Clinical Investigation of Local Anesthetics

Relative Potency of Amethocaine and Lignocaine

“…The literature is replete with evaluations of local anaesthetics; but application of the results of any particular study to a new situation is a doubtful undertaking because of the diversity of techniques and of experimental subjects (Bonica, 1957;Geddes, 1955). The first challenge in comparing drugs appears to be the formulation of an acceptable concept of the term "potency".…”

“…Crawford, (1964) has recently compared the latency, spread and duration of effect of lignocaine, mepivacaine, and prilocaine in epidural anaesthesia, enumerating certain advantages of the sedated patient and of extradural administration in such studies. Bonica (1957) has reviewed at some length the difficulties inherent in the evaluation of local anaesthetics; he has suggested the feasibility of designing proper studies, stressing the need for statistical evaluation of data. It has also been pointed out that failure to use "equipotent" doses leaves the comparison of different agents open to question (Phillips et al, 1958;Bonica, 1959).…”

“…Bonica (1957) has reviewed at some length the difficulties inherent in the evaluation of local anaesthetics; he has suggested the feasibility of designing proper studies, stressing the need for statistical evaluation of data. It has also been pointed out that failure to use "equipotent" doses leaves the comparison of different agents open to question (Phillips et al, 1958;Bonica, 1959).…”

“…That the latency, spread, intensity and duration of various forms of regional anaesthesia may all be influenced by the dose, concentration, and volume, as well as by the activity of an agent has been widely documented (Crawford, 1964;Bonica, 1957;Adriani, 1964). While it was clear, then, that as a comparison of lignocaine and amethocaine in obstetrical spinal anaesthesia could not be critically undertaken without using "equipotent" doses or concentrations, no pertinent technique for determining relative potency of spinal anaesthetics was found.…”

“…It seemed logical to determine relative potency of lignocaine and amethocaine on spinal administration, since this was the application in which comparison was proposed. Beyond this the intrathecal site appears favourable for comparison of local anaesthetic potency because the myelinated axons are exposed directly to the anaesthetic solution under essentially controllable conditions of dilution and spread, in contrast to variable conditions which prevail in other tissues (Bonica, 1957). The application of the above statistical method to the comparison of spinal anaesthetic potency in a homogeneous population of normal parturients, all moderately sedated according to the custom of the hospital, seemed a reasonable approach.…”