Opioid Analgesics in Anaesthesia

Opioids are the cornerstone of intra-operative analgesia and balanced anaesthesia, blunting the haemodynamic stress response to laryngoscopy and surgery. This note covers receptor pharmacology, the clinically vital concept of context-sensitive half-time, agent-by-agent comparison (morphine, fentanyl, remifentanil, tramadol), characteristic side-effects (rigidity, PONV, respiratory depression) and reversal with naloxone.

Definition & classification

An opioid is any agent that acts on opioid receptors, whether natural (opiate, e.g. morphine), semi-synthetic (e.g. buprenorphine) or fully synthetic (e.g. fentanyl, remifentanil, tramadol). In anaesthesia they provide analgesia, attenuate sympathetic responses, reduce the MAC of volatile agents (MAC-sparing effect) and contribute to "balanced anaesthesia."

Classification by receptor action:

Class	Examples	Action
Pure (full) agonists	Morphine, fentanyl, alfentanil, sufentanil, remifentanil, pethidine, methadone	Agonist at all opioid receptors, chiefly μ
Partial agonist	Buprenorphine	Partial μ agonist (high affinity, low efficacy, ceiling effect)
Agonist–antagonist	Pentazocine, nalbuphine, butorphanol	κ agonist + μ antagonist/partial; ceiling on respiratory depression
Pure antagonist	Naloxone, naltrexone, methylnaltrexone	Block all receptors
Atypical	Tramadol, tapentadol	Weak μ agonist + monoamine (NA/5-HT) reuptake inhibition

High-yield: Buprenorphine is a partial μ agonist with very high receptor affinity — it has a ceiling effect on respiratory depression but is difficult to reverse with naloxone because of its tight binding (large naloxone doses/infusion needed).

Opioid receptors & their effects

Opioid receptors are G-protein coupled (Gi/Go). Activation → ↓ adenylyl cyclase/cAMP, opening of K⁺ channels (hyperpolarisation, post-synaptic inhibition) and closing of voltage-gated Ca²⁺ channels (↓ pre-synaptic neurotransmitter release). Net effect is reduced neuronal excitability and reduced pain transmission.

Receptor (IUPHAR)	Older name	Key effects
MOP (μ, mu)	OP3	Supraspinal + spinal analgesia, respiratory depression, euphoria, miosis, bradycardia, constipation, physical dependence, pruritus
KOP (κ, kappa)	OP2	Spinal analgesia, dysphoria, sedation, miosis, less respiratory depression
DOP (δ, delta)	OP1	Analgesia, modulation of μ activity, mood effects
NOP (nociceptin/ORL-1)	—	Modulation of pain, anxiety; not classical analgesia

High-yield: Miosis ("pin-point pupils") is mediated by stimulation of the Edinger–Westphal nucleus and is one of the few opioid effects to which tolerance does NOT develop — along with constipation. Tolerance develops to analgesia, euphoria and respiratory depression.

The endogenous opioid peptides: β-endorphin (μ), enkephalins (δ), dynorphins (κ), and nociceptin/orphanin FQ (NOP).

Context-sensitive half-time (CSHT) — the most tested concept

The context-sensitive half-time is the time for plasma concentration to fall by 50% after stopping a continuous infusion, where the "context" is the duration of the infusion. It differs from the elimination half-life because it accounts for redistribution from peripheral compartments back into plasma.

• Drugs with large peripheral reservoirs (fentanyl) have a CSHT that rises steeply with infusion duration — accumulation occurs.
• Remifentanil's CSHT stays ~3–4 minutes regardless of infusion length because it is cleared by tissue/plasma esterases, not redistribution-limited.

Approach to choosing an infusion opioid → short procedure, want titratability → alfentanil/remifentanil; want prolonged post-op analgesia → morphine/fentanyl; renal failure → avoid morphine & pethidine, prefer fentanyl/remifentanil.

Opioid	Onset (IV)	CSHT behaviour	Notable
Remifentanil	~1 min	Flat ~3–4 min, infusion-independent	Esterase metabolism
Alfentanil	~1–2 min (fastest peak effect)	Short, modest rise	Low pKa → high unionised fraction
Sufentanil	~1–3 min	Moderate, plateaus	Most potent
Fentanyl	~3–5 min	Rises markedly with prolonged infusion	Accumulates

High-yield: Although fentanyl is more lipophilic than alfentanil, alfentanil has the fastest onset/peak effect because its low pKa (~6.5) means a large fraction is unionised at physiological pH, so more drug crosses the blood–brain barrier rapidly. A high unionised fraction, not lipid solubility alone, drives speed of onset.

Agent-by-agent clinical pharmacology

Morphine

• Prototype μ agonist; relatively hydrophilic → slow CNS penetration, delayed peak (~15–30 min IV).
• Metabolised by hepatic glucuronidation to morphine-3-glucuronide (M3G — inactive/neuroexcitatory) and morphine-6-glucuronide (M6G — active, more potent analgesic).
• M6G is renally excreted → accumulates in renal failure causing prolonged sedation and respiratory depression. Avoid morphine in renal impairment.
• Causes histamine release → hypotension, bronchospasm, pruritus, flushing. Avoid in asthmatics/large boluses.

