COMPARATIVE FLOW-RATES OF SALINE IN COMMONLY USED SPINAL NEEDLES INCLUDING PENCIL-TIP NEEDLES

Background and Objectives. Certain characteristics of spinal needles t hat can influence their clinical usefulness were examined, namely, flo w rates, internal diameters, and size of orifices of pencil-type needl es. Methods. Flow rates of 0.9% sodium chloride solution were measured in spinal needles with an infusion pressure of 10 mmHg at isothermic and steady-state conditions. In 15 different types, a new needle was u sed for each experiment, and 6 experiments were performed with each ne edle type for a total of 90 experiments. The Hagen-Poisseuille Law for laminar flow was used to calculate the internal diameters. Results. T he flow rates of needles of the same gauge varied considerably dependi ng on the manufacturer. Compared with 26-gauge ultrathin-wall spinal n eedle produced by the same manufacturer (Braun, Bethlehem, Pennsylvani a), the flow rate through 29-gauge spinal needle was 18% that of the f ormer needle. The flow rate through 27-gauge Quincke Whitacre needle ( Becton Dickinson, Rutherford, New Jersey) was not statistically differ ent from that through 26-gauge needle by the same manufacturer. By arb itrarily choosing less than or equal to 2 seconds to be the in vitro d esirable period for saline to traverse the needle from one end to the other, all needles with the exception of 29-gauge and standard 26-gaug e Quincke Braun needles met this criterion. The length and size of the orifices of Whitacre needles were found to be significantly smaller t han those of Sprotte needles (Havel, Cincinnati, Ohio). The orifice of Sprotte needle was longer than the known thickness of the dura while that of the Whitacre needle was smaller. Conclusions. Needles of the s ame gauge do not necessarily have the same flow rate. Flow rate correl ates well with internal, not external, diameter. By considering less t han or equal to 2 seconds to be the desirable time for saline to trave rse a spinal needle, all, except two, meet this criterion. The orifice of Whitacre needle has more desirable features than that of Sprottle needle. Anesthesiologists are encouraged to use 27-gauge needles since their flow rate is not slower than that of 26-gauge needles.