Keys to minimizing the risk of spinal cord trauma during a lumbar approach to thoracic epidural

Abstract

Thoracic epidurals provide effective analgesia for thoracic or upper abdominal procedures for both adult and pediatric patients; however, using a thoracic approach to place an epidural catheter is not without risk as it requires needle placement immediately posterior to the spinal cord. Consequently, some clinicians prefer to use a lumbar approach (caudal approach in pediatric cases), or if unsuccessful at a thoracic level, they may thread the epidural catheter cephalad from that point. This issue of the Journal features an interesting study by Gamble et al. in which the authors used a porcine model to address the safety of threading an epidural catheter via a lumbar approach. In their study, the authors show that styletted stimulating thoracic epidural catheters can be advanced in a predictable manner to target thoracic myotomes using a lumbar approach. However, damage to the spinal cord was observed in several animals. It is important to point out that, in humans, such perturbing findings are probably limited to high lumbar approaches (i.e., above L2-L3). This is partly because the human adult spinal cord, unlike the porcine model used in the Gamble et al. study, typically ends at approximately the L1-L2 level. Thus, if needle entry occurs either at or below the L2-L3 level, the risk of direct blunt trauma caused by the initial threading of the catheter from the needle tip is minimized. Nevertheless, the results of Gamble et al.’s study clearly show the risks involved when a catheter is advanced directly towards the spinal cord, as in thoracic and high lumbar approaches. Several key factors influence the risk and potential for spinal cord damage during these approaches, including the angle of catheter insertion, catheter stiffness, and applied force. To address this issue, the relationship between these three factors must be considered. Force must be applied to advance the catheter to its desired target myotome level. This threading force is the total applied force needed to insert the catheter through the Tuohy needle into the epidural space. Threading force itself involves two component forces that must be considered during catheter insertion: a bending force, needed to advance the catheter out of the needle, and a parallel force, needed to thread the catheter cranially past any obstacle within the epidural space (Fig. 1). Assuming there is minimal obstruction along the spinal cord, the initial total threading force is then directly related to the bending force. This is shown in Gamble et al.’s experiment where, in most animals, difficulty was experienced in advancing the catheter beyond the needle tip. The perpendicular angle of insertion used by the authors suggests the need for considerable bending force to advance the catheter out of the needle. Furthermore, the authors speculate that catheter stiffness and the presence of the stylet added to the difficulties in initial catheter advancement. Thus, the bending force required to advance the catheter out of the needle is influenced by two key factors: angulation of the needle and catheter stiffness (i.e., styletted or not). To show this relationship, we used a simple in vitro setup to demonstrate the peak (kilogram) force generated when threading a styletted catheter at various needle angles (Figure). As shown, the peak force increases dramatically as the angulation of the needle approaches 0 (i.e., perpendicular) relative to the surface of the force gauge transducer. With an insertion perpendicular to the plane of the back (i.e., no angulation), these results suggest that the B. C. H. Tsui, MD (&) Department of Anesthesiology and Pain Medicine, University of Alberta, 8-120 Clinical Sciences Building, Edmonton, AB T6G 2G3, Canada e-mail: btsui@ualberta.ca