In a spinal anaesthetic for a lower limb procedure, heavy bupivacaine 0.5% 3 mL is injected at L3-L4 with the patient in the right lateral decubitus position. The baricity of the solution and patient positioning determine spread. If the patient is kept lateral with right side down for 10 minutes, the block will be predominantly:

• A Bilateral due to rapid CSF diffusion overcoming baricity
• B Left-sided (non-dependent side) due to barometric pressure differences
• C Sacral distribution due to lumbar lordosis directing heavy solution caudally
• D Right-sided (dependent side) as heavy solution settles by gravity ✓

Explanation

Hyperbaric (heavy) bupivacaine has a specific gravity (1.026) greater than CSF (1.003–1.008) and therefore moves under gravity toward the dependent (lowest) portion of the subarachnoid space. With the right side down, the heavy solution pools on the right side, producing a predominantly right-sided unilateral spinal block. This technique is particularly useful for unilateral lower-extremity or perineal procedures, reducing the extent of sympathetic block and cardiovascular instability compared to bilateral blocks. The block becomes fixed after approximately 15–20 minutes.

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

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Written and medically reviewed by the StethoPrep medical team.