Adjuvants to Local Anesthetics — MCQs
A female was given morphine sulphate during labour for pain but she developed respiratory distress. Which of the following will be the correct antidote?
Anesthetic agent with vasoconstrictor is contraindicated in
A chronic smoker was on nicotine replacement therapy and clonidine tablets for smoking de-addiction. He stopped taking clonidine tablets and now presents with a headache. What is the reason behind this condition?
What is the maximum concentration allowed for epidural block?
All are vasodilators except –
The site of action of local anaesthetic in epidural anesthesia is
What is the effect of adding epinephrine to lignocaine (a local anesthetic)?
Which intravenous anaesthetic agent has analgesic effect also
The maximum dosage of a local anesthetic agent like lidocaine must be reduced when it is used in combination with a CNS and/or respiratory depressant because, it may result in?
Which of the following nerve fibers are first to get blocked by local anesthetic?
Adjuvants to Local Anesthetics Indian Medical PG Practice Questions and MCQs
Question 1: A female was given morphine sulphate during labour for pain but she developed respiratory distress. Which of the following will be the correct antidote?
- A. Naloxone (Correct Answer)
- B. Epinephrine
- C. Pralidoxime
- D. Atropine
Explanation: ***Naloxone*** - **Naloxone** is a pure opioid antagonist that rapidly reverses the effects of **opioid overdose** [1, 3], including **respiratory depression** [2], by competitively binding to opioid receptors [1]. - Its short half-life may necessitate repeated doses, especially with longer-acting opioids like morphine, to prevent recurrence of respiratory depression [1]. *Epinephrine* - **Epinephrine** is an adrenergic agonist used to treat **anaphylaxis** and severe allergic reactions, as it causes **vasoconstriction** and **bronchodilation**. - It is not an antidote for opioid-induced respiratory depression, which primarily results from central nervous system effects rather than allergic reactions. *Pralidoxime* - **Pralidoxime** is a **cholinesterase reactivator** used to treat poisoning by **organophosphates**, which inhibit acetylcholinesterase, leading to cholinergic crisis. - It works by restoring the function of the enzyme, thereby breaking down excess acetylcholine, and is not indicated for opioid overdose. *Atropine* - **Atropine** is an **anticholinergic agent** that blocks muscarinic acetylcholine receptors, used to treat **bradycardia** and **organophosphate poisoning**. - It would not reverse opioid-induced respiratory depression, as it primarily affects the parasympathetic nervous system and does not antagonize opioid receptor effects.
Question 2: Anesthetic agent with vasoconstrictor is contraindicated in
- A. Spinal block
- B. Regional anesthesia
- C. Digital block (Correct Answer)
- D. Epidural block
Explanation: ***Digital block*** - **Vasoconstrictors** in local anesthetics can cause severe **vasoconstriction** in tissues with limited collateral circulation, like digits. - This can lead to **ischemia**, **necrosis**, and even **gangrene** of the affected digit, making it a contraindication. *Spinal block* - **Vasoconstrictors** are sometimes added to local anesthetics for spinal blocks to prolong the duration of action and reduce systemic absorption. - While careful monitoring is needed, it is not an absolute contraindication and can be used cautiously. *Regional anesthesia* - In many forms of **regional anesthesia** (e.g., peripheral nerve blocks), vasoconstrictors like epinephrine are commonly added to prolong the block and reduce systemic toxicity. - This is a standard practice and generally safe outside of specific areas like digits. *Epidural block* - Similar to spinal blocks, **vasoconstrictors** are frequently used in **epidural anesthesia** to improve the quality, duration, and reduce systemic absorption of the local anesthetic. - While dose and patient factors must be considered, it is not a contraindication.
Question 3: A chronic smoker was on nicotine replacement therapy and clonidine tablets for smoking de-addiction. He stopped taking clonidine tablets and now presents with a headache. What is the reason behind this condition?
- A. Postural hypotension
- B. Receptor upregulation
- C. Rebound hypertension (Correct Answer)
- D. Receptor hypersensitivity
Explanation: ***Rebound hypertension*** - **Clonidine withdrawal** can cause a sudden surge in blood pressure due to increased sympathetic activity, leading to **rebound hypertension** and symptoms like headaches. - This occurs because chronic clonidine use suppresses sympathetic outflow, and its abrupt discontinuation unmasks this suppressed activity, causing a hypertensive crisis. *Postural hypotension* - **Postural hypotension** is a common side effect of clonidine due to its vasodilatory effects, causing blood pressure to drop when standing. - However, the patient's headache following clonidine cessation is more indicative of a **hypertensive event**, not hypotension. *Receptor upregulation* - **Receptor upregulation** refers to an increase in the number of receptors, often in response to prolonged antagonism or decreased ligand exposure. - While receptor changes occur, the primary mechanism of clonidine withdrawal is the **overcompensation** of the sympathetic nervous system, not simply an increased number of receptors. *Receptor hypersensitivity* - **Receptor hypersensitivity** implies an exaggerated response to a normal concentration of a neurotransmitter, which can contribute to withdrawal symptoms. - While it plays a role, the more immediate and critical cause of the headache is the rapid increase in blood pressure due to **rebound sympathetic activity**.
