To the Editor:—
The April 1999 issue of ANESTHESIOLOGY contained two articles that relate to the relative potencies of bupivacaine and ropivacaine in the respective settings of epidural analgesia in obstetrics 1and spinal anesthesia in volunteers. 2Comparing potencies of local anesthetic agents in the intact subject is difficult, and the findings in the article on obstetric epidural analgesia are qualified by an appropriate accompanying Editorial View. 3I concur with the Editorial, but I feel that the other article requires comment also.
“Potency” relates the degree of effect of a drug to its dose or concentration, but it seems that McDonald et al. 2were actually concerned with duration. In their first paragraph, three articles are cited (including two of my own 4,5) to support the statement that “clinical trials suggest that ropivacaine may be less potent.” In fact, we found that epidural ropivacaine and bupivacaine produced equally effective sensory block in equal concentrations (in milligrams), although the duration of ropivacaine was marginally less, and it produced significantly less motor block. McDonald et al. then confirm that they are really examining duration, not potency, by considering that ropivacaine may provide less interference with discharge criteria after outpatient surgery. Duration and potency of local anesthetic agents are inter-related, but we must not confuse them because there are differences.
I also have some general concerns about the study. Did it really justify the performance of two spinal anesthetics in volunteers who received preparations that have not been approved for intrathecal administration? It also seems that ropivacaine was given on the second occasion to each subject. Is this why there was more backache with ropivacaine, or was it simply the use of an inappropriate preparation? However, the difference in the incidence of backache was not, in fact, statistically significant, yet the conclusion of the article was that ropivacaine produced more side effects. There is little scientific rigor in this statement.
Finally, I am puzzled that the authors have concluded that ropivacaine is not worthy of further study for outpatient spinal anesthesia. Surely the data in their figure 1 shows that it is. Ropivacaine 12 mg produced the same mean height of block as bupivacaine 12 mg, but its time to complete regression was little more than half. I have concerns about several aspects, but this result is very interesting.