MORPHINE: OTHER ROUTES OF ADMINISTRATION-PHARMACOLOGY
Morphine injected into the subarachnoid space, or diffusing into the subarachnoid space after epidural injection, will diffuse into the dorsal horn of the spinal cord and produce analgesia by interaction with opioid receptors. The theoretic advantages of intraspinal morphine include:• the use of much smaller doses of morphine• delivered close to the site of action (the spinal opioid receptors)• less central side effects• less systemic side effects• no neurological deficits as occur with neurosurgery or anaesthetic blocks However, after epidural injection only 10-20% of the morphine diffuses into thesubarachnoid space and there is significant systemic absorption from the epidural space. Plasma levels may be equivalent to those following systemic administration, accounting for the central and systemic side effects. At the same time, the drug concentration in the CSF is much higher than after systemic administration, producing superior analgesia. There will be some rostral diffusion of morphine injected or diffusing into the subarachnoid space, predisposing to central side effects. *49\55\2*
