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The mechanism of the prolonged effect of dextran on the duration of local anaesthesia has been studied. Using radio-active mepivacaine it was found that dextran prolonged the duration of infiltration anaesthesia in guinea-pigs by delaying the absorption of the local anaesthetic agent. Experiments in vitro indicate that molecular complexes between the local anaesthetic and dextran may be formed and it is assumed that the delayed absorption might be due to the formation of such molecular complexes. This hypothesis was strengthened by experiments in which dental infiltration anaesthesias were performed in healthy volunteers.

Attempts have been made to correlate the duration of the local anesthetic effect with the amount of the local anesthetic compound remaining at the injection site at different times after an injection. In addition to delaying the absorption of the local anesthetic, adrenaline but not a vasopressin-derivate decreased the minimal anesthetic concentration of the local anesthetic agent. It is assumed that the pH value of the tissues at the injection site was kept down through the metabolic effects of adrenaline, thereby transforming the local anesthetic agent into cations, which is the form in which the amide-type of local anesthetics mainly exerts their nerve blocking activity.

In animals and humans the rate of resorption and the duration of the anesthesia have been investigated, using mepivacaine in different concentrations and volumes without addition of a vasoconstrictor. A linear relationship between duration of anesthesia and the logarithm to the concentration of mepivacaine was found.