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CASE STUDY
An 80-kg 35-year-old woman with a BMI of 32 is undergoing right knee surgery for a meniscus tear. The surgeon and the patient both request general anesthesia for the procedure. In addition to obesity, the patient has hypertension (treated with hydrochlorothiazide) and insulin-dependent diabetes, and she takes an oral contraceptive pill. She has no known drug allergies. Her physical examination is remarkable only for obesity and a Mallampati class 3 airway (indicating extremely limited space from tongue base to roof of mouth and probable difficulty in intubating). Because of her diabetes and risk for delayed gastric emptying, you elect to use endotracheal intubation to protect her airway during the procedure.
After induction of anesthesia with propofol, you administer a dose of rocuronium to achieve skeletal muscle relaxation and to facilitate endotracheal intubation. Once fully relaxed, you attempt direct laryngoscopy but are unable to visualize her airway. You make changes to the patient’s position, and use a different technique, but you are still unable to perform intubation.
You switch back to bag/mask ventilation, but it has now become more difficult to achieve adequate tidal volumes. You decide to reverse the neuromuscular blockade and wake the patient. (1) What agents could be used to reverse the neuromuscular blockade? (2) What would be the most appropriate agent to use in this scenario? (3) What problems may occur with your chosen agent?
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Drugs that affect skeletal muscle function include two different therapeutic groups: those used during surgical procedures and in the intensive care unit (ICU) to produce muscle paralysis (ie, neuromuscular blockers), and those used to reduce spasticity in a variety of painful conditions (ie, spasmolytics and antispasmodics). Neuromuscular blocking drugs interfere with transmission at the neuromuscular end plate and lack central nervous system (CNS) activity. These compounds are used primarily as adjuncts during general anesthesia to optimize surgical conditions and to facilitate endotracheal intubation in order to ensure adequate ventilation. Drugs in the spasmolytic group have traditionally been called “centrally acting” muscle relaxants and are used primarily to treat chronic back pain and painful fibromyalgic conditions. Dantrolene, an agent that has no significant central effects and is used primarily to treat a rare anesthetic-related complication, malignant hyperthermia, is also discussed in this chapter.
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NEUROMUSCULAR BLOCKING DRUGS
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During the 16th century, European explorers found that natives in the Amazon Basin of South America were using curare, an arrow poison that produced skeletal muscle paralysis, to kill animals. The active compound, d-tubocurarine, and its modern synthetic analogs have had a major influence on the practice of anesthesia and surgery and have proved useful in understanding the basic mechanisms involved in neuromuscular transmission.
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Normal Neuromuscular Function
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The mechanism of neuromuscular transmission at the motor end plate is similar to that described for preganglionic cholinergic nerves in ...