Anesthesia of the Surgical Patient
BASIC SCIENCE QUESTIONS
1. The therapeutic index is the
A. ratio of the sensitivity and efficacy of a drug
B. ratio of the lethal dose and effective dose of a drug
C. ratio of the efficacy and potency of a drug
D. ratio of the sensitivity and potency of a drug
The lethal dose (LD50) of a drug produces death in 50% of animals to which it is given. The average sensitivity to a particular drug can be expressed through the calculation of the effective dose; ED50would have the desired effect in 50% of the general population. The ratio of the lethal dose and effective dose, LD50/ED50, is the therapeutic index. A drug with a high therapeutic index is safer than a drug with a low or narrow therapeutic index.
The potency of a drug is the dose required to produce a given effect, such as pain relief or a change in heart rate. The efficacy of any therapeutic agent is its power to produce a desired effect. (See Schwartz 9th ed., p 1734.)
2. Local anesthetics block nerve conduction by their effect on the
A. Calcium channel
B. Sodium channel
C. Potassium channel
D. None of the above
The common characteristic of all local anesthetics is a reversible block of the transmission of neural impulses when placed on or near a nerve membrane. Local anesthetics block nerve conduction by stabilizing sodium channels in their closed state, preventing action potentials from propagating along the nerve. The individual local anesthetic agents have different recovery times based on lipid solubility and tissue binding, but return of neural function is spontaneous as the drug is metabolized or removed from the nerve by the vascular system. (See Schwartz 9th ed., p 1736.)
3. Which of the following induction agents does NOT work by its effect on the γ-aminobutyric acid (GABA) receptor?
Ketamine differs from the above IV agents in that it produces analgesia as well as amnesia. Its principal action is on the N-methyl-D-aspartate receptor; it has no action on the GAbA receptor. Propofol is an alkylated phenol that inhibits synaptic transmission through its effects at the GAbA receptor.
Etomidate is an imidazole derivative used for IV induction. Its rapid and almost complete hydrolysis to inactive metabolites results in rapid awakening. Like the above IV agents, etomidate acts on the GAbA receptor.
The most common barbiturates are thiopental, thiamylal, and methohexital. The mechanism of action is at the γ-GABA receptor, where they inhibit excitatory synaptic transmission. (See Schwartz 9th ed., p 1737.)
4. Reversal of a neuromuscular blockade is accomplished by
A. Direct antagonism of the agent
B. Increase in acetylcholine
C. Increasing breakdown of the neuromuscular blocking agent
D. None of the above
Reversal agents raise the concentration of the neurotransmitter acetylcholine to a higher level than that of the neuromuscular blocking agent. This is accomplished by the use of anticholinesterase agents, which reduce the breakdown of acetylcholine. The most commonly used agents are neostigmine, pyridostigmine, and edrophonium. (See Schwartz 9th ed., p 1749.)
5. Which of the following local anesthetic agents is an ester?
Local anesthetics are divided into two groups based on their chemical structure: the amides and the esters. In general, the amides are metabolized in the liver and the esters are metabolized by plasma cholinesterases, which yield metabolites with slightly higher allergic potential than the amides.
Lidocaine, bupivacaine, mepivacaine, prilocaine, and ropivacaine have in common an amide linkage between a benzene ring and a hydrocarbon chain that, in turn, is attached to a tertiary amine. The benzene ring confers lipid solubility for penetration of nerve membranes, and the tertiary amine attached to the hydro-carbon chain makes these local anesthetics water soluble.
Cocaine, procaine, chloroprocaine, tetracaine, and benzocaine have an ester linkage in place of the amide linkage mentioned above in the Amides section. (See Schwartz 9th ed., p 1736.)
1. Which of the following anesthetic agents has the longest duration?
Etidocaine is the longest acting local anesthetic. (See Schwartz 9th ed., p 1736, and Table 47-1.)
TABLE 47-1 Biologic properties of commonly used local anesthetics
2. What is the maximum number of cc’s of 1% lidocaine that can be used for local anesthesia in a 20-kg child?
A. 5 ml
B. 10 ml
C. 20 ml
D. 50 ml
The toxic dose of lidocaine is approximately 5 mg/kg; that of bupivacaine is approximately 3 mg/kg. Calculation of the toxic dose before injection is imperative. It is helpful to remember that for any drug or solution, 1% = 10 mg/mL.
