Remifentanil

How to use it (practical clinical structure)

Typical intra-op use: continuous infusion with/without bolus, titrate to effect with close monitoring
- Common contexts: TIVA (with propofol), volatile-based GA, awake fibreoptic intubation/sedation (careful), neurosurgery, ENT, bariatrics, short painful stimuli (e.g. laryngoscopy, pinning)
Dosing (adult, typical ranges, local protocols vary)
- Induction/maintenance analgesia: infusion often 0.05–0.3 micrograms/kg/min (3–18 micrograms/kg/h), adjust to haemodynamics and stimulus
- Bolus: generally avoid large boluses due to chest wall rigidity/apnoea, if used, small incremental doses with airway control and slow administration
- ICU/ventilated sedation-analgesia: infusion commonly 0.05–0.2 micrograms/kg/min with separate hypnotic as needed
Transition plan is mandatory (because offset is rapid)
- Give longer-acting opioid and/or multimodal analgesia before stopping (e.g. morphine/oxycodone/fentanyl, paracetamol, NSAID if appropriate, regional/LA infiltration)
- If high-dose/long infusion: anticipate acute opioid tolerance/hyperalgesia, consider ketamine, magnesium, alpha-2 agonist, and avoid abrupt cessation without cover
Monitoring and safety
- Continuous ECG, NIBP/arterial line as appropriate, SpO2, capnography (mandatory if any sedation), depth of anaesthesia where relevant
- Be ready to treat: bradycardia, hypotension, apnoea, rigidity, have naloxone available (but short duration—may need infusion)

Classification and presentation

Potent synthetic opioid, selective
Unique feature: contains an ester linkage → rapid metabolism by non-specific tissue and plasma esterases
Supplied as powder for reconstitution, commonly diluted for infusion (e.g. 50 micrograms/mL or 20–50 micrograms/mL depending on practice)

Mechanism of action (receptor + cellular effects)

Agonist at mu receptors (Gi/o-coupled GPCR)
- Decreases adenylyl cyclase activity → reduced cAMP
- Presynaptic: closes voltage-gated Ca2+ channels → ↓ neurotransmitter release (e.g. substance P, glutamate)
- Postsynaptic: opens K+ channels → hyperpolarisation
Clinical effects: profound analgesia, blunts sympathetic responses, respiratory depression, sedation, cough suppression

Pharmacokinetics (key FRCA points)

Onset: rapid (peak effect ~1–2 min) due to high potency and rapid effect-site equilibration
Offset: very rapid, context-sensitive half-time ~3–5 min and remains short even after prolonged infusions
Metabolism: ester hydrolysis by non-specific esterases in blood and tissues → inactive carboxylic acid metabolite
- Not dependent on hepatic blood flow or hepatic enzymes, minimal effect of liver failure on clearance
- Not dependent on plasma pseudocholinesterase (but still an esterase process), clinically significant prolongation is uncommon even with atypical pseudocholinesterase
Elimination: metabolite excreted renally, metabolite may accumulate in renal failure but has negligible opioid activity
Protein binding: moderate (commonly quoted ~70%), large Vd, crosses BBB rapidly

Pharmacodynamics and clinical effects

Respiratory: dose-dependent respiratory depression and apnoea, reduces ventilatory response to CO2, can cause chest wall rigidity (especially rapid bolus/high dose)
Cardiovascular: bradycardia and hypotension (vagotonia + reduced sympathetic tone), generally minimal direct myocardial depression
CNS: sedation, miosis, may cause postoperative shivering, can cause acute opioid tolerance/hyperalgesia after high-dose infusions
GI/urinary: nausea/vomiting, reduced gut motility, urinary retention possible (less relevant intra-op due to short duration)
Histamine release: negligible compared with morphine (so less flushing/bronchospasm from histamine)

Indications

Intra-operative analgesia where rapid titratability is valuable: short procedures, intense variable stimuli, neurosurgery (stable haemodynamics/rapid wake), ENT, bariatrics, shared airway cases
Facilitation of controlled hypotension (careful) and blunting pressor responses (laryngoscopy, pinning)
ICU: short-acting analgesia/sedation in ventilated patients where rapid neurological assessment is needed

