SN6 SHORT NOTE 5M

Describe Neuraxial Opioids in detail.

General Surgery · Final Professional

Rationale

Opioids are commonly added to local anaesthetic in neuraxial (spinal/epidural) blocks as adjuvants. They act on opioid receptors in the dorsal horn of the spinal cord to enhance and prolong analgesia, allowing a lower dose of local anaesthetic (less motor block and hypotension) — valuable in labour and postoperative analgesia.

Effects & Side-effects

A lipophilic opioid (fentanyl) acts quickly and briefly; a hydrophilic opioid (morphine) spreads in the CSF to give prolonged analgesia but with a risk of delayed respiratory depression. Side-effects of neuraxial opioids include pruritus (itching), nausea/vomiting, urinary retention and, importantly, respiratory depression (which can be early with lipophilic and delayed with hydrophilic agents — requiring monitoring).

DANGER / REMEMBER

Neuraxial opioids — especially hydrophilic ones (morphine) — can cause delayed respiratory depression hours after injection, so appropriate monitoring is needed. Pruritus, nausea and urinary retention are common.

CLINICAL PEARL

Adding an opioid to a neuraxial block improves analgesia and spares local anaesthetic (less motor block/hypotension); watch for itch, nausea, retention and — with morphine — delayed respiratory depression.

Choice of Opioid

The choice between a lipophilic and a hydrophilic opioid shapes both the benefit and the risk: a lipophilic agent such as fentanyl is taken up quickly into the cord and acts within minutes but for a relatively short time and with mainly early respiratory effects, whereas a hydrophilic agent such as morphine remains in the CSF and spreads rostrally, giving prolonged analgesia but carrying the risk of delayed respiratory depression some hours later. This difference dictates the monitoring: a patient given intrathecal morphine needs observation for respiratory depression well into the postoperative period, not merely in the first hour.

CLINICAL PEARL

Adding an opioid improves analgesia and spares local anaesthetic, but the price is itch, nausea, retention and — with hydrophilic morphine — delayed respiratory depression demanding prolonged monitoring.

Delayed respiratory depression is the risk with morphine.

Property	Lipophilic (fentanyl)	Hydrophilic (morphine)
Onset	Rapid (5–10 min)	Slow (30–60 min)
Duration	Short (2–4 h)	Long (12–24 h)
Spread in CSF	Segmental, limited	Extensive, rostral
Respiratory depression	Early	Delayed — up to 24 h

KEY POINT

Key points TO remember

Neuraxial opioids act on spinal dorsal-horn receptors to enhance/prolong analgesia & spare LA.
Fentanyl (lipophilic): fast, short; morphine (hydrophilic): prolonged but delayed respiratory depression.
Side-effects: pruritus, nausea/vomiting, urinary retention, respiratory depression.
Monitor for respiratory depression (early with lipophilic, delayed with hydrophilic).

EXAM TIP

Sources: Morgan & Mikhail’s Clinical Anesthesiology; Miller’s Anesthesia; Ajay Yadav’s Short Textbook of Anaesthesia.

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