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Using light touch to assess the level of spinal block during caesarean section: what dose of local anaesthetic should we be using?

We read with great interest the guidelines for the prevention and management of intra-operative pain during caesarean section [1]. Among the authors’ recommendations are that light touch be used as the primary testingmodality, aiming for a sensory block to T5 or higher. This recommendation is welcome given, as the authors point out, the “lack of consensus onwhat to test, how to test andwhat constitutes an adequate block.” A discussion of assessment of neuraxial block, we believe, cannot occur without a discussion of local anaesthetic dosing for neuraxial anaesthesia. The dose of local anaesthetic injected is a key determinant of block height [2]. In our own institution, and many others where we have worked, anaesthetists typically administer the doses of hyperbaric bupivacaine equivalent to, or slightly above, the ED95 of the drug. This is based on the findings that this will be effective in 95% of the population [3]. We believe it is important to point out that studies determining the ED95 of local anaesthetic doses for spinal anaesthesia for caesarean section have typically not used light touch as a determinant for block efficacy, but rather sharp pinprick [4]. In fact, these authors could not identify any dosing studies using light touch as a primary testing modality. This is important as loss of sharp pinprick sensation is typically observed several dermatomes higher than loss of light touch [5]. Many anaesthetists, including ourselves, who are using loss of sensation to cold or pinprick as the primary testing modality will likely change to use of light touch following publication of these guidelines. We wonder whether we should be increasing the dose of local anaesthetic administered in our spinal anaesthetics? Perhaps we should be performing more combined spinal–epidurals to allow epidural top-up in cases of low sensory block? Or perhaps we should be adjusting other factors which influence block height and quality, such as patient position or dose of intrathecal opioid [2]? The guidelines [1] are a significant step towards improved prevention and management of intra-operative pain during caesarean section under neuraxial anaesthesia. However, until such time when we have dosing studies using light touch as a primary testing modality, we believe anaesthetists switching to light touch should be cautious of the possible need to adjust their anaesthetic technique.