Current status and clinical relevance of studies of minimum local-anaesthetic concentration (MLAC)

Studies comparing the effects of epidural local anaesthetics have been limited by the lack of knowledge of their relative potencies. In 1995 the concept of the minimum local-anaesthetic concentration (MLAC) was introduced, this has been defined primarily as the median effective analgesic concentration in the first stage of labour. Pharmacologically, this model aims to determine equipotent analgesic concentrations of local anaesthetics, to compare motor effects and to evaluate the relative toxicity during labour. However, results of recent MLAC studies are not uniform and rather confusing, and thus, the basic validity of the MLAC concept for determining local-anaesthetic potency is increasingly discussed.

MLAC studies have postulated that ropivacaine is up to 40% less potent than bupivacaine, but as potent as levo-bupivacaine. Intriguingly, bupivacaine has been shown to be as effective as levo-bupivacaine in identical experimental protocols. Modified MLAC studies resulted additionally in local anaesthetic-sparing effects of epidural/intrathecal opioids, clonidine and epinephrine. MLAC studies have also been applied to compare the relative analgesic as well as relative motor-blocking potency of local anaesthetics.

Relative differences in local anaesthetics' potencies derived from MLAC examinations are meaningful and correct from the pharmacological point of view, but they cannot simply be transferred to daily clinical practice. Thus, MLAC values should not be misinterpreted as these data are not suggested to be suitable to define and quantify the pharmacodynamics of local anaesthetics, nor to unequivocally predict their toxicological profile in clinically relevant concentrations.