Approaching the anesthesia task with drugs
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.
At the 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).
The 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.
In this chapter, we will look at the drugs typically used in anesthesia. First, however, a word about the theories of anesthesia.