Twelve hours after uncomplicated cesarean delivery with epidural morphine, a parturient is found with a respiratory rate of 6 per minute and requires naloxone infusion. Why does neuraxial morphine uniquely produce this late respiratory depression?

• A It forms an inactive depot in epidural fat that releases morphine unpredictably
• B Its hydrophilicity keeps it in cerebrospinal fluid longer, permitting slow rostral spread to brainstem respiratory centers ✓
• C Its metabolites accumulate in the spinal cord over several hours
• D It induces systemic absorption peaks exactly at 12 hours

Explanation

Morphine is hydrophilic compared with fentanyl, so it persists in cerebrospinal fluid and migrates cephalad to reach the medullary respiratory centers many hours after epidural or intrathecal injection. Clinically significant respiratory depression can occur 6 to 12 hours later, mandating prolonged monitoring. Lipophilic fentanyl redistributes rapidly out of the CSF, producing earlier but briefer depression, which is precisely the discriminating contrast. Delayed systemic absorption is not the accepted mechanism.

Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 13th ed.

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Written and medically reviewed by the StethoPrep medical team.