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Summary neurology 3- usmlerx bricks
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General AnestheticsBy ScholarRx
Updated September 7, 2022
access_time15 min
Learning Objectives (3)
After completing this brick, you will be able to:
● Describe the objective of general anesthesia, the characteristics of an
ideal anesthetic, and the stages of general anesthesia.
● 1
● List the classes (and then names) of drugs that are used as
intravenous anesthetics.
● 2
● List the classes (and then names) of drugs that are used as inhaled
anesthetics.
● 3
CASE CONNECTION
“Thiopental or etomidate?” You are doing 2 weeks of anesthesia as
part of a surgery rotation, and the anesthesiologist ask you a question.
She is about to begin induction of GH, a 56-year-old man who is
having an emergent repair of a perforated duodenal ulcer. GH has
coronary artery disease and a depressed ejection fraction. “Which
medication do you think would be better for GH for induction?” she
asks you.How will you answer this question? What is the rationale for
this choice? Consider your answers as you read, and we’ll revisit at
the end of the brick.
,What Is General Anesthesia?
The concept of anesthesia as we know it today is the combination of
medications that keeps patients pain-free during surgery or other
potentially uncomfortable medical procedures. Different types of
anesthesia are chosen based on the procedure being done. Unlike
injections that numb only a small area of the body (local or regional
anesthesia), general anesthesia renders a patient unconscious.
Of course, achieving unconsciousness comes with risks, so
anesthesiologists have to take several factors into account to proceed
safely.
Ideal vs Balanced Anesthesia
The ideal anesthetic would meet all of our patients’ needs safely and
effectively. It should provide a smooth, rapid induction (onset of
anesthesia); muscle relaxation (akinesia); amnesia; easy titration; a
smooth, rapid reversibility; and emergence without adverse effects.
A single agent would hardly meet this ideal, so a combination of
drugs is often used to obtain general anesthesia without the
undesirable adverse effects that are associated with administering a
large amount of an anesthetic agent. This is balanced (aka,
combination) anesthesia.
Minimum Alveolar Concentration
Another important concept is minimal alveolar concentration (MAC).
MAC is the amount of inhaled anesthetic required to prevent 50% of
patients from responding to a noxious stimulus. This means that if all
patients were given a specific concentration for an inhaled anesthetic
that is equal to one MAC, half of them would not move when a
,surgeon makes an incision in their skin. We can think of MAC as
representing the potency of an inhaled anesthetic, and we’ll explore it
further in the discussion of inhaled agents.
Now, let’s break down step by step when and why we use a specific
combination of drugs in general anesthesia.
Stepwise Approach to General Anesthesia
Patients commonly experience anxiety prior to surgery, so the work of
the anesthesia team typically begins with agents that act to decrease
that anxiety and make the patient feel drowsy. Once the patient arrives
in the operating room, the anesthesiologist uses a medication (usually
intravenous [IV]) that causes the patient to lose the ability to form
memories and to fall asleep. If intubation is required, the
anesthesiologist administers a drug that induces paralysis. This
facilitates intubation because it relaxes the vocal cords so that they
will not be damaged during insertion of the endotracheal tube.
Paralysis is also useful for reducing muscle tension.
Once induction medications are administered, anesthetic gas is used to
keep the patient sedated during the remainder of the procedure.
Autonomic reflexes are dampened to prevent intraoperative
hypertension and tachycardia, which are normal sympathetic
responses to pain.
Stages of Anesthesia
Anesthesiologists track the stages of anesthesia to prevent
overmedication, because the distribution of anesthetic agents varies
from person to person. As blood concentration of an anesthetic
increases, we observe changes in body movement, respiration,
reflexes, and muscle tone (Table 1).
, Stage Respiratory Response to Eye Signs Cardiac
Response Stimulus Response
I. Analgesia Voluntary Voluntary Voluntary Adequate
control control movement output
II. Excitement: Irregular Actively Lash reflex lost, Tachycardia,
early breathing, purposeful variable movement, possibly
coughing, spasm disconjugate gaze arrhythmias
II. Excitement: Phonation, Weakly Tachycardia,
late breath holding purposeful possibly
arrhythmias
III. Surgical Regular Absent Lid reflex lost
Anesthesia:
light
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