Local Anesthetics Practice Questions

Q: What are the constituents of EMLA cream?

Lidocaine + Prilocaine. **Explanation:** **EMLA (Eutectic Mixture of Local Anesthetics)** is a topical anesthetic formulation designed to penetrate intact skin. A **eutectic mixture** is a combination of two substances that, when mixed in a specific ratio, results in a melting point lower than that of either individual component. 1. **Why Option B is Correct:** EMLA cream consists of a 1:1 mixture of **2.5% Lidocaine** and **2.5% Prilocaine**. Individually, both are crystalline solids at room temperature. However, when mixed, they form an oil that melts at 18°C. This liquid state allows for a high concentration of the drug to be absorbed through the stratum corneum, providing effective dermal analgesia. 2. **Why Other Options are Incorrect:** * **Option A (Cocaine):** Cocaine is a potent vasoconstrictor and is too toxic for routine topical mixtures like EMLA. It is occasionally used in TAC (Tetracaine, Adrenaline, Cocaine) for open wounds, but not EMLA. * **Options C & D (Bupivacaine):** Bupivacaine is a long-acting amide used primarily for regional blocks and epidurals. It does not possess the specific physical properties required to form a eutectic oil with lidocaine or prilocaine for skin penetration. **High-Yield Clinical Pearls for NEET-PG:** * **Application Time:** Requires application under an occlusive dressing for at least **45–60 minutes** to achieve effective anesthesia (peak effect at 120 mins). * **Depth of Anesthesia:** It penetrates to a depth of **3–5 mm**. * **Contraindication:** Avoid in infants <3 months or those with **methemoglobinemia**, as Prilocaine metabolites (o-toluidine) can exacerbate the condition. * **Common Use:** Ideal for needle-phobic patients, pediatric venipuncture, and split-thickness skin grafts.

Q: Which of the following statements is/are not true about Dibucaine?

Shorter acting than tetracaine. **Explanation:** **Dibucaine** (also known as Cinchocaine) is an amide-linked local anesthetic. It is historically significant in anesthesiology for being one of the most potent and long-acting local anesthetics ever synthesized. **1. Why Option A is the Correct Answer (The False Statement):** Dibucaine is **not** shorter acting than tetracaine. In fact, it is characterized by its exceptionally **long duration of action** and high toxicity. It is significantly more potent and longer-acting than tetracaine, which is an ester-linked anesthetic. In the NEET-PG context, identifying Dibucaine as "short-acting" is a factual error, making it the correct choice for a "not true" question. **2. Analysis of Other Options:** * **Option B (Longer acting than tetracaine):** This is a true statement. Dibucaine’s high lipid solubility allows it to persist in neural tissues longer than most other agents. * **Option C & D (More potent than tetracaine/bupivacaine):** These are true statements. Dibucaine is roughly 15–20 times more potent than procaine and exceeds the potency of both tetracaine and bupivacaine. However, its high systemic toxicity limits its modern clinical use primarily to topical applications and the "Dibucaine Number" test. **High-Yield Clinical Pearls for NEET-PG:** * **Dibucaine Number:** This is the most common way Dibucaine appears in exams. It measures the percentage inhibition of **pseudocholinesterase** (butyrylcholinesterase) by dibucaine. * **Normal (Typical):** 80% inhibition. * **Heterozygous atypical:** 40–60% inhibition. * **Homozygous atypical:** 20% inhibition (indicates risk of prolonged apnea after Succinylcholine). * **Chemical Class:** It is an **Amide**, despite having a complex quinoline structure. * **Primary Use:** Currently used mainly in spinal anesthesia (in some regions) and topical ointments for hemorrhoids.

Q: A patient receives a toxic dose of lignocaine intravenously. Which of the following is the patient likely to exhibit?

