A reply

Thank you for the opportunity to reply. I would like to respond to two points made by Drs Whittaker, Fortescue and Wee. Firstly, the conversion of labour epidural analgesia to surgical anaesthesia for Caesarean section. As chair of the Drug Related Incident Group of my hospital, I remain unconvinced that the uncertain benefits of constituting mixtures of lidocaine, bicarbonate and epinephrine on the ‘shop floor’ outweigh the risks. The authors cite ‘higher failure rates’ with plain bupivacaine top-ups. In the first study they referenced [1], 23% of women receiving bupivacaine 0.5% required supplementation, compared with 34% of women who had a mixture of bupivacaine 0.5% and lidocaine 2% with epinephrine, and 21% of women receiving lidocaine 2% with epinephrine. The differences between the groups were insignificant, although general anaesthesia was required for 11% of women who had top-ups with lidocaine and epinephrine, but not for any woman in the other groups (p = 0.04). It is incorrect therefore to claim a higher failure rate with plain bupivacaine. The second study [2] was a comparison of ropivacaine 0.75% with bupivacaine 0.5% (discussion of the relative potencies of ropivacaine and bupivacaine was conspicuous by its absence). There was indeed a greater need for supplementation in the bupivacaine group, in which one woman required general anaesthesia (ns). Although this study did suggest that 0.75% ropivacaine might be a useful agent for topping up labour epidural analgesia for Caesarean section, it does not provide evidence upon which to indict plain bupivacaine 0.5%. The second point is the issue of the dose of intrathecal diamorphine. I acknowledged in my review [3] that 0.25 mg was ‘rather conservative’. Although Drs Whittaker, Fortescue and Wee feel that diamorphine 0.25 mg is ‘unduly’ conservative, the potential benefits of exceeding this dose (augmentation of intra-operative anaesthesia and enhanced postoperative analgesia) have to be balanced against the (unknown) risk of postoperative respiratory depression in poorly staffed postnatal wards. It is easily forgotten that the routine use of intrathecal opioids is relatively new. As recently as 1997, the motion ‘Women must not receive spinal opiates for post-Caesarean section analgesia unless HDU nursing care is available’ was debated at an Obstetric Anaesthetists' Association Controversies meeting. The study which defined 0.4 mg diamorphine as the intrathecal dose to prevent intra-operative supplementation of spinal anaesthesia for Caesarean section in 95% of women [4] has to be reconciled with a study of opioid-free regional blocks in which there was a zero incidence of intra-operative pain, provided the block height (loss of touch sensation) was above T5 [5]. A more recent study from the Netherlands randomly allocated women to receive intrathecal hyperbaric bupivacaine alone or with clonidine (neither group had intrathecal opioid). Only 4% of women in the ‘bupivacaine alone’ group required intravenous alfentanil supplementation during Caesarean section [6]. An additional safety element of the diamorphine dose regimen I described is concurrence of the injectate volume (0.5 ml) with that for two other intrathecal opioids, preservative-free morphine 0.1 mg and fentanyl 25 μg, neither of which requires dilution.