Toxicity of Local Anesthetics — MCQs

10 questions

Which of the following signs of congestive cardiac failure constitute a major risk to the surgical patient undergoing anaesthesia ?

What is the preferred concentration range of lidocaine for topical anesthesia?

What is the percentage of halothane that is metabolized in the human body?

A young male was administered regional anesthesia with 0.25% bupivacaine. The patient became unresponsive, and the pulse became unrecordable. What is the best management in this situation?

What is the maximum dose of plain lignocaine (in mg) for adults?

What is the primary cardiotoxic effect of bupivacaine?

In ACLS, which antiarrhythmic drug can be given following ventricular fibrillation after cardiac arrest other than epinephrine?

Which is correct about the anesthetic drugs X and Y in the image shown? (Recent NEET Pattern 2016-17)

In ophthalmology, if a patient is allergic to aminoesters, which local anesthetic can be safely used?

Q10

A 25 year old male with roadside accident underwent debridement and reduction of fractured both bones right forearm under axillary block. On the second postoperative day the patient complained of persistent numbness and paresthesia in the right forearm and the hand. The commonest cause of this neurological dysfunction could be all of the following except –

Toxicity of Local Anesthetics Indian Medical PG Practice Questions and MCQs

Question 1: Which of the following signs of congestive cardiac failure constitute a major risk to the surgical patient undergoing anaesthesia ?

A. Pedal oedema
B. Jugular venous distention and third heart sound (Correct Answer)
C. Basal crepitations on auscultation
D. Cardiomegaly

Explanation: ***Jugular venous distention and third heart sound*** - **Jugular venous distention (JVD)** indicates elevated right atrial pressure and **central venous pressure**, signifying significant volume overload and potential right ventricular dysfunction. - A **third heart sound (S3)** is a strong indicator of **ventricular dysfunction** and high filling pressures, suggesting severe heart failure and poor cardiac reserve, which poses a major risk during anesthesia. *Pedal oedema* - **Pedal edema** can be a sign of fluid overload, but it is a relatively mild and chronic symptom of heart failure compared to acute decompensation. - While it reflects increased hydrostatic pressure, it doesn't alone signify the immediate, critical hemodynamic instability that poses a major perioperative risk. *Basal crepitations on auscultation* - **Basal crepitations** (rales) suggest **pulmonary congestion** due to left ventricular failure, indicating fluid in the small airways. - Although concerning, this sign primarily reflects lung involvement and not necessarily the critical, global hemodynamic compromise indicated by JVD and S3. *Cardiomegaly* - **Cardiomegaly** (enlarged heart) is an imaging finding that reflects chronic cardiac remodeling due to long-standing heart disease. - It indicates a history of heart failure but does not directly reflect the **acute hemodynamic status** and immediate risk of decompensation during surgery.

Question 2: What is the preferred concentration range of lidocaine for topical anesthesia?

A. 2-4% (Correct Answer)
B. 7-12%
C. 12-15%
D. <2%

Explanation: ***Correct Option: 2-4%*** - **Lidocaine** is an **amide-type local anesthetic** commonly used for topical anesthesia to numb localized areas before minor procedures. - The **preferred concentration range for topical application is 2-4%**, which provides effective anesthesia with an acceptable safety profile. - **2% lidocaine gel/cream** is commonly used for skin and genital mucosa. - **4% lidocaine** is standard for oral and respiratory mucous membranes. - **5% lidocaine patches** are used for post-herpetic neuralgia. - This concentration range balances **clinical efficacy** with **minimal systemic toxicity risk**. *Incorrect Option: <2%* - Concentrations below **2%** are generally **suboptimal** for achieving significant topical anesthesia. - These lower concentrations result in **insufficient pain relief** or require longer application times. - While 0.5-1% solutions exist, they are primarily used for infiltration anesthesia, not topical application. *Incorrect Option: 7-12%* - Concentrations in the **7-12%** range are **too high** for routine topical use and increase the risk of **systemic toxicity**. - These concentrations are not standard in clinical practice for general topical anesthesia. - Higher concentrations increase absorption without proportional improvement in efficacy. *Incorrect Option: 12-15%* - Concentrations in the **12-15%** range are **excessively high** and pose substantial **risk of systemic absorption and toxicity**. - Such high concentrations are **not recommended** for topical anesthesia in clinical practice. - Even 10% sprays (used for oropharyngeal anesthesia) require strict dose limitations to prevent toxicity.