Seizures and coma. ### Explanation **Correct Answer: D. Seizures and coma** Local anesthetic (LA) toxicity, particularly with Lignocaine, primarily affects the **Central Nervous System (CNS)** and the **Cardiovascular System (CVS)**. **Pathophysiology:** Lignocaine toxicity follows a predictable progression based on plasma concentration. Initially, LAs selectively inhibit **inhibitory cortical pathways** (GABAergic neurons). This leaves excitatory pathways unopposed, leading to signs of CNS excitation such as restlessness, tremors, and **tonic-clonic seizures**. As plasma levels rise further, both inhibitory and excitatory pathways are depressed, resulting in generalized CNS depression, **coma**, and respiratory arrest. **Analysis of Incorrect Options:** * **A & B (Salivation, Mydriasis, Diarrhea):** These are autonomic symptoms (cholinergic or adrenergic). Lignocaine toxicity does not typically present with these; instead, early signs include circumoral numbness, metallic taste, and tinnitus. * **C (Respiratory paralysis):** While respiratory arrest can occur in the terminal stages of toxicity due to medullary depression, it is preceded by CNS excitation (seizures). Seizures and coma are the classic hallmark sequence of systemic toxicity (LAST). **High-Yield Clinical Pearls for NEET-PG:** 1. **Sequence of Toxicity:** CNS symptoms (Seizures → Coma) usually appear *before* CVS symptoms (Arrhythmias → Cardiac arrest), except with Bupivacaine, which is highly cardiotoxic. 2. **Treatment of Choice:** **Intravenous Lipid Emulsion (20% Intralipid)** is the specific antidote for Local Anesthetic Systemic Toxicity (LAST). 3. **Seizure Management:** Benzodiazepines (e.g., Midazolam) are the first-line agents to control LA-induced seizures. 4. **Maximum Dose of Lignocaine:** 3 mg/kg (plain) and 7 mg/kg (with Adrenaline).

Q: A man presented with a cut wound over the scalp. What is the best anesthetic agent for wound repair?

Xylocaine with adrenaline. **Explanation:** The scalp is a highly vascular structure. When repairing a scalp laceration, the primary challenges are significant bleeding (which obscures the surgical field) and the rapid systemic absorption of local anesthetics due to high blood flow. **Why "Xylocaine with Adrenaline" is the correct answer:** Adrenaline (Epinephrine) acts as a potent **vasoconstrictor** when added to local anesthetics (usually in a 1:200,000 concentration). In scalp wounds, it provides two major benefits: 1. **Hemostasis:** It constricts local blood vessels, reducing bleeding and providing a "bloodless field" for precise suturing. 2. **Prolonged Action & Reduced Toxicity:** By slowing systemic absorption, it keeps the anesthetic at the site longer and reduces the risk of Systemic Local Anesthetic Toxicity (LAST). **Analysis of Incorrect Options:** * **1% and 2% Xylocaine (Plain):** While these provide anesthesia, they lack the vasoconstrictive benefit. Without adrenaline, the scalp will bleed profusely, and the duration of the block will be significantly shorter. * **Ketamine:** This is a dissociative general anesthetic. It is inappropriate for a simple scalp wound repair where local infiltration is safer and sufficient. **High-Yield Clinical Pearls for NEET-PG:** * **Maximum Dose of Lignocaine:** Plain = **3 mg/kg**; With Adrenaline = **7 mg/kg**. * **Contraindications:** Never use adrenaline-containing local anesthetics in areas supplied by **end-arteries** (e.g., fingers, toes, tip of the nose, ears, and penis) due to the risk of ischemia and gangrene. * **Scalp Anatomy:** The scalp is often cited as the site with the highest rate of systemic absorption of local anesthetics (mnemonic: **BICEPS** – Blood, Intercostal, Caudal, Epidural, Plexus, Sciatic/Subcutaneous).

