Regardless of how expertly surgical and anesthetic procedures are performed, appropriate prescription of analgesic drugs, especially opioids and cyclooxygenase (COX) inhibitors, can make the difference between a satisfied and an unsatisfied postoperative patient. Studies have shown that outcomes can be improved when analgesia is provided in a “multimodal” format (typically emphasizing COX inhibitors and local anesthetic techniques while minimizing opioid use) as one part of a well-defined and well-organized plan for postoperative care (see Chapter 48).
Opioids bind to specific receptors located throughout the central nervous system and other tissues. Four major opioid receptor types have been identified (Table 10-1): mu (μ, with subtypes μ1 and μ2), kappa (κ), delta (δ), and sigma (σ). All opioid receptors couple to G proteins; binding of an agonist to an opioid receptor causes membrane hyperpolarization. Acute opioid effects are mediated by inhibition of adenylyl cyclase (reductions in intracellular cyclic adenosine monophosphate concentrations) and activation of phospholipase C. Opioids inhibit voltage-gated calcium channels and activate inwardly rectifying potassium channels. Opioid effects vary based on the duration of exposure, and opioid tolerance leads to changes in opioid responses.
Table 10-1 Classification of Opioid Receptors.1 |Favorite Table|Download (.pdf)
Table 10-1 Classification of Opioid Receptors.1
- Supraspinal analgesia (μ1)
- Respiratory depression (μ2)
- Physical dependence
- Muscle rigidity
- Respiratory stimulation