Effect of prior anesthetic solution on epidural morphine analgesia

The quality and duration of analgesia and incidence of side effects following epidurally administered morphine after cesarean section are highly variable. Two suggested sources of this variability are prior use of epinephrine-containing solutions, which may enhance both analgesia and side effects of morphine, and prior use of 2-chloroprocaine, which may inhibit epidural morphine analgesia. To examine these proposed sources of this variability we performed two studies. In the first, designed to test epinephrine's effect, 30 women underwent testing for epidural catheter tip location with injection of 2-chloroprocaine, followed by either 0.5% bupivacaine alone or 0.5% bupivacaine with 5 micrograms/mL epinephrine for epidural anesthesia. Inclusion of epinephrine with bupivacaine decreased the bupivacaine dose needed to achieve a T-4 sensory block by 24% (P less than 0.05), but did not alter analgesic duration or the incidence of side effects of epidural morphine (5 mg). In the second study, designed to test the effect of 2-chloroprocaine, 30 women received 7 mL of either 2% 2-chloroprocaine or lidocaine for epidural catheter testing, followed by 0.5% bupivacaine for epidural anesthesia. Compared to lidocaine testing, 2-chloroprocaine decreased the duration of epidural morphine analgesia (median 16 h with 2-chloroprocaine vs 24 h with lidocaine; P less than 0.05) without altering the incidence of side effects. The authors conclude that addition of epinephrine to local anesthetic does not increase the incidence of side effects or the analgesic effect from epidurally administered morphine. 2-Chloroprocaine, even when administered in small doses remote to the time of morphine injection, interferes with the duration of epidural morphine analgesia.