Opioids act via G protein coupled opioid receptors. Several types of receptor have been characterised, but the μ opioid receptor is the most important in opioid-induced analgesia. Opioid receptors are located in synapses in areas of the brain, in the spinal cord and in the periphery intimately associated with pain processing [18]. Opioid receptors are found in many areas of the brain including the periaqueductal grey and locus coeruleus which are involved in the higher processing of pain perception, emotional response and localisation. Elements of the so-called pain matrix are here and the area’s importance in pain has been shown directly by functional imaging [19].

In the spinal cord opioid receptors are localised in the superficial dorsal horn, which modulates transmitter release and pain impulses, resulting in a spinally mediated analgesia. Release of the major excitatory transmitter glutamate at the spinal synapse of the primary afferent nociceptor is reduced by pre- and postsynaptic opioid mechanisms. Other transmitters are also modulated by opioid action such as substance P and calcitonin gene-related peptide [20].

Many different opioids are available for use in clinical practice such as opioids for moderate to severe pain including morphine, fentanyl and oxycodone. The opioids for mild to moderate pain (codeine, tramadol) may be used for less major palliative surgery. Choice of opioid will depend on local availability, local guideline or policy and patient tolerance to particular opioids (drug allergy or sensitivity to opioid side effects). The most common side effects are potential respiratory depression, sedation, nausea and vomiting and pruritus [21]. If a patient is on an effective dose of opioid preoperatively with a good efficacy/side effect balance, it is judicious to use the same opioid perioperatively.

Opioids can be delivered by many routes for acute pain management. The most common routes of opioid administration are intravenous, epidural or intrathecal, subcutaneous, intramuscular or oral. They can also be administered via a transdermal patch, sublingual or buccal mucosa and intranasal or rectal routes although these are more commonly used for chronic pain states. Intravenous patient-controlled analgesia (PCA) is the most frequently used method to deliver intravenous opioids postoperatively.

Intravenous opioids delivered using a patient-controlled analgesia (PCA) infusion device provide the most flexible form of opioid analgesia in the postoperative period [22]. Patients have been shown to prefer PCA compared to other routes of postoperative opioid administration [23]. PCA delivers a preset bolus dose on demand with a specified lockout period between each successful dose, and a background infusion can also be administered. These variables can be adjusted according to the patients’ opioid requirements. For patients on opioid therapy prior to surgery, PCA allows higher-than-normal bolus doses to be delivered and a background infusion rate equivalent to the patient’s baseline opioid requirement.

Intravenous opioid PCA provides better analgesia than conventional intramuscular and subcutaneous opioid regimens, although the magnitude of the difference in analgesia is small [24]. Although PCA administration was associated with greater opioid use, there are no differences in duration of hospital stay or opioid-related adverse effects other than pruritus, which is increased, and patient satisfaction is higher [24].

As previously discussed, for patients on preoperative opioids, there is a need to maintain a baseline level of opioid to ensure that an acute withdrawal syndrome is avoided and to ensure that pain that may be present, independent of surgery, is well managed. There are several approaches that can be taken to maintain the baseline opioid levels. As discussed previously, a background infusion can be added to a PCA and the bolus dose titrated to effect. In some cases, where the patient has a fentanyl patch, this could be left on (as long as all practitioners are aware) to maintain background requirements and PCA bolus added. Theoretically perioperative alterations in skin blood flow may result in erratic absorption from the fentanyl patch and some advocate removal of all transcutaneous medication. It should be remembered that absorption may continue after fentanyl patch removal [25]. In more minor palliative surgery, oral medications may be able to be re-established soon after surgery.