Cephalad Angulation of Epidural Needle Insertion May Be an Important Factor for Safe Epidural Space Approach: a Mathematical Model

Abstract

Epidural approach angle is one of the determining factors for needle traveling length to reach the epidural space. Likewise, the margin of safety against accidental dural puncture should be affected by epidural approach angle because the space available for epidural needle tip movement in the epidural space is depending on the inserting angle to the epidural space. The purpose of this study is to conjecture the effect of the angle formed with the skin and an epidural needle on the margin of safety against accidental dural puncture using a mathematical model. Suppose that the width of the epidural space is A mm and that the force advancing the epidural catheter is Ckgf and the diameter of the catheter is D mm. In this situation, assuming that the cephalad angle with the midline approach formed with the skin and an epidural needle is θ, the following parameters can be determined: the available distance for the needle tip in the epidural space = A/sinθmm; cephalad advancing force of catheter = C*cosθkgf; and the pressure at the site of dura matter where the epidural catheter is pushing = 400*C*sinθ.πD(-2)kgf.cm(-2). The longer the distance available for the needle tip, the larger the margin of safety for dura injury by the epidural needle. It is supposed that θ should be set smaller to make the margin of safety larger. Likewise, smaller θ could decrease the pushing pressure and more effectively create a distinct cephalad advancing force. Acute cephalad angulation should be considered to increase the margin of safety for epidural approach and catheterization.