Preprocedural Ultrasound Estimates of Epidural Depth: Transverse Median Plane is Comparable to Paramedian Sagittal Oblique Plane in Non-Pregnant Patients.

Objective: The aim of this study was to assess the accuracy and precision of the actual epidural depth (ND) and ultrasound (US) estimates of the distance from the skin to the epidural space in the transverse median plane (ED/TM) and paramedian sagittal oblique plane (ED/PSO) in patients who were not pregnant and who were undergoing unilateral inguinal hernia repair surgery under combined spinal epidural anaesthesia.

Methods: A total of 100 patients with the American Society of Anaesthesiology status I-III of either gender were recruited. All epidurals were inserted at the L3-4 intervertebral space, and the epidural needle was marked with a sterile marker. The ND was measured between the marker and the tip of the needle. The ED/TM and ED/PSO were assessed with a 2-5 MHz curved array probe at the L3-4 intervertebral space. The Pearson correlation coefficient and Lin's concordance correlation coefficient were used to assess the precision and the Bland-Altman test was used to assess the 95% limit of agreement.

Results: The ND was 48.98±6.91 mm, the ED/PSO was 48.92±6.91 mm and the ED/TM was 48.90±6.91 mm. The Pearson correlation coefficient between ND and ED/PSO was 0.995 (p<0.001) and 0.994 (p<0.001) with ED/TM. Lin's concordance correlation coefficient for ND and ED/PSO was 0.995 (95%CI [confidence interval]: 0.992-0.997), and 0.993 for ND and ED/TM (95% CI: 0.990-0.996). The 95% limit of agreement between ND and ED/PSO was 0.70-1.37, and for ND and ED/TM, it was 0.79-1.54.

Conclusion: Preprocedural ultrasound (US) scanning in both planes provides reliable estimates for actual epidural depth in non-pregnant patient population.