Factors that determine the rate of absorption of a local anesthetic were studied in canine and, when possible, in human subjects. The drugs used were tetracaine, cocaine, procaine, and benzocaine. Objective data was obtained in the form of actual concentrations of each drug in the blood, and the concentrations obtained by intravenous injection were compared with those obtained by infusion, infiltration, and topical application under various conditions. The concentrations reached in the blood after intravenous injection were much higher when infusion was rapid than when it was slow. Absorption from the mucous membranes of the pharynx and trachea gave blood concentrations comparable to those attained by intravenous injection. Absorption of detectable amounts occurred from the surface of the abraded skin and skin after third degree burns but not after first or second degree burns. Since it must be assumed that the danger of undesired systemic reactions is greater in proportion to the concentrations reached by the drug in the blood, anyone using local anesthetics should be well informed as to the factors that may hasten absorption.
Campbell D, Adriani J. ABSORPTION OF LOCAL ANESTHETICS. JAMA. 1958;168(7):873–877. doi:10.1001/jama.1958.03000070029006
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