Phase 2 block 10 mark answer anesthesia exam
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Phase 2 block succinylcholine 5 mark answer anaesthesia
Succinylcholine (suxamethonium) is a depolarizing neuromuscular blocking agent used for rapid sequence intubation. It acts on nicotinic acetylcholine receptors, causes initial fasciculations followed by flaccid paralysis, and has the fastest onset and shortest duration of all NMBDs.
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I now have comprehensive textbook content from Miller's Anesthesia and Morgan & Mikhail. Here is a complete 5-mark model answer:
Succinylcholine (Suxamethonium) - 5 Mark Model Answer
1. Introduction / Classification (0.5 marks)
Succinylcholine (suxamethonium) is the only depolarizing neuromuscular blocking drug (NMBD) in current clinical use. It is structurally composed of two acetylcholine molecules linked through the acetate methyl groups, which explains both its mechanism of action and metabolism.
2. Mechanism of Action (1 mark)
- Succinylcholine binds to nicotinic acetylcholine receptors (nAChRs) at the neuromuscular junction, mimicking acetylcholine.
- It causes persistent depolarization of the motor end plate, leading to initial muscle fasciculations followed by flaccid paralysis (Phase I block).
- Unlike acetylcholine, it is not rapidly hydrolyzed at the synapse, so the end plate remains depolarized.
- At high or repeated doses, the block can convert to a Phase II (desensitization) block, which resembles a non-depolarizing block.
3. Pharmacokinetics and Dosage (1 mark)
| Parameter | Value |
|---|---|
| Dose for intubation | 1-2 mg/kg IV |
| Onset of action | 60 seconds (complete block at 1 mg/kg) |
| Duration of action | 9-13 minutes (recovery to 90% muscle strength) |
| Elimination half-life | ~47 seconds |
| ED95 | 0.51-0.63 mg/kg |
Metabolism: Rapidly hydrolyzed by pseudocholinesterase (butyrylcholinesterase) in plasma to succinylmonocholine and choline. Only ~10% of the IV dose reaches the neuromuscular junction. Duration is prolonged by low pseudocholinesterase levels (pregnancy, liver disease, renal failure, hypothermia) or genetically atypical enzyme.
- Dibucaine number: A normal enzyme is inhibited 80% by dibucaine (dibucaine number = 80). Homozygous atypical enzyme has a dibucaine number of 20 and results in prolonged block of 4-8 hours.
4. Clinical Uses (0.5 marks)
- Rapid Sequence Intubation (RSI) - the primary indication due to fastest onset of all NMBDs
- Short procedures requiring brief paralysis (laryngoscopy, electroconvulsive therapy)
- Management of laryngospasm
5. Side Effects and Complications (1.5 marks)
| Side Effect | Mechanism / Notes |
|---|---|
| Hyperkalemia | Depolarization releases K+ from muscle cells; K+ rises ~0.5 mEq/L normally, but can be life-threatening (cardiac arrest) in burns, crush injury, denervation, prolonged immobility, myopathies |
| Bradycardia | Muscarinic stimulation; especially in children and with second bolus in adults |
| Fasciculations | Initial depolarization of motor units; can cause myalgia |
| Raised intraocular pressure | Due to fasciculations and contraction of extraocular muscles |
| Raised intragastric pressure | Counterbalanced by raised lower esophageal sphincter tone |
| Raised intracranial pressure | Secondary to hypercapnia/laryngospasm if intubation is difficult |
| Malignant hyperthermia | Rare, life-threatening; triggered in susceptible individuals |
| Rhabdomyolysis and cardiac arrest | In undiagnosed pediatric myopathies (e.g., Duchenne muscular dystrophy) |
6. Contraindications (0.5 marks)
- Burns, crush injury, denervation injuries (risk of life-threatening hyperkalemia)
- Personal or family history of malignant hyperthermia
- Myopathies / muscular dystrophies (especially in children)
- Hyperkalemia
- Open globe injury (raised IOP)
- Pseudocholinesterase deficiency
Key Exam Points:
- Succinylcholine is preferred for RSI because it has the fastest onset and shortest duration of all NMBDs.
- Its short duration depends on pseudocholinesterase - not present at the motor end plate, so succinylcholine must diffuse away from the junction into plasma to be broken down.
- Pre-treatment with a small dose of a non-depolarizing NMBD ("defasciculating dose") can reduce fasciculations.
