Chapter 159. Opioids

Naum Shaparin, MD

Bind to opioid receptors; specifically, analgesia takes place at the mu receptor.

Indications: moderate to severe pain.

Opioid receptor–specific agonists (see below for meperidine, methadone, and tramadol):

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System

Effects

Cardiovascular

HR and BP decrease usually due to decreased sympathetic tone with pain relief
Dose-dependent increase in vagal tone may cause bradycardia
Histamine release from morphine may cause hypotension and resultant tachycardia
Minimal effect on contractility
May enhance myocardial depressant effects of other medications

Respiratory

Decrease in RR due to decreased response to CO2
Increased tidal volume
Decreased minute ventilation
Suppression of cough reflex
Muscle rigidity with large IV boluses that can make ventilation difficult without paralysis (may be prevented with benzodiazepine premedication)

Ophthamological

Miosis via stimulation of the Edinger–Westphal nucleus

Gastrointestinal

Nausea/vomiting from direct stimulation of chemoreceptor trigger zone (usually happens at initiation of therapy or with dose changes)
Gastric stasis (may also be cause of nausea/vomiting)
Constipation; most patients never become tolerant to constipation (thus requiring laxatives and stool softeners)
Spasm of sphincter of Oddi

Endocrine

May block stress response at high doses
Decrease testosterone levels with long-term use at high doses (to the point that supplementation is needed to treat sexual dysfunction)

Obstetrical/neonatal

Neonatal CNS and respiratory depression (cross placenta)

Genitourinary

Urinary retention due to increased ureter tone (can be reversed with atropine)

Central nervous system

Analgesia
Sedation
Euphoria, dysphoria, or agitation
Unreliable amnesia (at high doses)
Decreased CBF and metabolic rate
Increased ICP (if opioid-induced hypoventilation is uncontrolled)
Withdrawal symptoms may occur with abrupt discontinuation
Addiction

Skin

Histamine release (from morphine and meperidine)
May cause pruritus, redness, and hives at injection site

Meperidine (in addition to those listed above for opioid receptor–specific agonists):

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System

Effects

Cardiovascular

Myocardial depressant
May cause increased HR and contractility due to anticholinergic effect
Histamine release may cause hypotension and resultant tachycardia

Gastrointestinal

Sphincter of Oddi spasm may be decreased compared with other opioids

Central nervous system

Seizures may result from accumulation of the metabolite normeperidine

Skin

Histamine release
May cause pruritus, redness, and hives at injection site

Methadone (in addition to those listed above for opioid receptor–specific agonists):

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System

Effects

Cardiovascular

QT prolongation usually above 100 mg daily dose
Review EKG if increasing dose over 100 mg per day

Respiratory

Unexpected and delayed respiratory depression due to prolonged biphasic elimination
Especially with rapid dose increases causing accumulation

Central nervous system

Unexpected and delayed sedation due to prolonged biphasic elimination (especially with rapid dose increases causing accumulation)
May have a role in the treatment of neuropathic pain due to NMDA receptor antagonist activity
Thought to cause less tolerance than other opioids
Useful in opioid maintenance therapy and detoxification due to long duration of action

Tramadol...

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