11: Opioids

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Last modified 06/02/2015

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CHAPTER 11 Opioids

Christopher L. Ciarallo, MD

1 What is an opiate? An opioid? A narcotic?

image Opiates are analgesic and sedative drugs that contain opium or an opium derivative from the poppy plant (Papaver somniferum). Opiates include opium, morphine, and codeine.
image An opioid is any substance with morphinelike activity that acts as an agonist or antagonist at an opioid receptor. Opioids may be exogenous or endogenous (such as the endorphins) and may be natural, derived, or completely synthetic.
image The term narcotic is not specific for opioids and refers to any substance with addictive potential that induces analgesia, euphoria, or altered sensorium.

2 What are endogenous opioids?

The endorphins, enkephalins, and dynorphins are the three classes of endogenous peptides that are derived from prohormones and are functionally active at opioid receptors. Although their physiologic roles are not completely understood, they appear to modulate nociception. Endorphins are not limited to the central nervous system and may even be expressed by activated leukocytes. A fourth class, the nociceptins, is currently being investigated.

3 Differentiate opioid tolerance, dependence, and abuse

Tolerance is a diminution in the physiologic effects of a substance resulting from repeated administration. Dependence may be physical or psychological and refers to the repeated use of a substance to avoid withdrawal symptoms. Tolerance may be necessary to establish the diagnosis of dependence. Abuse refers to the habitual use of a substance despite adverse consequences, including social and interpersonal problems.

4 Name the opioids commonly used in the perioperative setting, their trade names, equivalent morphine doses, half-lives, and chemical classes

See Table 11-1.

TABLE 11-1 Comparison of Commonly used Opioids

image

5 Describe the various opioid receptors and their effects

See Table 11-2.

TABLE 11-2 Opioid Receptor Subtypes

Opioid Receptor Subtype Agonists Agonist Response
Mu-1 (μ-1)
Enkephalin
β-endorphin
Phenanthrenes
Phenylpiperidines
Methadone

Supraspinal analgesia
Euphoria
Miosis
Urinary retention

Mu-2 (μ-2)
Enkephalin
β-endorphin
Phenanthrenes
Phenylpiperidines
Methadone
Spinal analgesia
Respiratory depression
Bradycardia
Constipation
Dependence
Kappa (κ)
Dynorphin
Butorphanol
Levorphanol
Nalbuphine
Oxycodone
Spinal analgesia (κ-1)
Supraspinal analgesia (κ-2)
Dysphoria
Sedation
Delta (δ)
Enkephalin
Deltorphin
Sufentanil
Spinal analgesia (δ-1)
Supraspinal analgesia (δ-2)
Respiratory depression
Urinary retention
Dependence
Sigma (σ)
(no longer classified as opioid receptor) Pentazocine
Dysphoria
Tachypnea
Hallucinations

Modified from Stoelting RK, Miller RD: Basics of Anesthesia, ed 4, New York, 2000, Churchill Livingstone, p 71.

6 What is an opioid agonist-antagonist?

Drugs such as pentazocine, butorphanol, buprenorphine, and nalbuphine were initially thought to be μ- and κ-receptor agonists. However, they are now classified as μ- and κ-receptor partial agonists. These drugs provide analgesia but with less euphoria and risk of dependence compared with pure agonists. Agonist-antagonists in general cause less respiratory depression than agonists and may reverse the respiratory depression and pruritus caused by pure agonists.

7 Explain the mechanism of action, duration, and side effects of the opioid antagonist naloxone

Naloxone is a μ-, κ-, and δ-receptor antagonist that will reverse the effects of agonist drugs. The peak effect occurs within 1 to 2 minutes of intravenous administration. The duration of action is between 30 and 60 minutes and may be shorter than the duration of the offending opioid agonist. Incremental doses of 0.5 to 1 mcg/kg should be used initially to reverse respiratory depression to minimize the side effects such as acute opioid withdrawal, severe hypertension, ventricular dysrhythmias, or pulmonary edema.

8 Describe the various routes of administration of opioids

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