- Succinylcholine cannot be reversed by neostigmine during Phase I block (cholinesterase inhibitors will prolong its action).
Sources: Miller's Anesthesia 10e, pp. 3228-3229; Morgan & Mikhail's Clinical Anesthesiology 7e, pp. 379-384
Phase 2 block 5 mark answer in anaesthesia exam
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Phase II block 5 mark answer
Phase II block (desensitization block) occurs with succinylcholine when the motor end plate is exposed to prolonged or repeated depolarization, causing the block to change character and resemble a non-depolarizing block.
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Excellent - I now have rich, authoritative content from Miller's Anesthesia (10e) and Barash's Clinical Anesthesia (9e). Here is your complete 5-mark model answer:
Phase II Block - 5 Mark Model Answer
1. Definition (0.5 marks)
Phase II block (also called dual block, desensitization block, or mixed block) is a complex phenomenon that occurs when the neuromuscular junction is exposed to prolonged or excessive depolarization by succinylcholine. The block transitions from the typical depolarizing (Phase I) pattern to a pattern that resembles non-depolarizing blockade, with fade on nerve stimulation.
2. Causes / Conditions for Development (1 mark)
Phase II block occurs under the following conditions:
- Single large dose of succinylcholine (≥10 × ED95, i.e., >5 mg/kg)
- Repeated bolus doses of succinylcholine
- Prolonged continuous infusion of succinylcholine
- Patients with genetically abnormal pseudocholinesterase - who experience prolonged end-plate exposure even with normal doses (same dose causes Phase II block due to slow metabolism)
3. Mechanism (1 mark)
The exact mechanism is complex and multifactorial:
- Prolonged depolarization keeps the end-plate sodium channels in an inactivated state - the membrane repolarizes but the receptor becomes unresponsive (desensitization)
- Succinylcholine acts on prejunctional (neuronal) nAChRs at high concentrations, blocking transmitter mobilization and release - this produces the characteristic fade
- Receptors shift from a normal "resting" state to a desensitized state in which they bind agonists tightly but fail to open ion channels
- Channel block by succinylcholine molecules entering the ion channel pore may also contribute
4. Characteristics and Diagnosis - Comparison with Phase I Block (1.5 marks)
| Feature | Phase I Block (Normal) | Phase II Block |
|---|---|---|
| Nature | Depolarizing | Resembles non-depolarizing |
| Train-of-Four (TOF) fade | Absent (TOF ratio = 1.0) | Present (fade, TOF ratio < 1.0) |
| Tetanic fade | Absent | Present |
| Post-tetanic potentiation | Absent | Present |
| Effect of anticholinesterases (neostigmine) | Potentiates block | May reverse (unpredictable) |
| Effect of prior non-depolarizing NMBD | Antagonizes blockade | Potentiates blockade |
| Recovery pattern | All 4 TOF responses return equally | TOF count recovers 1 → 2 → 3 → 4 with fade |
Monitoring: TOF stimulation is used clinically to distinguish Phase I from Phase II block. Fade on TOF = Phase II.
5. Management (1 mark)
- Prevention: Avoid large/repeated doses of succinylcholine; use infusions cautiously
- Discontinue succinylcholine - allow the block to dissipate
- Mechanical ventilation and sedation - continue until muscle function returns to normal (confirmed by clinical signs and TOF monitoring)
- Reversal with neostigmine:
- In patients with normal cholinesterase genotype: neostigmine can reverse Phase II block a few minutes after stopping succinylcholine
- In patients with abnormal cholinesterase genotype: neostigmine effect is unpredictable - it may potentiate, temporarily improve, then worsen the block. Use with extreme caution; avoid unless genotype is known to be normal
- After any reversal attempt: continue observation for at least 1 hour
- Sugammadex is not effective (it only reverses rocuronium/vecuronium)
Key Exam Points to Remember:
- Phase II block = succinylcholine behaving like a non-depolarizer
- The hallmark is fade on TOF and tetanic stimulation - same as non-depolarizing block
- Reversal is unpredictable - do NOT routinely give neostigmine
- The safest management is supportive ventilation until spontaneous recovery
Sources: Miller's Anesthesia 10e, pp. 1189-1192; Barash's Clinical Anesthesia 9e, pp. 1609; Miller's Anesthesia 10e (monitoring chapter), pp. 5788-5790
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