Question 4: What is the maximum concentration allowed for epidural block?
- A. Chlorprocaine (Correct Answer)
- B. Lidocaine
- C. Ropivacaine
- D. Bupivacaine
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.
Question 5: All are vasodilators except –
- A. Lidocaine
- B. Procaine
- C. Bupivacaine
- D. Cocaine (Correct Answer)
Explanation: ***Cocaine*** - Cocaine is unique among local anesthetics because it causes **vasoconstriction** rather than vasodilation. - This vasoconstrictive effect is due to its blocking of **norepinephrine reuptake** at adrenergic nerve terminals, leading to an accumulation of norepinephrine and subsequent adrenergic stimulation. *Lidocaine* - Lidocaine is a common **amide-type local anesthetic** known for its vasodilatory properties that contribute to its systemic absorption. - Its vasodilatory effect can lead to a **flushing** sensation and increased blood flow in the area of injection. *Procaine* - Procaine is an **ester-type local anesthetic** that causes vasodilation, which results in a relatively short duration of action. - This vasodilation increases **local blood flow**, speeding up the systemic absorption and metabolism of the drug. *Bupivacaine* - Bupivacaine is an **amide-type local anesthetic** with longer duration of action compared to lidocaine, and like most local anesthetics, it causes vasodilation. - The vasodilatory effect of bupivacaine can lead to increased **systemic absorption** and potential for systemic toxicity if not managed carefully.
Question 6: The site of action of local anaesthetic in epidural anesthesia is
- A. Anterior root of spinal nerve
- B. Spinal nerve root (Correct Answer)
- C. Epidural neural tissue
- D. Spinal cord
Explanation: ***Spinal nerve root*** - Local anesthetics injected into the epidural space primarily act on the **spinal nerve roots** as they exit the spinal cord. - They also affect the **dorsal root ganglia** and the unmyelinated axons within the epidural space. *Anterior root of spinal nerve* - While the **anterior roots containing motor fibers** are affected, the local anesthetic's action isn't limited exclusively to them. - Sensory fibers in the **dorsal roots** are also blocked, contributing significantly to the analgesic effect. *Epidural neural tissue* - "Epidural neural tissue" is a too broad and non-specific term; the primary targets are the **nerve roots** themselves, not just any neural tissue within the epidural space. - This option does not specify which neural structures within the epidural space are the primary site of action. *Spinal cord* - Local anesthetics do not directly act on the **spinal cord parenchyma** in epidural anesthesia, as they do not typically penetrate the meninges to reach the cord in significant concentrations. - The medication exerts its effect outside the dura mater, primarily on the **nerve roots** before they enter the subarachnoid space.
Question 7: What is the effect of adding epinephrine to lignocaine (a local anesthetic)?
- A. Increases distribution of local anesthetic
- B. Decreases absorption of local anesthetic (Correct Answer)
- C. Decreases duration of local anesthetic
- D. Increases metabolism of local anesthetic
Explanation: ***Decreases absorption of local anesthetic*** - Epinephrine causes **vasoconstriction** at the site of injection, which reduces the rate at which the local anesthetic is absorbed into the systemic circulation. - This slower absorption leads to a **higher concentration of the anesthetic** at the nerve fibers, prolonging its effect and reducing systemic toxicity. - This is the primary mechanism by which epinephrine enhances local anesthetic efficacy. *Increases distribution of local anesthetic* - The primary effect of epinephrine is to **localize the anesthetic** by reducing its systemic distribution. - This localization is achieved through **vasoconstriction**, which keeps the drug at the desired site rather than allowing it to distribute widely. *Decreases duration of local anesthetic* - By slowing absorption, epinephrine effectively **increases the duration of action** of the local anesthetic. - The anesthetic remains at the site of action for a longer period, providing **extended pain relief**. *Increases metabolism of local anesthetic* - Epinephrine does not directly affect the **metabolic rate** of local anesthetics. - The primary mechanism of metabolism for amides like lignocaine is in the **liver** by cytochrome P450 enzymes.
Question 8: Which intravenous anaesthetic agent has analgesic effect also
- A. Thiopentone
- B. Ketamine (Correct Answer)
- C. Propofol
- D. Etomidate
Explanation: ***Ketamine*** - Ketamine acts as an **N-methyl-D-aspartate (NMDA) receptor antagonist**, providing significant **analgesia** in addition to its anaesthetic effects. - It induces a state of **dissociative anaesthesia**, where the patient appears awake but is unresponsive to pain, making it unique among intravenous anaesthetics. *Thiopentone* - Thiopentone is a **barbiturate** that acts as a potent hypnotic and anaesthetic but provides no significant analgesic properties. - It can even cause **anti-analgesia** (hyperalgesia) at sub-hypnotic doses, increasing sensitivity to pain. *Propofol* - Propofol is a potent intravenous anaesthetic that works primarily as a **GABA-A receptor agonist**, but it lacks intrinsic analgesic properties. - While it can cause some sedation and reduced pain perception due to CNS depression, it does not directly modulate pain pathways in the way an analgesic would. *Etomidate* - Etomidate is a hypnotic agent highly valued for its **cardiovascular stability**, making it suitable for patients with compromised cardiac function. - Like propofol and thiopentone, etomidate primarily acts on **GABA-A receptors** to induce unconsciousness and offers no significant analgesic effects.
