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University of California, San Diego
Mechanical and Aerospace Engineering
MAE 156B: Senior Design Project (Spring 2023)
Project Description
This project aims to design an automated device that will attach to an epidural needle which can accurately detect the resistance the needle feels and notify the doctor once the needle enters into the epidural space. This device utilizes two differential pressure sensors and a piezoelectric pump to measure the pressure drop within the system as the needle advances through different layers of a patient’s tissue.
Figure 1: Schematic demonstrating epidural needle injection into the epidural space located around the spinal cord and nerve roots.
Motivation
The epidural space is referred to as “potential space” that exists in the posterior region (back), in between the outermost layer of the spinal cord (dura mater) and the surrounding vertebral wall. Medication can be injected into the epidural space to provide anesthesia during medical procedures, such as childbirth, or to provide relief to patients suffering from chronic back and neck pain. Epidural injections involve the insertion of an epidural needle through multiple tissue layers until it penetrates the ligamentum flavum and enters the epidural space (Figure 1).
The ligamentum flavum is a robust and dense layer of ligament that forms the boundary of the epidural space and poses the greatest resistance to needle penetration. A high resistive force against needle insertion is felt when advancing through the ligamentum flavum, which contrasts the nearly resistance-free environment of the epidural space. Currently, healthcare professionals rely on the subjective sensation of the "loss of resistance" experienced when the needle punctures the ligamentum flavum to determine successful entry into the epidural space. However, this technique heavily relies on the intuition and experience of the administrator and can yield inconsistent results.
The goal of this project is to develop a device that can be attached to an epidural needle, capable of precisely detecting the loss of resistance during needle advancement. Once the epidural space is reached, the device will provide the attending physician with immediate notification via measurements displayed on an LCD screen.
Electronics
This device incorporates a piezoelectric pump to facilitate the movement of saline from an external reservoir, which is then directed through tubing integrated within the device and eventually reaches the epidural needle. An integral component of the device is a differential pressure sensor that measures the pressure drop across a specific section of the tubing. As the needle traverses different tissue layers, the pressure differential undergoes variations, enabling the device to discern these changes.
The recorded pressure alterations are used to calculate fluid flow rate through fluid dynamics. This enables the device to continuously provide real-time updates on the fluid flow rate within the tubing, correlating with the insertion of the needle through tissue layers with distinct resistances. Notably, the transition from the ligamentum flavum to the epidural space will provoke a significant change in the flow rate registered by the device, thus serving as an indicator that the epidural space has been successfully reached.
Plumbing
Device Testing and Results
Testing of the device was conducted at the Simulation Training Center at UCSD Medical School. A spine model made of ballistic gel and silicon simulated the various tissue layers (skin, subcutaneous fat, and muscle) that the needle will pierce through. A plastic tube running through the model simulates the ligamentum flavum and the epidural space.
The device demonstrated its effectiveness in accurately detecting variations in flow rate across different tissue layers. A notable contrast in flow rate is evident when the needle penetrates from the simulated ligamentum flavum into the epidural space. This successful detection of a substantial flow rate discrepancy serves as a validation of the device's functionality and its ability to identify the epidural space.
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