Thank you for allowing us to respond to Dr. Wildsmith’s concerns regarding our article in the April 1999 issue of ANESTHESIOLOGY. 1His first comment refers to our explanation of potency, which he defines as the “degree of effect of a drug to its dose.” We studied several “effects” of the two drugs, including height of block, intensity of block (defined as tolerance to transcutaneous electrical stimulation [TES]), and duration of sensory and motor block. A comparison of potencies can be found in the data listed in our table 3, which gives the characteristics of the two spinal anesthetic drugs. For example, 4 mg bupivacaine produces tolerance to TES at T10 and T12 dermatomes, but 4 mg ropivacaine does not (duration not a consideration). Although the strictest definition of potency may be found by testing an isolated nerve in a controlled laboratory setting, to the clinician, who deals with “intact subjects,” duration is an important measure of potency.
His concern regarding our use of volunteers (which included one of the authors [S. B. M.] and her spouse) should be allayed not only by our Institutional Review Board approval, but by full disclosure to the volunteers that although ropivacaine is not approved at this time for intrathecal use, its safety has been studied in case of inadvertent intrathecal injection of an epidural test dose. 2,3We chose to use volunteers for two reasons: (1) asking patients to receive an anesthetic in doses not yet proven to produce surgical anesthesia is unethical; and (2) our crossover method of control is unsuitable for patients who require only one anesthetic.
The association of back pain and ropivacaine in our study was not statistically significant (as stated, P = 0.098), but the trend is concerning. Our conclusion was that “the possibility of side effects… warrants further investigation.” We apologize if the study design was not clearly stated, but the order of drug injection was randomized as well. In our interpretation of the results, we commented that there was no association whether ropivacaine was given first or second and, in addition, that all three doses were represented.
Finally, Dr. Wildsmith suggested that our figure 1, which depicted block height to pin prick, was encouraging evidence that ropivacaine may be a suitable spinal drug for outpatients. However, sensory block to pinprick does not equal surgical anesthesia. Our results regarding TES, which was designed to simulate surgical incision, were far from encouraging. 4Again, referring to our table 3, the 12-mg dose of ropivacaine produced tolerance to TES at the ankle for 48 ± 40 min versus 153 ± 54 min for 12 mg bupivacaine or 46 ± 30 min for 8 mg bupivacaine. Ropivacaine 8 mg did not produce any tolerance to TES. This data coupled with its unfavorable recovery profile brought us to the conclusion that ropivacaine is unlikely to offer an advantage over presently available drugs such as bupivacaine. Other researchers have also come to a similar conclusion for patients undergoing outpatient surgery. 5