Spinal sufentanil in rats: Part I: Epidural versus intrathecal sufentanil and morphine.

Male Wistar rats were injected epidurally or intrathecally with increasing doses of sufentanil or morphine in order to determine differences in potency, onset and duration of analgesia and supra-spinal side-effects. For sufentanil, only small differences in the lowest ED50-values for analgesia and supra-spinal side-effects were observed between the two spinal routes. Given intrathecally, sufentanil had a somewhat faster onset but a shorter duration of action than did epidural sufentanil. However, intrathecal morphine when compared to epidural morphine had a faster onset with a greater potency and a longer duration of action. The stronger opioid activity of intrathecal morphine was also reflected in a reduced safety ratio for the blockade of the cornea reflex. These differences between the two opioids, with regard to their optimal route of spinal administration, are discussed in terms of lipophilicity and optimal clinical use.