calculate the value of resistance (R) from the differences between Paw, {dot over (V)} and V at T_O and at T_I and where the change in patient generated pressure (Pmus) in the interval T_O→

T_I (Δ

Pmus (T_O→

T_O)) is estimated, by extrapolation, from the differences between Paw, {dot over (V)} and V at T_O and at T<sub>−

1</sub>, in accordance with equation (8).

25. The apparatus of claim 24 wherein Δ

Pmus (T_O→

T_I) is estimated, by extrapolation, from the differences between Paw, {dot over (V)} and V values obtained at two time points preceding T_O, as opposed to T_O and a single preceding time point.

26. The apparatus of claim 24 wherein Δ

Pmus (T_O→

T_I) is estimated, by extrapolation, using regression coefficients obtained from regression analysis of Paw, {dot over (V)} and V values measured at multiple (>

2) points preceding the pulse.

27. The apparatus of claim 24 wherein Δ

Pmus (T_O→

T_I) is estimated, by interpolation, from the differences between Paw, {dot over (V)} and V values obtained at T_O and at a second point (T₂) beyond T_I in addition to, or instead of, as opposed to extrapolation of differences between values at T_O and T<sub>−

1</sub>.

28. The apparatus of claim 24 or 26 wherein Δ

Pmus (T_O→

T_I) is estimated by back entrapolation of values obtained beyond T_I.

29. The apparatus of any one of claims 24 to 28 wherein the single R value in the equations is replaced by mathematical functions that allow for non-linear pressure-flow relations.

30. The apparatus of claim 29 wherein said mathematical function is given by R=K₁+, wherein K1 and K2 are coefficients defining the non-linear pressure-flow relation.

31. The apparatus of claim 30 wherein K₂ is replaced by a known or assumed K₂ value of the endotracheal tube of the patient.

32. The apparatus of claim 29 or 30 wherein the coefficients defining the non-linear pressure-flow relation (K₁, K₂) are obtained by regression analysis performed on the results of two or more pulses applied in separate breaths.

33. The apparatus of any one of claims 24 to 31 wherein a default elastance value (E) is used in lieu of an actually measured elastance value for the sake of computing differences in elastic recoil pressure between T_O, T₁ and T₋

I.

34. The apparatus of any one of claims 24 to 33 wherein positive pulses are delivered instead of, or in addition to, negative pulses and the T_I values of Paw, {dot over (V)} and V are measured at or near peak Paw or flow of the positive pulse.

35. The apparatus of any one of claims 24 to 34 including algorithms to automatically adjust the amplitude of the pulse depending on response to previous pulses.

36. The apparatus of any one of claims 24 to 35 including algorithms which automatically adjust the timing of pulse application during the inflation phase.

37. The apparatus of any one of claims 24 to 36 including algorithms that automatically adjust the shape of the pulse to insure the presence of a flat segment in the Paw/flow signal during the pulse for use in measuring T_I values.

38. The apparatus of any one of claims 24 to 37 wherein the pulses are delivered at random intervals.

39. The apparatus of any one of claims 24 to 38 including a user interface that permits the user to select one or more pulse characteristics.

40. The apparatus of any one of claims 24 to 39 wherein the resistance results are reported as averages of the results of several pulses.

41. The apparatus of any one of claims 24 to 40 wherein resistance results are stored over minutes, hours or days to permit the display of time dependent trends.

42. The apparatus of any one of claims 24 to 41 including algorithms which identify pulses with results that fall outside the normal variability of the data and which exclude the results of these pulses from analysis.

43. The apparatus of any one of claims 24 to 42 including algorithms which delete early data as new data are acquired reporting the results of a specified number of pulses.

44. The apparatus of claim 43 wherein the specified number of pulses is selected by user or, in absence of user input, as a default value (e.g. 20).

45. The apparatus of any one of claims 24 to 44 wherein some or all necessary components are incorporated within the main body of the ventilator.

46. The apparatus of any one of claims 24 to 45 wherein the step decrease or increase in Paw or {dot over (V)} are produced by an electromechanical system attached to the external tubing of the ventilator as opposed to directly interfacing with the ventilator control system.

47. The apparatus of any one of claims 24 to 46 wherein the results of the resistance values are used in closed loop control of the assist level provided by the ventilator.