Ropivacaine has greater cardiosafety compared to bupivacaine for regional anaesthesia. Which pharmacological property explains this differential toxicity?

• A Ropivacaine is an ester; bupivacaine is an amide, with different hydrolysis rates
• B Ropivacaine has a higher protein binding, reducing free drug levels
• C Ropivacaine does not cause vasoconstriction, allowing rapid redistribution
• D Ropivacaine is the S(-) enantiomer with lower potency at cardiac sodium channels and faster dissociation (kinetics of unbinding), less lipophilic than bupivacaine ✓

Explanation

Ropivacaine is formulated as a pure S(-) enantiomer (levoisomer), whereas bupivacaine is a racemic mixture. The S(-) enantiomer has lower affinity for cardiac sodium channels and faster dissociation from them (fast-in, fast-out kinetics compared to bupivacaine's slow-in, slow-out cardiac sodium channel binding). Ropivacaine is also less lipophilic than bupivacaine, reducing cardiac and CNS penetration. Additionally, ropivacaine has intrinsic vasoconstrictive properties (unlike bupivacaine which is vasodilatory at low concentrations). Levobupivacaine (pure S-enantiomer of bupivacaine) also has improved cardiac safety compared to racemic bupivacaine.

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

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