Background and Objectives. Approximately 5-10% of attempted myelograms are spoiled by partial subdural injection. Similar spillage of local anesthetics may underlie the wide variability of segmental spread reported for spinal anesthesia. A technique of applied negative pressure to avoid accidental subdural injection and to facilitate subarachnoid puncture with narrow gauge needles is described.

Methods. Hydraulic circuitry was designed to measure saline flow rates through spinal needles at hub-to-tip differential pressures of 55 mm Hg and 650 mm Hg. A model of dura and arachnoid membranes was constructed to demonstrate the effects of applied negative pressure on the subdural space during the passage of Quincke-tip and pencil-point needles. Cisternal puncture in 38 dogs and thoracic or lumbar subarachnoid puncture in 680 patients were performed using applied negative pressure to the needle hub.

Results. A sustained vacuum of −650 mm Hg at the hub of air-filled spinal needles was transmitted beyond the tip but fell to zero at the tip as the shaft filled with saline. Flow rates through the Quincke needles, ranging from 22-gauge to 29-gauge, rose 22- to 33-fold when a vacuum of −650 mm Hg was applied at the hub. Model dura and arachnoid membranes were separated by air-filled tips at atmospheric pressure but were juxtaposed by negative pressure applied to the hub.

Conclusions. Negative pressure applied to the needle hub is transmitted to the tip without decrement in air-filled needles but not in fluid-filled needles. Clinical application of this principle by exertion of strong negative pressure at the hub during needle advancement facilitates rapid identification of cerebrospinal fluid, avoids unintentional subdural injection of local anesthetics or contrast media, and increases the safety of subarachnoid punctures above the termination of the spinal cord.