Method and apparatus for improved flow limitation detection of obstructive sleep apnea

1. A respiratory apparatus comprising:

an air source adapted to selectively provide pressurized breathable air to a patient,

a flow detecting device configured to sense respiratory airflow from the patient and to generate an airflow signal indicative of airflow of a breathing cycle,

an obstruction detector coupled to said flow detecting device and configured to detect partial obstruction of a patient's airway by:

measuring respiratory flow from the patient;

detecting inspiratory flow from the measured respiratory flow;

trimming the inspiratory flow to remove any expiratory pause;

normalizing an effect of overshoot by estimating a peak flow value of a central portion of the inspiratory flow and by resetting each point outside the central portion that has a flow value exceeding the peak flow value;

scaling the inspiratory flow to a reference value;

calculating a mean deviation of the central portion of the inspiratory flow;

calculating a flattening index from the mean deviation; and

filtering the flattening index with a moving average filter, and

a controller coupled to the flow detecting device and arranged to control the operation of the air source and to receive an obstruction signal indicative of a partial obstruction.

2. The apparatus of claim 1 wherein in response to a positive obstruction signal the controller is configured to increase a pressure of the breathable air delivered to the patient.

3. An apparatus for monitoring and/or treating a patient having a sleep disorder, the apparatus comprising

a flow detecting device that senses patient respiration and generates a corresponding flow signal; and

an obstruction detector coupled to the flow detecting device and adapted to detect partial obstruction of a patient's airway by:

measuring respiratory flow from the patient;

detecting inspiratory flow from the measured respiratory flow;

trimming the inspiratory flow to remove any expiratory pause;

normalizing an effect of overshoot by estimating a peak flow value of a central portion of the inspiratory flow and by resetting each point outside the central portion that has a flow value exceeding the peak flow value;

scaling the inspiratory flow to a reference value;

calculating a mean deviation of the central portion of the inspiratory flow;

calculating a flattening index from the mean deviation; and

filtering the flattening index with a moving average filter,

wherein the obstruction detector is configured to generate a signal indicative of an airway obstruction based on the flattening index.

4. An apparatus for treating a patient having a sleep disorder, the apparatus comprising

a mask,

a gas source selectively supplying pressurized breathable air to the patient through the mask,

a flow detecting device that senses airflow and generates a flow signal indicative of respiration,

an obstruction detector coupled to the flow detecting device and adapted to detect partial obstruction of a patient's airway by:

measuring respiratory flow from the patient;

detecting inspiratory flow from the measured respiratory flow;

trimming the inspiratory flow to remove any expiratory pause,

normalizing an effect of overshoot by estimating a peak flow value of a central portion of the inspiratory flow and resetting each point outside the central portion that has a flow value exceeding the peak flow value,

scaling the inspiratory flow to a reference value,

calculating a mean deviation of the central portion of the inspiratory flow,

calculating a flattening index from the mean deviation, and

filtering the flattening index with a moving average filter, and

a controller generating a command for activating the gas source in response to the detecting of the partial obstruction.

5. A method for detecting a partial obstruction in a patient's airway, the method comprising the steps of:

measuring respiratory flow from the patient;

detecting, with a processor, inspiratory flow from the measured respiratory flow;

trimming the inspiratory flow to remove any expiratory pause;

normalizing an effect of overshoot by estimating a peak flow value of a central portion of the inspiratory flow and by resetting each point outside the central portion that has a flow value exceeding the peak flow value;

scaling the inspiratory flow to a reference value;

calculating a mean deviation of the central portion of the inspiratory flow;

calculating a flattening index from the mean deviation; and

filtering the flattening index with a moving average filter.

6. The apparatus of claim 1 , wherein the obstruction detector includes a signal generator that generates the obstruction signal based on the flattening index.

7. The apparatus of claim 1 , wherein the controller is configured to alter the operation of the air source in response to the received obstruction signal.

8. The apparatus of claim 1 , wherein the obstruction detector trims the inspiratory flow by finding an earliest time at which inspiration could have started, including:

locating a first time where the inspiratory flow first reaches a first predetermined percentage of a peak value of the inspiratory flow,

searching backwards to a second time where the inspiratory flow last reached a second predetermined percentage of the peak value, and

extrapolating backwards to find the earliest time where the inspiratory flow should be zero.

9. The apparatus of claim 8 , wherein the first predetermined percentage is approximately 75% and the second predetermined percentage is approximately 25%.

