Narcotics affect the CNS via their binding to the |-opiod receptor, which results in an analgesic effect as well as euphoria. Opiates also cause respiratory depression in a dose-dependent manner that may be reversed by narcotic antagonists. Respiratory depression can occur with doses smaller than the dose needed to achieve altered consciousness. Narcotics depress both the hypoxic and hyper-carbic respiratory drive.
Fentanyl is the narcotic of choice for sedation because of its rapid onset of action (almost immediate when the drug is given by IV), as well as shorter duration of action (30
to 60 minutes), and less emetic effect as either meperidine or morphine. The initial dose is 50 to 100 |g administered over a 2-minute period. Titration to desired level of sedation should be performed with adequate time interval (3 to 4 minutes) between doses. Maximum total dose should not exceed 3 |ig/kg; however, larger doses may be necessary in long procedures and in the narcotic intolerant patient (Johns Hopkins, 2001). The elderly, debilitated or chronically ill patients, and patients with chronic obstructive pulmonary disease require smaller doses. Fentanyl is primarily transformed in the liver and demonstrates a high first pass clearance with approximately 75% of an IV dose excreted in urine, mostly as metabolites with < 10% as unchanged drug. The risk of respiratory depression and hypotension is potentiated when fentanyl is combined with sedatives such as midazolam and droperidol.
Meperidine (Demerol) is still widely used as the narcotic analgesic for sedation. Meperidine is a synthetic analgesic structurally very dissimilar to morphine, but with many of the same pharmacologic properties. Its onset of action is 3 to 5 minutes, with duration of action of 2 to 4 hours. The initial dose is 1 to 2 mg/kg, with maximum initial dose of 100 mg, titrating to desired effect with 2 to 3 minute intervals between doses.
Meperidine is metabolized in the liver, resulting in a toxic metabolite of normeperidine, which may cause seizures. Other adverse effects include nausea, vomiting, and sphincter of Oddi spasm. The elderly, debilitated, or chronically ill patients require smaller doses. Meperidine has catastrophic interactions with monoamine oxidase inhibitors. Respiratory depression is potentiated when meperidine is combined with sedatives.
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