Question 3: What is the percentage of halothane that is metabolized in the human body?

A. 50%
B. 5%
C. 2.50%
D. 25% (Correct Answer)

Explanation: **Correct: 25%** - Approximately **25%** of administered halothane is metabolized in the liver, which is a relatively high percentage compared to other volatile anesthetics. - This extensive metabolism can lead to the formation of reactive intermediates, contributing to its potential for **hepatotoxicity** (halothane hepatitis). *Incorrect: 50%* - **50%** metabolism is significantly higher than what is observed for halothane and would imply even greater risk of significant metabolic byproduct accumulation and toxicity. - Most volatile anesthetics are metabolized to a much lesser extent, with desflurane having the least metabolism (<0.02%). *Incorrect: 5%* - **5%** metabolism is too low for halothane; while some volatile anesthetics like isoflurane fall into this range (~0.2-2%), halothane is known for its considerably higher metabolic rate. - A 5% metabolism rate would result in less concern for and incidence of **halothane hepatitis**. *Incorrect: 2.50%* - **2.50%** metabolism is an underestimation of halothane's metabolic activity within the body. - Anesthetic agents such as **enflurane** have a metabolism rate closer to this value (~2-5%), whereas halothane is much higher.

Question 4: A young male was administered regional anesthesia with 0.25% bupivacaine. The patient became unresponsive, and the pulse became unrecordable. What is the best management in this situation?

A. ECPR with calcium
B. ECPR with dobutamine
C. ECPR with 20% intralipid (Correct Answer)
D. ECPR with sodium bicarbonate

Explanation: ***ECPR with 20% intralipid*** - The scenario describes **Local Anesthetic Systemic Toxicity (LAST)**, likely due to bupivacaine, leading to cardiovascular collapse. - **Intralipid 20%** is the first-line treatment for LAST-induced cardiovascular toxicity, as it acts as a lipid sink for the lipophilic local anesthetic. *ECPR with calcium* - While calcium may be used in certain cardiac arrest scenarios, it is **not the primary treatment for bupivacaine-induced cardiovascular collapse** and LAST. - Calcium might offer some cardiac support but does not directly neutralize the local anesthetic's toxic effects. *ECPR with dobutamine* - **Dobutamine is an inotropic agent** used to improve cardiac contractility but is not indicated as a primary rescue therapy for severe LAST. - It would not address the underlying toxicity caused by bupivacaine and could potentially worsen the situation by increasing myocardial oxygen demand without reversing toxin effects. *ECPR with sodium bicarbonate* - **Sodium bicarbonate** is used to treat metabolic acidosis and can be beneficial in certain drug overdoses to enhance excretion or stabilize cardiac membranes. - However, it is **not the primary or most effective treatment for bupivacaine-induced LAST** and cardiovascular collapse compared to lipid emulsion therapy.

Question 5: What is the maximum dose of plain lignocaine (in mg) for adults?

A. 300 (Correct Answer)
B. 500
C. 700
D. 1000

Explanation: ***300 mg*** - The maximum recommended dose of **plain lidocaine** (without epinephrine) for adults is typically **300 mg** or 4.5 mg/kg, whichever is less. - Exceeding this dose can increase the risk of **systemic toxicity**, including central nervous system and cardiovascular effects. *500 mg* - This dose is generally considered the maximum for **lidocaine with epinephrine** in adults, as **epinephrine** causes vasoconstriction and delays systemic absorption of lidocaine. - For **plain lidocaine**, 500 mg would be an overdose and significantly increase the risk of toxicity. *700 mg* - This is well above the recommended maximum dose for both plain lidocaine and lidocaine with epinephrine, posing a **severe risk of toxicity**. - Such a high dose could lead to **seizures**, cardiac arrhythmias, and even **cardiac arrest**. *1000 mg* - This dose is extremely dangerous and far exceeds any recommended maximum for lidocaine, regardless of whether it contains epinephrine. - Administration of 1000 mg of lidocaine would almost certainly result in **severe and potentially fatal toxicity**.