10. The apparatus of claim 1 , wherein the obstruction detector

resets each point outside the central portion that exceeds the peak flow value to the peak flow value.

11. The apparatus of claim 1 , wherein the obstruction detector

resets each point outside the central portion that has a flow value exceeding the peak flow value to a moving average of flow values outside the central portion.

12. The apparatus of claim 11 , wherein the resetting continues until no point outside the central portion has a flow value greater than the peak flow value.

13. The apparatus of claim 1 , wherein the obstruction detector scales the inspiratory flow to a unit length and a unit mean height.

14. The apparatus of claim 1 , wherein the obstruction detector determines the mean deviation using the formula

MeanDeviation

=

∑

t

=

24

40

⁢

(

fs

⁡

(

t

)

-

M

)

16

wherein fs(t) is a sample function of the inspiratory flow.

15. The apparatus of claim 14 , wherein the obstruction detector determines the flattening Index (FI) using the formula

FI

=

MeanDeviation

M

.

16. The apparatus of claim 1 , wherein the obstruction detector determines the flattening Index (FI) by:

dividing the inspiratory flow into n parts; calculating mean amplitudes for each part;

determining what sort of flow the pattern indicates; and

marking the flattening index as low, when overshoot is indicated.

17. The apparatus of claim 1 , wherein the obstruction detector determines the flattening Index (FI) by:

calculating an estimate of flattening for every n samples in a middle half of the inspiratory flow; and

marking the flattening index as a value that corresponds to the lowest value obtained for all sample permutations.

18. The apparatus of claim 1 , wherein the moving-average filter creates a pointwise moving average of the inspiratory flow over multiple breaths.

19. The apparatus of claim 1 , wherein the moving-average filter is based on a 5 breath average.

20. The method of claim 5 , wherein the processor trims the inspiratory flow by finding an earliest time at which inspiration could have started, including:

locating a first time where the inspiratory flow first reaches a first predetermined percentage of a peak value of the inspiratory flow;

searching backwards to a second time where the inspiratory flow last reached a second predetermined percentage of the peak value, and

extrapolating backwards to find the earliest time where the inspiratory flow should be zero.

21. The method of claim 20 , wherein the first predetermined percentage is approximately 75% and the second predetermined percentage is approximately 25%.

22. The method of claim 5 , wherein the processor

resets each point outside the central portion that exceeds the peak flow value to the peak flow value.

23. A method for detecting a partial obstruction in a patient's airway, the method comprising the steps of:

measuring respiratory flow from the patient;

detecting, with a processor, inspiratory flow from the measured respiratory flow;

trimming the inspiratory flow to remove any expiratory pause;

normalizing an effect of overshoot;

scaling the inspiratory flow to a reference value;

calculating a mean deviation of a central portion of the inspiratory flow;

calculating a flattening index from the mean deviation; and

filtering the flattening index with a moving average filter, wherein the processor normalizes the effect of overshoot by:

estimating a peak flow value of the central portion of the inspiratory flow; and

resetting each point outside the central portion that has a flow value exceeding the peak flow value to a moving average of flow values outside the central portion.

24. The method of claim 23 , wherein the resetting continues until no point outside the central portion has a flow value greater than the peak flow value.

25. The method of claim 5 , wherein the processor scales the inspiratory flow to a unit length and a unit mean height.

26. The method of claim 5 , wherein the processor determines the mean deviation using a formula

MeanDeviation

=

∑

t

=

24

40

⁢

(

fs

⁡

(

t

)

-

M

)

16

wherein fs(t) is a sample function of the inspiratory flow.

27. The method of claim 26 , wherein the processor determines the flattening Index (FI) as

FI

=

MeanDeviation

M

.

28. The method of claim 5 , wherein the processor determines the flattening Index (FI) by:

dividing the inspiratory flow into n parts;

calculating mean amplitudes for each part;

determining what sort of flow the pattern indicates; and

marking the flattening index as low, when overshoot is indicated.

29. The method of claim 5 , wherein the processor determines the flattening Index (FI) by:

calculating an estimate of flattening for every n samples in a middle half of the inspiratory flow; and

marking the flattening index as a value that corresponds to the lowest value obtained for all sample permutations.

30. The method of claim 5 , wherein the moving-average filter creates a pointwise moving average of the inspiratory flow over multiple breaths.

31. The method of claim 5 , wherein the moving-average filter is based on a 5 breath average.