14. BLOOD PRESSURE MEASUREMENT USING CO-EXTRUDED CATHETER

A. Problem/Issue

An Implantable Telemetry Company was developing a smaller device that measured blood pressure by inserting a tiny tube inside a blood vessel. A pulse was transmitted up the fluid inside the tubing and sensed inside the electronic enclosure. They were having difficulties getting a small enough catheter that would keep a viable signal until it reached the sensing unit.

B. Analysis

I joined the development team on a regular meeting schedule to ascertain their needs. We developed a design criteria based on various design inputs from team members. Ongoing tests indicated that the tubing was too soft to transmit a good signal. Increasing the durometer to help this situation encountered kinking problems that would stop the signal altogether. I began to research possibilities from extrusion companies that were knowledgeable with medical tubing in a very small diameter. We found that tubing of this size was very difficult to do and the implantable urethanes exhibit "gel" inconsistencies that further complicate the extrusion process.

C. Solution/Results

We researched many medical extrusion companies to find one that would give it a try. I wanted a tubing that would have an interior hard enough to pass an adequate signal and a softer exterior to support the harder core and be resistant to kinking. We wanted to try a .007" ID X .014" OD tubing with 1.5 mils of 70D on the inside and 2 mils of 55D on the outside. One company agreed to try it on a "best effort" basis. I flew out and assisted as they extruded the material. The concentricity was slightly out of spec, but it was not important to the client. The new tubing worked extrememly well, and the client company decided to patent the application. I signed as the co-patenter and the company continues to produce cutting-edge blood pressure measuring units using the same method of co-extrusion.