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AMIDATE Clinical Pharmacology (etomidate injection, USP)

CLINICAL PHARMACOLOGY

Etomidate is a general anesthetic without analgesic activity. Intravenous injection of etomidate produces anesthesia characterized by a rapid onset of action, usually within one minute. Duration of anesthesia 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 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 Adrenocorticotropic hormone (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 experience, 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% to 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 anesthetic plasma levels of unchanged drug are equal to or higher than 0.23 mcg/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.

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 to 8 hours and appear to be unresponsive to ACTH stimulation. This probably represents blockage of 11 beta-hydroxylation within the adrenal cortex.

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