Accuracy of Ultrasound-Estimated Epidural Depth in Transverse Median Versus Parasagittal Oblique Planes for Lower Thoracic Epidural Anesthesia: A Prospective Observational Comparative Study.

Background Lower thoracic epidural anesthesia is widely used for perioperative analgesia in thoracic and upper abdominal surgeries. However, accurate localization of the epidural space remains technically challenging because of the complex thoracic anatomy. Preprocedural ultrasonography has emerged as a useful adjunct for identifying spinal landmarks and estimating epidural depth before needle insertion. Among the available ultrasound scanning techniques, the transverse median (TM) and parasagittal oblique (PSO) planes are commonly employed, although comparative evidence regarding their accuracy in lower thoracic epidural anesthesia remains limited. This study compared ultrasound-estimated epidural depth with the actual needle depth obtained using the conventional loss-of-resistance (LOR) technique in the TM and PSO planes and evaluated the correlation between body mass index (BMI) and epidural depth measurements. Methods This prospective observational comparative study included 70 adult patients (20 to 65 years) with American Society of Anesthesiologists physical status I or II and BMI between 25 and 29.9 kg/m² who underwent elective upper abdominal surgery requiring lower thoracic epidural anesthesia. Patients were equally allocated to the TM group (n=35) and the PSO group (n=35). Preprocedural ultrasound was performed to estimate the skin-to-epidural depth in both planes. Thoracic epidural catheterization was subsequently performed using the conventional LOR technique by an anesthesiologist blinded to the ultrasound measurements. The primary outcome was the accuracy of ultrasound-estimated epidural depth compared with the actual needle depth obtained using the LOR technique. Secondary outcomes included comparisons of epidural depth between the two approaches, correlations with BMI, first-attempt success, needle manipulations, and procedure-related complications. Results Baseline demographic and clinical characteristics were comparable between the groups. In the TM group, ultrasound-estimated depth (3.445 ± 0.54 cm) closely matched the actual needle depth (3.449 ± 0.55 cm; p=0.908). Similarly, in the PSO group, ultrasound-estimated depth (4.096 ± 0.66 cm) was comparable to the actual needle depth (4.111 ± 0.61 cm; p=0.524). Both ultrasound-estimated and actual needle depths were significantly greater in the PSO group than in the TM group (p<0.001). Ultrasound-estimated depth showed a very strong positive correlation with actual needle depth in both the TM group (r=0.930, p<0.001) and the PSO group (r=0.977, p<0.001). BMI demonstrated a significant positive correlation only with ultrasound-estimated depth in the TM group (r=0.327, p=0.017), whereas no significant correlations were observed for the remaining comparisons. The PSO group showed a higher first-attempt success rate (94.3% vs. 82.9%) and fewer needle manipulations and complications than the TM group; however, these differences were not statistically significant. Conclusions Preprocedural ultrasound accurately estimated epidural depth in both the TM and PSO approaches, showing close approximation to the actual needle depth. While both techniques demonstrated comparable accuracy, the PSO approach demonstrated numerically higher first-attempt success and fewer needle manipulations, although these differences were not statistically significant.