Background and Objectives. Certain characteristics of spinal needles that can influence their clinical usefulness were examined, namely, flow rates, internal diameters, and size of orifices of pencil-type needles.

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 used for each experiment, and 6 experiments were performed with each needle type for a total of 90 experiments. The Hagen-Poisseuille Law for laminar flow was used to calculate the internal diameters.

Results. The flow rates of needles of the same gauge varied considerably depending on the manufacturer. Compared with 26-gauge ultra-thin-wall spinal needle produced by the same manufacturer (Braun, Bethlehem, Pennsylvania), the flow rate through 29-gauge spinal needle was 18% that of the former needle. The flow rate through 27-gauge Quincke Whitacre needle (Becton Dickinson, Rutherford, New Jersey) was not statistically different from that through 26-gauge needle by the same manufacturer. By arbitrarily choosing ≤2 seconds to be the in vitro desirable period for saline to traverse the needle from one end to the other, all needles with the exception of 29-gauge and standard 26-gauge Quincke Braun needles met this criterion. The length and size of the orifices of Whitacre needles were found to be significantly smaller than 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 same gauge do not necessarily have the same flow rate. Flow rate correlates well with internal, not external, diameter. By considering ≤2 seconds to be the desirable time for saline to traverse 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.