Surgery – Diagnostic testing – Detecting brain electric signal
Reexamination Certificate
2001-05-21
2003-10-07
Bennett, Henry (Department: 3743)
Surgery
Diagnostic testing
Detecting brain electric signal
Reexamination Certificate
active
06631291
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is directed to a method and apparatus for controlling the administration of an hypnotic drug in “closed loop” fashion.
An hypnotic drug may comprise an anesthetic agent and the hypnotic state induced in a patient by the administration of such a drug is one of anesthetization. An hypnotic drug typically acts on the brain to produce a lessening or loss of consciousness in the patient. The extent to which the patient is anesthetized is often termed the “hypnotic level” or “depth of anesthesia.” In the present invention, the existing hypnotic level, or depth of anesthesia, in the patient is sensed and used to control the hypnotic drug administration to the patient in the manner of a closed loop, or feedback, regulator to achieve and maintain a desired level in the patient.
More particularly, the present invention employs the complexity of electroencephalographic (EEG) data obtained from the patient as a sensed indication of the hypnotic level of the patient for use in controlling hypnotic drug administration. The use of such an indication provides closed loop control of drug administration that is based on an accurate assessment of the hypnotic condition of the patient and one that is highly responsive to changes in that condition. Such an indication can be made rapidly responsive to changes in the hypnotic condition of the patient.
Hypnotic drugs, or anesthetic agents, are administered by inhalation or intravenously. When administration is by inhalation, the anesthetic agent comprises a volatile liquid that is vaporized in a vaporizer. The vaporized anesthetic agent is entrained in breathing gases for the patient. The concentration of the anesthetic agent supplied by the vaporizer is determined by the anesthesiologist by manipulating appropriate controls on the vaporizer. The concentration of anesthetic agent in the lungs of the patient may be measured by measuring the amount of anesthetic agent contained in the breathing gases exhaled by the patient at the end of the exhalation phase of the respiratory cycle, i.e. the end tidal concentration (ET
conc
). Typical inhaled anesthetic agents are sevoflurane, enflurane, and desflurane, among others.
In a simple form, intravenous administration of an hypnotic drug may employ a syringe that injects the drug into a vein of the patient. For extended administration, a motor driven syringe or a motor driven infusion pump may be employed. A commonly used, intravenously administered, anesthetic agent is propofol.
In addition to hypnosis, high quality anesthesia must also consider loss of sensation (analgesia), muscle relaxation, suppression of the autonomous nervous system, and blockage of the neuro muscular function. This may require administration of a number of different drugs via the same or different routes. Further, different hypnotic drugs and/or different administration routes may be used at different stages of an anesthetization or a medical procedure. For example, hypnosis may be introduced by an intravenously administered drug and maintained by an inhaled drug.
In the process by which a drug, including a hypnotic drug, takes its effect in the body, two aspects are important: pharmacokinetics and pharmacodynamics. Pharmacokinetics deals with the effect of the body on the drug, such as the body's absorption, distribution or diffusion, metabolism, and excretion of the drug. Pharmacokinetics describes how the drug is distributed in the course of time from the site of delivery to different parts of the body and to a particular organ in which the drug is supposed to have its effect.
For use in the study of drugs, the determination of dosages, and the like, mathematical models have been developed for the pharmacokinetics of a drug. Because of the complexity of the physiology of the body, the models are typically based on theoretical compartments, such as plasma, fat, or the brain. Pharmacokinetic models typically allow for consideration of anthropometric data, such as patient height, weight, age, sex, etc. Pharmacokinetic models are available for hypnotic drugs, or anesthetic agents, including propofol, based on two or more different compartments. See Shafer, et al. Anesthesiology, 1998; 69:348-356 describing a two compartment model for propofol.
When a specific effect of a drug can be directly or indirectly measured, such data can be used to define a pharmacodynamic model of the drug with respect to its concentration at the site at which it is effective, i.e. effect-site concentration. Such models may also use anthropometric data. For hypnotic drugs the effect is the hypnotic state of the patient and the effect-site in the brain.
In a broad sense, all hypnotic drug administration is of a controlled loop nature. In a basic form, an anesthesiologist administers such a drug to a patient, observes the state of the patient resulting from the administration of the drug, and then maintains or alters the dose based on his/her observations. However, in a more specific sense, reflecting recent work in the field of anesthesia, closed loop control relates to the sensing of the hypnotic state of the patient by some form of instrumentation and automatically controlling the administration of the drug responsive to a feedback signal from the instrumentation. The term is used herein in the more specific sense.
The interest in closed loop control is posited, at least in part, on a desire to accurately establish the hypnotic level or depth of anesthesia of the patient. If the anesthesia is not sufficiently deep, the patient may maintain or gain consciousness during a surgery, or other medical procedure, resulting in an extremely traumatic experience for the patient, anesthesiologist, and surgeon. On the other hand, excessively deep anesthesia reflects an unnecessary consumption of hypnotic drugs, most of which are expensive. Anesthesia that is too deep requires increased medical supervision during the surgery recovery process and prolongs the period required for the patient to become completely free of the effects of the drug.
Rapidity is another desirable feature of an hypnotic drug administration control system. Fast response is particularly desirable should the patient approach consciousness since, as noted above, unexpected emergence is to be avoided, but is rendered more likely as excessively deep anesthesia is avoided.
A closed loop hypnotic drug delivery system has been described using the bispectral index as a control parameter. See Mortier E., et al. Anesthesia, 1998, August; 53 (8):749-754. See also published European Patent Application No. EP 959,921 to authors of this article. The bispectral index is proprietary to Aspect Medical Systems of Farmingham, Mass. and is described in one or more of the following U.S. Pat. Nos.: 4,907,597; 5,101,891; 5,320,109; and 5,458,117. The bispectral index is an effort to form a single variable, termed the bispectral index (BIS), that correlates behavioral assessments of sedation and hypnosis over a range of anesthesia for several hypnotic drugs.
The bispectral index comprises three components that are combined in various ways to provide an indication over a range of hypnotic levels from light sedation to deep anesthesia. See Ira R. Rampil, “A Primer for EEG Signal Processing in Anesthesia”, Anesthesiology 89 (1998), 980-1002. See also U.S. patent application, Ser. No. 09/688,891 to an inventor named herein and another, assigned to a common assignee, also containing a description of this index.
In order to compute a BIS value, measured EEG data over a period of fifteen seconds is used. During anesthesia, the level of painful stimulation can vary drastically and cause rapid changes in the hypnotic level of the patient, i.e. wake the patient up. Because of the time required to compute a BIS value, the bispectral index may not be sufficiently rapid to warn the anesthesiologist that this is occurring. Also, the bispectral index is contaminated by electromyographic (EMG) activity which may lead to misjudgment of the hypnotic level of a patient. See Bruhn J., et
Cohen-Laroque Emmanuel-S
Viertïo-Oja Hanna
Basichas Alfred
Instrumentarium Corp.
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