Which opioid has poor lipid solubility, leading to delayed onset of neuraxial analgesia?

Which opioid has poor lipid solubility, leading to delayed onset of neuraxial analgesia?

• A. Fentanyl
• B. Remifentanil
• C. Morphine
• D. Sufentanil

Answer: (C)

Lipid solubility dictates how quickly an opioid reaches its receptors after neuraxial administration. Morphine is relatively hydrophilic (poor lipid solubility), so it stays in the cerebrospinal fluid and diffuses slowly into spinal tissue, leading to a delayed onset of analgesia. In contrast, highly lipophilic opioids like fentanyl and sufentanil rapidly cross membranes to produce quick onset, and remifentanil is also rapidly acting with a short duration due to fast metabolism. The hydrophilic nature of morphine also means a longer duration of action and greater potential for cephalad spread within the CSF.

Lipid solubility dictates how quickly an opioid reaches its receptors after neuraxial administration. Morphine is relatively hydrophilic (poor lipid solubility), so it stays in the cerebrospinal fluid and diffuses slowly into spinal tissue, leading to a delayed onset of analgesia. In contrast, highly lipophilic opioids like fentanyl and sufentanil rapidly cross membranes to produce quick onset, and remifentanil is also rapidly acting with a short duration due to fast metabolism. The hydrophilic nature of morphine also means a longer duration of action and greater potential for cephalad spread within the CSF.