Understanding Anesthesiology - The Global Regional Anesthesia ...

10 mg Morphine
=100 mg Meperidine
=100 µg Fentanyl
=10 µg Sufentanil
Onset of action is determined by lipid solubility and
ionization (pKa). Duration of action is determined by
both clearance and volume of distribution. Table 14
summarizes the clinically useful pharmacology of the
opioids most commonly used in anesthesia.
Table 14 Pharmacology of commonly used opioids
Fentanyl Sufentanil Remifentanil
Induction dose (µg/kg)* 4-20 0.25-2 0.5-1
Intra-op dose for
maintenance*
2-5 µg/kg/hr 0.3-2.0 µg/kg/hr 0.1-1 µg /kg/min
Additional boluses* 25-150 µg 2.5-20 µg 0.1-1 µg/kg
Onset of action (min) 5-8 4-6 1-2
Duration after bolus (min) 45 45 1-4
Elimination
The relatively short duration of action of these agents is
in part attributable to their lipid solubility. This leads to
rapid redistribution away from the central nervous system
to inactive tissue sites. Fentanyl and sufentanil are
metabolized in the liver to (mostly) inactive metabolites
which are then excreted in the urine. Remifentanil
on the other hand, is susceptible to metabolism by
blood and tissue esterases which accounts for its ultrashort
duration of action.
Effects
Opioids have effects on almost every system in the
body. The reader is referred to Chapter 6 for a detailed
discussion.
The most important side effect of the opioids manifests
on the respiratory system. Minute ventilation is reduced
due to a reduction in respiratory rate. (Tidal volume
actually increases.) The responsiveness to raised
PCO2 is diminished such that apnea occurs until the
dose-dependent “apneic threshold” of PCO2 is
reached.
Opioids cause nausea and vomiting due to stimulation
of the chemoreceptor trigger zone. They also cause constipation
due to decreased GI motility.
*Loading and maintenance requirements depend greatly on the patient
age and status, the doses of other anesthetic agents given, and the nature
of the surgical procedure being performed.
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