Fentanyl

At-a-glance clinical use

Potent synthetic opioid used for intraoperative analgesia, blunting sympathetic responses, and as an adjunct in anaesthesia and sedation
- Common settings: induction/maintenance of GA, TIVA adjunct, ICU analgesia/sedation, procedural sedation (with appropriate monitoring), neuraxial/epidural (specialist use)
Typical IV bolus dosing (adult): 0.5–2 micrograms/kg for analgesia; 2–5 micrograms/kg for intubation response attenuation; higher doses (10–50 micrograms/kg) for cardiac anaesthesia/“high-dose opioid” techniques (context dependent)
- Titrate to effect; reduce dose in elderly, frail, hypovolaemia, co-administered sedatives/volatile agents
Onset/offset (IV): onset ~1–2 min; peak ~3–5 min; clinical duration after small bolus ~30–60 min (context sensitive with repeated dosing/infusion)
- Offset after infusion depends on context-sensitive half-time (increases with duration due to redistribution and tissue uptake)
Key adverse effects: respiratory depression, bradycardia, chest wall rigidity, nausea/vomiting, pruritus, urinary retention, ileus; tolerance and dependence with prolonged use
- Histamine release is minimal compared with morphine (less hypotension/bronchospasm from histamine)

Immediate management pearls

Respiratory depression/apnoea: airway support + ventilation; consider naloxone titration if clinically indicated (beware re-sedation due to shorter naloxone duration)
- Naloxone typical IV titration: 20–40 micrograms increments every 1–2 min to adequate ventilation (avoid abrupt full reversal in opioid-tolerant patients)
Chest wall rigidity (often with rapid/high-dose IV, especially with co-administered nitrous/volatile): treat with neuromuscular blockade and ventilation; consider naloxone if appropriate
- Prevention: slower administration; avoid large rapid boluses; consider small dose muscle relaxant if high-dose technique
Bradycardia: treat causes; anticholinergic (e.g., glycopyrrolate/atropine) if clinically significant; consider interaction with other vagotonic agents (propofol, remifentanil, beta-blockers)
- Mechanism: increased vagal tone and reduced sympathetic tone

Classification and chemistry

Synthetic phenylpiperidine opioid (same broad class as alfentanil, sufentanil, remifentanil)
Highly lipid soluble; largely unionised at physiological pH (pKa ~8.4) but high lipophilicity drives rapid CNS effect
Potency: ~100 times morphine (approximate; depends on endpoint and route)

Mechanism of action

Predominantly μ-opioid receptor agonist (Gi/o-coupled): inhibits adenylate cyclase → ↓cAMP; presynaptic ↓Ca2+ influx; postsynaptic ↑K+ efflux → hyperpolarisation
- Effects: analgesia (spinal/supraspinal), sedation, respiratory depression (brainstem), cough suppression, reduced GI motility
Synergistic sedation/respiratory depression with benzodiazepines, propofol, volatile agents, gabapentinoids, alcohol

Pharmacokinetics (PK)

Absorption/routes: IV (common), IM (less predictable), transdermal patches (chronic pain), buccal/intranasal preparations (breakthrough cancer pain; specialist prescribing)
Distribution: rapid redistribution from brain to muscle/fat; large volume of distribution; high protein binding (mainly α1-acid glycoprotein)
- Clinical implication: repeated boluses/infusions accumulate in peripheral compartments → prolonged offset
Metabolism: hepatic CYP3A4 N-dealkylation to inactive metabolites (e.g., norfentanyl)
- Drug interactions: CYP3A4 inhibitors (e.g., macrolides, azoles, protease inhibitors, grapefruit) can increase effect; inducers (e.g., carbamazepine, phenytoin, rifampicin) reduce effect
Elimination: mainly renal excretion of metabolites; clearance reduced in severe hepatic impairment; renal failure has less effect than morphine (no active analgesic metabolites) but sensitivity may still increase
Context-sensitive half-time: increases with infusion duration (slower offset than remifentanil; generally shorter than morphine for single bolus but can become prolonged with long infusions)

Pharmacodynamics (PD) and organ effects

CNS: analgesia, sedation, euphoria/dysphoria; miosis (pinpoint pupils) unless severe hypoxia/hypercapnia
Respiratory: dose-dependent respiratory depression; reduced ventilatory response to CO2 and hypoxia; can cause apnoea; may cause chest wall rigidity
- Particularly hazardous with co-administered sedatives and in OSA, obesity hypoventilation, COPD, elderly
Cardiovascular: generally haemodynamically stable; bradycardia common; minimal myocardial depression; minimal histamine release
- High-dose opioid technique can reduce stress response and catecholamine surge (useful in cardiac surgery)
GI/GU: nausea/vomiting, reduced gastric motility, constipation/ileus, biliary spasm (less prominent than morphine but possible), urinary retention
Endocrine/immune: chronic opioids may suppress HPA axis and immune function; acute perioperative relevance usually limited

