Opioids advances in pharmacokinetics and pharmacodynamics

Drugs are often chosen on the basis of differing elimination half-lives. Opioids with long elimination half-lives (e.g. morphine) may be chosen for long procedures and infusions, whereas opioids such as alfentanil with a shorter elimination half-life are used for brief procedures ( Hugheseia/ 1992). This is simplistic as all intravenous drugs are described by multicompartmental pharmacokinetics. Rational selection of an opioid for use in critical care should utilize new pharmacokinetic concepts such as biophase (i.e. effect site) and context-sensitive half-life (the time required for the central compartment drug concentration to decrease by 50 per cent). Pharmacodynamic variables are also important. The principal pharmacodynamic differences between the opioids are in the half-time of equilibration between the biophase and plasma (e.g. 5-6 min for fentanyl and 1.5 min for alfentanil) and their potency.

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