Esters
The kidneys are responsible for excreting ester local anesthetics and their metabolites. However, because esters are hydrolyzed so rapidly in the plasma, only small amounts of the parent drug are excreted unchanged in the urine. For instance, only about 2% of a dose of procaine is excreted unchanged, with the majority being eliminated as PABA (about 90%). Cocaine, an ester with a different metabolism, has approximately 10% of its dose excreted unchanged.
Patients with renal impairment may have difficulty eliminating the metabolites of ester local anesthetics, which could lead to slightly elevated blood levels and an increased risk of toxicity. However, due to the rapid metabolism of esters, this risk is generally low except in the case of significant renal disease (ASA 4 to 5), where caution is advised.
Amides
Amide local anesthetics are also excreted by the kidneys, but a larger percentage of the parent drug is found unchanged in the urine compared to esters, due to their slower and more complex metabolism. For example, less than 3% of lidocaine and 1% of mepivacaine and etidocaine are excreted unchanged.
Patients with significant renal impairment may be at risk for toxicity due to the accumulation of the parent drug or its active metabolites. This risk is particularly relevant in patients with chronic kidney disease or those undergoing renal dialysis. Amide local anesthetics should be used with caution in these patients, with careful monitoring of drug levels and adjustments in dosing as needed.
