3. Old stuff
          3.1. Old pharm stuff (pre 2009)
              3.1.3. Pharmacology
                  3.1.3.5. Opioids
                      3.1.3.5.3. Opioid agonists
 3.1.3.5.3.8. Remifentanil 

Remifentanil

Quick summary

 

 

Usage

What is the drug used for?

Structure

Structure

  • Structurally unique because of the ester linkage
    --> Allows hydrolysis by nonspecific plasma and tissue esterase

Thus,

  • Short duration of action
  • Rapid and easy titration
  • Noncumulative effects
  • Rapid recovery after discontinuation

Pharmacodynamics

Potency

Analgesic potency is similar to fentanyl
* About 15-20 times as potent as alfentanil

Mechanisms of action

  • Selective mu opioid agonist

Effects by systems

CNS

  • High dose remifentanil decreases CBF and CMRO2
    * No impairment of CBF autoregulation in response to pCO2
  • Seizure-like activity
    * All fentanyl analogs, including remifentanil

Respiratory

After 0.5 microgram/kg IV remifentanil
--> Decrease in slope and downward shift of the CO2 ventilatory response curve
--> Nadir after about 150 seconds after injection
--> Recovery within 15 minutes

 

Propofol and remifentanil
--> Synergistic respiratory depression effect

Acute opioid tolerance

  • May be associated with acute opioid tolerance
    * Not all study supports acute opioid tolerance
  • Activation of NMDA receptors may be a possible mechanism
    --> NMDA receptor antagonists (ketamine, Mg2+) block opioid tolerance

Others

  • PONV
  • No histamine release
  • No change in ICP or IOP
  • Muscle rigidity [PI]

Pharmacokinetics (PK)

Characterised by

  • Small Vd
  • Rapid clearance
  • Low interindividual variability

Absorption

IV infusion

Distribution

  • Vd = 0.2 - 0.3 L/kg
    * Small Vd
    * Limited accumulation
    * Steady state is reached within 10 minutes of the start of infusion
  • Protein-binding
    = 66-93% [SH4:p93]
    = ?80% [RDM6:p401]

Metabolism

  • Ester link in remifentanil
    --> Susceptible to hydrolysis by nonspecific plasma and tissue esterase
    --> Inactive metabolites

Metabolism is by

  • Hydrolysis
    * By nonspecific plasma esterase and tissue esterase
    * Remifentanil acid is formed (300 to 4600 fold less potent than remifentanil)
    * The major pathway of metabolism of remifentanil in human
  • N-dealkylation
    * Minor pathway

NB:

  • Remifentanil is NOT metabolised by butyrylcholinesterases (pseudocholinesterase, plasma cholinesterase)
    --> Remifentanil clearance is not affected by cholinesterase deficiency or anticholinergics
  • Esterase metabolism appears to be well-preserved metabolic system and has little interindividual variability

Elimination

  • Clearance = 30 - 40 mL/kg/min
    * Very fast clearance
    * Contribute to the lack of accumulation
  • Metabolites are excreted by kidneys

Action profile

  • Elimination half-time = 0.17 - 0.33 hr
  • Context-sensitive half-time (4 hours) = 4 min
    * Nearly independent of infusion duration
  • Effect-site equilibration time = 1.1 min

Physicochemical properties

  • Weak base
  • pKa = 7.3
    --> 58% non-ionised at pH 7.4

Pharmaceutics

[PI on MIMs]

Presentation

  • 1mg, 2mg, and 5mg vials
  • Diluted to 20, 25, 50, or 250 microgram/mL
  • 50 microgram/mL is the recommended concentration in adults
  • 20-25 microgram/mL is the recommended concentration for paediatrics

Composition

  • Active = remifentanil
  • Inactive = glycine

Clinical

Administration

Induction of anaesthesia

  • Remifentanil 1 microgram/kg IV over 60-90 seconds

Analgesia

  • Remifentanil 0.05 - 2.00 microgram/kg/min

NB:

  • Clinical dosing of remifentanil should be based on ideal body mass

 

Contraindications

Spinal or epidural
  • Not recommended because the safety of the opioid and/or the vehicle (glycine) have not been determined.

Special consideration

Renal and hepatic disease

  • Renal and hepatic disease is unlikely to affect metabolism of remifentanil
    * Esterase metabolism is usually well-preserved

Hypothermic cardiopulmonary bypass

  • Decrease in temperature
    --> Decreased blood and tissue esterase activity
    --> Remifentanil clearance is reduced by 20%

Trivia

History

 

Others