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Neuromuscular Blocker - NEET PG Anesthesia

Feb 15, 2023

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Acts on NMJ (Neuromuscular Junction)

Train of 4 Response

Succinylcholine

Side effects of Succinylcholine

Used in the following condition

Non – Depolarizing NMB

Gantacurium 

Atracurium and Cis-atracurium

Reversal Agents

Previous Year Questions

Neuromuscular Blocker - NEET PG Anesthesia

Neuromuscular blocking agents, also known as muscle relaxants, play a crucial role in the field of anesthesia. Neuromuscular blockers are essential for various surgical procedures, as they allow for a patient to remain immobile and without pain during surgery. A good understanding of neuromuscular blockers is important for a comprehensive understanding of anesthesia and its role in surgical procedures.

In this blog post, we discuss this important anesthesia topic briefly for the NEET PG exam Preparation. Read on.


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Acts on NMJ (Neuromuscular Junction)

Depolarizing 

Non - Depolarizing 

  • Succinylcholine
  • Rest All 
  • Persistent / prolonged depolarization of Nc (Nicotinic cholinergic Receptor).
  • Acts by Competitive antagonism.
  • Fasciculation is seen.
  • Postoperative myalgia may be seen.
  • Not reversed by the Neostigmine.
  • No post tetanic facilitation seen.
  • No fasciculations seen.
  • No myalgia seen.
  • Reversal by Neostigmine.
  • post tetanic facilitation seen.

Depolarising

Non-Depolarising

Aminosteroids

Benzylisoquinolines

Oximes

Succinylcholine

Pancuronium

D-tubocurarine

Gantacurium

Pipecuronium

Doxacurium

Vecuronium

Atracurium

Rocuronium

Cis-atracurium

Rapacuronium

Mivacurium

Train of 4 Response

DepolarizingDefinitionNon - Depolarizing
MoA

Prolonged or persistent depolarization of nicotinic cholinergic receptor  

Example

Succinylcholine

Causes: Depolarization
Competitive inhibition


Fasciculation are seen-No Fasciculations
Lesser amplitude of contraction produced.-It will fade.
Post-operative myalgia-No Postoperative myalgia
Not reversed by Neostigmine -Reversed by Neostigmine
No post tetanic fasciculation seen -Post tetanic fasciculation seen
No train of 4 responses was seen.

A - Response by First stimulus.

B - Response by Last stimulus

B/A = TOF Ratio TOF ratio = 1.Neuromuscular monitor is not used.
4 successive stimuli given at 2 Hz at the same interval of time i.e., 0.5 sec and of same amplitude.

In normal muscle it will produce 4 contractions.
TOF Ratio < 1

Neuromuscular monitor is used.

Important Information

  • Ulnar Nerve: M/C nerve to see the adequacy of muscle paralysis.
  • For complete recovery TOF ratio should be more than 0.9.

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Succinylcholine

  • Mode of action
    • Prolonged depolarization of nicotinic cholinergic receptor
  • Fastest Onset: 30-45 seconds. 
  • Shortest Duration: 45 minutes. 
  • Metabolized by Pseudocholinesterase / Butyrylcholinesterase / Plasma Cholinesterase.
  • Suxamethonium Apnea 
    • Due to deficiency of Pseudocholinesterase or due to atypical pseudocholinesterase.
    • There is Prolonged Apnea. 
    • Management: Mechanical Ventilation (IPPV).
  • Two phases of block. 
    • Phase I block: Normal activity of Succinylcholine: TOF not seen in it.
    • Phase Il block: Desensitization block: Start resembling the property of Non – depolarizing block (competitive block): TOF seen in it. 

Also Read: Drugs for General Anesthesia - NEET PG Anesthesia

Side effects of Succinylcholine

  • Fasciculation, Post- Op Myalgia 
  • ↑ IOP, ↑ ICP , ↑ IGP 
  • ↑ Exaggerated hyperkalemia seen in conditions like 
    • Muscle Dystrophy 
    • Neuromuscular Disease 
    • Paraplegia
    • Burn
    • Trauma 
    • Stroke 
  • Bradycardia 
  • Can trigger Malignant Hyperthermia.
  • Due to all these side effects succinylcholine is not used routinely.

Used in the following condition

  • Rapid sequence induction and intubation- because of its fastest onset.
  • Very short general anesthesia, e.g. in Electroconvulsive therapy.
  • Difficult airway intubation attempt.

Also Read:  Alternative Airway Devices and Adjuncts - NEET PG Anesthesia

Non – Depolarizing NMB

  • Routinely used 

Aminosteroids

Benzylisoquinolines

Pancuronium

Pipecuronium

Long Acting

d- tubocurarine

Doxacurium 

Rocuronium 

M/c used

Vecuronium 

Intermediate Acting

Atracurium 

Cis-atracurium

Rapacuronium

(Faster onset)

Short Acting

Mivacurium (Shortest Duration)

  • No Histamine Release
  • CVS Stable

-

  • Histamine Release
  • CVS Unstable
  • Bronchospasm
  • BP ↓
  • HR ↑
  • Flushing
  • Metabolized by liver, kidney

-

  • Few drugs have unique Metabolism as given below

Gantacurium 

  • Non depolarizing NMB but it is an Oxime compound 
  • Benefits
    • Fast onset.
    • Duration very short. 
    • Metabolized by L – Cysteine.

Atracurium and Cis-atracurium

  • Metabolized by Hoffmann's degradation i.e., self-destruction of drug. Also metabolized by Nonspecific Ester hydrolysis. Produces by product Laudanosine: which is epileptogenic 

Important Information

  • Cis-atracurium > Atracurium
    • Because CVS stable.
    • More potent.
    • Less laudanosine production.
    • No histamine release.
  • Gantacurium (fastest & shortest) > Rapacuronium (Faster Onset 75 sec) > Rocuronium 90 sec (Mivacurium (Shortest Duration ), if Gantacurium is given in option go with it, but it not that much clinical use
    • So overall succinylcholine is fastest and shortest

Rest all have 3 – 5 min onset

Reversal Agents

  • Neostigmine + glycopyrrolate/atropine
    • Neostigmine is an indirect reversal agent; it acts by increasing the quantity of acetylcholine.
  • Sugammadex: Direct reversal agent for aminosteroids (rocuronium and vecuronium).
  • Calabadion: Direct reversal agent for all non depolarizing agents (still in inception phase, and not in the market).

Previous Year Questions

Q. Cis-atracurium is better than atracurium because? (NEET Jan 2019)

A. Less histamine release

B. Less half life

C. Low Drug Dose

D. Low cost

Q. A person was given a muscle relaxant that competitively blocks nicotinic receptors. Which of the following drugs is used for reversal of muscle relaxation after surgery? (NEET Jan 2020)

A. Neostigmine

B. Carbachol

C. Succinylcholine

D. Physostigmine

To study this topic in detail along with other important, high-yield topics from Anesthesia, download the PrepLadder app now.


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