During spinal anaesthesia a patient develops sudden hypotension after intravenous morphine is given for shivering control. Compared with fentanyl, morphine is more likely to cause this haemodynamic change because morphine:

• A Directly depresses myocardial contractility at therapeutic doses
• B Induces histamine release from mast cells causing vasodilation ✓
• C Blocks alpha-adrenergic receptors on vascular smooth muscle
• D Increases vagal outflow more strongly than other opioids

Explanation

Morphine releases histamine from mast cells, producing venodilation, arteriolar dilation and a fall in blood pressure, occasionally with bronchospasm. Fentanyl and its congeners cause little or no histamine release and are therefore haemodynamically more stable. Opioids at therapeutic doses do not significantly depress contractility, they do not block adrenergic receptors, and enhanced vagal tone contributes to bradycardia but does not explain the histamine-mediated vasodilation seen characteristically with morphine.

Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.