Perioperative Opioids and Antagonists

Mu-opioid receptor agonism provides analgesia and contributes to sedation, respiratory depression, miosis, reduced gastrointestinal motility, nausea, pruritus, urinary retention, and physical dependence. Kappa and delta receptors contribute additional effects, but most perioperative opioid activity is dominated by mu signaling.

Opioids blunt responses to painful stimulation but do not reliably produce amnesia or unconsciousness. They are usually components of balanced anesthesia rather than sole general anesthetics.

Morphine is less lipid soluble than fentanyl and has a slower effect-site equilibration. Histamine release can contribute to vasodilation, and morphine-6-glucuronide is an active metabolite that can accumulate in renal impairment.

Fentanyl is highly lipid soluble, potent, and rapidly acting. Large or rapidly administered doses can cause chest-wall rigidity, and CYP3A interactions may alter exposure.

Remifentanil is hydrolyzed by nonspecific blood and tissue esterases, producing rapid offset that is comparatively independent of infusion duration. Its rapid disappearance means postoperative analgesia must be planned before stopping it.

Meperidine can reduce shivering, but its metabolite normeperidine is neurotoxic and accumulates in renal impairment. Serotonergic interactions and seizure risk limit routine use.

Nalbuphine is a kappa agonist and mu antagonist or partial antagonist. It can reduce opioid-induced pruritus but may precipitate withdrawal in a patient dependent on a full mu agonist.

Respiratory depression is dose related and is amplified by benzodiazepines, propofol, alcohol, sleep-disordered breathing, advanced age, and other CNS depressants. Continuous reassessment is required because sedation can precede ventilatory failure.

• Rapid opioid administration can cause bradycardia, hypotension, or rigidity.
• Tolerance to analgesia does not guarantee equal tolerance to respiratory depression.
• Renal or hepatic dysfunction can prolong the effects of parent drugs or active metabolites.
• Constipation and sphincter of Oddi effects are class effects, although magnitude varies by agent and context.

Naloxone is a competitive opioid antagonist used to reverse clinically important opioid-induced respiratory or CNS depression. Titration to adequate ventilation can reduce abrupt pain, sympathetic activation, and acute withdrawal.

Naloxone may wear off before the opioid, so recurrence of respiratory depression is possible. Continued monitoring and repeated dosing or infusion may be required according to the opioid, exposure, and clinical response.

Primary teaching source: Akram Amer, Pharmacology of Anesthesia Drugs, pages 24-28.

Opioid safety statements were checked against current fentanyl and naloxone prescribing information, with special attention to respiratory depression and CNS-depressant interactions.

This material is educational and remains a draft until reviewed by a qualified clinician. Local protocols and current product information take precedence for patient care.