Contraindications and cautions

Hypersensitivity to opioid or formulation components
Use with extreme caution in non-intubated sedation: high risk of apnoea/airway obstruction, requires skilled airway practitioner and capnography
Bradyarrhythmias, hypovolaemia, severe aortic stenosis: haemodynamic instability risk
Raised ICP: opioids can increase PaCO2 if ventilation inadequate, remifentanil itself not contraindicated if ventilation controlled

Drug interactions

Synergy with hypnotics (propofol, volatile agents, benzodiazepines) → increased hypotension and respiratory depression
Other vagotonic drugs (beta-blockers, dexmedetomidine) increase bradycardia risk
Neuromuscular blockers: remifentanil can cause rigidity that may mimic inadequate paralysis, ensure adequate NMBD if needed

Adverse effects and management (high yield)

Apnoea/respiratory depression
- Stop/reduce infusion, support ventilation/airway, consider naloxone titration if needed (beware short naloxone duration vs remifentanil offset is usually rapid)
Bradycardia/hypotension
- Reduce infusion, treat with fluids/vasopressors, atropine/glycopyrrolate for bradycardia, consider ephedrine/metaraminol depending on context
Chest wall rigidity (esp. rapid bolus/high dose)
- Stop opioid, ventilate, give NMBD if severe, consider naloxone if needed, avoid rapid bolus administration
Acute opioid tolerance / opioid-induced hyperalgesia
- Minimise very high doses, provide multimodal analgesia, consider low-dose ketamine, ensure adequate longer-acting opioid before stopping
PONV, pruritus, urinary retention: treat symptomatically

Special populations

Elderly: increased sensitivity, reduce starting doses and titrate carefully
Obesity: dosing often based on lean body weight/adjusted body weight for infusion, titrate to effect, beware airway/OSA-related respiratory risk
Hepatic failure: clearance largely preserved (non-specific esterases), still titrate due to altered sensitivity and haemodynamics
Renal failure: parent drug unaffected, inactive metabolite accumulates but minimal clinical effect, still plan postoperative analgesia carefully
Pregnancy/neonates: crosses placenta, can cause neonatal respiratory depression if used near delivery, specialist use only

Comparisons (commonly asked)

Remifentanil vs fentanyl/alfentanil
- Remifentanil: esterase metabolism, ultra-short context-sensitive half-time, requires infusion and transition analgesia
- Fentanyl: longer context-sensitive half-time with infusion, hepatic metabolism, more postoperative respiratory depression risk if repeated/infused
- Alfentanil: faster onset than fentanyl (lower pKa), shorter than fentanyl but longer than remifentanil, hepatic metabolism

Test yourself…

Describe the pharmacology of remifentanil.

Structure your answer: class → mechanism → PK (what makes it unique) → PD effects → adverse effects → clinical implications.

Class: potent synthetic opioid, selective
Mechanism: mu receptor (Gi/o) → ↓cAMP, ↓presynaptic Ca2+ influx, ↑postsynaptic K+ efflux → reduced nociceptive transmission
PK: rapid onset, ultra-short offset, context-sensitive half-time ~3–5 min even after long infusions
Metabolism: ester hydrolysis by non-specific tissue/plasma esterases to inactive metabolite, not reliant on hepatic metabolism
PD: analgesia, respiratory depression, bradycardia/hypotension, minimal histamine release, risk of rigidity
Clinical implication: needs infusion and a postoperative analgesia plan before stopping

Why does remifentanil have a short context-sensitive half-time, and why is this clinically important?

Short context-sensitive half-time because clearance is high and metabolism is rapid via non-specific esterases, it does not rely on slow redistribution or saturable hepatic pathways
Clinically: rapid wake-up and rapid return of spontaneous ventilation once infusion reduced/stopped, useful for neuro cases and short procedures
Downside: abrupt loss of analgesia → severe pain, sympathetic surge, and risk of hyperalgesia unless longer-acting analgesia is established

A patient becomes bradycardic and hypotensive shortly after starting a remifentanil infusion. How do you manage this?

