Chapter 31
- Measuring Depth of Anesthesia
- Donald R. Stanski
- Steven L. Shafer
Anesthesiologists appear to define the state of "anesthesia" the
same way that Justice Potter Stewart, without disclosing his research, defined pornography
in 1964: "I know it when I see it." This definition is inadequate to quantitatively
measure the anesthetic state. Anesthesiologists proffer multiple and sometimes inconsistent
views on quantitative measures of the anesthetic state. Fortunately, an integrated,
quantitative concept of the anesthetic state can now be built on the incremental
knowledge developed since the introduction of anesthesia over a century ago.
The chapter is divided into three general sections. The first
section discusses two related topics: the evolving definitions of depth of anesthesia
and the problem of recall (memory) and awareness. The history of attempts to define
"depth of anesthesia" make one realize that our knowledge of the specific attributes
of the drugs used in clinical practice has to be incorporated into our understanding
of anesthetic depth.
The second section of this chapter involves basic pharmacologic
concepts relevant to the scientific investigation of depth of anesthesia. This brief
review, which includes an overview examining the interaction of two anesthetic drugs,
prepares the reader for the third section, which discusses specific drugs and clinical
situations in two subsections. The first subsection discusses clinical measures
of anesthetic depth that derive directly from the patient. Each discussion begins
with a review of the scientific approaches available to quantitate the relationship
of drug dose or plasma concentration to the resulting drug effects. This fundamental
anesthetic clinical pharmacology is needed to then integrate the different drugs
into the clinical administration of an anesthetic and subsequently interpret the
drug interactions relative to the fundamental "depth of anesthesia." The second
subsection discusses electrophysiologic approaches to assessing depth of anesthesia.
These approaches represent the application of technology to the issue of quantitation
of anesthetic drug effect and depth of anesthesia. Depth of anesthesia is the interaction
of two drug effects fundamental to clinical anesthesia. One drug effect involves
the hypnotic component, which creates unconsciousness. The second drug effect involves
the analgesic component, which decreases the body's reflex response to noxious stimuli.
The interaction of these two components allows anesthesia to be delivered to patients
in a safe and effective manner over a broad range of clinical situations.
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