VALO'S ARCHIVE

Anyone who has ever popped a cholesterol-lowering drug, or an antidepressant, or an anti-seizure medication, and expected to wake up the next day with a changed life knows that taking medicine is often a waiting game. And anyone who has experienced an adverse side effect from a drug, and is waiting anxiously for it to clear the system, knows the same. Both the time it will take for a drug to be effective, and the time it will take for it to leave the body completely, depend on a drug's  half-life. The half-life is the amount of time necessary for the concentration of the drug in the bloodstream of the body to be reduced by one-half. The time it will take for a drug to reach a steady state, or full effectiveness, in the system is based on that half-life. And sometimes it just isn't all that fast. A presentation from the University of Florida entitled " Basic Biopharmaceutics " clarifies that, "It takes [a medication] one half-life to reach 50%, 2 half-...

Dr Mahesh Trivedi, Dr Shafee Shaikh, Specialist Registrars Dr Carl Gwinnutt, Consultant Department of Anaesthesia, Hope Hospital, Salford M6 8HD, UK. E mail: mptrivedi85@yahoo.com   PHARMACOKINETICS OF OPIOID AGONISTS There is substantial variability (3-5 fold) in the clinical response to opioids due to their pharmacokinetics and pharmacodynamics. Pharmacokinetic properties of the opioids commonly used in anaesthesia are displayed in table 3. Table 3: Pharmacokinetics of commonly used opioids Morphine Pethidine Fentanyl Alfentanil Remifentanil pKa 8.0 8.5 8.4 6.5 7.1 Unionised at pH 7.4 (%) 23 5 9 90 68 Plasma protein bound (%) 30 40 84 90 70 Terminal half life (hrs) 3 4 3.5 1.6 0.06 Clearance (ml/min/kg) 15-30 8-18 0.8-1.0 4-9 30-40 Volume of distribution (L/kg) 3-5 3-5 3-5 0.4-1.0 0.2-0.3 Relative lipid solubility 1 28 580 90 50 Opioids are weak bases (pKa 6.5-...

Pharmacology of Opioids Dr Mahesh Trivedi, Dr Shafee Shaikh, Specialist Registrars Dr Carl Gwinnutt, Consultant Department of Anaesthesia, Hope Hospital, Salford M6 8HD, UK. E mail: mptrivedi85@yahoo.com Before reading the tutorial try answering the following: What is an opioid and how do they work? How can opioid analgesics be classified? What are the adverse effects of opioids? Why is it useful to know the lipid solubility of an opioid? How does ionisation of opioids (at plasma pH) affects their onset of action. What are opioid antagonists, their uses and adverse effects? DEFINITIONS Opium : A mixture of alkaloids from the poppy plant- Papaver Somniferum. Opioid : Any naturally occurring, semi-synthetic or synthetic compounds that bind specifically to opioid receptors (see below) and share the properties of one or more of the naturally occurring endogenous opioids. Opiate : Any naturally occurring opioid derived from opium (eg morphine). Narcotic : ...