Intermittent vs continuous extradural infusion of bupivacaine during labour.

Sir,—The absolute significance of the findings of Eddleston and colleagues [1] would have been much clearer if they had compared the two extradural groups with a control group which did not receive extradural bupivacainc. Although this control group would have been self-selected and not subject to randomization, the group could have been matched as closely as possible to the two study groups. In addition, although the incidence of oxytocin use was recorded in this study, an analysis of the association between oxytocin use and the fetal heart rate abnormalities would have provided additional information. This association is recognized widely, and ascribing heart rate abnormalities to different extradural techniques irrespective of the use of oxytocin may not be valid. Whilst it is tempting to attribute fetal heart rate abnormalities to uptake of bupivacaine by the fetal myocardium, the evidence for this is far from conclusive. Although Abboud and colleagues [2] also observed fetal heart rate abnormalities after extradural bupivacaine, other factors may have been contributory. In the large majority of cases, abnormalities arc associated with aortocaval compression which is not always diagnosed or recognized [3]. The umbilical vein:maternal vein concentration ratio of bupivacaine is only an index of placental transfer in that it describes the equilibrium concentration ratio of bupivacaine across the placenta at one moment in time. Although Kuhnert and colleagues [4] suggested that low fetal: material ratios were caused by extensive fetal tissue uptake, more recent work in animals by Laishley, Carson and Reynolds [5] does not support this. In our study, in the presence of a constant maternal arterial concentration of bupivacaine, fetal plasma and brain concentrations increased slowly over 120 min and only approached a fetal: maternal ratio of 0.3 at rhat time. Moreover, despite bupivacaine being lipophilic, the fetal brain:fetal blood concentration ratio diminished over this time. This latter rinding indicates that there is not extensive tissue uptake by fetal brain.