A parturient receives 0.1 mg of morphine intrathecally as part of a combined spinal-epidural for caesarean delivery. She has been well overnight and is being prepared for discharge the next morning when she is noted to have a respiratory rate of 8 per minute with sedation. What feature of intrathecal morphine best explains this event?
- A Its hydrophilicity permits slow cephalad spread in cerebrospinal fluid with delayed respiratory depression up to 24 hours ✓
- B Its lipophilicity allows rapid brainstem uptake within the first hour
- C Its active metabolite accumulates progressively over several days
- D Its effect depends entirely on systemic absorption from the epidural space
Correct answer: A. Its hydrophilicity permits slow cephalad spread in cerebrospinal fluid with delayed respiratory depression up to 24 hours
Explanation
Morphine is hydrophilic, so it lingers in cerebrospinal fluid and spreads slowly upward to reach the medullary chemoreceptor areas in the fourth ventricle. Respiratory depression after intrathecal morphine is therefore delayed and biphasic, with a late nadir possible 6 to 24 hours after injection, mandating prolonged monitoring. Lipophilic opioids such as fentanyl act quickly but carry far less risk of late depression. Naloxone reverses the event while preserving analgesia if infused carefully.
Reference: Miller's Anesthesia, 9th ed.
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