Local Anesthetics

Local Anesthetics Indian Medical PG Question 1: Which inhalational agent increases intracranial pressure most significantly?

• A. Halothane (Correct Answer)
• B. Sevoflurane
• C. Isoflurane
• D. Desflurane

Local Anesthetics Explanation: ***Halothane*** - **Halothane** causes a greater increase in **cerebral blood flow** and thus **intracranial pressure (ICP)** compared to newer volatile anesthetics due to its more potent cerebral vasodilation. - Its use has largely declined due to concerns about its effects on ICP and potential for **hepatotoxicity**. *Sevoflurane* - While sevoflurane can cause **cerebral vasodilation** and increase ICP, its effect is generally less pronounced than halothane, especially when normocapnia is maintained. - It is often favored in neuroanesthesia due to its rapid onset and offset, allowing for quicker adjustments in anesthetic depth. *Isoflurane* - Isoflurane causes less cerebral vasodilation and a smaller increase in ICP compared to halothane, particularly at lower concentrations. - It maintains **cerebral vascular autoregulation** better than halothane, helping to preserve a more stable ICP. *Desflurane* - Desflurane also causes cerebral vasodilation and can increase ICP, but its effect is typically less significant than halothane. - Rapid increases in desflurane concentration can lead to sympathetic stimulation and transient increases in blood pressure, which can indirectly affect ICP.

Local Anesthetics Indian Medical PG Question 2: Which of the following is not an ESTER?

• A. Benzocaine
• B. Bupivacaine (Correct Answer)
• C. Amethocaine
• D. Procaine

Local Anesthetics Explanation: ***Bupivacaine*** - Bupivacaine is an **amide-type local anesthetic** because it contains an amide linkage in its chemical structure. - Amide-type local anesthetics are metabolized in the liver, generally having a **longer duration of action** and a lower risk of allergic reactions compared to esters. *Benzocaine* - Benzocaine is an **ester-type local anesthetic**, characterized by an ester linkage that connects the aromatic ring and the amine group, though it lacks a tertiary amine. - It is typically used for **topical applications** due to its poor water solubility and rapid metabolism by plasma esterases. *Amethocaine* - Amethocaine (also known as tetracaine) is an **ester-type local anesthetic**, identifiable by its ester bond. - It has a relatively **long duration of action** for an ester and is often used for spinal anesthesia and topical applications. *Procaine* - Procaine is an **ester-type local anesthetic**, containing the characteristic ester linkage in its chemical structure. - It was one of the first local anesthetics discovered and has a **short duration of action** due to rapid metabolism by plasma pseudocholinesterase.

Local Anesthetics Indian Medical PG Question 3: Which of the following nerve fibre types is least susceptible to LA blockade?

• A. B fibers
• B. A beta
• C. C fibers
• D. A alpha (Correct Answer)

Local Anesthetics Explanation: **A alpha** - **A alpha fibers** are the **largest** and most heavily myelinated nerve fibers, responsible for **motor function** and **proprioception**. - Due to their large diameter and thick myelination, they have the **highest conduction velocity** and are the **least susceptible to local anesthetic blockade**, requiring higher concentrations or longer exposure times. *B fibers* - **B fibers** are **preganglionic autonomic fibers** that are myelinated but of relatively small diameter. - They are generally **highly sensitive to local anesthetics**, often being blocked even before A-delta and C fibers, due to their specific physiologic properties like repetitive firing and length of node of Ranvier. *C fibers* - **C fibers** are **unmyelinated** and have the **smallest diameter**, transmitting **pain, temperature, and autonomic information**. - Despite being unmyelinated, their small diameter makes them **highly sensitive to local anesthetics**, as the drug can easily penetrate to block sodium channels. *A beta* - **A beta fibers** are large, myelinated fibers involved in transmitting **touch and pressure sensations**. - While myelinated, they are **smaller than A-alpha fibers** and thus more susceptible to local anesthetic blockade than A-alpha, but less so than C or B fibers.

Local Anesthetics Indian Medical PG Question 4: What is the maximum concentration allowed for epidural block?