Question 6: What is the primary cardiotoxic effect of bupivacaine?

A. Depressed pacemaker activity (Correct Answer)
B. Toxic compound damaging myocardial cells
C. Depressed neural control on heart
D. Vascular thrombosis and Myocardial ischemia

Explanation: ***Depressed pacemaker activity*** - **Bupivacaine** is a potent **local anesthetic** that blocks voltage-gated **sodium channels** in myocardial cells with **high affinity** and **slow dissociation kinetics**. - This prolonged channel blockade leads to decreased cardiac excitability and **depressed automaticity** of pacemaker cells, particularly affecting the **SA node** and **His-Purkinje system**. - Results in slowing of the **heart rate**, **bradyarrhythmias**, **conduction blocks**, and potentially **ventricular arrhythmias** or **asystole**. - Bupivacaine is **more cardiotoxic** than other local anesthetics due to its **lipophilicity** and prolonged binding to cardiac sodium channels. *Toxic compound damaging myocardial cells* - While **bupivacaine** is cardiotoxic, its primary mechanism is not direct **cellular damage** through cytotoxic effects, oxidative stress, or cell membrane lysis. - The toxicity is predominantly due to **electrophysiological effects** on ion channels, interfering with normal cardiac conduction and contractility. *Depressed neural control on heart* - **Bupivacaine's** cardiotoxicity primarily affects the **myocardium directly** through sodium channel blockade, rather than indirectly through the **autonomic nervous system**. - Although high systemic concentrations can affect the **central nervous system** (causing seizures and CNS depression), the direct cardiac effects occur independently of neural influence. *Vascular thrombosis and Myocardial ischemia* - **Bupivacaine** cardiotoxicity does not typically involve formation of **thrombi** or mechanisms leading to **myocardial ischemia** through coronary artery occlusion. - Its effects are predominantly on the **electrical conduction system**, **myocardial contractility**, and **cardiac ion channels**, not the vascular supply to the heart.

Question 7: In ACLS, which antiarrhythmic drug can be given following ventricular fibrillation after cardiac arrest other than epinephrine?

A. Amiodarone (Correct Answer)
B. Dopamine
C. Adenosine
D. Atropine

Explanation: ***Amiodarone*** - **Amiodarone** is a Class III antiarrhythmic agent recommended in ACLS for **refractory ventricular fibrillation (VF)** or pulseless ventricular tachycardia (pVT) after initial defibrillation and epinephrine. - It works by blocking potassium channels, prolonging repolarization, and increasing the **refractory period** in the heart. *Dopamine* - **Dopamine** is a **vasopressor** used to improve **hemodynamics** in patients with symptomatic hypotension, not primarily as an antiarrhythmic for VF. - Its effects include increasing heart rate, myocardial contractility, and blood pressure. *Adenosine* - **Adenosine** is a drug of choice for **supraventricular tachycardia (SVT)** to interrupt reentry pathways in the AV node. - It is not indicated for ventricular fibrillation, as it would be ineffective in this rhythm. *Atropine* - **Atropine** is an **anticholinergic agent** used to treat **symptomatic bradycardia** by increasing heart rate. - It has no role in the management of ventricular fibrillation.

Question 8: Which is correct about the anesthetic drugs X and Y in the image shown? (Recent NEET Pattern 2016-17)

A. Drug X and Y have equally fast onset of action
B. Drug X and Y have equally fast onset of action but potency of X is more than Y
C. Drug Y is more fast acting than X
D. Drug X is more fast acting than Y (Correct Answer)

