Force and pressure feedback during epidural needle insertion in the ligamentum flavum of piglets.

Abstract

Epidural anesthesia is a commonly used anesthesia method during surgery or giving birth to a child. During the insertion of the needle in the back of the patient, the anesthetist has to interpret the different forces which requires extensive skills. To obtain these skills the residents train on patients, which is not without risk. Due to a lot of variation in anatomy of the human body, the identification of the tissues can be very difficult. An epidural anesthesia simulator can reduce this risk by offering anesthetists the opportunity to experience the differences in force and pressure feedback of the different tissues before moving on to patients. In literature only very little is available about needle-tissue interation around the epidural space. To collect data about tissue resistance during the insertion of an epidural needle, tests were done on living piglets. These animal tests were done at Erasmus MC, Rotterdam. During needle insertion, the position of the needle was captured along with the force on the needle and the pressure in the needle. Pressure measurements were used for the epidural space identification. The most important force feedback during the insertion is a force-peak which occurs by puncturing the ligamentum flavum. At the moment the needle tip enters the epidural space, a pressure-drop in the needle occurs which is the most important pressure feedback. The measured force-peak with a 18G epidural needle can be described by an lognormal distribution and lies in the 68.3% confidence interval of 5.45 till 11.03 N and has an geometric mean of 7.754 N. The distance between the force-peak event and the pressure-drop event can be described with a normal distribution with a mean of 1.17 mm (s=1.217), where a positive value means that the force event occurs before the pressure-drop. The experimental results showed that there is no systematic difference in force peaks between piglets and neither between vertebrea. The measured pressure data showed that the pressure gradient is time-dependent when using a constant flow of liquid. Since it is unknown if an eventually ’tissue absorption’ ratio of the ligamentum flavum is velocity dependent, no valid statements can be done based on this data. Another observation was that the force-peaks measured in deceased tissue (1-5h) are in the same range as the force-peaks in living tissue.