Development of a system for studying subcutaneous absorption kinetics of local anesthetic amines.

In the search for optimum conditions (pH 7.40, isotonic solution) for the study of subcutaneous absorption of local anesthetics using the system of Ballard and Menczel, unexpectedly wide pH variations (6.26-8.40) were consistently observed. All isotonic buffer systems examined failed to maintain a constant pH in the absorption cell for the desired duration (5-7 hr.). It was demonstrated that subcutaneous tissue liberates and absorbs ions (Na+, K+, Cl−, and HCO3−), apparently in an attempt to produce an ionic composition in the solution similar to that of interstitial extracellular fluid. Additional experiments indicate that the high pH (above 7.40) results from the loss of carbon dioxide from the cell and that the liberation of bicarbonate ion by the tissue is time dependent. Constancy of pH was achieved using a slightly hypertonic tromethamine buffer (pH 7.95 ± 0.05, buffer capacity = 0.225).