Q: Toxicity of local anaesthesia is reversed by:

IV Barbiturates. **Explanation:** The correct answer is **IV Barbiturates**. Local Anesthetic Systemic Toxicity (LAST) primarily affects the Central Nervous System (CNS) and the Cardiovascular System. CNS toxicity typically manifests as excitation, tremors, and generalized tonic-clonic seizures. **Why IV Barbiturates are correct:** Barbiturates (like Thiopental) and Benzodiazepines (like Midazolam) are the drugs of choice to terminate seizures caused by local anesthetic toxicity. They act by enhancing GABAergic inhibition, thereby raising the seizure threshold and suppressing cortical excitability. **Analysis of Incorrect Options:** * **A. IV Epinephrine:** While used in ACLS protocols for cardiac arrest, it is not the reversal agent for toxicity. In fact, high doses of epinephrine can worsen arrhythmias in the setting of bupivacaine toxicity. * **B. IV Nalorphine:** This is an older opioid antagonist/mixed agonist-antagonist. It has no role in treating local anesthetic toxicity, which involves sodium channel blockade, not opioid receptors. * **D. IV Sodium Bicarbonate:** While used to treat arrhythmias in TCA overdose or to alkalinize urine, it is not the primary treatment for reversing LA toxicity. **NEET-PG High-Yield Pearls:** 1. **Gold Standard Treatment:** The definitive treatment for LAST today is **Intravenous Lipid Emulsion (ILE) 20%** (Lipid Rescue). It acts as a "lipid sink," sequestering the lipophilic anesthetic molecules away from the heart and brain. 2. **Order of Toxicity:** CNS symptoms (tinnitus, metallic taste, perioral numbness, seizures) usually precede Cardiovascular symptoms (hypotension, arrhythmias, cardiac arrest). 3. **Bupivacaine:** It is the most cardiotoxic local anesthetic due to its slow dissociation from cardiac sodium channels ("fast in, slow out" kinetics). 4. **Management Priority:** Airway management (100% $O_2$) is the first step, as hypoxia and acidosis worsen toxicity.

Q: Which of the following is a naturally occurring local anesthetic?

Cocaine. **Explanation:** **Correct Answer: A. Cocaine** Cocaine is the only **naturally occurring** local anesthetic (LA). It is an alkaloid derived from the leaves of the *Erythroxylum coca* plant. Historically, it was the first local anesthetic used in clinical practice (by Karl Koller for ophthalmic surgery). Chemically, it is an ester of benzoic acid. **Analysis of Incorrect Options:** * **B. Procaine:** This is a **synthetic** ester-type local anesthetic. It was the first synthetic substitute for cocaine, developed to reduce toxicity and addiction potential. * **C. Lignocaine (Lidocaine):** This is a **synthetic** amide-type local anesthetic. It is the most widely used LA and serves as the prototype for the amide group. * **D. Bupivacaine:** This is a **synthetic** amide-type local anesthetic known for its long duration of action and high lipid solubility. **High-Yield Clinical Pearls for NEET-PG:** * **Vasoconstriction:** Cocaine is unique among local anesthetics because it causes **intrinsic vasoconstriction** by inhibiting the reuptake of norepinephrine at sympathetic nerve endings. All other LAs (except ropivacaine and levobupivacaine to a lesser extent) are vasodilators. * **Classification:** Remember the "i" rule: Amides have two "i"s in their name (L**i**doca**i**ne, Bup**i**vaca**i**ne, Pr**i**loca**i**ne), while Esters have only one (Coca**i**ne, Proca**i**ne, Tetraca**i**ne). * **Metabolism:** Esters (like Cocaine) are metabolized by **plasma pseudocholinesterase**, whereas amides are metabolized in the **liver**. * **Toxicity:** Cocaine is highly addictive and can cause significant cardiovascular stimulation (hypertension, arrhythmias).

Q: What is the maximum safe dose of bupivacaine?

