Minimum effective local anaesthetic dose for spinal anaesthesia

Editor—As a comparison of the dose of local anaestheticrequired for spinal anaesthesia with either levobupivacaineor ropivacaine the study by Sell and colleagues¹ is both interestingand informative. However, I have a number of concerns aboutthe paper that has been based on that study. In the title, and subsequently, the authors describe plain solutionsof both drugs as being ‘isobaric’ yet the densityfigures they quote in the paper are well below the normal rangefor CSF. These solutions are not isobaric with cerebral spinalfluid and they spread in a manner that is different from thosethat are truly isobaric, the primary difference being much widervariability