8.3 : Local Anesthetics: Pharmacokinetics

The potency and duration of action of local anesthetics (LAs) are determined by their pharmacokinetics. Pharmacokinetics describes how LAs are absorbed, distributed, metabolized, and eliminated from the body. When administered to the vascular tissues, LAs are quickly absorbed and enter the systemic circulation, reducing their localized effects. Adding vasoconstrictors such as epinephrine to LAs reduces their absorption into the systemic circulation, making them clinically effective. The vasoconstrictors localize the anesthetics at the desired site and enhance neuronal uptake to produce a more prolonged blockade. Vasoconstrictors also promote LA metabolism and elimination to reduce their systemic toxicity. Furthermore, vasoconstrictors reduce the total anesthetic requirement and extend anesthesia duration, making them suitable for lengthy surgical procedures and postoperative pain control.

However, the inclusion of vasoconstrictors can also have unwanted effects. It increases the probability of necrosis and edema in local tissues. It may reduce the local oxygen supply, which results in delayed wound healing. In susceptible individuals, LAs may increase blood pressure and cause arrhythmia.