Closed-loop non-invasive oxygen saturation control system

• US 5,365,922 A
• Filed: 03/19/1991
• Issued: 11/22/1994
• Est. Priority Date: 03/19/1991
• Status: Expired due to Term

• Current Assignee: Brigham and Women's Hospital Incorporated
• Original Assignee: Brigham and Women's Hospital Incorporated
• Inventors: Raemer, Daniel B.
• Primary Examiner(s): Burr, Edgar S.
• Assistant Examiner(s): Lewis, Aaron J.
• Application Number:
  US07/671,586
• Time in Patent Office: 1,344 Days
• Field of Search: 128/204.18, 128/204.21, 128/204.23, 128/716, 128/719, 128/202.22, 128/205.23, 128/205.11
• US Class Current: 128/204.23
• CPC Class Codes: A61B 5/0833 Measuring rate of oxygen consumption
• CPC Class Codes: A61B 5/097 Devices for facilitating collection of breath or for directing breath into or through measuring devices
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: A61M 16/0051 with alarm devices
• 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 2230/205 partial oxygen pressure (P-O2)

Abstract

An adaptive controller for delivering a fractional amount of oxygen to a patient. The controller utilizes an oximeter coupled by a non-invasive sensor to the patient for measuring the blood hemoglobin saturation in the patient. The oximeter generates a plurality of blood saturation output signals over a given period of time which are sequentially representative of the patient'"'"'s blood hemoglobin saturation. A processing means evaluates a plurality of the oximeter output signals and, based on the evaluation, provides a pseudo blood saturation signal. A feedback control means responsive to the pseudo output signal sets the fractional amount of oxygen to be delivered to the patient. When deviations of the oximeter output signal are excessive, the pseudo output signals cause a gradual increase in the fractional amount of oxygen for the patient. Furthermore, the feedback control means is periodically disconnected, and the response of the patient to random changes in the fractional amount of oxygen delivered to the patient is used to adapt the response characteristics of the feedback control means in a manner tailored to the needs of the patient.

Claims

1. An adaptive controller for delivering a fractional amount of oxygen to a patient, said controller comprising:

oximeter means coupled by a non-invasive sensor to said patient for measuring blood hemoglobin saturation in the patient, said oximeter generating a plurality of blood hemoglobin saturation output signal values over a given period of time, sequentially representative of said blood hemoglobin saturation;

means for generating a running average of said blood hemoglobin saturation output values;

means for generating pseudo-output signals that are a function of said running average;

processing means including means for identifying possibly invalid output signal values and being responsive to said blood hemoglobin saturation output signal values for evaluating a plurality of said blood hemoglobin saturation output signal values and, based on said evaluation, providing a processed output signal;

means for substituting respective pseudo-output signals for each of said possibly invalid output signal values thereby forming a sequence of valid output signals.

2. The adaptive controller of claim 1 wherein said oximeter is a pulse oximeter and said plurality of blood hemoglobin saturation output signal values are oxygen saturation of hemoglobin values as measured by a pulse oximeter (S_p O₂ values).

3. The adaptive controller of claim 1 wherein said processing means includes:

artifact recognition means for identifying possibly invalid output signal values and for providing a sequence of valid output signal values, exclusive of said identified possibly invalid output signal values; and

means for generating a running average of said sequence of valid output signal values and for providing said running average as said processed output signal.

4. The adaptive controller of claim 3 wherein said artifact recognition means includes means for comparing each of said plurality of output signal values to said running average and for identifying as possibly invalid any output signal value which differs from said running average by more than a predetermined amount.

5. The adaptive controller of claim 3 wherein said means for generating said running average includes means for calculating a mean value and a standard deviation value for m of said plurality of valid output signal values most recently provided by said artifact recognition means, where m is an integer, and for providing said calculated mean value as said processed output signal.

6. The adaptive controller of claim 5 wherein said artifact recognition means includes means for comparing each of said plurality of output signal values to said mean value and for identifying as possibly invalid any output signal value which differs from said mean value by an amount greater than two time said standard deviation value.

7. The adaptive controller of claim 5 wherein the feedback control means includes:

means for generating a difference signal having a value which represents the difference between the processed output signal and a value representing a desired blood hemoglobin saturation value; and

means, responsive to said difference signal, for changing the fractional amount of oxygen delivered to the patient in a sense which tends to reduce the value of said difference signal.

8. The adaptive controller of claim 7 wherein the means for changing the fractional amount of oxygen delivered to the patient includes a proportional-integral-derivative controller.

