Twelve hours after epidural morphine for caesarean section, a parturient is found drowsy with a respiratory rate of 7 per minute. Why is this complication characteristically delayed with neuraxial morphine?

• A Morphine is highly lipophilic and remains sequestered in spinal cord fat before releasing gradually
• B Morphine is hydrophilic, so it spreads rostrally in cerebrospinal fluid to reach brainstem respiratory centres slowly ✓
• C Epidural fat converts morphine to a long-acting ester prodrug
• D Morphine induces local inflammation that delays its vascular absorption

Explanation

Hydrophilic opioids like morphine dissolve poorly into epidural fat and instead remain in cerebrospinal fluid, where they migrate cephalad over hours to the medullary respiratory centres. Delayed respiratory depression up to 24 hours is therefore characteristic. Lipophilic agents such as fentanyl are taken up rapidly by spinal cord and vasculature, giving quick onset but short duration and less rostral spread.

Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.