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Anatomy of the Epidural Needle
Abstract Number: S1B-3
Abstract Type: Original Research
Introduction: An abrupt interruption in the supply of epidural kits (Weiss-1, table) necessitated acquisition of kits from different sources. This change led to an observational increase in dural puncture requiring blood patch. No study analyzed physical characteristics of epidural needles which could affect neuraxial analgesia. We evaluated the characteristics of six different epidural needle bevels, and ability of the catheter to negotiate the aperture against a resistance.
Methods: A Canon® REBEL T3i digital SLR camera with a macro lens was used to obtain magnified images of the epidural needles. Pictures of the front and lateral views of the bevel, and lateral views with an epidural catheter threaded at select distances (13cm and 14cm) were obtained. Adobe Photoshop® CC measurement scale was used to measure the dimensions and angulation of each bevel. Minimum length of the bevel required for the catheter to exit was analyzed (MLB). Exit characteristics of catheter through the bevel was studied using in-vitro elastic band model.
Results: Weiss-1 (table) has the largest area (3.80) and shortest MLB (1.07). Tuohy-4 has longest bevel and MLB compared to others. The catheter angulation varied from 16.4o (Hustead-6) to 12.4o (Tuohy-4). The catheter insertion through the longest bevel encountered the least obstruction against fixed elastic band resistance.
Discussion: If the MLB is longer, more bevel length must be in the epidural space for catheter insertion. If the dura is abutting, or closer to ligamentum (normal range of epidural space, 2mm-2.5cm) indentation of dura would be greater with longer MLB, and consequent resistance to catheter insertion. The Weiss-1 seems superior in this situation with shorter MLB. The longer bevels with higher MLB, such as with the Tuohy-4, may run the potential risk of the tip of the bevel touching the dura when in small epidural spaces. This may create increased resistance while threading, and potential of increased risk of dural puncture.
Conclusion: In addition to concentration, volume, and mode of administration of neuraxial medications, morphological characters of epidural needles are critical components for successful neuraxial analgesia. The morphological variations require readjustment in tactile perception, force and angulation entry into the epidural space.
1. Int J Obstet Anes. 2017 May;31:115-117.
2. Int J Obstet Anesth. 2014;23:113-117.
3. J. Reg Anesth Pain Med. 1999 Jul-Aug;24(4):303-10.