Lidocaine hydrochloride absorption from a subcutaneous site

Subcutaneous disappearance of lidocaine hydrochloride was followed as a function of time using a specially designed “closed” subcutaneous absorption cell affixed to anesthetized rats. Unbuffered, stirred lidocaine hydrochloride solutions in cells open to the atmosphere were previously shown to increase in pH with time because of carbon dioxide loss. The closed cell was designed to prevent this loss, but pH shifts still occurred, making the derivation of a simple pharmacokinetic absorption model impossible. Because the pH of the solution shifted to higher pH values, the data suggest that precipitation of lidocaine base may have occurred in some experiments.