Approaching the anesthesia task
Many different approaches to general anesthesia are possible. Often, pre-operative preparation includes the administration of drugs to (i) minimize the chance of aspiration of gastric juice, (ii) minimize anxiety and – if necessary – provide analgesia. Once the patient is in the operating room, we aim to deni-trogenate the patient’s lungs, followed by induction of anesthesia. One technique is to induce sleep with thiopental, give a paralyzing dose of succinylcholine to facilitate intubation of the trachea, and then maintain anesthesia with a halo-genated anesthetic vapor administered together with nitrous oxide and, of course, oxygen. Muscle relaxation during the operation might be accomplished with one of the non-depolarizing neuromuscular blockers, frequently called “muscle relax-ants.” Another technique might start with propofol instead of thiopental and it might rely on large doses of an opiate, such as fentanyl and, to assure amne-sia, a low concentration of a halogenated inhalation anesthetic. Many different combinations of these approaches are in use.
end of anesthesia and if the patient is still weakened from the mus-cle
relaxant, the neuromuscular blockade has to be reversed with, for exam-ple,
neostigmine given together with an anticholinergic drug. When the patient
responds to commands, we remove the endotracheal tube and return the patient to
the post-anesthesia care unit (PACU).
practice of anesthesia involves the administration of several drugs, some of
them with overlapping effects. For example, premedication with midazo-lam
(Versed®, a benzodiazepine) will make the patient more sensitive to the side
effects of narcotic analgesics; neuromuscular blockade can be more readily
achieved if the patient is in surgical anesthesia from a halogenated vapor than
if anesthesia relies on nitrous oxide and narcotics. The degree of surgical
stimula-tion will influence the patient’s response to anesthetic drugs. During
a small bowel anastomosis, which does not represent major noxious stimulation,
less anesthe-sia will be required than when stimulating the carina with a
suction catheter, for example. An elderly or debilitated or abstentious patient
will require less depres-sant drug for the same effect than a young and
vigorous person accustomed to regular alcohol intake. Drugs that undergo
biotransformation with the help of enzymes that had been induced may have a
shorter duration of action (some barbiturates) or more side effects, e.g.,
halothane biotransformation liberating hepatotoxins, than in the absence of
induced enzymes. Repeated exposure to a drug can induce marked tolerance to the
drug as is well known for narcotics. In other words, our brief discussion of
pharmacology cannot cover all factors that might influence the patient’s
response to a cited dose.
chapter, we will look at the drugs typically used in anesthesia. First,
however, a word about the theories of anesthesia.