Epidural opioid analgesia
Epidural opioid analgesia is a technique to reduce pain from labour. Opioids act by interacting with specific receptors in the dorsal horn and dorsal roots of the spinal cord. Most often, it is given in combination with bupivacaine as opioids alone are not able to provide adequate pain-relief. In combination with local anaesthetics, pain relief is rapid, shivering is decreased and motor blockade is more dense.The common side effects are urinary retention and pruritus.
Mechanism of action
Pain is transmitted in the dorsal horn of spinal cord via C fiber neurons. At the pre-synapse of these neurons, neuropeptides are released. Tachykinin, a neuropeptide, binds to the post-synaptic neurokinin receptors and cause depolarization and changes in second messengers. In the spinal cord, opioids act by reducing the release of neuropeptides at presynaptic level and by hyperpolarizing the membrane of dorsal horn neurons at the post-synaptic level.
Properties of opioids in epidural space
Epidural opioids can produce analgesia without motor or sympathetic blockade. Lipid insoluble opioids stay in the cerebrospinal fluid for a long time and hence give a longer supply of analgesia. However, in this case, the onset of analgesia is delayed. The lipophilicity of an opioid is determined by its octanol-buffer partition coefficient. The concentration of an opioid in the spinal cord is determined by the net difference between its rate of uptake and distribution in the vascular and subarachinoid spaces.