The baricity of plain bupivacaine

The use of ‘Cochrane’ methodology gives a study an aura of statistical quality, but authors must ensure that the clinical and scientific insights are to the same standard. In describing the plain solution of bupivacaine as ‘isobaric’ Sng et al. made an error that I thought had been identified and recognised widely many years ago 1. This solution is actually marginally hypobaric, behaves differently to a truly isobaric solution 2 and is better described as ‘plain’, a term they seem to dismiss with a single mention in their paper. Given that their aim was to compare the efficacy of two different solutions for caesarean section it might be argued that this is mere nominal pedantry especially because, in the female patient at term, the effects of pregnancy on vertebral canal physiology overwhelm the effect of baricity on solution spread. Extensive sensory blocks are produced by all solutions, but the quality of block at various levels may be different with each solution, and only be shown by the need for supplementary medication, the very topic being studied. Thus, precise definitions of baricity do matter, and a counsel of perfection would be to require all papers on spinal anaesthesia to quote the specific gravities of the solutions being described. The difference between plain bupivacaine and truly isobaric solutions is more overt in the non-pregnant patient where the bupivacaine solution produces a wider range of blocks 2, some being excessively extensive and others inadequate in spread. Use of the term ‘isobaric’ for this solution can lead the inexperienced practitioner astray in the non-obstetric setting because it can occasionally produce a ‘failed’ spinal 3. Finally, I am perplexed by Sng et al.s’ statement 1 that work from my own department was excluded from their review because we ‘used hypobaric bupivacaine’. As I note above, one of their subject solutions is hypobaric, but the solutions (specific gravities quoted) we compared were the commercial, hyperbaric solution of bupivacaine and one manufactured locally to contain a lower concentration of glucose, but still be (marginally) hyperbaric 4.