Flow characteristics of long spinal needles

A 120 mm, 27-gauge needle has recently been introduced for subarachnoid anaesthesia when using a single-space, combined spinal-extradural technique. The flow characteristics of this needle were compared with those of a 120 mm, 26-gauge needle using a pressure monitor infusion pump. Surprisingly, the resistance to fluid flow of the 26-gauge needle was twice that of the 27-gauge needle. The results of this experiment were confirmed by the findings of a clinical study undertaken in 100 obstetric patients who required regional anaesthesia. The time taken from dural puncture to appearance of cerebrospinal fluid at the hub of the needle was on average three times greater with the 26-gauge needle.