The Unmet Need
The early and most current CPNB devices are a modification of the epidural catheter placement method which involves insertion of a large bore needle adjacent to a targeted nerve. A smaller gauge catheter is then advanced through the needle until the catheter exits the needle to reside near the nerve. The needle is then withdrawn while simultaneously advancing the catheter all the while trying to keep the catheter in the same location. The risk of pulling the catheter out along with the needle as it is being withdrawn is a crucial skill to learn and requires the use of two hands to perform.
The original devices used a stimulating needle to locate the nerve where an injection of fluid through the needle blindly generates a pocket of fluid near the nerve for the catheter to reside. The next generation of devices used both a stimulating needle and a stimulating catheter to verify location of the tip of the catheter close to the nerve. These devices always require a large setup with sterile drapes, probe covers, and gloves for the operator because the catheters are very long (need to be twice as long as the needle to be able to withdraw the needle without dislodging the catheter). These longer catheters have to be handled so the operator must have sterile gloves on both hands which mandate the need for an assistant. The assistant is required to operate the nerve stimulator, provide medications, as well as accessories onto the sterile field.
Since the introduction of ultrasound guidance, the use of ultrasound has become the standard of care for nerve block procedures. In order to use an ultrasound machine for imaging, the operator must dedicate one of their hands to control the ultrasound probe. This leaves only one hand to perform the procedure if one wants to maintain constant imaging of the needle and /or catheter to improve efficiency, effectiveness, and reduce time. The only way to use the epidural style of catheter placement with ultrasound imaging is to put the probe down to advance the catheter/withdraw the needle. The image must be re-obtained to see if the catheter is still adjacent to the nerve. This is very difficult at times because once the rigid needle conduit is removed, the flexible catheter often is contorted out of a perfectly linear orientation due to the surrounding tissue. Also there is high probability of moving the catheter away from the nerve as the needle is withdrawn - the catheter often pulls back with the needle.
The ultrasound beam is less than 1 millimeter in width and visualizing the tip or the length of the small bore catheter is technically difficult if the catheter is not perfectly straight. The goal is to verify that the tip of the catheter is close enough to the nerve, and indirect methods are used to achieve this. It could involve injecting fluid to watch tissue movement or fluid dispersion around the nerve, or injecting air to see a hyperechoic ultrasound effect around the nerve, or using Doppler imaging to see where the flow from the catheter tip may be spreading. All of these maneuvers require an injection hub to be connected to the distal end of the catheter in order to inject. The hub can only be attached once the needle has been withdrawn.
Time for insertion is the eminent obstacle for CPNB programs. Surgeons do not want to feel that they are being delayed by an anesthesia procedure even if they take an additional hour to do the surgery. They will frequently rationalize pain pills over nerve catheters to save time. These are the real constraints that anesthesiologists face. Most people should expect a CPNB procedure with these tradition devices to take on average 20 – 30 min at best from start to finish. This could prolong a typical surgical schedule by an additional 2 to 3 hours. In an era of efficiency, no program that adds hours to the day will survive.