In spinal anaesthesia for a Caesarean section, hyperbaric bupivacaine 0.5% 2.2 mL is injected at L3–4 with the patient sitting upright. The patient is immediately placed supine. What determines the final spread of the block?

• A The protein-binding affinity of bupivacaine for spinal cord tissue
• B Total dose of bupivacaine injected, independent of baricity or position
• C Rate of injection determining turbulence and mixing within the CSF column
• D Baricity of the solution relative to CSF and patient position during the fixation period ✓

Explanation

Spread of spinal anaesthesia is primarily governed by baricity (specific gravity relative to CSF) and patient position during the 10–15 minute fixation window before the drug is absorbed into neural tissue. Hyperbaric solutions (heavier than CSF) settle to dependent areas; with the patient supine, the lumbar lordosis creates a 'valley' at L3–4 and a 'peak' at T5–6 that can limit thoracic spread, while Trendelenburg position would increase thoracic spread. After fixation, patient position no longer influences the block. Total dose influences maximum height of block but cannot override the baricity-position interaction during the fixation period.

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

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Written and medically reviewed by the StethoPrep medical team.