Tachyphylaxis associated with repeated epidural injections of lidocaine is not related to changes in distribution or the rate of elimination from the epidural space.

The relationship between tachyphylaxis (measured as a decrease in the rate of regression of sensory levels of analgesia) during repeated epidural injections of lidocaine and both the distribution of lidocaine within the epidural space (as measured by spread of simultaneous injection of the tracer technetium-99m diethylenetriaminepentaacetate [99mTc-DTPA]) and elimination of lidocaine from the epidural space (as measured by serum concentrations of lidocaine) was investigated in 18 patients undergoing minor surgery during lumbar epidural analgesia. Twelve patients received four injections of 20 mL of 2% lidocaine at 2-hr intervals. Epidural distribution was assessed by injection of 99mTc-DTPA diluted in saline on the preoperative day and diluted in an equal volume of 2% lidocaine on the morning before surgery and again after the fourth injection of lidocaine 6 hr later. The distribution of 99mTc-DTPA in the epidural space was unchanged during the three measurements despite significant tachyphylaxis in both sensory analgesia and motor blockade (11 of 12 patients had sensory analgesia 2 hr after the first injection in contrast to only 3 of 12 patients during the third injection). In another six patients 20 mL of 2% lidocaine were injected three times at 2-hr intervals before surgery, with measurements of serum concentrations of lidocaine after the first and last injections. Despite tachyphylaxis (no patient had sensory analgesia 2 hr after the third injection), there was no difference in the rate of disappearance of lidocaine from the epidural space as assessed by plasma lidocaine concentration curves during the first and third injection (0.5 +/- 0.1 and 0.3 +/- 0.04 microgram.mL-1.min-1, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)