Predictors of Epidural Depth and the Role of Technology

Epidural anesthesia and analgesia are commonly used for a number of medical and surgical procedures. Although the blind loss of resistance is usually successful in experienced hands, it can be challenging at times. Imaging technology is increasingly used to assist in the accurate localization of the epidural depth (ED). When compared to the blind loss of resistance technique, ultrasound measurements slightly underestimated; computed tomography slightly overestimated; while MRI depth measurement was more accurate, although it occasionally overestimated the ED. Ultrasound imaging resulted in fewer placement trials and increased the chances of success. The strongest predictor of epidural depth is body weight and body mass index. The epidural space is deeper in men than in women and in pregnant than in non-pregnant women. The role of ethnicity was studied in parturients and found to be deeper in patients of African origin and shallowest in the east Asian parturients. Regarding the level of insertion, it is deepest at the cervicothoracic and lumbosacral areas and the shallowest at the thoracolumbar area. It is also deeper when using the lateral, compared to the sitting position, and when using the paramedian or off-the-midline approach than the midline. The height, age, and technique probably have no effect or are controversial at best. The posterior epidural distance is smallest at the cervical level, enlarges as one moves caudally, and is largest at L4–5.