Distance from Skin to Epidural Space: Correlation with Body Mass Index (BMI)

BACKGROUND: Epidural anaesthesia is being increasingly used to provide anaesthesia for surgery on the lower abdomen, perineum and lower extremities. However success of the epidural technique depends upon the correct identification of epidural space.2 We conducted a study to find the distance from skin to the epidural space and its correlation with body mass index, to improve the success rate. PATIENT AND METHODS: 120 adults patients belonging to ASA physical status I and II in the age group of 18-70 years, scheduled for surgery and or pain relief under epidural block, were taken up for the study. 60 patients of either sex were further subdivided into 2 subgroups of 30 patients each having BMI less than 30 or more than 30. The distance from skin to epidural space was measured as the distance between rubber marker and tip of Tuohy's needle. RESULTS: It was found that with increase in Body mass Index, the distance from skin to the epidural space also increases. The distance from the skin to the epidural space does not depend on the age or the sex of the patients. CONCLUSIONS: We formulated predictive equation of depth of epidural space from skin in relation to BMI based on linear regression analysis as: Depth (mm) = a + b (BMI). Where a = 17.7966 and b = 0.9777.