Correlation between Pre-procedural Ultrasound-estimated and Actual Procedural Epidural Depth in Lumbar Epidural Anaesthesia Taking BMI into Consideration: An Observational Study

Introduction: Ultrasound guidance has become a popular tool for regional anaesthesia due to its ease of use and ability to accurately identify anatomical landmarks. This technique has been shown to improve safety and reduce complications. In recent years, there has been growing interest in using ultrasound for lumbosacral regional anaesthesia, even as a preoperative assessment tool. However, the variability in the distance between the skin and the epidural space can make it difficult to accurately locate the space, thereby increasing the risk of complications such as nerve injuries, paresthesia, haematoma formation, and post-dural puncture headache. Aim: To evaluate the skin-to-epidural space distance using Ultrasound-derived (UD) measurements compared to the conventional Needle Depth (ND) using the Loss of Resistance (LOR) technique, and to investigate their correlation with Body Mass Index (BMI). Materials and Methods: This observational study was conducted in the Amala Institute of Medical Sciences, Kerala, India from November 2020 to November 2021 with 60 patients aged 18-70 years, classified as American Society of Anaesthesiologists (ASA) class 1-2 and having a BMI between 18-30 kg/m2 , who were undergoing elective lumbar epidural anaesthesia. The epidural space depth was assessed using transverse axial plane UD, and the actual ND was measured using a sterile linear scale. The correlation with BMI was studied. Data analysis was performed using Statistical Package for Social Sciences (SPSS) version 23.0, and the agreement between UD and ND was evaluated using the Intraclass Correlation Coefficient (ICC). Results: The mean age of the patients was 50.77±9.12 years. The study revealed a strong agreement between UD and ND in assessing the epidural space, with an ICC of 0.963 (p-value=0.0001) and a Pearson’s correlation coefficient (r) of 0.960 (p-value=0.0001). A high correlation was observed between UD and ND in patients with a BMI of 18-22.9 kg/m2 (ICC=0.983, p-value <0.0001) and 23-24.9 kg/m2 (ICC=0.953, p-value=0.0001), while a moderate correlation was found in the BMI group of 25-29.9 kg/m2 (ICC=0.627, p-value=0.008). Conclusion: The present study demonstrated a strong correlation between ultrasound-estimated and actual needle depths in patients with a BMI under 30 kg/m2 . This correlation was associated with more accurate puncture site identification, higher success rates, fewer attempts for lumbar epidurals, and a reduced rate of complications.