Local Anesthetics — MCQs

Local Anesthetics Indian Medical PG Practice Questions and MCQs

Question 11: What are the constituents of EMLA cream?

A. Lidocaine + Cocaine
B. Lidocaine + Prilocaine (Correct Answer)
C. Lidocaine + Bupivacaine
D. Bupivacaine + Prilocaine

Explanation: **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.

Question 12: Which of the following statements is/are not true about Dibucaine?

A. Shorter acting than tetracaine (Correct Answer)
B. Longer acting than tetracaine
C. More potent than tetracaine
D. More potent than bupivacaine

Explanation: **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.

Question 13: In which patient population is local anesthesia generally feasible during dental treatment?

A. Child
B. Adult (Correct Answer)
C. Mentally retarded individual
D. Physically challenged individual

Explanation: **Explanation:** The feasibility of local anesthesia (LA) in dental procedures depends primarily on the patient’s ability to cooperate, follow instructions, and remain still during the administration and the subsequent procedure. **1. Why Adult is the Correct Answer:** Adults are generally the most suitable candidates for local anesthesia because they possess the **cognitive maturity** to understand the procedure, provide informed consent, and maintain the necessary physical cooperation. Unlike general anesthesia, LA requires the patient to be conscious and communicative, making the adult population the most predictable demographic for successful application. **2. Analysis of Incorrect Options:** * **Child (Option A):** While LA is used in children, it is often challenging due to anxiety, fear of needles, and limited attention spans. Pediatric patients frequently require sedation or general anesthesia for complex dental work to prevent movement-related injuries. * **Mentally Retarded Individual (Option C):** Patients with cognitive impairments may struggle to understand the necessity of the procedure or the sensation of numbness. Lack of cooperation and potential combativeness often necessitate general anesthesia for safety. * **Physically Challenged Individual (Option D):** While many physically challenged patients can receive LA, those with involuntary movements (e.g., severe cerebral palsy or tremors) pose a risk for needle-stick injuries or procedural failure, often requiring systemic management. **Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Local anesthetics block **voltage-gated sodium channels** from the intracellular side, preventing depolarization. * **Sensitivity:** Small, myelinated fibers (A-delta) and unmyelinated fibers (C-fibers) carrying pain are blocked before larger motor fibers. * **Maximum Dose (Lidocaine):** 4.5 mg/kg (plain) and 7 mg/kg (with epinephrine). * **Contraindication:** Avoid LA in areas of **acute infection/inflammation** because the acidic pH (low pH) ionizes the drug, preventing it from crossing the neuronal membrane.

Question 14: A patient receives a toxic dose of lignocaine intravenously. Which of the following is the patient likely to exhibit?

A. Excessive salivation
B. Mydriasis and diarrhea
C. Respiratory paralysis
D. Seizures and coma (Correct Answer)

Explanation: ### 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).

Question 15: A man presented with a cut wound over the scalp. What is the best anesthetic agent for wound repair?

A. 1% xylocaine
B. 2% xylocaine
C. Ketamine
D. Xylocaine with adrenaline (Correct Answer)

Explanation: **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).

Question 16: Which of the following local anesthetics is suitable for a patient with hypertension and heart disease?

A. Lignocaine without adrenaline
B. Prilocaine
C. Lignocaine with adrenaline 1:1000
D. Prilocaine and felypressin (Correct Answer)

Explanation: **Explanation:** The primary concern in patients with hypertension and heart disease is the use of **Adrenaline** (Epinephrine) as a vasoconstrictor. Adrenaline acts on $\beta_1$ and $\alpha_1$ receptors, leading to increased heart rate, myocardial oxygen demand, and peripheral resistance, which can trigger arrhythmias or hypertensive crises. **Why Prilocaine and Felypressin is correct:** **Felypressin** (Octapressin) is a synthetic analogue of vasopressin (ADH). Unlike adrenaline, it acts primarily on the **V1 receptors** in vascular smooth muscle. At clinical doses, it provides effective local vasoconstriction to prolong anesthesia without significant effects on heart rate or blood pressure. It lacks the sympathomimetic cardiac stimulatory effects of adrenaline, making it the safest choice for cardiac patients. **Analysis of Incorrect Options:** * **Lignocaine with adrenaline (1:1000):** This concentration is extremely high (standard dental/local use is 1:100,000 or 1:200,000). Adrenaline is contraindicated in severe hypertension and unstable ischemic heart disease due to its cardiac stimulatory effects. * **Lignocaine without adrenaline:** While safer than the adrenaline combination, the lack of a vasoconstrictor leads to rapid systemic absorption, shorter duration of action, and a higher risk of systemic toxicity (LAST). * **Prilocaine (alone):** Similar to plain lignocaine, without a vasoconstrictor, its efficacy is reduced and systemic absorption is faster. **High-Yield Clinical Pearls for NEET-PG:** * **Felypressin Caution:** It should be avoided in **pregnancy** as it has a theoretical oxytocic effect (though minimal). * **Prilocaine Side Effect:** High doses (>600mg) can cause **Methemoglobinemia** due to its metabolite, *o-toluidine*. The treatment is Methylene Blue. * **Maximum Dose:** Lignocaine with adrenaline (7mg/kg); Lignocaine plain (3-4mg/kg). * **Cocaine:** The only local anesthetic that is a natural vasoconstrictor (blocks NE reuptake).

