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CSF Volume Block Height Determinant — FRCA Final MCQ

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HardRegional AnaesthesiaCSF Volume Block Height DeterminantFRCA Final

Which factor most influences spinal block height?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: C — Volume of lumbar CSF

Lumbosacral CSF volume is the single most important determinant (varies 2-3 fold, 28-81 mL on MRI). Patients with reduced CSF (obesity, pregnancy, raised IAP) achieve higher blocks with identical doses. Baricity determines gravitational distribution but CSF volume determines effective drug concentration.

Reference: BJA Education – 2022 – Determinants of spinal block height