Background and objectives: The objective of the present study was to evaluate the skin-epidural space distance as assessed by ultrasonography and conventional loss of resistance (LOR) technique and to find the correlation of epidural depth with body mass index (BMI).
Methods: Ninety-eight patients of either sex, American Society of Anesthesiology I/II, BMI <30 kg/m2 requiring lumbar epidural for surgery were enrolled. The epidural space was assessed with a curvilinear ultrasound (US) probe, 2-5 MHz, in the transverse plane at L3-L4 intervertebral space. Thereafter, the epidural depth from skin was assessed with conventional LOR method while performing the epidural. The needle depth (ND) was measured using a sterile linear scale, and any change in the needle direction or intervertebral space was noted.
Results: The patients were demographically similar. Depth of epidural space measured by US depth (UD) was 3.96 ± 0.44 cm (range 3.18-5.44 cm) and by ND was 4.04 ± 0.52 cm (range 2.7-5.7 cm). The Pearson's correlation coefficient (r) between UD and ND was 0.935 (95% confidence interval: 0.72-0.92, r2 = 0.874, P < 0.001), and Bland-Altman analysis revealed the 95% limits of agreement -0.494-0.652 cm.
Conclusion: The present study demonstrates a good correlation between UD and ND and shows that the preprocedural US scan in transverse plane provides accurate needle entry site with a high success rate in single attempt for lumbar epidurals in patients with a BMI <30 kg/m2.
Keywords: Lumbar epidural; needle depth; transverse plane; ultrasound; ultrasound depth.