Duration of cuteneous analgesia induced by bupivacaine and ketamine in female Wistar rats; role of epinephrine

This study was designed to investigate the cumulative effect of bupivacaine and ketamine on rats' cutaneous trunci muscle reflex (CTMR) and the effect of epinephrine on the duration of analgesia. Fifteen rats were randomly selected into three groups of five rats each. Each rat served as its own control with the right and left thoracolumbar regions being the experimental and control sites respectively. Rats in the first group (group A) were administered Bupivacaine alone, Group B rats were given Ketamine alone while Group C rats were infiltrated with a combination of Bupivacaine and Ketamine. The experiment was repeated seven days after with the addition of epinephrine to all the 3 categories of infiltrate. 0.6 ml total volume of drugs in each group was injected subcutaneously following which CTMR was stimulated via forceps bite on the injection and control sites alternately. The duration of induced analgesia (period of negative responses) in each group was recorded. The result showed a statistically significant difference ( P < 0.05) in the duration of analgesia among the three groups. Bupivacaine and ketamine injections individually produced analgesia of 160 min and 120 min, respectively while a combination of both drugs acted for less duration. The duration of analgesia was further prolonged in all three groups following the addition of epinephrine. Bupivacaine and ketamine have a combined analgesic effect that is permissive on one another rather than additive and these effects are prolonged by concurrent administration with epinephrine due to its local vasoconstrictive effect.