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INABILITY TO THREAD THE EPIDURAL CATHETER: A PROVOCATIVE HYPOTHESIS
Abstract Number: F-32
Abstract Type: Original Research
BACKGROUND: Occasionally, despite a convincing loss of resistance (LOR), an epidural catheter will not thread into the epidural space. Flushing saline through the epidural needle has been advocated  with the premise that it will displace dura or epidural contents from the tip and facilitate catheter insertion. Alternatively, the benefits of cephalad angulation of the epidural needle  have been examined. However, no study has evaluated how much of the needle orifice must traverse the ligamentum flavum in order for the catheter to thread into the epidural space. We sought to create a model to identify how much of the epidural needle must pass through the ligamentum flavum to allow insertion of the catheter.
METHODS: We created a durable cardboard model of the ligamentum flavum. We advanced the epidural needle through the model and attempted to obtain LOR to saline, as well as ability to pass an epidural catheter into a virtual “epidural space.” We photographed our attempts using a macro lens and Canon Rebel T3i digital SLR camera. We used photo-editing software to examine the high-definition images and measure the epidural needle tip at various points of insertion.
RESULTS: Using this model, we demonstrated that it is possible to obtain a convincing LOR to saline (Figure 1A) when the needle tip has barely traversed the ligamentum flavum, but it would be impossible to thread a 19 gauge epidural catheter. We determined that passage of the epidural catheter could not occur (Figure 1B) unless the point at which the width of the elliptical opening of the needle equals the diameter of the catheter traversed the ligamentum flavum (Figure 1C). Measurement of the 19 gauge epidural catheter diameter demonstrated that it was approximately 1 mm. Analysis of our photographs revealed that the point at which the 17 gauge epidural needle orifice measures 1 mm in length occurs at a width of 1.1 mm, 1.3 mm from the tip (Figure 1D). We also examined the effects of needle angulation and close proximity of the dura, as well as the flexibility of different epidural catheters.
CONCLUSION: Our model demonstrates that passage of the epidural needle tip of less than 1.3 mm through the ligamentum flavum will not allow epidural catheter insertion despite LOR. Cephalad angulation facilitates flexible epidural catheter insertion; stiff catheters pass regardless of insertion angle.
1. Sviggum H, Farber M. IJOA, 2013.
2. Inoue S et. al. Rev Bras Anestesiol, 2011.