For a 50-kg person, the toxic dose of bupivacaine would be approximately 3 mg/kg = 150 mg. A 0.5% solution of bupivacaine is 5 mg/mL, so 150 mL/5 mg/mL = 30 mL as the upper limit for infiltration. For lidocaine in the same patient, the calculation is 50 kg × 5 mg/mL = 250 mg toxic dose. If a 1% solution is used, the allowed amount would be 250 mg/10 mg/mL = 25 mL. (See Schwartz 9th ed., p 1736.)
3. Succinylcholine should NOT be used for induction in a patient with
A. An open femur fracture
B. A crush injury to the lower extremity
C. Atherosclerotic occlusion of the femoral artery
D. A burn to the foot
Although the rapid onset (60 seconds) and rapid offset (5 to 8 minutes) make succinylcholine ideal for management of the airway in certain situations, total body muscle fasciculations can cause postoperative aches and pains, an elevation in serum potassium levels, and an increase in intraocular and intragastric pressure. Its use in patients with burns or traumatic tissue injuries may result in a high enough rise in serum potassium levels to produce arrhythmias and cardiac arrest. (See Schwartz 9th ed., p 1738.)
4. Which of the following is the LEAST potent inhalational agent?
D. Nitrous oxide
Minimum alveolar concentration (MAC) is a measure of anesthetic potency. It is the ED50 of an inhaled agent (i.e., the dose required to block a response to a painful stimulus in 50% of subjects). The higher the MAC, the less potent an agent is. The potency and speed of induction of inhaled agents correlates with their lipid solubility and is known as the Meyer-Overton rule. Nitrous oxide has a low solubility and is a weak anesthetic agent, but has the most rapid onset and offset. The ‘potent’ gases (e.g., desflurane, sevoflurane, enflurane, and halothane) are more soluble in blood than nitrous oxide and can be given in lower concentrations, but have longer induction and emergence characteristics. (See Schwartz 9th ed., p 1739, and Table 47-2.)
TABLE 47-2 Advantages and disadvantages of common inhalational agents
5. Which of the following nerves are NOT blocked by spinal or epidural anesthesia?
As in spinal anesthesia, local anesthetic delivered to the epidural space bathes the spinal nerves as they exit the dura; the patient achieves analgesia from the sensory block, muscle relaxation from blockade of the motor nerves, and hypotension from blockade of the sympathetic nerves as they exit the spinal cord. (See Schwartz 9th ed., p 1737.)
6. The Mallampati classification is a risk stratification that evaluates a patient’s
A. Overall health
B. Airway status
C. Pulmonary status
D. Circulatory status
The amount of the posterior pharynx one can visualize preoperatively is important and correlates with the difficulty of intubation. A large tongue (relative to the size of the mouth) that also interferes with visualization of the larynx on laryngoscopy will obscure visualization of the pharynx. The Mallampati classification (Fig. 47-1, Table 47-3) is based on the structures visualized with maximal mouth opening and tongue protrusion in the sitting position. (See Schwartz 9th ed., p 1740.)
FIG. 47-1. The Mallampati classification.
TABLE 47-3 Mallampati classification
Class I: soft palate, fauces, uvula, pillars
Class II: soft palate, fauces, portion of uvula
Class III: soft palate, base of uvula
Class IV: hard palate only
7. The risk of “spinal headache” after a spinal anesthetic can be decreased by
A. Using a small needle
B. Slower injection of the anesthetic agent
C. A traumatic lumbar puncture
D. Increasing the narcotics instilled into the spinal fluid
Possible complications of spinal anesthesia include hypotension, especially if the patient is not adequately prehydrated; high spinal block requires immediate airway management; and postdural puncture headache sometimes occurs. Spinal headache is related to the diameter and configuration of the spinal needle, and can be reduced to approximately 1% with the use of a small 25- or 27-gauge needle. (See Schwartz 9th ed., p 1737.)
8. Which of the following is a potential triggering agent for malignant hyperthermia?
Triggering agents for malignant hyperthermia include all volatile anesthetics (e.g., halothane, enflurane, isoflurane, sevoflurane, and desflurane), and the depolarizing muscle relaxant succinylcholine. (See Schwartz 9th ed., p 1750.)
9. Which of the following agents produces the least respiratory depression?
D. None of the above
Although opioids have differing potencies required for effective analgesia, equianalgesic doses of opioids result in equal degrees of respiratory depression. Thus, there is no completely safe opioid analgesic. (See Schwartz 9th ed., p 1738.)
