In Reply:-We thank Dr. Wulf [1]for his interest in our epidural minimum local analgesic concentration (MLAC) studies [2]and welcome the opportunity to comment further.
Regarding his comments on differences in MLAC estimates, these are neither surprising nor new. I (M.O.C.) have previously presented to the Society of Obstetric Anesthesiology and Perinatology (SOAP) [3,4]an overview of the first seven MLAC bupivacaine studies that were conducted in the United Kingdom, the United States, France, and Italy. There was significant heterogeneity (P < 0.0001) in the MLAC estimates. The most significant factor affecting MLAC was, not surprisingly, cervical dilatation. Indeed, within the same institution, Capogna et al. [4]have shown an almost threefold increase in MLAC as labor progresses. Other significant factors included are initial pain score, gestation, and use of oxytocin. Also, as the studies have evolved, it can be noticed that the methodology has been further simplified to include subjects with "missed segments" as having ineffective analgesia to reduce the number of repeat tests. This also will tend to increase MLAC. It is of course possible that there are institutional, ethnic, or racial demographic factors involved; hence, the simplicity of the model encourages it to be used by any interested practitioner in many clinical settings.
I agree with his comments regarding terminology, I prefer median to minimum. However, because the model is analogous to the minimum alveolar concentration (MAC) or minimum infusion rate (MIR) for inhalation and intravenous anesthetics, the convention was continued. The fixed 20-ml volume does, of course, imply variations in dose, and I refer Dr. Wulf to previous comments, where I question the "pivotal" role of dose and discuss in detail the possibility of concentration-dependent pharmacodynamics for epidural local anesthetics. [5]Incidentally his suggestion of the acronym "MLAD," I understand is already in use for similar intrathecal studies!
Malachy O. Columb, F.R.C.A.
Consultant in Anesthesia and Intensive Care Medicine; South Manchester University Hospital Trust; Withington Hospital; Manchester M20 2LR, United Kingdom; columbmo@msn.com
Linda S. Polley, M.D.
Clinical Assistant Professor; Section of Obstetric Anesthesiology; University of Michigan Medical Center; Ann Arbor, Michigan
(Accepted for publication January 12, 1999.)
