Background and Goal of Study:
The loss of resistance (LOR) technique is the most used for the positioning of the epidural catheter in pregnant women but the anatomic landmarks are not always clear. [1]
The aim of our study was to verify the accuracy and precision of the transverse echographic approach for the definition of the depth of the epidural space.

Materials and methods:
Prospectic observational study conducted in the Hospital “S. Maria delle Croci” in Ravenna from February to June 2014. The epidural set used was B. Braun Perifix® Filter Soft Tip set. US imaging was performed using a portable SonoSite NanoMax® equipped with a 52 MHz C60n Convex probe.
The distance between the skin and the first hyperechoic line (posterior dura) was named "ultrasound depth”, UD.
Tuohy needle was then introduced with LOR technique and a mark was made at the skin level. The distance from the needle tip to the mark was as “real depth” (RD).
Data regarding the whole procedure of epidural analgesia, RD, UD were collected.

Results and discussion:
We examined 30 parturient women candidate for epidural analgesia, mean age 30.3 (±4.6), actual BMI 27.2 (±2.4). We found a correlation between RD and UD, with an index of Pearson correlation of 0.88, in line with the literature.[23]
RD was 5.08 cm ± 0.62 cm while UD was 4.94 cm ± 0.60 cm.
At the BlandAltman analysis the mean difference between UD and RD was 0.15
cm (95% C.I. of 5.9 mm).

Conclusion:
US-assisted epidural catheter positioning shows a great degree of correlation between UD and RD, but US measure tends to underestimate the RD of the epidural space. This discrepancy may be explained by the different needle trajectory, or the different tissue compression exerted by the probe.

References:
1. Borges B, et al. Reg Anesth Pain Med.2009;34:581585.
2. Shaikh F, et al. BMJ. 2013;346:f1720.
3. Arzola C, et al. Anesth Analg. 2007;104:118892.

Epidural catheter positioning in labour: Is the transverse echo-graphic approach accurate and precise for the definition of the epidural space?