Fig. 1. Comparison of the time course and percent inhibition of the cutaneous trunci muscle reflex, expressed as the graded value of maximum possible effect (MPE) after subcutaneous injections (at time = 0 h) of 0.05% (A ), 0.25% (B ), and 0.5% (C ) concentrations of amitriptyline or bupivacaine with and without epinephrine (n = 8 in all groups compared). In (A ), 0.05% amitriptyline's sustained partial analgesia (approximately 40–45% MPE) for a prolonged duration is markedly enhanced by epinephrine. (B ) Epinephrine retards the rate of block regression of amitriptyline (50% MPE reached at 12.5 h without epinephrine and at 17.5 h with epinephrine) but does not significantly (P > 0.05) change the duration until complete recovery of nociception. (C ) Differences between the time course of amitriptyline and bupivacaine remain statistically significant but become smaller (compared with the lower concentrations in A and B ) as the concentration of the drugs increases. (D ) The comparitive inhibition of the cutaneous trunci muscle reflex after subcutaneous injections (at time = 0 h) of the combination of amitriptyline, bupivacaine at varying concentrations, and epinephrine (n = 6 in all groups compared). The drugs potentiate their analgesic effects as the time courses of the complete nociceptive blockade and the duration until full recovery of nociception are significantly (P < 0.05) longer than the time courses of the equivalent total amount (i. e. , compared with the total amount of drugs in the combination) of either amitriptyline alone with epinephrine or bupivacaine alone with epinephrine (C ). EPI = epinephrine.