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Instrumentation and ultrasound imaging for epidural anesthesia Hor, King Wei

Abstract

The loss-of-resistance technique in epidural anesthesia is the accepted standard for indicating the entry of the needle into the epidural space. In conventional epidurals, it is also the only feedback mechanism to confirm needle entry. Unsuccessful epidurals due to the technical difficulties can result in mild to severe complications. These difficulties include correctly choosing the puncture site and needle trajectory, which are determined solely by palpation and the experience of the anesthesiologist. Instrumentation of the thumb's force on the plunger of the syringe, displacement of the plunger and fluid pressure is developed for laboratory and clinical trials to study the dynamics of the loss-of- resistance technique. Instrumentation of the loss-of-resistance technique was performed on culled domestic pigs using standard epidural procedures. A static and decay model, based on physical properties and empirical data, are used for estimating the pressure from the force and displacement values. The decay model is shown to be reasonably accurate and allows the omission of the pressure sensor in clinical trials. Furthermore, the accuracy of decay model is further improved for the "smooth" protocol performed by the anesthesiologist, over the "bouncing" protocol. The loss-of-resistance, indicated orally by the anesthesiologist, is consistent with the rapid fall in all three measurements. The oral indication of the loss-of-resistance slightly lags that of the measured values and is consistent with the lag in oral communication. The instrumentation of the loss-of-resistance is further confirmed by direct and indirect measurements from ultrasound images of the epidural space and needle. However, obtaining good image quality is difficult due to the steep needle angle and the surrounding bone structures. An adaptive spatial compounding algorithm is developed to improve important features such as the bone and epidural space. A specially constructed phantom with speed-of-sound distortion is used to compare several variations of the algorithm. The adaptive spatial compounding using median-based averaging produced image quality with the best balance for point resolution, edge resolution and noise reduction in homogeneous regions. In porcine studies, the technique shows visible improvements of the epidural space and surrounding features.

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