Fentanyl

• Synthetic, ~75–100× more potent than morphine, highly lipophilic, rapid onset, short single-dose action (redistribution).
• Minimal histamine release → haemodynamically stable (favoured in cardiac anaesthesia).
• Hepatic metabolism (CYP3A4) to inactive norfentanyl. Accumulates with repeated dosing/infusion.
• Available transdermal (chronic pain), transmucosal, intrathecal/epidural.

Remifentanil — exam favourite

• Ultra-short-acting μ agonist with an ester linkage, metabolised by non-specific plasma and tissue esterases (NOT plasma cholinesterase/pseudocholinesterase).
• Organ-independent metabolism → safe in hepatic and renal failure; metabolite GR90291 is essentially inactive and renally excreted.
• Constant CSHT (~3–4 min) → rapid offset, ideal for titratable infusions, neuro-anaesthesia, day-care.
• No accumulation even in renal/hepatic failure and unaffected by pseudocholinesterase deficiency (contrast with suxamethonium/mivacurium).
• Drawbacks: profound but brief respiratory depression, bradycardia/hypotension, acute opioid tolerance / opioid-induced hyperalgesia with high-dose infusions, and no residual post-op analgesia — a longer-acting opioid must be given before the infusion stops.

High-yield: Remifentanil is metabolised by non-specific (tissue/plasma) esterases, not by plasma pseudocholinesterase — therefore its action is NOT prolonged in pseudocholinesterase deficiency or organ failure. This single fact is repeatedly examined.

Tramadol

• Atypical, centrally acting analgesic: weak μ agonist plus inhibition of noradrenaline and serotonin reuptake (descending inhibitory pathway).
• Active metabolite O-desmethyltramadol (M1) via CYP2D6 carries most μ activity → poor metabolisers get less analgesia, ultra-rapid metabolisers risk toxicity.
• Low respiratory depression and abuse potential; useful for moderate pain and shivering.
• Risks: lowers seizure threshold, serotonin syndrome (with SSRIs/SNRIs/MAOIs), nausea/vomiting. Naloxone only partially reverses tramadol toxicity.

Other agents (rapid)

• Pethidine (meperidine): anticholinergic (tachycardia, mydriasis), anti-shivering (κ effect), metabolite norpethidine is neurotoxic/proconvulsant and accumulates in renal failure. Avoid with MAOIs → excitatory serotonin syndrome.
• Alfentanil: fastest peak effect (low pKa), short procedures.
• Sufentanil: most potent (~1000× morphine), used in cardiac/neuraxial anaesthesia.
• Methadone: long-acting, NMDA antagonism, also blocks 5-HT reuptake; risk of QT prolongation.

Opioid-induced muscle rigidity

A potentially life-threatening complication of rapid, high-dose IV administration of potent lipophilic opioids (fentanyl, sufentanil, alfentanil, remifentanil).

• Chest wall ("wooden chest") and glottic/laryngeal rigidity → difficult or impossible mask ventilation, even before loss of consciousness.
• Mechanism: central, involving μ-mediated effects on GABA/dopamine pathways in the brainstem — not a direct action on muscle.
• Risk factors: large bolus, rapid injection, elderly, neonates, concomitant N₂O.
• Management → stop injection → muscle relaxant (suxamethonium/rocuronium) + control airway/ventilate → naloxone can reverse it. Prevent by slow injection ± prior small dose of a muscle relaxant or benzodiazepine.

High-yield: Opioid-induced chest-wall rigidity ("wooden chest") classically follows rapid fentanyl boluses and is best managed acutely with a neuromuscular blocker, not just naloxone.

Postoperative nausea & vomiting (PONV)

Opioids are a major patient/anaesthetic risk factor for PONV via stimulation of the chemoreceptor trigger zone (area postrema) and the vestibular apparatus.

• Apfel score (4 risk factors): female sex, non-smoker, history of PONV/motion sickness, postoperative opioid use — each adds ~20% risk (0→~10%, 4→~80%).
• Prophylaxis/treatment: ondansetron (5-HT₃ antagonist), dexamethasone, droperidol, metoclopramide, antihistamines, scopolamine; reduce opioid load with multimodal analgesia/regional techniques.

High-yield: All four Apfel factors are mnemonic-worthy — "Female, non-smoker, prior PONV, post-op opioids." Each adds roughly 20% to baseline risk.

