In comparing hyperbaric versus isobaric bupivacaine for spinal anaesthesia in a patient positioned in the lateral decubitus position, which statement is MOST accurate?

• A Isobaric bupivacaine spreads symmetrically regardless of patient position
• B Hyperbaric bupivacaine spreads to non-dependent (upper) side in lateral position
• C Both preparations have identical spread because CSF mixing overrides baricity within 2 minutes
• D Hyperbaric solution spreads preferentially to the dependent (lower) side due to denser-than-CSF property ✓

Explanation

Hyperbaric bupivacaine has a density greater than CSF (by addition of glucose 80 mg/mL), causing it to sink to the dependent (gravitationally lower) side when injected with the patient in the lateral decubitus position. This allows selective unilateral spinal anaesthesia for procedures on the lower limb. Isobaric bupivacaine (density ≈ CSF) spreads more by barometric pressure and patient positioning is less predictive; spread tends to be somewhat random and positional effects less reliable. Baricity differences persist well beyond 2 minutes in clinical practice.

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

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Written and medically reviewed by the StethoPrep medical team.