System for identifying patient response to anesthesia infusion
A generic model structure captures basic characteristics in BIS-based patients' responses to anesthesia and surgical stimulation, the model being used in combination with the insight of an anesthesiologist. The model structure represents the patient response with a time delay, a time constant for response speed, and a nonlinear function for drug sensitivity. Clinical data confirms the model structure and is used to establish parameters and function forms for individual patients. A feedback and predictive control strategy for anesthesia drug infusion is then introduced on the basis of the patient model. Feedback control alone cannot avoid large fluctuations in BIS values when significant surgical stimulation is imposed, as a result of time delays in a patient's response to drug infusion. Predictive control attenuates fluctuations of BIS levels from surgical stimulation.
1. An apparatus for assisting a human expert during the administration of a medical anesthesia drug to a patient, the apparatus comprising:
a computing machine having:
an input that receives data corresponding to a plurality of coefficients C 1 , C 2 , C 3 , as well as time periods τ p (initial time delay after infusion of the anesthesia drug) and T p (time constant representing speed of response), the time periods being initially determined in response to an assessment of the patient by the human expert;
a memory that stores the plurality of coefficients C 1 , C 2 , C 3 , and the time periods τ p and T p ;
a processor that solves the formula:
y
=
f
p
(
x
)
=
C
1
x
x
1
Φ
1
(
x
)
+
C
2
x
x
2
Φ
2
(
x
)
+
C
3
x
x
3
Φ
3
(
x
)
and
a display that displays resulting data to the human expert.
2. The apparatus of claim 1 , where a relative value between 1 and 10 is assigned to represent the patient's response to infusion of the anesthesia drug, where 1 represents the slowest and 10 represents the fastest, and the memory of the computing machine is configured to store the relative value.
3. The apparatus of claim 1 , wherein typical set points are selected to be x 1 ≅50, x 2 ≅100, and x 3 ≅150, and are stored in the memory of the computing machine.
4. A method of using a computing machine to assist a human expert in the administration of anesthesia to a patient, the computing machine having a memory to determine a model that corresponds to a predicted response of the patient to anesthesia drug delivery, the method comprising the steps of:
determining an initial time delay τ p after drug infusion for the patient;
entering a time delay value corresponding to the initial time delay τ p into the memory of the computing machine;
determining a time constant T p representing speed of response of the patient;
entering a time constant value corresponding to the time constant T p into the memory of the computing machine;
determining a nonlinear static function ƒ p representing the sensitivity of the patient to a dosage of the anesthesia drug at steady state; and
displaying data corresponding to a solution of the formula:
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=
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p
(
x
)
=
C
1
x
x
1
Φ
1
(
x
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+
C
2
x
x
2
Φ
2
(
x
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+
C
3
x
x
3
Φ
3
(
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.
5. The method of claim 4 , wherein said steps of determining an initial time delay τ p , determining a time constant T p , and determining a nonlinear static function ƒ p , are implemented in a Weiner structure that is computed in the computing machine.
6. The method of claim 4 , wherein said steps of determining an initial time delay τ p , determining a time constant T p , and determining a nonlinear static function ƒ p , are implemented in a Hammerstein structure that is computed in the computing machine.
7. Apparatus for determining a predicted response of a patient to the administration of an anesthesia drug, the apparatus comprising:
a first memory for storing patient dynamics information relating to the infusion of a bolus dosage of anesthesia drug, said first memory having a first output for producing a first output signal corresponding to a first anesthesia level;
a second memory for storing patient dynamics information relating to the infusion of a titrated dosage of anesthesia drug, said second memory having a second output for producing a second output signal corresponding to a second anesthesia level;
a third memory for storing patient dynamics information relating to the patient's predicted response to events of surgical stimulation, said third memory having a third output for producing a third output signal corresponding to an anesthesia effect level;
a signal combiner arrangement for receiving the first and second output signals and the anesthesia effect level, and producing at an output thereof a combined anesthesia effect signal;
a limiter coupled to the output of said signal combiner for establishing maximum and minimum values of the combined anesthesia signal;
a processor for generating a virtual anesthesia monitor that produces an anesthesia value responsive to the combined anesthesia signal; and
a display that displays resulting data.
8. The apparatus of claim 7 , wherein the first and second anesthesia levels correspond to respective bi-spectrum levels of the patient's electroencephalogram signal, the anesthesia effect level is a bi-spectrum level, and the combined anesthesia signal is a combined bi-spectrum level signal.
9. The apparatus of claim 8 , wherein the virtual anesthesia monitor is a virtual bi-spectrum monitor for producing a bi-spectrum value responsive to the combined bi-spectrum signal.
10. The apparatus of claim 7 , wherein there is further provided a source of known noise to compensate for an unpredictable disturbances signal, and said signal combiner arrangement is arranged to receive the unpredictable disturbances signal and the combined anesthesia effect signal is responsive to the unpredictable disturbances signal.