Question 17: Toxicity of local anaesthesia is reversed by:

A. IV epinephrine
B. IV nalorphine
C. IV Barbiturates (Correct Answer)
D. IV sodium bicarbonate

Explanation: **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.

Question 18: Which of the following is NOT a symptom of epinephrine overdose following a local anesthetic injection?

A. Restlessness
B. Hypotension (Correct Answer)
C. Apprehension
D. Palpitations

Explanation: **Explanation:** The correct answer is **Hypotension**. Epinephrine is a potent sympathomimetic amine that acts as an agonist at both alpha (α) and beta (β) adrenergic receptors. When an overdose occurs—often due to accidental intravascular injection or rapid absorption—it triggers an intense "fight or flight" response. **Why Hypotension is the correct answer:** Epinephrine overdose typically causes **hypertension** (due to α1-mediated vasoconstriction and β1-mediated increased cardiac output) rather than hypotension. Hypotension is more characteristic of the cardiovascular collapse seen in severe **Local Anesthetic Systemic Toxicity (LAST)**, where drugs like Bupivacaine cause direct myocardial depression and vasodilation, rather than the epinephrine additive itself. **Analysis of Incorrect Options:** * **Restlessness & Apprehension:** These are early central nervous system manifestations of epinephrine's stimulatory effect. The surge in sympathetic activity leads to acute anxiety, tremors, and a sense of impending doom. * **Palpitations:** Epinephrine acts on **β1 receptors** in the heart, causing positive inotropy (increased force) and chronotropy (increased rate). This results in tachycardia and forceful contractions, perceived by the patient as palpitations. **NEET-PG High-Yield Pearls:** * **Standard Concentration:** Epinephrine is typically added to local anesthetics in a concentration of **1:200,000** (5 µg/mL) to prolong the duration of action and decrease systemic absorption via vasoconstriction. * **Maximum Dose:** With the addition of epinephrine, the maximum safe dose of Lidocaine increases from **3 mg/kg to 7 mg/kg**. * **Contraindications:** Avoid epinephrine-containing LAs in "end-artery" areas (fingers, toes, penis, nose, pinna) to prevent ischemic necrosis. * **Clinical Tip:** If a patient develops sudden tachycardia and hypertension immediately after injection, suspect epinephrine overdose; if they develop bradycardia and hypotension, suspect LAST.

Question 19: Which of the following is a naturally occurring local anesthetic?

A. Cocaine (Correct Answer)
B. Procaine
C. Lignocaine
D. Bupivacaine

Explanation: **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).

Question 20: What is the maximum safe dose of bupivacaine?

A. 1 mg/kg
B. 2 mg/kg (Correct Answer)
C. 3 mg/kg
D. 5 mg/kg

Explanation: **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.

Practice by Chapter

Chemistry and Mechanism of Action

Practice Questions

Pharmacokinetics of Local Anesthetics

Practice Questions

Amide Local Anesthetics

Practice Questions

Ester Local Anesthetics

Practice Questions

Clinical Uses of Local Anesthetics

Practice Questions

Toxicity of Local Anesthetics

Practice Questions

Management of Local Anesthetic Systemic Toxicity

Practice Questions

Adjuvants to Local Anesthetics

Practice Questions

Maximum Safe Doses

Practice Questions

Local Anesthetics in Special Populations

Practice Questions

Allergic Reactions to Local Anesthetics

Practice Questions

Future Developments in Local Anesthetics

Practice Questions

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