Beyond selective spinal anesthesia: A flow pattern analysis of a hyperbaric dye solution injected in a lower-density fluid

INTRODUCTION: Spinal anesthesia is a technique performed since more than a century and the introduction of hyperbaric anesthetics allowed the anesthesiologists to be more selective when using this technique. The aim of this study is to show the in vitro flow patterns of a hyperbaric dye solution through 27 G Quincke and Sprotte spinal needles, injected at different speeds, in a lower-density fluid. METHODS: A simulator was made using a gummy-like sponge and a disposable plastic urine glass, filled with saline solution, which has a similar density to cerebrospinal fluid (CSF). A hyperbaric dye solution was composed by mixing 3 ml of plain methylene blue with 1 ml of glucose 33%. We used both 27 G Quincke and Sprotte spinal needles to perform a bevel up and a bevel down injection with both slow (15 s) and fast (4 s) injection speed of 0.5 mL hyperbaric dye solution. All the injections were performed using a preset syringe pump and recorded by a camera. RESULTS: The least selectivity was observed after a bevel up-fast injection through the 27 G Sprotte needle, followed by both bevel up and down fast injections through the 27 G Quincke needle. On the contrary, the best selectivity was observed after a bevel down-slow injection through the 27 G Sprotte needle, followed by both bevel up and down slow injections through the 27 G Quincke needle. CONCLUSION: When a 27 G Sprotte needle is used to inject a hyperbaric solution in a lower-density fluid-like CSF, the spread depends on both the bevel direction and the injection speed.