Which property of morphine explains its greater risk of delayed respiratory depression after epidural administration?

Which property of morphine explains its greater risk of delayed respiratory depression after epidural administration?

• A. High lipid solubility
• B. Low lipid solubility (hydrophilic)
• C. Rapid clearance from CNS
• D. Strong protein binding

Answer: (B)

Hydrophilicity (low lipid solubility) explains the delayed respiratory depression after epidural morphine. Because morphine is relatively water-loving, it stays in the cerebrospinal fluid rather than crossing into brain tissue rapidly. In the CSF it can slowly diffuse upward to the brainstem, where respiratory centers reside, causing respiratory depression that can appear hours after the dose. Lipophilic opioids, by contrast, cross membranes quickly and are cleared from the CSF more rapidly, leading to quicker onset and a shorter window for delayed brainstem effects. While other properties can influence duration, the key reason for the delayed, possibly prolonged respiratory depression with epidural morphine is its hydrophilic nature and resultant rostral diffusion in the CSF.

Hydrophilicity (low lipid solubility) explains the delayed respiratory depression after epidural morphine. Because morphine is relatively water-loving, it stays in the cerebrospinal fluid rather than crossing into brain tissue rapidly. In the CSF it can slowly diffuse upward to the brainstem, where respiratory centers reside, causing respiratory depression that can appear hours after the dose. Lipophilic opioids, by contrast, cross membranes quickly and are cleared from the CSF more rapidly, leading to quicker onset and a shorter window for delayed brainstem effects. While other properties can influence duration, the key reason for the delayed, possibly prolonged respiratory depression with epidural morphine is its hydrophilic nature and resultant rostral diffusion in the CSF.