20110215

Opioids

Opioids
Mechanism of Action
  • Opioids bind to specific receptors.
  • Mu (mu-1 and mu-2).
  • Kappa.
  • Delta.
  • Sigma.
  • Agonist opioids : capable of receptor activation.
  • Antagonist opioids : inactivation of receptor.
  • Agonist-antagonist opioids : have opposite actions at different receptor types.
  • Endorphins, enkephalins, and dynorphins are endogenous peptides that bind opioids receptors.
  • Activation of opioids receptor.
  • Inhibits the presynaptic release and postsynaptic response to excitatory neurotransmitters from nociceptive neurons.
Pharmacokinetics
  • Absorption.
  • Intramuscular injection of morphine and meperidine; peak plasma 20-60 minutes.
  • Transdermal; fentanyl reach plateu concentration within 14-24 hrs and remain constant for 72 hrs.
  • Intravenous.
Biotransformation
  • Most opioids depends primarily on liver biotransformation.
  • Morphine : morphine 3-glucoronide and morphine 6-glucoronide.
  • Meperidine : normeperidine.
Excretion
  • End products of morphine and meperidine biotransformation are eliminated by the kidneys.
Effects on Organ System
Cardiovascular
  • Meperidine : increase heart rate.
  • Morphine, fentanyl high dose cause vagus-mediated bradycardi.
  • No opioids depress cardiac contractility.
  • Blood pressure fall due to bradycardia, venodilatation and decrease sympathetic reflexes.
Respiration
  • Depress ventilation, particularly respiration rate.
  • Resting PaCO2 increases and the response to CO2 challenge is blunted.
  • Apneic threshold increase.
  • Hypoxic drive decreased.
  • Fentanyl high dose cause chest wall rigidity.
  • Cerebral.
  • Reduce CMRO2, CBF and ICP.
  • Stimulation of medullary chemoreceptor trigger zone responsible for high incidence of nausea and vomiting.
  • Do not cause amnesia.
  • Meperidine can decrease shivering.
  • Gastrointestinal.
  • Slow gastric emptying time by reducing peristalsis.
  • Biliary colic sphincter of Odd.
  • Endocrine.
  • Opioids block catecholamine, antidiuretic hormone, and cortisol.
Drug Interactions
  • Combination of opioids and MAO inhibitors may result in respiratory arrest, hipertension or hypotension, coma and hyperpyrexia.
  • Barbiturates, benzodiazepines can have synergistic cardiovascular, respiratory and sedative effects with opioids.





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