Immediate assessment: depth of anaesthesia, surgical stimulus, ECG rhythm, volume status, other drugs (propofol/volatile/beta-blocker), exclude anaphylaxis/bleeding
Reduce/stop remifentanil temporarily, consider reducing hypnotic if appropriate
Treat bradycardia: glycopyrrolate/atropine, if severe with instability consider adrenaline bolus per local practice
Treat hypotension: fluids if hypovolaemic, vasopressor (metaraminol/phenylephrine) or ephedrine depending on HR and cause
Restart at lower rate with careful titration once stable

Explain opioid-induced chest wall rigidity with remifentanil and how you would prevent and treat it.

Mechanism: central mu receptor effects increase muscle tone, more likely with rapid bolus/high dose and in neonates/elderly
Prevention: avoid large/rapid boluses, use infusion with gradual titration, ensure adequate hypnotic and consider NMBD when appropriate
Treatment: stop opioid, support ventilation, give NMBD if severe, consider naloxone if needed (but airway control is priority)

How would you provide postoperative analgesia after a remifentanil-based anaesthetic?

Plan early: administer longer-acting opioid before stopping remifentanil (timing depends on drug and patient factors)
Use multimodal analgesia: paracetamol ± NSAID, regional techniques/LA infiltration, consider ketamine for high-dose remifentanil cases
Titrate opioid to effect in recovery, monitor for respiratory depression from the longer-acting opioid (not from remifentanil once stopped)
If concern about hyperalgesia: avoid abrupt cessation at very high rates, consider gradual down-titration while establishing alternative analgesia

Compare remifentanil with fentanyl for TIVA.

Remifentanil: very rapid titration and offset, stable and predictable recovery, requires infusion pump and careful transition analgesia
Fentanyl: longer duration and accumulation with infusion, less need for immediate transition but more risk of delayed respiratory depression and slower wake-up
Haemodynamics: both blunt sympathetic response, remifentanil more likely to cause bradycardia/hypotension if over-titrated due to potency and rapid effect-site changes

What happens to remifentanil pharmacokinetics in hepatic and renal failure?

Hepatic failure: minimal change in clearance because metabolism is via non-specific esterases, still increased sensitivity possible so titrate
Renal failure: parent drug clearance largely unchanged, inactive metabolite accumulates but has negligible opioid effect

A patient on a remifentanil infusion becomes apnoeic during monitored anaesthesia care. What are your immediate steps?

Call for help, stop/reduce remifentanil, apply airway manoeuvres and 100% oxygen
Assess ventilation with capnography, support with bag-mask ventilation, consider supraglottic airway/intubation if not rapidly reversible
Consider naloxone in titrated doses if ventilation cannot be maintained or prolonged respiratory depression suspected from other opioids/co-administered sedatives
Review contributing factors: co-administered sedatives, OSA, positioning, airway obstruction, local anaesthetic toxicity (if regional), anaphylaxis

Explain opioid-induced hyperalgesia and its relationship to remifentanil.

Opioid-induced hyperalgesia: paradoxical increased pain sensitivity after opioid exposure, distinct from tolerance (need more opioid for same effect)
Associated with high-dose/long remifentanil infusions, proposed mechanisms include NMDA receptor activation, descending facilitation, and neuroinflammatory changes
Mitigation: avoid excessive dosing, use multimodal analgesia, consider ketamine/magnesium, ensure adequate longer-acting analgesia before stopping

What are the key differences between remifentanil and alfentanil that explain onset and offset?

Alfentanil has a lower pKa → higher fraction unionised at physiological pH → rapid onset, remifentanil also has rapid onset due to fast effect-site equilibration
Offset: remifentanil is metabolised rapidly by esterases → very short context-sensitive half-time, alfentanil relies on hepatic metabolism and redistribution → longer offset and potential accumulation