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Chapter 9. Local Anesthetic Mixtures for Peripheral Nerve Blocks

In general, local anesthetics (LAs) with the following property tend to have shorter latency of action:

A. High lipid solubility

B. Low pKa

C. High ratio of ionized state

D. Large molecular weight

B is correct. Low pKa. LAs with lower pKa tend to have faster onset of action. Because local anesthetics are weak bases, a lower pKa will favor the nonionized form at physiologic pH. The more nonionized drug present at the lipid membrane, the more the drug can permeate the sodium channel of the nerve.

A is incorrect. Highly lipid soluble LAs tend to have slower latency (eg, bupivacaine). An exception to the rule is etidocaine.

C is incorrect. The greater the ratio of ionized state of drug, the more difficult it is to cross lipid membranes and inhibit the sodium channel of the nerve axons.

D is incorrect. While molecular weight inversely affects the aqueous diffusion rate and thus onset of action, most LAs are similar in size.

Which property of local anesthetics (LAs) most significantly prolongs duration of action?

A. Protein binding

B. High lipid solubility

C. High molecular weight

D. Greater vascularity at site of injection

B is correct. High lipid solubility. Block duration is mostly determined by the extent that the LA remains in the vicinity of the nerve. High lipid solubility allows the drug to remain in or near lipid membranes rather than be absorbed to continue its action on the membrane sodium channels.

A is incorrect. Degree of protein binding has little clinically relevant effect on block duration. This is due to the fact that dissociation times of LAs from sodium channels are measured in seconds.

C is incorrect. Molecular weight does not factor into duration of action.

D is incorrect. Greater vascularity of block site increases systemic absorption of the LA and shortens its duration. Decreasing vascularity or vascular uptake through the addition of vasoconstrictors such as epinephrine will prolong the drug’s presence at the site and increase its duration.

Previous studies of landmark-based techniques of peripheral nerve blocks (PNBs) examining mixtures of local anesthetics (LAs) have shown the following:

A. Mixing of LAs was shown to be ideal with the benefits of faster onset ...

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