• A. Chlorprocaine (Correct Answer)
• B. Lidocaine
• C. Ropivacaine
• D. Bupivacaine

Local Anesthetics Explanation: ***Chlorprocaine*** - **Chlorprocaine** is an ester-type local anesthetic that can be safely used in higher concentrations for epidural blocks up to **3%**, due to its rapid hydrolysis by plasma pseudocholinesterase, leading to a very short half-life and reduced systemic toxicity. - Its rapid metabolism minimizes the risk of accumulation and systemic toxicity, making it a suitable choice when a dense block is needed and a short duration of action is acceptable. *Lidocaine* - **Lidocaine** is an amide-type local anesthetic commonly used in epidural blocks, but its maximum concentration for this application is typically limited to **2%** to avoid systemic toxicity. - Higher concentrations of lidocaine are associated with an increased risk of neurological and cardiovascular adverse effects. *Ropivacaine* - **Ropivacaine** is an amide-type local anesthetic that is less cardiotoxic than bupivacaine, with common concentrations for epidural use ranging from **0.2% to 1%**. - Its maximum concentration is significantly lower than chlorprocaine due to its longer duration of action and potential for systemic toxicity at higher doses. *Bupivacaine* - **Bupivacaine** is a potent amide-type local anesthetic with a high risk of cardiotoxicity, and its maximum concentration for epidural use is generally restricted to **0.5%** or even less for continuous infusions. - Using concentrations above this limit significantly increases the risk of severe cardiovascular complications, including arrhythmias and cardiac arrest.

Local Anesthetics Indian Medical PG Question 5: Which of the following is FALSE about ropivacaine?

• A. More cardiotoxic than lignocaine
• B. Contains only R enantiomer (Correct Answer)
• C. Less cardiotoxic than bupivacaine
• D. Onset of action is faster than bupivacaine

Local Anesthetics Explanation: ***Contains only R enantiomer*** - Ropivacaine is a **pure S-enantiomer** (S-(-)-enantiomer) preparation, NOT the R-enantiomer. - This single enantiomer formulation contributes to its **reduced cardiotoxicity** and improved safety profile compared to racemic bupivacaine. - The statement "contains only R enantiomer" is **FALSE** - this is the correct answer. *Less cardiotoxic than bupivacaine* - Ropivacaine has **lower lipid solubility** and faster dissociation from cardiac sodium channels compared to bupivacaine. - This results in **significantly reduced cardiotoxicity**, making it safer for large-volume regional blocks. - This statement is **TRUE**. *More cardiotoxic than lignocaine* - Ropivacaine is indeed **more cardiotoxic than lidocaine** due to its greater potency and longer duration of action. - Amide local anesthetics with higher potency (bupivacaine > ropivacaine > lidocaine) carry greater cardiac risk. - This statement is **TRUE**. *Onset of action is faster than bupivacaine* - Ropivacaine and bupivacaine have **similar onset times** (10-15 minutes for epidural/nerve blocks). - Both have similar pKa values (ropivacaine 8.1, bupivacaine 8.1), resulting in comparable onset characteristics. - While ropivacaine may have marginally faster onset in some contexts, clinically they are considered equivalent. - This statement is generally **FALSE or equivocal**, making it potentially the second-best answer, but the R-enantiomer statement is definitively false.

Local Anesthetics Flashcard 1 of 5

Question: Bupivacaine has a tendency to provide more _____ block over motor block (sensory/motor)

Answer: sensory

Local Anesthetics Flashcard 2 of 5

Question: _____ is the mainstay rx for cardiac arrest due to bupivacaine

Answer: Epinephrine

Local Anesthetics Flashcard 3 of 5

Question: Neurotoxicity from neuro-axial (epidural or spinal) placement of local anesthetic solution is maximum with _____

Answer: lidocaine

Local Anesthetics Flashcard 4 of 5

Question: 2 mL of 1% _____ in 18 mL propofol can be used to decrease pain on injection of propofol

Answer: lidocaine

Local Anesthetics Flashcard 5 of 5

Question: Alkalinization of local anesthetics can lead to a _____ onset of action and improved quality of block

Answer: faster