• Remifentanil

     
       

    Daniel Jolley.

    Created February 7, 2021, last updated about 3 years ago.


    Collection: 132, Score: 980, Trend score: 0, Read count: 1155, Articles count: 0, Created: 2021-02-07 11:16:22 UTC. Updated: 2021-02-07 11:24:38 UTC.

    Notes

    reference
    1

    A. Physiochemistry

    • pKa - 7.3 (58% nonionised @ 7.4)
    • Octanol water coeff - 18
    • phenylpiperidine opioid
    • contain 2 ester bonds so hydrolysed by non-specific tissue esterases.
    • Preparation contains 'glycine', so cannot be used epidurally.
    • White powder for reconstitution with water - 1, 2, 5 mg packs

    B. Pharmacokinetics

    1. Dose: (100x morphine potency, ~equal to fent)   - TCI: 3-8 ng/mL     - (up to 15 ng/mL for very stimulating procedures)     - Spontaneous ventilation returns @ 1-2 ng/mL   - 0.1-0.3 mcg/kg/min infusion (with propofol 80 mcg/kg/min (= 34 mL/h for 70 kg).
      • 0.01-0.05 mcg/kg/min spont vent
      • dilute 1 mg to 50 mL = 20 mcg/mL, or 5 mg in 50 mL = 100 mcg/mL.
      • paeds: 0.03 mg/kg in 50 mL then 1 mL/h = 0.01 mcg/kg/min.
      • Or paediatric whole-ampoule dilutions when advanced pumps are unavailable:
      • 1mg in 16.7mLs
      • or 2mg in 33.3 mLs
      • or 3mg in 50mLs 
      • → to give a dilution of 60mcg/mL
      • then for a patient of XYkg running at X.Y mLs/hr is 0.1mcg/kg/min. eg. for a 42kg patient running at 0.1mcg/kg/min will be 4.2mLs/hr which over 4 hrs uses 16 mL so a 1mg ampoule would be sufficient.
      • 1 mcg/kg IV bolus to blunt pressor resp to intubation, better than fentanyl. (equiv. fent 2 mcg/kg, alfent 20 mcg/kg)
      • 3-5 mcg/kg for intubation with propofol 2 mg/kg.
      • 0.2-0.8 mcg/kg bolus for PCA analgesia (++SEs: sedation, desaturation)
    2. Absorption - IV
    3. Distribution - 0.5 L/kg (small)
    4. Protein binding - 70-90%
    5. Onset 1-4 min; Offset 4 min (offset due to metab not redist)
    6. Metabolism - ß½ ~10 min. (CSHT-8h only 4 min!) Metabolised by non-specific plasma esterases to almost-inactive metabolites (GR90291: 1/4600 activity! / t½ 2h).
      • Minor pathway - N-dealkylation. NOT metabolised by plasma cholinesterase.
    7. Clearance - 42 mL/min/kg (30-50% CO)

    C. Pharmacodynamics

    1. Mech - highly selective mu agonist.
    2. CVS - dec MAP & HR 20-30%. (? low dose glycopyrrolate to attenuate brady).
      • No histamine release.
    3. CNS
      • max MAC reduction ~ 85% (0.1-0.2 mcg/kg/min = 60-70% MAC reduction).
      • To avoid awareness keep propofol @ at least 80 mcg/kg/min or volatile 0.3 MAC.
      • Sedation.
      • Beware rapid Opioid Induced Hyperalgesia.
    4. Resp - ⇣ RR & MV; apnoea. Spontaneous respiration occurs at blood concentrations of 4 to 5 nanogram/mL in the absence of other anaesthetic agents; for example, after discontinuation of a 0.25 microgram/kg/minute infusion of remifentanil, these blood concentrations would be reached in two to four minutes.
    5. GIT - dec CTZ stimulation as rapidly metabolised; no ion trapping.
    6. Muscle - muscle rigidity similar to alfentanil, though more than fentanyl.
      • May cause chest wall rigidity (inability to ventilate) after single doses of > 1 microgram/kg administered over 30 to 60 seconds or infusion rates > 0.1 microgram/kg/minute.
      • Administration of doses < 1 microgram/kg may cause chest wall rigidity when given concurrently with a continuous infusion of remifentanil.
    7. Foetal - little effect as rapidly metabolised by foetus.
    Daniel Jolley  Daniel Jolley
     
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