30 Questions
Which of the following is a potential CNS effect of local anesthetics?
Nystagmus
What is a possible cardiovascular effect of bupivacaine?
Arrhythmias
How might repeated epidural injections in anesthetic doses affect the individual?
Tachyphylaxis
Which local anesthetic is known to be less cardiotoxic compared to bupivacaine?
Levobupivacaine
What type of anesthesia involves slow infusion at low concentrations for postoperative pain relief?
Epidural anesthesia
Besides cardiovascular effects, what other toxic effects can local anesthetics produce?
Respiratory depression
What determines the differential sensitivity of various types of nerve fibers to local anesthetics?
Fiber diameter, myelination, physiologic firing rate, and anatomic location
Why are smaller fibers blocked more easily by local anesthetics compared to larger fibers?
Smaller fibers have a lower activation threshold
What will be the sequence of sensory block occurrence from proximal to distal when anesthetic is placed outside a nerve bundle?
From proximal to distal
How does local anesthetic affect pain sensation in relation to activated pain fibers?
Pain sensation is blocked selectively
Which fibers located within a thick nerve bundle are blocked sooner by local anesthetics?
Fibers at the periphery of the bundle
What effect does local anesthetic have on the upstroke of sodium-dependent action potentials in the heart?
It slows the upstroke
What is the primary mechanism of action of local anesthetics?
Blockage of sodium channels of excitable membranes
How can the duration of action of shorter-acting local anesthetics be extended?
Administration of vasoconstrictor
Which local anesthetic is considered an exception due to its intrinsic sympathomimetic action?
Cocaine
How do longer-acting local anesthetics differ from shorter-acting ones in terms of vasoconstrictor dependency?
Shorter-acting ones require vasoconstrictors, while longer-acting ones do not
What effect does cocaine have on norepinephrine reuptake into nerve terminals?
Blocks reuptake
How do local anesthetics restrict their effect to a localized area?
By targeting only specific nerve endings
What is the primary goal of sustained-release formulations in local anesthesia?
To provide prolonged analgesia
Why are less toxic and more selective agents explored in the development of local anesthesia formulations?
To improve therapeutic indexes
What is Neosaxitoxin primarily known for in the context of local anesthesia?
Providing prolonged block without catheter placement
What does sustained-release delivery have as an added advantage compared to catheter administration?
Reduced risk of systemic toxicity
Why does anesthetic neurotoxicity not result from blockade of the voltage-gated sodium channel?
It is unrelated to tissue toxicity
What is the primary advantage of drug delivery systems that slowly release anesthetic?
Prolonged analgesia without drawbacks of a catheter
Which of the following is a cardiovascular toxicity associated with the use of cocaine?
Arrhythmias
Why has the popularity of cocaine as a topical anesthesia diminished recently?
It may induce methemoglobinemia
Why is bupivacaine often avoided for techniques that require high concentrations of concentrated anesthetic?
It can induce cardiotoxicity
What is a reason why spinal bupivacaine is not well suited for outpatient or ambulatory surgery?
It delays recovery due to its long duration of action
Why are relatively low concentrations of bupivacaine (≤ 0.25%) used for prolonged peripheral anesthesia and analgesia?
To reduce its cardiotoxic effects
Why is there no specific antidote for local anesthetic (LA) toxicity mentioned in the text?
Because LA toxicity is treated symptomatically
Study Notes
Local Anesthetics
- Block sensory transmission from a local area of the body to the CNS
- Accomplished by disrupting afferent neural traffic via inhibition of impulse generation or propagation
- Chemically similar agents (esters and amides) that block sodium channels of excitable membranes
Mechanism of Action
- Injection or topical application restricts effect to localized area
- Inhibit impulse generation or propagation in afferent neural traffic
- Block sodium channels of excitable membranes
Pharmacokinetics
- Most shorter-acting local anesthetics are readily absorbed into the blood from the site of administration
- Duration of action is limited unless blood flow to the area is reduced
- Vasoconstrictors can be used to reduce blood flow and increase duration of action
- Cocaine is an exception due to its intrinsic sympathomimetic action (inhibition of norepinephrine reuptake into nerve terminals)
Pharmacologic Effects
- Differential sensitivity of various types of nerve fibers to local anesthetics depends on:
- Fiber diameter
- Myelination
- Physiologic firing rate
- Anatomic location
- Smaller fibers are blocked more easily than larger fibers
- Myelinated fibers are blocked more easily than unmyelinated fibers
- Activated pain fibers fire rapidly, and pain sensation appears to be selectively blocked by local anesthetics
- Fibers located in the periphery of a thick nerve bundle are blocked sooner than those in the core
Cardiovascular Toxicity
- Cocaine:
- Contributes to cardiovascular toxicity due to its vasoconstricting actions and ability to block norepinephrine reuptake
- Can cause severe hypertension, arrhythmias, and myocardial infarction
- Bupivacaine:
- Can produce severe cardiovascular toxicity including arrhythmias and hypotension
- Levobupivacaine is less cardiotoxic
- Ropivacaine:
- Can produce cardiotoxicity when used for peripheral nerve block
Commonly Used Local Anesthetics
- Bupivacaine:
- Agent of choice for epidural infusions used for postoperative pain control and labor analgesia
- Has a relatively unblemished record as a spinal anesthetic
- Chloroprocaine:
- Used for postoperative pain control and labor analgesia
- Articaine:
- Used for dental anesthesia
Toxicity
- CNS:
- Can produce a spectrum of central effects, including light-headedness, sedation, restlessness, nystagmus, and tonic-clonic convulsions
- Severe convulsions may be followed by coma with respiratory and cardiovascular depression
- Cardiovascular:
- All local anesthetics are capable of producing cardiovascular toxicity, with the exception of cocaine
- Patients with pre-existing cardiovascular disease may develop heart block and other disturbances of cardiac electrical function at high plasma levels of anesthetics
Future Developments
- Sustained Release Formulations:
- Can provide prolonged analgesia or anesthesia without the drawbacks of a catheter
- Reduced risk of systemic toxicity
- Less Toxic Agents:
- Developing compounds with considerably better therapeutic indexes
- Neosaxitoxin:
- A site 1 Na⁺ channel biotoxin explored as a method to provide prolonged block, with the goal of obviating the need for catheter placement and continuous anesthetic infusion
Test your knowledge on the uses and administration of local anesthetics in medical practice, including spinal anesthesia, autonomic blockade, postoperative analgesia, and pain management in neuropathic states.
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