A 55-year-old man receives a spinal anaesthetic with 0.5% hyperbaric bupivacaine 2.5 mL at L3–L4 for a right total knee replacement. Ten minutes later his blood pressure drops from 130/80 to 80/50 mmHg and he feels nauseated. His block level is T6. What is the PRIMARY mechanism of this hypotension?

• A Blockade of cardiac accelerator fibres from T1–T4
• B Direct myocardial depression by absorbed bupivacaine
• C Bradycardia from parasympathetic dominance
• D Vasodilation from sympathetic blockade extending to thoracic dermatomes ✓

Explanation

Spinal anaesthesia blocks sympathetic preganglionic fibres, causing arteriolar and venous dilation, which reduces systemic vascular resistance and venous return. With a T6 block level, the splanchnic sympathetics (T5–L1) are blocked, producing profound vasodilation and the primary cause of hypotension. Cardiac accelerator fibre blockade (T1–T4) contributes bradycardia but is not the primary driver of hypotension at T6. Direct bupivacaine myocardial depression requires systemic absorption that does not occur with standard spinal doses.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.