Voler developed a circuit to measure the impedance of tissue inside the heart to determine which part of the tissue is infarcted (dead) for use in a prototype cardiac catheter. We worked with a medical device start-up to implement an innovative method to reliably detect the minute differences between healthy and infarcted heart tissue.
Our design was able to deal with the normal variations in measurement between patients and to distinguish between blood, healthy tissue, and infarcted tissue. It represents a significant advancement in diagnostic accuracy of cardiac catheterization for detecting infarcted heart muscle.
“This lays the groundwork for better strategies in identifying damage from heart attacks, and for the development of new ways of optimizing interventions. We are obviously awaiting the results of the animal trials,” says Walt Maclay, President of Voler.
Although Voler has been part of the design team for catheters in the past, this was the first time we had been asked to measure impedance using a catheter. In the heart chamber this is a complicated measurement, because the probe is immersed in blood–which has its own impedance–and the device needs to give the cardiologist feedback when it is in contact with muscle tissue that has lower impedance than blood. We also had to design a stable quality source of AC signal that fit within budget and within a small space.The need to fit the detector with its multiple output wires into a very thin catheter made the design constraints even more challenging, as did the need to work from battery power.
This project required a considerable amount of analog design expertise. The analog electronics include an oscillator, a detector that does a rectification, several amplifiers,and filters. The catheter contained multiple extremely tiny contacts, which added a high resistance connection to the tissue or blood that complicated our ability to measure impedance. The biggest challenge was determining the range of impedance of blood, healthy tissue, and infarcted tissue and adjusting for the impedance of the probe to find the right measurement range. Because a single battery powered it we only had two to three volts to work with. For the data acquisition and display we were able to use off-the-shelf software on a standard PC for the animal trials. In the future this will be embedded in an on-board microprocessor.