CLINICAL PHARMACOLOGY
Etomidate is a hypnotic drug without analgesic activity.
Intravenous injection of etomidate produces hypnosis characterized by a rapid
onset of action, usually within one minute. Duration of hypnosis is dose
dependent but relatively brief, usually three to five minutes when an average
dose of 0.3 mg/kg is employed. Immediate recovery from anesthesia (as assessed
by awakening time, time needed to follow simple commands and time to perform
simple tests after anesthesia as well as they were performed before
anesthesia), based upon data derived from short operative procedures where
intravenous etomidate was used for both induction and maintenance of anesthesia,
is about as rapid as, or slightly faster than, immediate recovery after similar
use of thiopental. These same data revealed that the immediate recovery period
will usually be shortened in adult patients by the intravenous administration
of approximately 0.1 mg of intravenous fentanyl, one or two minutes before
induction of anesthesia, probably because less etomidate is generally required
under these circumstances (consult the package insert for fentanyl before
using).
The most characteristic effect of intravenous etomidate
on the respiratory system is a slight elevation in arterial carbon dioxide
tension (PaCO2). See also ADVERSE REACTIONS.
Reduced cortisol plasma levels have been reported with
induction doses of 0.3 mg/kg etomidate. These persist for approximately 6 to 8
hours and appear to be unresponsive to ACTH administration.
The intravenous administration of up to 0.6 mg/kg of
etomidate to patients with severe cardiovascular disease has little or no
effect on myocardial metabolism, cardiac output, peripheral circulation or
pulmonary circulation. The hemodynamic effects of etomidate have in most cases
been qualitatively similar to those of thiopental sodium, except that the heart
rate tended to increase by a moderate amount following administration of
thiopental under conditions where there was little or no change in heart rate
following administration of etomidate. However, clinical data indicates that
etomidate administration in geriatric patients, particularly those with
hypertension, may result in decreases in heart rate, cardiac index, and mean
arterial blood pressure. There are insufficient data concerning use of
etomidate in patients with recent severe trauma or hypovolemia to predict
cardiovascular response under such circumstances.
Clinical experience and special studies to date suggest
that standard doses of intravenous etomidate ordinarily neither elevate plasma
histamine nor cause signs of histamine release.
Limited clinical experiene, as well as animal studies,
suggests that inadvertent intra-arterial injection of etomidate, unlike
thiobarbiturates, will not usually be followed by necrosis of tissue distal to
the injection site. Intra-arterial injection of etomidate is, however, not
recommended.
Etomidate induction is associated with a transient 20-30%
decrease in cerebral blood flow. This reduction in blood flow appears to be
uniform in the absence of intracranial space occupying lesions. As with other
intravenous induction agents, reduction in cerebral oxygen utilization is
roughly proportional to the reduction in cerebral blood flow. In patients with
and without intracranial space occupying lesions, etomidate induction is
usually followed by a moderate lowering of intracranial pressure, lasting
several minutes. All of these studies provided for avoidance of hypercapnia.
Information concerning regional cerebral perfusion in patients with
intracranial space occupying lesions is too limited to permit definitive
conclusions.
Preliminary data suggests that etomidate will usually
lower intraocular pressure moderately.
Etomidate is rapidly metabolized in the liver. Minimal
hypnotic plasma levels of unchanged drug are equal to or higher than 0.23
μg/mL; they decrease rapidly up to 30 minutes following injection and
thereafter more slowly with a half-life value of about 75 minutes.
Approximately 75% of the administered dose is excreted in the urine during the
first day after injection. The chief metabolite is R-(+)-1-(1-
phenylethyl)-1H-imidazole-5-carboxylic acid, resulting from hydrolysis of etomidate,
and accounts for about 80% of the urinary excretion. Limited pharmacokinetic
data in patients with cirrhosis and esophageal varices suggest that the volume
of distribution and elimination half-life of etomidate are approximately double
that seen in healthy subjects.
(Reference: H. Van Beem, et. al., Anaesthesia 38 (Supp
38:61-62, July 1983).
In clinical studies, elderly patients demonstrated
decreased initial distribution volumes and total clearance of etomidate.
Protein binding of etomidate to serum albumin was also significantly decreased
in these individuals.
Reduced plasma cortisol and aldosterone levels have been
reported following induction doses of etomidate. These results persist for
approximately 6-8 hours and appear to be unresponsive to ACTH stimulation. This
probably represents blockage of 11 beta-hydroxylation within the adrenal
cortex.
(References: 1. R.J. Fragen, et. al., Anesthesiology 61:652-656, 1984.
2. R.L. Wagner & P.F. White, Anesthesiology 61:647-651, 1984. 3. F.H.
DeJong, et. al., Clin. Endocrinology and Metabolism 59:(6):1143-1147, 1984, and
three additional drafts of Metabolic Studies, all submitted to NDA 18-228 on
April 1, 1985).