Indications and practical use

Intraoperative analgesia and attenuation of laryngoscopy/intubation response; supplement to volatile/TIVA; analgesic component of balanced anaesthesia
Adjunct in regional/neuraxial techniques (specialist practice): epidural/spinal opioids provide segmental analgesia but carry risk of delayed respiratory depression (less than morphine due to lipophilicity; still clinically important)
- More rapid onset and shorter duration neuraxially than morphine; more rostral spread risk is lower than hydrophilic opioids but not zero
ICU: analgesia/sedation adjunct; consider accumulation with prolonged infusion and organ dysfunction; daily sedation holds and analgesia-first strategies

Contraindications and cautions

Absolute: hypersensitivity (rare); avoid transdermal initiation for acute pain/opioid-naïve patients
Relative/caution: severe respiratory disease, OSA, raised intracranial pressure (CO2 retention), severe hepatic impairment, elderly/frail, concomitant CNS depressants
Serotonin toxicity risk: fentanyl has serotonergic properties; caution with SSRIs/SNRIs/MAOIs and other serotonergic drugs (rare but important)
- Features: agitation, hyperreflexia/clonus, autonomic instability, hyperthermia; management is supportive, stop serotonergic agents, consider cyproheptadine

Adverse effects and management

Respiratory depression: support ventilation; titrated naloxone if needed; monitor for re-sedation
Chest wall rigidity: ventilate; give neuromuscular blocker; consider naloxone; prevent with slow administration
Bradycardia/hypotension: treat with anticholinergic/vasopressors as required; consider depth of anaesthesia and co-drugs
PONV/pruritus: antiemetics; consider opioid-sparing multimodal analgesia; low-dose naloxone infusion sometimes used in specialist settings for pruritus (balance analgesia)

Comparisons (high-yield)

Vs morphine: fentanyl is more potent, more lipid soluble, faster onset, shorter duration after single bolus, minimal histamine release, no active metabolites (better in renal failure than morphine)
Vs alfentanil: fentanyl is more potent and longer acting; alfentanil has faster onset (lower pKa → more unionised at pH 7.4)
Vs remifentanil: fentanyl has longer offset and accumulation with infusion; remifentanil is ester-metabolised by non-specific plasma/tissue esterases → very short context-sensitive half-time

Describe fentanyl: class, mechanism of action, and key clinical effects.

Structure your answer: classification → receptor/pharmacology → organ effects → practical implications.

Class: synthetic phenylpiperidine opioid; potent μ-receptor agonist
Mechanism: Gi/o-coupled μ receptor → ↓adenylate cyclase/↓cAMP; presynaptic ↓Ca2+ → ↓neurotransmitter release; postsynaptic ↑K+ efflux → hyperpolarisation
Clinical effects: analgesia, sedation, respiratory depression, bradycardia, cough suppression; minimal histamine release
Implications: haemodynamic stability but significant ventilatory risk; synergistic depression with other sedatives

Give the pharmacokinetics of fentanyl and explain why its duration can be prolonged after infusions.

Examiners want redistribution, lipophilicity, metabolism, and context-sensitive half-time.

Highly lipid soluble → rapid CNS uptake and rapid redistribution from brain to muscle/fat
Large Vd and high protein binding (α1-acid glycoprotein)
Hepatic metabolism via CYP3A4 to inactive metabolites; metabolites excreted renally
After prolonged infusion: peripheral compartment saturation (muscle/fat) → drug returns to plasma → prolonged offset; context-sensitive half-time increases with infusion duration

A patient becomes difficult to ventilate shortly after a rapid fentanyl bolus. What is the diagnosis and how do you manage it?

This is a classic viva on opioid-induced rigidity.

Diagnosis: opioid-induced chest wall (and laryngeal) rigidity causing impaired ventilation; more likely with rapid/high-dose IV fentanyl
Immediate actions: call for help; ensure airway patency; attempt ventilation with 100% O2; deepen anaesthesia as appropriate
Definitive management: administer neuromuscular blocker (e.g., suxamethonium/rocuronium) and ventilate; consider naloxone if clinically appropriate
Prevention: avoid large rapid boluses; give slowly and titrate; consider alternative opioid strategy

Discuss the cardiovascular effects of fentanyl and why it is often considered haemodynamically stable.

