Method and apparatus for improved flow limitation detection of obstructive sleep apnea
In a respiratory apparatus for treatment of sleep apnea and other disorders associated with an obstruction of a patient's airway and which uses an airflow signal, an obstruction index is generated which detects the flattening of the inspiratory portion of the airflow. The flattening index serves as an obstruction index used to differentiate normal and obstructed breathing. The obstruction index is based upon comparison of values of airflow in different sectors of the inspiratory peak of the wave function and is particularly suitable for distinguishing M shaped or square shaped respiratory patterns indicative of partially obstructed airways.
1. A respiratory apparatus, comprising:
a flow detecting device to sense respiration and generate an inspiratory wave pattern; and
a controller coupled to the flow detecting device, the controller configured to:
decompose the inspiratory wave pattern into a matrix pair of basis vectors that are orthogonal;
fit the basis vectors to the inspiratory wave pattern;
determine factors associated with the basis vectors from the fitting; and
determine a presence of a flow limited shape in the inspiratory wave pattern based on a function of the factors, the flow limited shape being indicative of an M-shaped breath.
2. The respiratory apparatus of claim 1 , wherein each basis vector normalizes a time interval of the inspiratory wave pattern such that inspiration begins at zero and ends at a predetermined number of radians.
3. The respiratory apparatus of claim 2 , wherein the basis vectors are sinusoidal functions of the predetermined number of radians.
4. The respiratory apparatus of claim 2 , further comprising a motor operated blower, the motor operated blower coupled to the controller, wherein the controller is configured to adjust a setting for a treatment pressure generated with the motor operated blower in response to the determined presence of the flow limited shape.
5. The respiratory apparatus of claim 2 wherein the controller is configured to log an event based on the determined presence of the flow limited shape.
6. The respiratory apparatus of claim 1 , wherein the basis vectors are sinusoidal.
7. The respiratory apparatus of claim 1 , wherein the controller is configured to map the factors associated with the basis vectors to a first point in a space.
8. The respiratory apparatus of claim 7 , wherein the space comprises a second point representing an M-shaped breath and a third point representing a non-M-shaped breath.
9. The respiratory apparatus of claim 8 , wherein the controller is configured to calculate a measure of distance between the first point and the second point representing the M-shaped breath.
10. The respiratory apparatus of claim 1 , wherein the controller is further configured to detect inspiration by:
provisionally taking start of inspiration as a time that inspiratory airflow exceeds a predetermined threshold,
rejecting the start of inspiration if the airflow then falls below zero again before a total volume reaches a predetermined value,
provisionally taking end of inspiration as a time that the airflow falls below zero after the total volume reaches a predetermined value, and
rejecting the end of inspiration if the airflow then goes above a second predetermined value in an inspiratory direction before the total volume expired reaches a third predetermined value.
11. A respiratory apparatus, comprising:
a flow detecting device to sense breath; and
a controller coupled to the flow detecting device, the controller configured to:
interpolate an inspiratory portion of the breath;
calculate a first factor as a function of a first basis function and the interpolated inspiratory portion of the breath;
calculate a second factor as a function of a second basis function and the interpolated inspiratory portion of the breath; and
determine whether the breath is flow limited based on the first and second factors, wherein the flow limited breath is indicative of an M-shaped breath.
12. The respiratory apparatus as claimed in claim 11 , wherein the basis functions are sinusoidal.
13. The respiratory apparatus of claim 11 , wherein the first basis function is sin(itt), and the second basis function is sin(3πt).
14. The respiratory apparatus of claim 11 , wherein the controller is configured to store the basis functions for use with a subsequent calculation.
15. The respiratory apparatus of claim 11 , wherein the controller is configured to derive a shape index based on the first and second factors.
16. The respiratory apparatus as claimed in claim 15 , wherein the shape index is computed by a formula:
shape
index
=
F
2
F
1
2
+
F
2
2
,
wherein F 1 and F 2 represent the first factor and the second factor, respectively.
17. The respiratory apparatus of claim 15 , wherein the controller is configured to determine whether the breath is flow limited by comparing the shape index to a threshold indicative of a non-flow-limited breath.
18. The respiratory apparatus of claim 11 , wherein the controller is further configured to detect inspiration by:
provisionally taking start of inspiration as a time that inspiratory airflow exceeds a predetermined threshold,
rejecting the start of inspiration if the airflow then falls below zero again before a total volume reaches a predetermined value,
provisionally taking end of inspiration as a time that the airflow falls below zero after the total volume reaches a predetermined value, and
rejecting the end of inspiration if the airflow then goes above a second predetermined value in an inspiratory direction before the total volume expired reaches a third predetermined value.
19. The respiratory apparatus of claim 11 , further comprising a motor operated blower, the motor operated blower coupled to the controller, wherein the controller is configured to adjust a setting for a treatment pressure generated with the motor operated blower in response to the determined flow limited breath.
20. The respiratory apparatus of claim 11 wherein the controller is configured to log an event based on the determined flow limited breath.
21. A method for determining whether an inspiratory wave pattern has an M-shape, comprising:
sensing respiration and generating an inspiratory wave pattern;
decomposing the inspiratory wave pattern into a matrix pair of basis vectors that are orthogonal;
fitting the basis vectors to the inspiratory wave pattern;
determining factors associated with the basis vectors from the fitting; and
determining a presence of a flow limited shape in the inspiratory wave pattern based on a function of the factors, the flow limited shape being indicative of an M-shaped breath.
22. A method for determining whether an inspiratory wave pattern has an M-shape, comprising:
sensing breath;
interpolating an inspiratory portion of the breath;
calculating a first factor as a function of a first basis function and the interpolated inspiratory portion of the breath;
calculating a second factor as a function of a second basis function and the interpolated inspiratory portion of the breath; and
determining whether the breath is flow limited based on the first and second factors, wherein the flow limited breath is indicative of an M-shaped breath.