Just some fun facts to refresh our memory for NCE certification/recertification in case you need to take it soon.
- Morphine: 10mg IV = 1mg Epidural = 0.1mg Intrathecal (1/10 ratio; very hydrophilic)
- Hydromorphone: 1mg IV = 0.2mg Epidural = 0.04 Intrathecal (1/5 ratio; intermediate)
- Fentanyl: 100mcg IV = 33mcg Epidural = 6-10mcg Intrathecal (between 1/3 to 1/5 ratio; very lipophilic)
Intrathecally, hydrophilic opioids are more likely to bind specific receptors in the dorsal horn, and the intrathecal concentration will remain elevated for longer periods of time as compared to lipophilic opioids. This gradual effects are more likely to cause delayed respiratory depression via rostral spread. Conversely, hydrophobic opioids bind on the white matter of the spinal cord, and are more likely to be systemically absorbed. Therefore, the intrathecal spread of lipophilic opioids is less, and hence a smaller area of analgesia. Respiratory depression is likely due to short duration of action in intrathecal space.