9. An adaptive controller for delivering a fractional amount of oxygen to a patient, said controller comprising:

oximeter means coupled by a non-invasive sensor to said patient for measuring blood hemoglobin saturation in the patient, said oximeter generating a plurality of blood saturation output signal values over a given period of time, sequentially representative of said blood hemoglobin saturation;

processing means responsive to said blood saturation output signal values for evaluating a plurality of said output signal values and, based on said evaluation, providing a processed output signal; and

feedback control means continuously responsive over a full range of patient activity to said processed output signal for determining the fractional amount of oxygen to be delivered to the patient;

wherein the processing means includes;

artifact recognition means for identifying possibly invalid output signals values and for providing a sequence of valid output signal values, exclusive of said identified possibly invalid output signal values; and

means for generating a running average of valid output signal values and for providing said running average as said processed output signal; and

wherein the artifact recognition means further includes means for substituting respective pseudo output signal values for each of said identified possibly invalid output signal values to generate said sequence of valid output signal values, wherein the value of said pseudo output signal values is a function of said running average of said oximeter means output signal values.

10. The adaptive controller of claim 9 wherein said processing means further includes:

monitoring means, coupled to said artifact recognition means, for determining the frequency of occurrence of said possibly invalid output signal values; and

means, coupled to said monitoring means, for decreasing the value of said pseudo output signal values in proportion to the frequency of occurrence of said possibly invalid output signal values to condition the feedback control means to gradually increase the fractional amount of oxygen to be delivered to the patient.

11. An adaptive controller for delivering a fractional amount of oxygen to a patient, said controller comprising:

measuring means, coupled to the patient, for measuring blood oxygen level in the patient and for providing a plurality of output signal values, sequentially representative of said measured blood oxygen level;

processing means, coupled to said measuring means, for generating a running average of said plurality of output signal values and for subtracting said running average from a target value, representing a desired blood oxygen level for the patient, to produce a difference signal;

feedback control means, coupled to receive said difference signal, for adjusting the fractional amount of oxygen to be delivered to the patient to minimize said difference signal in magnitude.

12. An adaptive controller for delivering a fractional amount of oxygen to a patient, said controller comprising:

measuring means, adapted to be coupled to the patient, for measuring blood oxygen level in the patient and for providing a plurality of output signal values, sequentially representative of said measured blood oxygen level;

processing means, coupled to said measuring means, for generating a running average of said plurality of output signal values and for subtracting said running average from a target value, representing a desired blood oxygen level for the patient, to produce a difference signal;

feedback control means, coupled to receive said difference signal, for adjusting the fractional amount of oxygen to be delivered to the patient to minimize said difference signal in magnitude;

wherein the feedback controller is defined by a transfer function, having a plurality of adjustable coefficients, which establishes how the fractional amount of oxygen delivered to the patient is to be changed, both in time and in magnitude, in response to a change in the difference signal.

13. The adaptive controller of claim 12, further including:

means for measuring a plurality of physiological parameters related to respiration to produce a respective plurality of output signals; and

adjustment means responsive to said plurality of output signals for adaptively changing the values of said coefficient values to change the transfer function of said feedback controller.

14. The adaptive controller of claim 13 wherein said physiological parameters include one or more of minute ventilation, respiratory rate and tidal volume;

and

said adjustment means includes means for automatically adjusting said coefficient values as a function of said plurality of output signal.

15. The adaptive controller of claim 12 further including means for periodically and automatically varying the coefficient values to change the transfer function of the feedback controller to track changing needs of the patient.

16. the adaptive controller of claim 15 wherein the means for periodically and automatically varying the coefficient values includes:

means for randomly changing the fractional amount of oxygen to be delivered to the patient within predetermined minimum and maximum limiting values; and

coefficient adapting means, responsive to the respective measured blood oxygen level values resulting from the changes in the fractional amount of oxygen, for adjusting the coefficient values to match the response of the control system to the response of the patient.

17. The adaptive controller of claim 16 wherein the coefficient adapting means includes:

means for recording the random sequence of changes in fractional amount of oxygen delivered to the patient and the respective measured blood oxygen levels resulting from the sequence of changes in the fractional amount of oxygen;

means for calculating, based on the recorded changes in the fractional amount of oxygen, the recorded blood oxygen levels and the transfer function of the feedback controller, an expected present blood oxygen level; and

means for repeatedly changing the coefficient values and recalculating the expected present blood oxygen level to minimize any difference in magnitude between the expected present blood oxygen level and the measured blood oxygen level currently provided by the measuring means.

18. The adaptive controller of claim 17 wherein the means for repeatedly changing the coefficient values includes means for evaluating the recorded changes in the fractional amount of oxygen, the recorded blood oxygen levels and the transfer function of the feedback controller according to a recursive least squares optimization method.

19. The adaptive controller of claim 12 wherein the feedback controller is a proportional-integral-derivative (PID) controller.

