Background and Objectives The object of this study was to measure the clinically relevant depth of the posterior epidural space (ES) while placing subarachnoid catheters.

Methods “Hanging drop” technique was used to locate the ES via a midline approach in the lumbar area. The distance from the skin to the ES was measured, and the needle was advanced until free flow of cerebrospinal fluid was observed. The distance between entry into the ES and the subarachnoid space—the posterior ES depth—was then measured. Regression analysis was used to investigate relationships between skin-to-ES distance, posterior ES depth, and patient characteristics (height, weight, body surface area [BSA] and age).

Results The 55 patients studied had a mean skin-to-ES distance of 50.9 ± 12 mm (range, 27-94) and a mean posterior ES depth of 6.9 ± 4 mm (range 2-25). Skin-to-ES distance was related to BSA (r = .597, P < .0001) and weight (r = .572 P < .0001). No correlation was found between posterior ES depth and any other variable.

Conclusions The posterior ES has been found to be somewhat larger and more variable than previously described. The findings provide clinical confirmation of recent radiologic and cadaveric studies, which portray a posterior ES of variable size and complex shape. These findings have implications for cannulation and use of epidural therapy as well as for the combined catheter epidural and single-dose spinal technique.