Instrumentation and ultrasound for epidural anesthesia

Lumbar epidural anesthesia is used for alleviating the pain of labor and for surgery. Here, a catheter is threaded through a Tuohy needle that is traditionally inserted using the loss-of-resistance technique to confirm entry into the epidural space. This research begins with a study of the loss-of-resistance through instrumentation. Sensors measure 1)the force applied at the plunger by the anesthesiologist, 2)the pressure at the needle tip and 3)the position of the plunger relative to the syringe. The “feel” in different tissues is quantified for porcine subjects ex vivo and human subjects in vivo. A vertebra counting protocol is developed to identify the desired vertebral interspaces. Ultrasound is then used to measure anatomical distances such as the distance between the skin and ligamentum flavum and surrogate measures compared to the actual needle insertion depth. Good correlation is only found between skin-to-ligamentum flavum and the actual needle insertion depth. Next, a real-time in-plane ultrasound technique is developed with a needle guide fixing the needle trajectory to the ultrasound transducer. This allows the anesthesiologist to guide the insertion of the epidural needle as an “aim-and-insert” method. In 18 of 19 subjects, the procedure was successfully performed. The key limitation of ultrasound in this application is the image quality that inhibits interpretation of the images. A median-based spatial compounding with warping is performed to align the anatomical features of different beam-steered images and combine them to obtain a single enhanced image. This method is tested on image sets of phantoms and lumbar anatomy of 23 human subjects and shows a significant improvement in noise reduction and clarity. Another limitation is the interpretation of ultrasounds of the spinal anatomy requires understanding of ultrasound. An automatic detection algorithm is developed based on the experienced sonographer’s method of detecting the ligamentum flavum in ultrasounds. This novel method is tested on ultrasounds of the lumbar anatomy in 20 human subjects and shows the method successfully detects the ligamentum flavum in 34 of 39 cases. The main conclusion is that specialized ultrasound tools and protocols are needed to accomodate the range of patients and levels of experience of practitioners.Applied Science, Faculty ofElectrical and Computer Engineering, Department ofGraduat