Local anaesthetics block voltage-gated sodium channels from inside nerve axons and prevent conduction along the nerve.

Individual nerve fibres are lipid membranes and the unionized form of the local anaesthetic must cross this lipid membrane to reach the inside. The more lipid soluble the agent, the more potent it is (Fig 1).

Question: Which is more potent, lidocaine or bupivacaine?

Answer

Local anaesthetics block voltage-gated sodium channels from inside nerve axons and prevent conduction along the nerve.

Individual nerve fibres are lipid membranes and the unionized form of the local anaesthetic must cross this lipid membrane to reach the inside. The more lipid soluble the agent, the more potent it is (Fig 1).

Question: Which is more potent, lidocaine or bupivacaine?

Answer: Bupivacaine: it is approximately four times more potent than lidocaine.

Question: Does this fit with their relative lipid solubilities?

Answer

Local anaesthetics block voltage-gated sodium channels from inside nerve axons and prevent conduction along the nerve.

Individual nerve fibres are lipid membranes and the unionized form of the local anaesthetic must cross this lipid membrane to reach the inside. The more lipid soluble the agent, the more potent it is (Fig 1).

Question: Which is more potent, lidocaine or bupivacaine?

Answer: Bupivacaine: it is approximately four times more potent than lidocaine.

Question: Does this fit with their relative lipid solubilities?

Answer: Yes, in terms of the order of potency. However, bupivacaine is more lipid soluble than lidocaine by a factor of about nine; other factors also affect potency.