Remifentanil

1) Receptor-level action → clinical effects

Primary:

μ-opioid receptor full agonist, ultra–short acting.

Same receptor subtype profile as fentanyl 

(μ₁ → analgesia, μ₂ → respiratory depression, bradycardia, rigidity).

Unique feature:

Rapid hydrolysis by non-specific plasma and tissue esterases. 

Hence, independent of liver or renal function → predictable and context-insensitive offset.

Other receptor effects (minor):

* Weak δ and κ agonism (minimal clinical effect).

* No histamine release → stable haemodynamics.

Clinical effects derived from above

* CNS: profound, titratable analgesia and hypnosis supplement; rapid recovery after discontinuation.

* CVS: dose-dependent bradycardia and hypotension (from vagal tone and reduced sympathetic drive).

* Respiratory: potent depression and apnoea at high doses.

* No residual analgesia after infusion stops → requires overlap with longer-acting analgesic.

* No histamine release or direct myocardial depression — excellent for cardiac surgery and TIVA.

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2) Vial strength, preparation & basic pharmacokinetics (+ disadvantages)

Formulation

* Powder vials 1 mg, 2 mg, or 5 mg; reconstitute with saline → 50 mcg/mL or 20–25 mcg/mL for infusion.

Pharmacokinetics

* Onset:30–60 seconds.

* Peak effect: 1–2 minutes.

* Steady state: within 3–5 minutes of infusion start.

* Context-sensitive half-time: ~3–5 minutes regardless of infusion duration (key advantage).

* Metabolism: by non-specific esterases in blood & tissues (not pseudocholinesterase).

* Elimination: inactive remifentanil acid via urine.

Disadvantages:

1. No residual analgesia → abrupt pain surge if not bridged.

2. Rigidity and bradycardia if bolused rapidly.

3. Costly compared to other opioids.

4. Requires continuous infusion for effect.

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3) Practical dosing

A. Anaesthesia (cardiac / general)

Induction & maintenance

* Bolus:

- avoid rapid large boluses;

- if required, ≤ 0.5–1 mcg/kg slowly over 60–90 s.

* Infusion:

- Induction: 0.5–1 mcg/kg/min for intubation (combined with hypnotic).

- Maintenance: 0.05–2 mcg/kg/min

- (typical 0.1–0.3 mcg/kg/min in cardiac surgery).

- Titrate to surgical stimulus; taper before end to prevent abrupt awakening pain.

**TIVA protocols:

- Commonly combined with propofol 50–150 mcg/kg/min, producing smooth, titratable anaesthesia.

- For off-pump CABG or minimally invasive cardiac surgery: start low (0.05–0.15 mcg/kg/min) and titrate to haemodynamic response.

**Extubation or emergence:

- Gradually wean over 10–15 min while introducing morphine/fentanyl for postoperative analgesia.

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B. ICU Analgosedation

Indications:

short-term analgesia during ventilator support when rapid offset needed (e.g., after cardiac surgery).

* Loading dose:

avoid bolus; start infusion directly.

* Infusion:

0.05–0.2 mcg/kg/min (3–12 mcg/kg/h).

* For longer sedation (>24 h): taper slowly to avoid withdrawal and bridge with longer opioid.

* Contraindicated for prolonged sedation (>48–72 h) due to tolerance and hyperalgesia risk.

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4) Special populations — dosing cautions

Pregnancy

* Crosses placenta → neonatal respiratory depression; not preferred for labour analgesia.

* Acceptable for induction in cardiac/obstetric anaesthesia if benefit outweighs risk (short half-life useful).

Lactation

* Negligible transfer; safe for short procedures.

Hepatic impairment

* No adjustment needed — metabolism independent of liver function.

Renal impairment

* No adjustment needed— inactive metabolite, rapid clearance. Ideal in renal failure.

Obesity

* Dose by Ideal or Adjusted Body Weight (not Total BW); avoid overshoot due to high potency.

Paediatrics

* Bolus:0.5–1 mcg/kg IV slowly.

* Infusion: 0.05–0.3 mcg/kg/min.

* Neonates: higher clearance variability; titrate carefully.

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5) Drug interactions (clinically key)

1. CNS depressants 

(propofol, volatile agents, benzos): synergistic → reduce doses of both.

2. Neuromuscular blockers:

may mask chest wall rigidity; slow titration preferred.

3. β-blockers / calcium channel blockers:

additive bradycardia & hypotension → start low, titrate cautiously.

4. MAOIs / SSRIs:

rare serotonin toxicity risk.

5. Nitrous oxide / volatiles:

potentiate analgesia; lower infusion requirement.

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6) Significant complications & management

1. Respiratory depression / apnoea

- Due to Potent μ₂ effect; rapid onset

🟢 Airway/ventilation support;

🟢 titrate down; 

🟢 naloxone (40–80 mcg IV q2 min)

2. Bradycardia / asystole

- Due to High vagal tone, esp. in β-blocked cardiac patients 

🟢 Atropine 0.3–0.6 mg IV or glycopyrrolate; 

🟢 small adrenaline if severe.

3. Chest wall rigidity

- Due to Rapid bolus

🟢 Prevent by slow administration or pre-muscle relaxant.

4. Acute tolerance / hyperalgesia

- Due to NMDA activation with prolonged infusion

🟢 Taper gradually, 

🟢 co-administer low-dose ketamine or morphine transition.

5. Rebound pain on cessation 

- No residual effect

                  

🟢 Overlap with longer-acting opioid 20–30 min before stopping. 

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Pocket viva line

Remifentanil is an ultra–short acting μ-opioid agonist metabolised by non-specific esterases, giving a context-insensitive half-life (~3–5 min).

It provides intense, titratable analgesia with rapid recovery, minimal accumulation, and is ideal for cardiac anaesthesia and short ICU sedation.

Induction 0.5–1 mcg/kg/min; maintenance 0.05–0.3 mcg/kg/min; ICU 0.05–0.2 mcg/kg/min. 

Must bridge with a longer opioid before stopping.