Local Anesthetics: Pharmacokinetics

Pharmacokinetics describes how rapidly local anesthetics or LAs are absorbed, distributed, metabolized, and eliminated from the body, which affects their potency and duration of action.

When LAs are administered in vascular tissues, they escape into the systemic circulation leading to unwanted effects. So, LAs are often administered with vasoconstrictors that reduce the local blood flow and limit the anesthesia to the target area.

The distribution of LAs depends on the type of target tissue. For example, if an LA injected into the subarachnoid space has the same density as the cerebrospinal fluid, it will remain static, allowing localized anesthesia as long as the patient remains upright.

The effects of LA are reversed when ester-linked LAs like procaine are metabolized by serum cholinesterases, while most amide-linked LAs are metabolized in the liver by cytochrome P450 enzymes.

The resulting metabolites are water-soluble and can be eliminated by the kidneys.