10. A 55-year-old man is scheduled to undergo an elective resection of a sessile polyp of the colon. He has chronic obstructive pulmonary disease and coronary artery disease, both of which are relatively well controlled by medication. He would be classified as
A. ASA Class 2
B. ASA Class 3
C. ASA Class 4
D. ASA Class 5
Research into quantifying preoperative factors that correlate with the development of postoperative morbidity and mortality has recently gained great interest. Originally designed as a simple classification of a patient’s physical status immediately before surgery, the ASA physical status scale is one of the few prospective scales that correlate with the risk of anesthesia and surgery (Table 47-4). (See Schwartz 9th ed., p 1740.)
TABLE 47-4 American Society of Anesthesiologists physical status classification system
11. Inhalational anesthetic agents
A. Dilate the airways
B. Constrict the airways
C. Increase the production of sputum
D. Decrease the production of sputum
General anesthesia can be performed safely in patients with pulmonary disease. Inhaled anesthetics are often used due to their bronchodilating properties. (See Schwartz 9th ed., p 1741.)
12. Which of the following techniques for induction of general anesthesia is most commonly used in children?
A. Intravenous induction
B. Rapid sequence induction
C. Inhalational induction
D. Combined induction
Patients undergoing inhalation induction progress through three stages: (a) awake, (b) excitement, and (c) surgical level of anesthesia. Adult patients are not good candidates for this type of induction, as the smell of the inhalation agent is unpleasant and the excitement stage can last for several minutes, which may cause hypertension, tachycardia, laryngospasm, vomiting, and aspiration. Children, however, progress through stage 2 quickly and are highly motivated for inhalation induction as an alternative to the IV route. The benefit of postinduction IV cannulation is the avoidance of many presurgical anxieties, and inhalation induction is the most common technique for pediatric surgery. (See Schwartz 9th ed., p 1743, and Fig. 47-2.)
FIG. 47-2. Techniques for the induction of general anesthesia.
13. Patients with inherited atypical pseudocholinesterase should be advised to avoid
Some patients have a genetic disorder manifesting as atypical plasma cholinesterase; the atypical enzyme has less-than-normal activity, and/or the patient has extremely low levels of the enzyme. The incidence of the homozygous form is approximately one in 3000; the effects of a single dose of succinylcholine may last several hours instead of several minutes. Two separate blood tests must be drawn:pseudocholinesterase level to determine the amount of enzyme present, and dibucaine number, which indicates the quality of the enzyme. Patients with laboratory-confirmed abnormal pseudocholinesterase levels and/or dibucaine numbers should be counseled to avoid succinylcholine as well as mivacurium, which is also hydrolyzed by pseudocholinesterase. First-degree family members should also be tested. (See Schwartz 9th ed., p 1738.)
14. Which of the following nondepolarizing neuromuscular blockers has the longest duration of action?
There are several competitive nondepolarizing agents available for clinical use. The longest acting is pancuronium, which is excreted almost completely unchanged by the kidney. Intermediate duration neuromuscular blockers include vecuronium and rocuronium, which are metabolized by both the kidneys and liver, and atracurium and cis-atracurium, which undergo breakdown in plasma known asHofmann elimination. The agent with shortest duration is mivacurium, the only nondepolarizer that is metabolized by plasma cholinesterase, and like succinylcholine, is subject to the same prolonged blockade in patients with plasma cholinesterase deficiency. (See Schwartz 9th ed., p 1738, and Table 47-5.)
TABLE 47-5 Advantages and disadvantages to common nondepolarizing neuromuscular blocking agents
15. Which of the following inhalational agents would be the best choice for a patient with chronic liver disease?
Halothane, enflurane, isoflurane, and desflurane all yield a reactive oxidative trifluoroacetyl halide and may be cross-reactive, but the magnitude of metabolism of the volatile anesthetics is a probable factor in the ability to cause hepatitis. Halothane is metabolized 20%, enflurane 2%, isoflurane 0.2%, and desflurane 0.02%; desflurane probably has the least potential for liver injury. Sevoflurane does not yield any trifluoroacetylated metabolites and is unlikely to cause hepatitis. (See Schwartz 9th ed., p 1742.)
16. Inhalational agents provide all of the following EXCEPT
D. Muscle relaxation
Unlike the IV agents, the inhalational agents provide all three characteristics of general anesthesia: unconsciousness, analgesia, and muscle relaxation. However, it would be impractical to use an inhalational technique in larger surgical procedures, because the doses required would cause unacceptable side effects, so IV adjuncts such as opioid analgesics and neuromuscular blockers are added to optimize the anesthetic. (See Schwartz 9th ed., p 1739.)