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9 AnalgesicsPresentation
Remifentanil is presented as a crystalline white powder in glass vials containing 1,
2or5mg remifentanil hydrochloride. The preparation also contains glycine and is
not licensed for spinal or epidural administration.Uses
Remifentanil is administered intravenously by infusion. It should be diluted before
use with 5% dextrose, 0.9 or 0.45% saline, in which it is stable for 24 hours. It is not
recommended for use as a sole induction agent and is given as an initial bolus of
1 μg.kg−^1 over not less than 30 seconds, followed by an infusion that will vary accord-
ing to the choice of supplemental anaesthesia. The usual dose range is 0.05–2.00
μg.kg−^1 .min−^1.
While it is capable of producing intense analgesia during administration, it should
be remembered that additional post-operative analgesia is required following painful
procedures due to its short duration of action.Effects
Remifentanil shares many of morphine’s effects including respiratory depression
and chest wall rigidity. However, due to its ultra-short duration of action nausea and
vomiting seem to be less common. It characteristically causes a fall in heart rate and
blood pressure, which may be reversed by glycopyrrolate. Its analgesic effects are
reversed by naloxone.Kinetics
Remifentanil is rapidly broken down by non-specific plasma and tissue esterases
resulting in an elimination half-life of 3–10 minutes. Its duration of action is, there-
fore, determined by metabolism and not distribution (cf. alfentanil and fentanyl).
Owing to the abundance of these esterases the duration of administration does
not effect the duration of action, that is, the context-sensitive half-time (c.f. p77)
does not change. This is in contrast to the other opioids whose half-time is context-
sensitive, being dependent on the duration of infusion. It is a poor substrate for
plasma cholinesterases and as such is unaffected by cholinesterase deficiency. Anti-
cholinesterase drugs do not alter its metabolism. An essentially inactive carboxylic
acid metabolite (1/4600th as potent) is excreted in the urine. The half-life of this
metabolite in the healthy adult is 2 hours. Impaired hepatic and renal function do
not prolong its effects.Tramadol
Tramadol is a cyclohexanol derivative. It is a racemic mixture, each enantiomer
producing specific actions.