In-vivo measurements of heart ischemia using transoesophageal electrical impedance

Previous studies have proved that a heart infarction produces an electrical impedance change in the ischemic area that could be measured with four electrodes inserted in the tissue. Our objective is to evaluate the feasibility of detecting the changes of the ischemic area with a less invasive technique based on a esophageal probe. A further objective is to demonstrate the capability of this technique to detect heart graft rejection in humans to avoid repetitive biopsies. Impedance measurement is performed using a four electrode method. Current is injected between the distal electrode of the esophageal probe and one pregeled electrode on the thorax. Voltage measurement is taken between the proximal electrode of the probe and another electrode on the thorax. We used a custom system based on an optically isolated patient interface that acquires two ECG channels (standard I derivation and a unipolar esophageal derivation) and electrical impedance. After isolation, the raw impedance signals are digitalized and processed to calculate impedance magnitude and phase angle. Impedance measurements are taken between 10 kHz and 1 MHz. The ischemia was produced in animals using a balloon inflated in the Left Anterior Descending artery. We compared different probes in animals and selected the one with the greatest stability, showing lower electrode impedance and fewer motion artifacts due to esophagus contractions. The resolution of the designed system is not enough to detect the changes produced by ischemia on the tissue impedance when the ischemic area is small, however the death of the animal produces a significant change in the measured impedance. Nevertheless, the detection of a global change of the myocardial tissue, like the one produced by a heart rejection in transplanted patients, could not be discarded