EFFECT OF ORIFICE-AREA REDUCTION ON FLOW CHARACTERISTICS DURING INJECTION THROUGH SPINAL NEEDLES

A reduction in hole size for certain side-port spinal needles has been advocated in recent reports. While the influence of orifice-area redu ction on the aspiration capability of the needle has been studied, the influence on the anaesthetic delivery properties is relatively unknow n. As a first step in understanding the effects of hole-size reduction on anaesthetic distribution within the subarachnoid space, we studied flows emanating from isolated needles using computer simulations. Fol lowing validation of the numerical model using experimental particle v isualisation, trajectories of anaesthetic particles injected through 2 5G Whitacre needles of various orifice areas were computed and used to determine the orientation and rate of spread of the anaesthetic jet e xiting the needle. Two factors impacting the concentration distributio n were observed: the rate of spread of the anaesthetic jet increases m arkedly with decreasing orifice area and the jet alignment shifts towa rd perpendicular to the needle axis.