BLOOD LEVELS OF BUPIVACAINE IN OBSTETRIC ANALGESIA

Local analgesic agents have long been used to provide analgesia during labour. Lumbar epidural, caudal epidural and paracervical blocks are used commonly to-day. In the last 15 years, lignocaine has held a prominent place in regional analgesia. The principal disadvantage of lignocaine has been that the duration of action, even when administered with adrenaline, is relatively short, somewhere between 60 and 90 minutes. The introduction of continuous techniques has improved the situation, but the phenomenon of tachyphylaxis soon became apparent. However, since the introduction of the long-acting analgesic bupivacaine it has been possible to produce pain relief for about 150-200 minutes after each dose, and tachyphylaxis has not been demonstrated. The purpose of this paper is to present the blood concentrations that were found following the use of bupivacaine during lumbar epidural and paracervical block. Whole blood rather than plasma concentrations were analysed. Bupivacaine was found to be present almost entirely in the plasma, either protein-bound or free. A negligible amount enters the red blood cells. The measurement of blood concentrations of local analgesics was carried out by gas chromatography, using a modification of the method described by REYNOLDS and BECKETT. The sensitivity and specificity afforded by gas chromatography was found to be essential in this study.