Methods and apparatus for the systemic control of ventilatory support in the presence of respiratory insufficiency
A method and apparatus for providing ventilatory assistance to a spontaneously breathing patient an error signal ( 56 ) is computed that is the difference between a function of respiratory airflow ( 54 ) over a period of time and a target value ( 52 ). Using a servo loop, air is delivered to the patient at a pressure that is a function of the error signal, the phase of the current breathing cycle, and a loop gain that varies depending on the magnitude of the error signal. The loop gain increases with the magnitude of the error signal, and the gain is greater for error signals below a ventilation target than for error signals above the ventilation target value. The target value ( 52 ) is an alveolar ventilation that takes into account the patient's physiologic dead space.
1. An apparatus for providing pressurized ventilatory assistance to a patient, the apparatus comprising:
a blower for producing pressure support ventilation for a patient,
a controller coupled with the blower to control the blower,
a flow sensor for deriving an estimate of respiratory flow; and
wherein the controller is configured to:
change a rate of adjustment of the pressure support ventilation by applying a difference function to a determined pressure support amplitude, the difference function comprising a variable representing an extent to which leak is changing rapidly, the determined pressure support amplitude being determined from an error between a target ventilation and measured ventilation;
filtering the estimate of respiratory flow with a filter having a peak hold tendency to produce an estimate of residual leak; and
compute the variable of the difference function by applying a function to the estimate of residual leak,
wherein the filter is a multi-stage filter comprising a first stage and a second stage; and
wherein the controller is configured to vary a variable time constant for the filtering with a ratio of an output of the first stage of the filtering and the target ventilation.
2. The apparatus of claim 1 wherein the difference function comprises a subtraction of the variable from one to produce a difference value.
3. The apparatus of claim 2 wherein the difference function is applied to the determined pressure support amplitude by multiplying the different value and the determined pressure support amplitude.
4. The apparatus of claim 1 wherein the function applied to the estimate of residual leak produces a value in a range from 0 to 1.
5. The apparatus of claim 4 wherein the value in the range is 0 when an absolute value of the estimate of residual leak is less than the target ventilation.
6. The apparatus of claim 5 wherein the value in the range is 1 when an absolute value of the estimate of residual leak is twice the target ventilation.
7. The apparatus of claim 6 wherein the value in the range is linearly interpolated based on the absolute value of the estimate of residual leak and the target ventilation.
8. The apparatus of claim 1 wherein the first stage comprises a low pass filter.
9. The apparatus of claim 8 wherein the low pass filter is a 4-pole Bessel lowpass filter.
10. The apparatus of claim 9 wherein the 4-pole Bessel lowpass filter has a corner frequency of 3.2/60 hertz.
11. The apparatus of claim 1 wherein the second stage is an instant-rise, variable time constant, first order lowpass fall filter.
12. The apparatus of claim 1 , wherein output of the second stage is weighted toward retaining values closer to output of the first stage when output of the first stage is further from zero.
13. A method of a controller for apparatus for providing pressurized ventilatory assistance to a patient, the method comprising:
controlling a blower for producing pressure support ventilation for a patient;
receiving a signal from a flow sensor to derive an estimate of respiratory flow;
determining a pressure support amplitude from an error between a target ventilation and measured ventilation;
changing a rate of adjustment of the pressure support ventilation by applying a difference function to the determined pressure support amplitude, the difference function comprising a variable representing an extent to which leak is changing rapidly;
filtering the estimate of respiratory flow with a filter having a peak hold tendency to produce an estimate of residual leak; and
computing the variable of the difference function by applying a function to the estimate of residual leak,
wherein the filter is a multi-stage filter comprising a first stage and a second stage; and
wherein the controller varies a variable time constant for the filtering with a ratio of an output of the first stage of the filtering and the target ventilation.
14. The method of claim 13 wherein the difference function comprises subtracting the variable from one to produce a difference value.
15. The method of claim 14 wherein the difference function is applied to the determined pressure support amplitude by multiplying the different value and the determined pressure support amplitude.
16. The method of claim 13 wherein the function applied to the estimate of residual leak produces a value in a range from 0 to 1.
17. The method of claim 16 wherein the value in the range is 0 when an absolute value of the estimate of residual leak is less than the target ventilation.
18. The method of claim 17 wherein the value in the range is 1 when an absolute value of the estimate of residual leak is twice the target ventilation.
19. The method of claim 18 wherein the value in the range is linearly interpolated based on the absolute value of the estimate of residual leak and the target ventilation.
20. The method of claim 13 wherein the first stage comprises a low pass filter.
21. The method of claim 20 wherein the low pass filter is a 4-pole Bessel lowpass filter.
22. The method of claim 21 wherein the 4-pole Bessel lowpass filter has a corner frequency of 3.2/60 hertz.
23. The method of claim 22 wherein the second stage is an instant-rise, variable time constant, first order lowpass fall filter.
24. The method of claim 13 , wherein controller weights output of the second stage toward retaining values closer to output of the first stage when output of the first stage is further from zero.