Experimental analysis of the influencing factors on the response of a tool for epidural space detection

Epidural injection is a local-regional anesthesia technique used both for analgesic and anesthetic purposes. During the procedure, a needle is inserted between two spinous processes, and it is advanced through interspinous ligament, supraspinous ligament skin, ligamentum flavum, and finally it reaches the epidural space. It is crucial the optimal positioning of the needle's tip within this space to avoid incorrect medication administration or dural puncture. Nowadays, the anaesthetist detects the epidural space by a sense of a resistance loss due to the different density of the mentioned anatomical tissues. In a previous work, we described a bespoke system called Epidural Sensing Management Tool (ESMT) designed to support the anaesthetist in the detection of the epidural space. The system is based on a piezoresistive sensor which measures the pressure exerted by the clinician on the syringe plunger during the procedure. The resistance of the sensor is converted in voltage by a Wheatstone bridge, then is recorded and visualized on a laptop. The aim of this study is twofold: i) to assess the influence of the sensor's active area on the ESMT response; ii) to assess the influence of the resistance value of the Wheatstone bridge on the ESMT response. At this scope, two sensors with different circular area and Wheatstone bridge with three different resistance values were used. The influence of the two mentioned parameters on the ESMT response was investigated by applying known force in a wide range of values. These experiments were performed considering two different configurations: i) the first set of tests was carried out with a 3 mm-thick cylinder of silicon material (taken from an upper limb of cosmetic prosthetic gloves) interposed between the indenter and the sensor; ii) The second set of tests was carried out by placing the thumb of a volunteer between the indenter and the sensor. Also, the sensor is placed on a syringe plunger used during epidural puncture. This configuration is very close to the clinical settings (force applied to the whole area of the plunger by the thumb). As expected, both the sensor model and the Wheatstone bridge resistance significantly influence the metrological characteristics of the proposed system.