Explanation: ***Drug X is more fast acting than Y*** - The **oil:gas partition coefficient** for Drug X is lower than for Drug Y. A lower oil:gas partition coefficient typically correlates with a **faster onset of action** for inhaled anesthetics as it indicates lower solubility in blood and tissues, allowing for quicker equilibration in the brain. - While MAC is plotted against oil:gas partition coefficient, the question specifically asks about **onset of action**, which is primarily influenced by blood-gas solubility rather than oil-gas solubility. However, an anesthetic with lower oil-gas solubility (like X) would generally also have lower blood-gas solubility, leading to faster onset. *Drug Y is more fast acting than X* - Drug Y has a **higher oil:gas partition coefficient** compared to Drug X, indicating greater lipid solubility. - A higher oil:gas partition coefficient generally correlates with a **slower onset of action** for inhaled anesthetics, as more drug dissolves in lipids before reaching the brain. *Drug X and Y have equally fast onset of action* - The graph clearly shows that Drug X and Drug Y have different **oil:gas partition coefficients**. - Since the partition coefficients are different, their **solubility characteristics** and therefore their clinical onset of action would also be different. *Drug X and Y have equally fast onset of action but potency of X is more than Y* - Onset of action is **not equal** for X and Y due to their differing oil:gas partition coefficients. - Potency, represented by **MAC** (Minimum Alveolar Concentration), is inversely related to the oil:gas partition coefficient for many inhaled anesthetics. From the graph, Drug X has a higher MAC value than Drug Y (meaning it is **less potent** but acts faster).

Question 9: In ophthalmology, if a patient is allergic to aminoesters, which local anesthetic can be safely used?

A. Procaine
B. Cocaine
C. Prilocaine (Correct Answer)
D. Tetracaine

Explanation: **Local anesthetics are classified into two chemical groups: esters (aminoesters) and amides. Allergies to esters typically do not cross-react with amides.** ***Prilocaine*** - **Prilocaine** is an **amide-type local anesthetic**, and allergies to **aminoesters** typically do not cross-react with **amides**. - It is a safe alternative in patients with a known allergy to **ester-type local anesthetics**. *Cocaine* - **Cocaine** is an **ester-type local anesthetic**, sharing a similar chemical structure with **aminoesters**. - Patients allergic to **aminoesters** are likely to experience a **cross-reaction** with **cocaine**. *Procaine* - **Procaine** is a classic **ester-type local anesthetic** (an aminoester). - An allergy to aminoesters directly implies an allergy to **procaine** due to its chemical classification. *Tetracaine* - **Tetracaine** is also an **ester-type local anesthetic** (an aminoester). - It is contraindicated in patients with an allergy to **aminoesters** due to the high risk of **allergic reaction**.

Question 10: A 25 year old male with roadside accident underwent debridement and reduction of fractured both bones right forearm under axillary block. On the second postoperative day the patient complained of persistent numbness and paresthesia in the right forearm and the hand. The commonest cause of this neurological dysfunction could be all of the following except –

A. A tight cast or dressing
B. Systemic toxicity of local anaesthetics (Correct Answer)
C. Tourniquet pressure
D. Crush injury to the hand and lacerated nerves

Explanation: ***Systemic toxicity of local anaesthetics*** - **Systemic toxicity** of local anesthetics would typically manifest with symptoms like **seizures**, **cardiac arrhythmias**, or **respiratory depression** within minutes to hours of administration, not persistent numbness and paresthesia on the second postoperative day. - While local anesthetic toxicity can occur, its acute nature and generalized systemic effects make it an unlikely cause for delayed, localized neurological symptoms. *A tight cast or dressing* - A **tight cast or dressing** can cause **compression neuropathy**, leading to persistent numbness and paresthesia by putting pressure on nerves. - This is a common cause of **neurological dysfunction** following orthopedic procedures, especially in the forearm where nerves like the median or ulnar nerve can be vulnerable. *Tourniquet pressure* - **Tourniquet-induced nerve injury** can occur if the tourniquet is applied for too long or at excessive pressure, leading to **ischemia and direct compression** of nerves. - This can result in **postoperative numbness and paresthesia** that persists for days or weeks after surgery. *Crush injury to the hand and lacerated nerves* - A **crush injury** or laceration during the initial trauma, especially if severe, could directly damage nerves, causing **immediate and persistent numbness** and paresthesia. - Even after debridement and reduction, existing nerve damage from the trauma itself would manifest as prolonged neurological deficits.

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