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SUCCINYLCHOLINE / SUXAMETHONIUM (Depolarizing NM blocker)

Class / Use

Depolarizing neuromuscular blocking agent
Rapid sequence intubation
Very fast onset

Mechanism

Binds nicotinic ACh receptors → persistent depolarization → fasciculations → flaccid paralysis

Onset / Duration

Onset: 30–60 seconds
Duration: 3–5 minutes (very short)

Metabolism

Pseudocholinesterase (plasma cholinesterase)
Prolonged effect in deficiency

Adverse effects

Hyperkalemia (burns, neuromuscular disease risk)
Malignant hyperthermia trigger
Bradycardia
Raised ICP/IOP
Muscle pains (fasciculations)

FENTANYL (Opioid analgesic)

Class / Use

Potent synthetic opioid
Analgesia + adjunct for induction
Can cause respiratory depression

Mechanism

μ-opioid receptor agonist

Onset / Duration

Onset: 2–3 minutes
Duration: 30–60 minutes

Metabolism

Hepatic, CYP3A4
No active metabolites

Key effects / disadvantages

Strong analgesia
Chest wall rigidity (rapid high dose use)
Bradycardia
Mild ↓ BP
Respiratory depression

both fentanyl and remifentanil cross the placenta.

Why?

Because both are:

highly lipid-soluble
unionized fraction present at physiologic pH
small molecular size
not strongly protein-bound enough to prevent transfer

REMIFENTANIL (Ultra-short opioid)

Class / Use

Ultra short-acting μ-opioid agonist
TIVA, short intense surgical stimulation
Useful in neurosurgery/critical airway cases

Mechanism

μ-opioid receptor agonist

Onset / Duration

Onset: ~1 minute
Duration: 3–5 minutes (context-insensitive)

Metabolism

Unique: plasma esterases
Not dependent on liver or kidney
Safe in renal/hepatic impairment

Key effects / disadvantages

Profound analgesia
Severe respiratory depression
Apnea risk
Requires infusion pump
Rapid offset → no residual analgesia

KEY DIFFERENCES – HIGH-YIELD EXAM PEARLS

Feature	Succinylcholine	Fentanyl	Remifentanil
Class	NM blocker	Opioid	Opioid
Duration	Very short	Intermediate	Ultra-short
Metabolism	Plasma cholinesterase	Liver	Plasma esterases
Major risk	Hyperkalemia, MH	Chest wall rigidity	Apnea, acute tolerance
Use	RSI	Analgesia during anesthesia	TIVA, neuro cases

🧠 ULTRA-MASTER TABLE — ANESTHESIA CORE DRUGS

DOMAIN	SUCCINYLCHOLINE (SUXAMETHONIUM)	FENTANYL	REMIFENTANIL	PROPOFOL
Pharmacologic class	Depolarizing neuromuscular blocker	Potent synthetic opioid	Ultra-short acting opioid	IV general anesthetic (sedative-hypnotic)
Primary clinical role	Rapid skeletal muscle paralysis	Analgesia + induction adjunct	Profound short-term analgesia	Hypnosis / induction / maintenance
Analgesia	❌ None	✅ Strong	✅ Very strong	❌ None
Hypnosis / amnesia	❌	❌	❌	✅ Profound
Sedation	❌	Mild	Mild	✅
Muscle relaxation	✅ Complete	❌	❌	❌
Typical uses	RSI, emergency intubation	Induction, maintenance adjunct	TIVA, neuro, critical airway,epidural	Induction, TIVA, ICU sedation
Primary receptor / target	Nicotinic ACh receptor (Nm)	μ-opioid receptor	μ-opioid receptor	GABA-A receptor
Core mechanism	Persistent depolarization → fasciculations → flaccid paralysis	Central μ-receptor activation	Same μ-activation with rapid offset	↑ GABA-A chloride conductance → neuronal inhibition
Additional mechanisms	Phase I depolarization block	↓ sympathetic tone	Same as fentanyl	↓ NMDA excitation, ↓ sympathetic tone
Onset of action	30–60 s	2–3 min	~1 min	30–45 s
Peak effect	~1 min	3–5 min	1–2 min	~1 min
Duration (single bolus)	3–5 min	30–60 min	3–5 min	5–10 min
Context sensitivity	None	Yes	No (context-insensitive)	Low–moderate
Termination of effect	Plasma hydrolysis	Redistribution + metabolism	Rapid esterase hydrolysis	Redistribution + metabolism
Primary metabolism	Pseudocholinesterase (plasma)	Hepatic (CYP3A4)	Plasma & tissue esterases	Liver + lungs
Active metabolites	❌	❌	❌	❌
Dependence on liver	❌	✅	❌	✅
Dependence on kidney	❌	❌	❌	❌
Effect in organ failure	Prolonged in pseudocholinesterase deficiency	Prolonged in liver disease	Unaffected	Mild prolongation
Placental transfer	❌ Minimal clinical effect	✅ Yes	✅ Yes-best for epidural	✅ Yes
Why placental transfer?	—	Lipid soluble, unionized fraction	Same properties	Lipid soluble, rapid diffusion
Neonatal effect	Minimal	Respiratory depression possible	Apnea possible	Minimal (rapid redistribution)
Respiratory effects	Apnea via paralysis	Respiratory depression	Severe respiratory depression / apnea	Apnea common
Cardiovascular effects	Bradycardia (esp children)	Bradycardia, mild ↓ BP	Bradycardia, hypotension	Marked hypotension
SVR effect	—	↓	↓	↓↓ SVR
Myocardial depression	❌	Mild	Mild	Mild–moderate
ICP effect	↑ ICP	Neutral	Neutral	↓ ICP
IOP effect	↑ IOP	Neutral	Neutral	↓ IOP
CMRO₂ effect	—	↓	↓	↓↓ CMRO₂
Muscle effects	Fasciculations → myalgia	❌	❌	❌
Electrolyte effect	↑ K⁺ (hyperkalemia)	❌	❌	❌
Temperature / muscle risk	Triggers malignant hyperthermia	❌	❌	❌
Unique adverse effect	Malignant hyperthermia	Chest wall rigidity	No residual analgesia	Injection pain
Other major adverse effects	Bradycardia, myalgia	Nausea, vomiting	Acute tolerance	Bradycardia
Infusion pump required	❌	❌	✅	Optional
Residual effect after stop	None	Analgesia persists	None (pain rebound)	Sedation fades
ICU use	❌	Limited	Limited	Yes
Neurosurgery suitability	❌	Adjunct	Excellent	Excellent
OB anesthesia role	RSI only	Adjunct	best for epidural	Common induction agent
Absolute contraindications	Burns, NM disease, crush injury	Severe respiratory compromise	No ventilatory support	Hemodynamic instability
High-risk exam associations	Hyperkalemia + MH	Chest wall rigidity	Apnea	Hypotension
Mnemonic identity	Fast paralysis	Strong opioid	On–off opioid	Hypnosis without analgesia

🧠 FINAL EXAM LOCK (Do NOT forget)

Succinylcholine → fastest paralysis, hyperkalemia + malignant hyperthermia
Fentanyl → strong opioid, chest wall rigidity, placental transfer
Remifentanil → ultra-short, esterase metabolism, no residual analgesia
Propofol → hypnosis only, hypotension + apnea, neuro-friendly