Respiratory depression & other side-effects

• Respiratory depression is the most feared effect: μ-mediated reduction in brainstem responsiveness to CO₂ → ↓ respiratory rate first, then tidal volume, shifting the CO₂ response curve right/down. Dose-dependent, potentiated by sedatives, sleep, and old age.
• Cardiovascular: generally stable; bradycardia (vagal) — except pethidine (tachycardia). Morphine causes histamine-mediated hypotension.
• GI: constipation (no tolerance), ↑ biliary/Oddi sphincter tone (caution in biliary colic), delayed gastric emptying.
• GU: urinary retention (↑ sphincter tone).
• Pruritus: especially neuraxial opioids (centrally mediated, not histamine) → treat with low-dose naloxone/nalbuphine, not antihistamines.
• Opioid-induced hyperalgesia (OIH): paradoxical ↑ pain sensitivity with high-dose remifentanil; mitigated by ketamine (NMDA antagonist).

Naloxone reversal

Naloxone is a pure competitive μ (and κ, δ) antagonist — the antidote for opioid overdose/respiratory depression.

• Onset IV ~1–2 min; short duration (~30–60 min) — frequently shorter than the agonist (e.g. morphine, methadone) → risk of re-narcotisation; may need repeat doses or an infusion.
• Titrate in small increments (e.g. 0.04–0.1 mg IV) to restore ventilation without abolishing analgesia.
• Hazards of over-rapid full reversal: acute pain, hypertension, tachycardia, arrhythmias, pulmonary oedema, withdrawal in dependent patients.
• Naltrexone = long-acting oral antagonist (de-addiction). Methylnaltrexone/alvimopan = peripheral antagonists for opioid-induced constipation (do not cross BBB, spare analgesia).

High-yield: Because naloxone's duration (~30–60 min) is shorter than morphine's, the patient can re-narcotise; always monitor and consider a naloxone infusion after reversing a long-acting opioid.

Flow — suspected intra/postoperative opioid overdose: Recognise (low RR, pin-point pupils, sedation) → support airway + ventilate/O₂ → titrated IV naloxone → reassess → repeat/infuse if relapse → treat precipitated withdrawal/pain.

Key differentials & distinguishing pearls

Scenario	Distinguishing point
Opioid overdose vs other coma	Triad: coma + respiratory depression + miosis; responds to naloxone
Opioid vs serotonin syndrome	Serotonin syndrome (e.g. tramadol/pethidine + SSRI/MAOI): hyperthermia, clonus, hyperreflexia, agitation, mydriasis — not miosis
Wooden chest vs anaphylaxis	Rigidity follows rapid fentanyl bolus, normal skin/CVS; anaphylaxis → rash, hypotension, bronchospasm
Renal failure opioid choice	Avoid morphine (M6G), pethidine (norpethidine); prefer remifentanil/fentanyl
PONV vs raised ICP vomiting	PONV multifactorial/Apfel; raised ICP → projectile, neuro signs

Recently asked / exam angle

• Remifentanil metabolism — non-specific esterases, not plasma pseudocholinesterase; organ-independent; flat context-sensitive half-time (~3–4 min). Repeatedly asked.
• Context-sensitive half-time definition and why it differs from elimination half-life; remifentanil's constancy.
• Which opioid causes maximum histamine release → morphine.
• Drug with fastest onset/peak effect among opioids → alfentanil (low pKa → high unionised fraction).
• Effects with NO tolerance → miosis and constipation.
• Opioid receptor mediating respiratory depression and analgesia → MOP (μ); dysphoria/sedation → κ.
• Tramadol mechanism → weak μ + NA/5-HT reuptake inhibition; CYP2D6 → M1; serotonin syndrome risk.
• Pethidine + MAOI interaction; norpethidine convulsions in renal failure.
• Naloxone duration shorter than morphine → re-narcotisation.
• Apfel score components for PONV.
• Buprenorphine — partial agonist, ceiling effect, hard to reverse.

Rapid revision

1. Opioid receptors are Gi-coupled → open K⁺, close Ca²⁺, ↓ cAMP → neuronal inhibition.
2. μ (MOP) = analgesia + respiratory depression + miosis + euphoria; κ = spinal analgesia + dysphoria + less respiratory depression.
3. No tolerance develops to miosis and constipation.
4. Remifentanil — esterase-metabolised, CSHT flat (~3–4 min), safe in organ failure, NOT affected by pseudocholinesterase deficiency; no post-op analgesia + risk of hyperalgesia.
5. Alfentanil has the fastest peak effect (low pKa, high unionised fraction).
6. Fentanyl ~100× morphine, haemodynamically stable, but CSHT rises with prolonged infusion.
7. Morphine → histamine release; active renally-cleared M6G accumulates in renal failure — avoid.
8. Tramadol = weak μ + NA/5-HT reuptake inhibition; CYP2D6 → M1; lowers seizure threshold, serotonin syndrome risk; minimal respiratory depression.
9. Pethidine → norpethidine (convulsant), anticholinergic, anti-shivering, avoid with MAOIs.
10. Wooden chest (chest-wall rigidity) after rapid fentanyl → treat with muscle relaxant ± naloxone.
11. Naloxone is short-acting (~30–60 min) → re-narcotisation; titrate to ventilation, beware withdrawal/pulmonary oedema.
12. Apfel PONV factors: female, non-smoker, prior PONV/motion sickness, post-op opioids — each ~20% risk.