A 48-year-old man with a body mass index (BMI) of 33 (obesity) and history of obstructive sleep apnea presents to the emergency department with severe back pain following a fall from a ladder. He complains of severe pain without loss of consciousness or focal neurologic deficits. What is the most appropriate immediate treatment for his pain? Are any special precautions needed?
Morphine, the prototypic opioid agonist, has been used throughout history to relieve acute severe pain with remarkable efficacy.* The opium poppy is the source of crude opium from which Sertürner in 1803 isolated morphine, the pure alkaloid, naming it after Morpheus, the Greek god of dreams. It remains the standard against which all drugs that have strong analgesic action are compared. These drugs are collectively known as opioids and include not only the natural and semisynthetic alkaloid derivatives from opium but also synthetic surrogates, other opioid-like drugs whose actions are blocked by the nonselective antagonist naloxone, plus several endogenous peptides that interact with the different subtypes of opioid receptors.
BASIC PHARMACOLOGY OF THE OPIOIDS
Opium, the source of morphine, is obtained from the poppy, Papaver somniferum and P album. After incision, the poppy seed pod exudes a white substance that turns into a brown gum that is crude opium. Opium contains many alkaloids, the principal one being morphine, which is present in a concentration of about 10%. Codeine can also be found in opium and is synthesized commercially from morphine.
Classification & Chemistry
The term opioid describes all compounds that work at opioid receptors. The term opiate specifically describes the naturally occurring alkaloids: morphine, codeine, thebaine, and papaverine. In contrast, narcotic was originally used to describe sleep-inducing medications, but in the United States, its usage has shifted into a legal term.
Opioid drugs include full agonists, partial agonists, and antagonists–measures of intrinsic activity or efficacy. Morphine is a full agonist at the µ (mu)-opioid receptor, the major analgesic opioid receptor (Table 31–1). Opioids may also differ in receptor-binding affinity. For example, morphine exhibits a greater binding affinity at the μ-opioid receptor than does codeine. Other opioid receptor subtypes include ẟ (delta) and κ (kappa) nociception/opioid-receptor-like subtype 1 (ORL-1) receptors. Simple substitution of an allyl group on the nitrogen of the full agonist morphine plus addition of a single hydroxyl group results in naloxone, a strong μ-receptor antagonist. The structures of some of these compounds are shown later in this chapter. Some opioids, eg, nalbuphine, a mixed agonist-antagonist, are capable of producing an agonist (or partial agonist) effect at one opioid receptor subtype and an antagonist effect at another. The receptor-activating properties and affinities of opioid analgesics can be manipulated ...