Focus on HR, BP, histamine, and stress response.

Minimal direct myocardial depression at clinical doses; maintains cardiac output if HR maintained
Bradycardia is common (vagal predominance); can reduce CO in fixed stroke volume states
Minimal histamine release → less vasodilation/hypotension compared with morphine
Attenuates sympathetic responses to surgical stimulation/laryngoscopy → reduced catecholamine surge

Compare fentanyl with morphine in terms of onset, duration, metabolism, and suitability in renal failure.

A frequent FRCA comparison theme: lipophilicity and metabolites.

Onset: fentanyl faster (high lipophilicity); morphine slower
Duration: fentanyl shorter after single bolus but can accumulate with infusion; morphine longer and more predictable for longer analgesia
Metabolism: fentanyl CYP3A4 to inactive metabolites; morphine glucuronidation to active metabolites (M6G) and neurotoxic metabolites (M3G)
Renal failure: fentanyl generally preferred over morphine (no active metabolites), but still titrate carefully due to increased sensitivity and potential accumulation with prolonged use

Explain the concept of context-sensitive half-time and apply it to fentanyl.

Define it clearly, then relate to tissue uptake and infusion duration.

Definition: time for plasma concentration to fall by 50% after stopping an infusion, dependent on the duration (context) of infusion
For fentanyl: increases with infusion duration due to distribution into and slow release from peripheral compartments (muscle/fat)
Clinical relevance: delayed emergence/respiratory depression after long cases or ICU infusions; plan weaning and postoperative monitoring

Outline important drug interactions with fentanyl relevant to anaesthesia and ICU.

Cover pharmacodynamic synergy and CYP3A4 interactions.

Pharmacodynamic: synergistic sedation/respiratory depression with benzodiazepines, propofol, volatile agents, gabapentinoids, alcohol
Pharmacokinetic: CYP3A4 inhibitors increase fentanyl effect (e.g., macrolides, azoles, protease inhibitors, grapefruit); inducers reduce effect (e.g., carbamazepine, phenytoin, rifampicin)
Serotonergic drugs: rare serotonin toxicity with SSRIs/SNRIs/MAOIs and other serotonergic agents

A patient in PACU is sedated with a low respiratory rate after intraoperative fentanyl. How would you assess and manage this?

Think ABCDE, reversible causes, and careful reversal.

Assessment: airway patency, RR, SpO2, ETCO2 if available, level of consciousness, pupil size; review total opioid dose and co-sedatives; consider residual anaesthetic, hypothermia, metabolic causes
Immediate management: stimulate, oxygen, airway manoeuvres, support ventilation as needed
Naloxone if inadequate ventilation: titrate small IV boluses (e.g., 20–40 micrograms) to adequate ventilation; avoid complete reversal if pain is severe or opioid tolerant
Ongoing: monitor for re-sedation (naloxone shorter duration); consider infusion or repeat doses; ensure appropriate postoperative observation

Discuss fentanyl use in the elderly and in obesity/OSA.

Dose reduction and postoperative risk are key.

Elderly: increased sensitivity and reduced clearance; reduce dose and titrate slowly; higher risk of delirium, respiratory depression, hypotension/bradycardia
Obesity/OSA: high risk of postoperative airway obstruction and opioid-induced ventilatory impairment; use lean body weight for bolus dosing (common practice), multimodal opioid-sparing analgesia, enhanced monitoring
Consider alternatives/adjuncts: regional techniques, paracetamol/NSAIDs (if appropriate), ketamine, dexmedetomidine, local infiltration

What are the key differences between fentanyl, alfentanil, and remifentanil that influence choice for intubation and short cases?

Examiners want onset/offset and metabolism.

Fentanyl: rapid onset, intermediate duration; accumulates with infusion; CYP3A4 metabolism
Alfentanil: faster onset than fentanyl (more unionised at pH 7.4 due to lower pKa); shorter duration; useful for brief intense stimuli
Remifentanil: ultra-short acting; metabolised by non-specific esterases; context-sensitive half-time ~constant and short; requires postoperative analgesia plan

Explain why fentanyl causes miosis and how pupil size can mislead in severe opioid toxicity.

A small physiology/pharmacology crossover.

Miosis: μ-receptor mediated stimulation of Edinger–Westphal nucleus → parasympathetic outflow to pupil
In severe hypoxia/hypercapnia: pupils may dilate despite opioid toxicity, so pupil size must not be used in isolation