Question 9: The maximum dosage of a local anesthetic agent like lidocaine must be reduced when it is used in combination with a CNS and/or respiratory depressant because, it may result in?
- A. Seizures
- B. Coma
- C. Death
- D. All of the above (Correct Answer)
Explanation: ### Explanation **1. The Underlying Medical Concept** Local anesthetics (LAs) like lidocaine are CNS depressants when they reach toxic systemic levels. While they initially cause excitatory symptoms (due to inhibition of inhibitory pathways), higher concentrations lead to generalized CNS depression. When lidocaine is administered alongside other **CNS or respiratory depressants** (such as opioids, benzodiazepines, or barbiturates), an **additive or synergistic effect** occurs. Furthermore, respiratory depressants increase arterial $PCO_2$ (hypercapnia) and decrease pH (acidosis). Acidosis decreases the seizure threshold and increases the fraction of ionized drug, while hypercapnia increases cerebral blood flow, delivering more lidocaine to the brain. This potentiation significantly lowers the threshold for **Systemic Toxicity (LAST)**. **2. Analysis of Options** * **A. Seizures:** This is the classic sign of moderate-to-severe CNS toxicity. The combined effect of drugs lowers the seizure threshold, making neurotoxicity more likely even at "standard" doses. * **B. Coma:** As drug levels rise or are potentiated by other depressants, the initial excitatory phase (seizures) is rapidly followed by profound CNS depression, leading to a comatose state. * **C. Death:** Severe toxicity results in respiratory arrest (due to medullary depression) and cardiovascular collapse (negative inotropy and arrhythmias). Without immediate resuscitation, this progression leads to fatality. * **D. All of the above:** Since the toxic progression follows a continuum from excitation to depression to death, all outcomes are possible when lidocaine is combined with other depressants. **3. High-Yield Clinical Pearls for NEET-PG** * **Maximum Dose of Lidocaine:** 3 mg/kg (plain) and 7 mg/kg (with adrenaline). * **Early Signs of Toxicity:** Perioral numbness, metallic taste, and tinnitus. * **Treatment of Choice for LAST:** **Intravenous Lipid Emulsion (20% Intralipid)**. * **Hypercapnia & Toxicity:** Always remember that an increase in $PaCO_2$ is the most potent factor in increasing the CNS toxicity of local anesthetics.
Question 10: Which of the following nerve fibers are first to get blocked by local anesthetic?
- A. A alpha
- B. A beta
- C. A delta (Correct Answer)
- D. A gamma
Explanation: ### Explanation The susceptibility of nerve fibers to local anesthetics (LA) is determined by the **size (diameter)** and the presence of **myelination**. Generally, smaller fibers and myelinated fibers are blocked more easily than larger or unmyelinated ones. **Why A-delta is the correct answer:** According to the **Gasser-Erlanger classification**, nerve fibers are blocked in a specific sequence. While **B fibers** (preganglionic autonomic) are technically the first to be blocked due to their small diameter and light myelination, among the **A-group** fibers listed in the options, **A-delta** fibers are the smallest and thinnest. They are responsible for transmitting fast pain and temperature. Because they have a smaller diameter than other A-type fibers, the critical length of the node that must be exposed to the LA is shorter, making them the first among the choices to reach the threshold for a conduction block. **Analysis of Incorrect Options:** * **A-alpha:** These are the largest, most heavily myelinated fibers responsible for motor function and proprioception. They are the most resistant and the last to be blocked. * **A-beta:** These are large fibers responsible for touch and pressure. They are blocked after A-delta and A-gamma fibers. * **A-gamma:** These fibers are responsible for muscle spindle tone. While smaller than A-alpha and A-beta, they are still larger than A-delta fibers and thus blocked later. **NEET-PG High-Yield Pearls:** 1. **Sequence of Blockade (Clinical):** Autonomic (B) > Pain/Temperature (A-delta & C) > Touch/Pressure (A-beta) > Motor (A-alpha). 2. **The "C-fiber" Paradox:** Although C-fibers are the smallest, they are **unmyelinated**. Myelinated fibers (like A-delta) are often blocked *before* unmyelinated C-fibers because the LA concentrates at the Nodes of Ranvier. 3. **Differential Block:** This refers to the clinical phenomenon where sensory loss occurs without complete motor loss, a principle used extensively in labor analgesia.
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