Comparison Of Skin To Epidural Space Distance Using BMI-Derived Predictive Equation Versus Ultrasound Guidance In Elective Surgery Patients: A Prospective Randomized Study

Abstract

Background: Accurate identification of the epidural space is essential to the safety and efficacy of neuraxial anaesthesia. Conventionally, the depth of the epidural space is identified using the loss of resistance technique, which is highly operator-dependent. Ultrasound (USG) provides a more reliable, real-time estimation of epidural depth. In resource-limited settings, a predictive equation based on body mass index (BMI) may serve as an alternative. This study aimed to compare the skin-to-epidural space distance obtained using a BMI-based predictive formula with the distance measured by ultrasound in patients undergoing elective surgery.

Materials and Methods: Prospective, cross-sectional study conducted at a tertiary care hospital. Sixty adult patients (ASA I–III) scheduled for elective lower abdominal or lower limb surgery were enrolled. The depth of the epidural space at the L3–L4 interspace was measured using ultrasound guidance. A BMI-based predictive equation—Depth (mm) = 17.7966 + (0.9777 × BMI)—was also applied. Mean values from both methods were compared.

Results: The mean skin-to-epidural space distance measured by USG was 4.7 ± 0.4 cm, while the mean predicted depth using the BMI equation was 4.3 ± 0.3 cm. The difference was statistically significant (p < 0.0001).

Conclusion: The BMI-based predictive equation underestimates the actual epidural depth measured by ultrasound. While useful as an approximate guide in settings where USG is unavailable, ultrasound remains the superior tool for accurate epidural space localization.