2 mg/kg. **Explanation:** The maximum safe dose of **Bupivacaine** is **2 mg/kg** (without epinephrine). Bupivacaine is an amino-amide local anesthetic known for its high potency and long duration of action. However, it is significantly more **cardiotoxic** than lidocaine because it dissociates slowly from cardiac sodium channels during diastole (the "fast-in, slow-out" kinetics) [1]. Exceeding the 2 mg/kg threshold increases the risk of Local Anesthetic Systemic Toxicity (LAST), which can lead to refractory ventricular arrhythmias and cardiac arrest [1]. **Analysis of Options:** * **A (1 mg/kg):** This is an underestimation. While conservative, it is not the standard maximum dose defined in clinical guidelines. * **B (2 mg/kg):** **Correct.** This is the standard maximum recommended dose for plain bupivacaine. When combined with epinephrine, some texts suggest a slight increase (up to 2.5 mg/kg), but 2 mg/kg remains the high-yield "safe" limit for exams. * **C (3 mg/kg):** This is the maximum dose for **Ropivacaine**, which is a S-enantiomer of bupivacaine designed to be less cardiotoxic. * **D (5 mg/kg):** This is the maximum dose for **Lidocaine** (plain). If lidocaine is used with adrenaline, the limit increases to 7 mg/kg. **High-Yield Clinical Pearls for NEET-PG:** * **Cardiotoxicity:** Bupivacaine has a low **CC/CNS ratio** (3.0), meaning the dose required to cause cardiovascular collapse is very close to the dose that causes seizures [1]. * **Antidote:** In case of bupivacaine-induced cardiac arrest, **20% Intravenous Lipid Emulsion (ILE)** is the specific treatment of choice (the "Lipid Sink" theory). * **Levobupivacaine:** The S-isomer of bupivacaine is preferred in modern practice as it is less cardiotoxic and neurotoxic.

Question 1

What are the constituents of EMLA cream?

Accepted Answer

Lidocaine + Prilocaine

Answer

Lidocaine + Cocaine

Answer

Lidocaine + Bupivacaine

Answer

Bupivacaine + Prilocaine

Question 2

Which of the following statements is/are not true about Dibucaine?

Accepted Answer

Shorter acting than tetracaine

Answer

Longer acting than tetracaine

Answer

More potent than tetracaine

Answer

More potent than bupivacaine

Question 3

In which patient population is local anesthesia generally feasible during dental treatment?

Accepted Answer

Adult

Answer

Child

Answer

Mentally retarded individual

Answer

Physically challenged individual

Question 4

A patient receives a toxic dose of lignocaine intravenously. Which of the following is the patient likely to exhibit?

Accepted Answer

Seizures and coma

Answer

Excessive salivation

Answer

Mydriasis and diarrhea

Answer

Respiratory paralysis

Question 5

A man presented with a cut wound over the scalp. What is the best anesthetic agent for wound repair?

Accepted Answer

Xylocaine with adrenaline

Answer

1% xylocaine

Answer

2% xylocaine

Answer

Ketamine

Question 6

Which of the following local anesthetics is suitable for a patient with hypertension and heart disease?

Accepted Answer

Prilocaine and felypressin

Answer

Lignocaine without adrenaline

Answer

Prilocaine

Answer

Lignocaine with adrenaline 1:1000

Question 7

Toxicity of local anaesthesia is reversed by:

Accepted Answer

IV Barbiturates

Answer

IV epinephrine

Answer

IV nalorphine

Answer

IV sodium bicarbonate

Question 8

Which of the following is NOT a symptom of epinephrine overdose following a local anesthetic injection?

Accepted Answer

Hypotension

Answer

Restlessness

Answer

Apprehension

Answer

Palpitations

Question 9

Which of the following is a naturally occurring local anesthetic?

Accepted Answer

Cocaine

Answer

Procaine

Answer

Lignocaine

Answer

Bupivacaine

Question 10

What is the maximum safe dose of bupivacaine?

Accepted Answer

2 mg/kg

Answer

1 mg/kg

Answer

3 mg/kg

Answer

5 mg/kg

Local Anesthetics — MCQs

Local Anesthetics — MCQs

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