Epidural Catheter Removal in Patients on Warfarin Thromboprophylaxis: A More Cautious Interpretation of Results Required?

To the Editor: P recise dexterity is an essential technical skill to properly align and track the block needle when performing ultrasoundguided regional anesthesia. This skill is most effectively acquired through hands-on practice. Several training phantoms have been developed to shorten the learning curve and provide learners with the opportunity to refine their needle guidance skills. However, most phantoms do not provide feedback for learners to help distinguish whether the needle tip or the shaft is contacting the target. We have developed a simple and economical phantom that allows learners to practice maneuvering the needle tip under ultrasound guidance. This model uses a common commercial nerve stimulator to provide immediate feedback when the tipVnot the needle shaftVcomes in contact with the target wire. The phantom on its own is an incomplete electrical circuit made up of metal wire embedded in a liquid gel. When the tip of the needle comes in contact with the target wire, the circuit is complete. This triggers a signal that lets the learner know that they have contacted their target with the appropriate part of the needle. To construct this phantom (Fig. 1), we obtained a heat-resistant glass container and suspended a metal wire, formed into the appropriate shape, in an empty container with one end of the wire positioned outside the area of the container. Melted gel (Gel Wax; Yaley Enterprises, Redding, Calif) colored with dye (Blue Liquid Candle Dye; Yaley Enterprises) to achieve the desired opacity was poured into the container to cover the suspended wire, leaving one end of the wire exposed. The ground electrode was then connected to the exposed wire while the insulated needle (Sonoplex Needle; Pajunk Medizintechnologie, Geisingen, Germany) was connected to the nerve stimulator (Stimuplex HNS 12; B-Braun, Melsugen, Germany). This system uses a similar setup to that of the nerve stimulator used in practice. Using ultrasound visualization, the embedded metal appears predominantly hyperechoic, round in short axis, and cable-like in long axis (Fig. 2). The target size can be varied by changing the diameter of the wire, providing a challenge appropriate to the skill level of the learner. This phantom can then be seated with a lid and stored for an extended period. This nonorganic phantom has the following advantages over other training models3,4: (1) it is affordable and simple to maintain; (2) after repeated needle punctures, needle-track artifacts can be easily removed by remelting the phantom in any conventional oven; (3) the gel can be dyed to a desired opacity to allow the learner to visualize the target under different conditions; and (4) there is immediate feedback for the learner when the needle tip touches the target, which is independent from the ultrasound image. However, the model has disadvantages, one of which is that the gel medium does not accurately represent human tissue because of different echogenic properties when visualized using ultrasound. This may result in the needle appearing to be of different brightness in the gel when compared with actual human tissue. Nevertheless, the described setup, using metal wire in conjunction with a nerve stimulator, can be inserted into other tissue types, such as pork, to provide a more realistic simulation.