Studies on the peripheral action of opioids in postoperative pain in man

Using simultaneous measurements of pain in both limbs after bilateral orthopaedic surgery, peripheral and central components of regionally administered drugs can be separated. Perineural local anesthetic action is readily shown in both upper and lower limbs. Perineural morphine had no effect in ankle blocks, although an effect superior to that of local anesthetics has been claimed in man. Similarly, no effect was seen from morphine or buprenorphine given by intravenous regional administration in the upper limbs, although opioids have shown local antinociceptive action in animal models. There is little evidence to suggest that the usual parenteral opioids have any significant peripheral action clinically, but further human studies are needed because of the theoretical importance of even a small peripheral component.