20. The adaptive controller of claim 19 wherein the transfer function, Y(nT)/E(nT), defining the feedback controller is given by an equation:

space="preserve" listing-type="equation">Y(nT)/E(nT)=K.sub.3 +K.sub.2 T(Z+1)/(2(Z-1))+K.sub.1 (Z-1)/(TZ)

where K₃, K₂ and K₁ are respective proportional, integral and derivative coefficient values, Z is the Z-transform operator, T is a sampling interval in seconds, n is the number of current sample values, Y(nT) is the current output value and E(nT) is the current value of the difference signal.

21. In a system for automatically providing a controlled fractional amount of oxygen to a patient, including an oximeter for measuring blood oxygen levels of the patient and a feedback controller, coupled to the oximeter for adjusting the fractional amount of oxygen delivered to the patient to maintain the measured blood oxygen levels within predetermined limits, a safety subsystem to prevent the system from erroneously administering a hypoxic mixture, said safety subsystem comprising:

means for measuring the fractional amount of oxygen being delivered to the patient and for providing a signal indicative thereof including means for sensing the oxygen level of the air being provided to the patient using a plurality of sensors;

means for correlating the measured fractional amount of oxygen to a preferred fractional amount of oxygen as determined by the feedback control system and for providing an alarm signal when the measured and preferred fractional amounts of oxygen are determined to be uncorrelated including means for correlating the preferred fractional amount of oxygen to each of the plurality of sensed oxygen levels and for generating said alarm signal if any of the sensed oxygen levels is not correlated to said preferred fractional amount of oxygen; and

means, responsive to said alarm signal, for adjusting the fractional amount of oxygen to be delivered to the patient to a maximum value said safety subsystem further including means for monitoring the blood oxygen level signals provided by the oximeter including means to identify possibly invalid signal values and means to provide a sequence of valid signal values, exclusive of said identified possibly invalid signal values; and

said safety subsystem includes means for setting a first threshold value representing a minimum desirable level of said blood oxygen level signal and a second threshold value representing a minimum desirable level of said preferred fractional amount of oxygen and said measured fractional amount of oxygen; and

means for producing said alarm signal when one of said sequence of valid signal values is less than said first threshold value;

means for producing said alarm signal when one of said preferred fractional amount of oxygen and said measured fractional amount of oxygen is less than said second threshold value.

22. the safety subsystem of claim 21, further comprising:

first monitoring means for detecting errors in the blood oxygen level signal provided by said oximeter;

second monitoring means for detecting errors in the signal provided by said means for measuring the fractional amount of oxygen being delivered to the patient; and

means for generating said alarm signal in response to an error being detected by one of said first and second monitoring means.

23. the safety subsystem of claim 22 wherein said first and second monitoring means detect missing values of said respective blood oxygen level signal and said measured fractional amount of oxygen signal as the errors in the respective signals.

24. The safety subsystem of claim 21 further comprising, non-volatile memory means for storing the measured blood oxygen level values provided by said oximeter and the measured fractional amount of oxygen being delivered to the patient.

25. The safety subsystem of claim 24 wherein values representing said preferred fractional amount of oxygen are stored in said non-volatile memory means.

26. A method for adaptively controlling the fractional amount of oxygen delivered to a patient comprising the steps of:

a) measuring the blood hemoglobin saturation in the patient during a plurality of intervals over a given period of time and providing said measured values as an output signal;

b) evaluating each of the measured values of said output signal to identify possibly invalid output signal values;

c) eliminating said identified possibly invalid output signal values from said output signal to produce a processed output signal;

d) adjusting the fractional amount of oxygen delivered to the patient in a sense to minimize any difference between said processed output signal and a predetermined desired blood hemoglobin saturation signal.

27. A method for adaptively controlling the fractional amount of oxygen delivered to a patient comprising the steps of:

a) measuring the blood hemoglobin saturation in the patient during a plurality of intervals over a given period of time and providing said measured values as an output signal;

b) evaluating each of the measured values of said output signal to identify possibly invalid output signal values;

c) eliminating said identified possibly invalid output signal values from said output signal to produce a processed output signal by

c1) generating a sequence of valid output signal values by substituting a pseudo value for each of said identified possibly invalid output signal values;

c2) generating a running average of successive values of said sequence of valid output signal values, wherein the instantaneous value of said running average is said pseudo value;

c3) providing said running average as said processed output signal; and

d) adjusting the fractional amount of oxygen delivered to the patient in a sense to minimize any difference between said processed output signal and a predetermined desired blood hemoglobin saturation signal.

28. The method of claim 27 wherein step c1) includes the steps of:

monitoring the frequency of occurrence of said identified possibly invalid output signal values; and

decreasing the value of said substituted pseudo values in proportion to the frequency of occurrence of said identified possibly invalid output signal values.