ELECTRICAL IMPEDANCE PHASE SPECTROGRAM OF THE ISCHEMIC MYOCARDIUM IN PIGS

Authors:Bragós, R., Riu P.J., Casas O., Rosell J. , Warren M., Tresànchez M., Carreño A. and Cinca J.

Journal: Medical & Biological Engineering & ComputingVolume 35 Supplement Part 1, p.335

Conf. title:World Congress on Medical Physics and Biomedical Engineering, Nice, France, September 14-19,1997

Abstract: The electrical impedance of the myocardium shows large variations during the first hour of acute ischemia. This effect has been related to several pathological circumstances, i.e. cell swelling and the closing of gap-junctions [1]. Most of the published measurements have been performed at a single frequency (1 kHz) and display an increase of the impedance magnitude and a decrease of its phase angle. The measurement of the impedance spectrum (1 kHz-1MHz) allows the identification of changes in two impedance relaxation mechanisms, the first with a central frequency around 300 kHz and the second around 10 kHz. The latter becomes appreciable after the first 25 minutes of ischemia [2]. There is a need of a simple graphical representation that synthesizes the large amount of data acquired. Methods: In-vivo measurements were made in six open chest pigs anesthetized with (-chloralose and artificially ventilated. The infarction was produced by ligation of the left anterior descending coronary artery. The myocardium impedance of the ischemic area was measured with a four platinum needle electrode array connected to a HP4192A impedance analyzer through a remote front end. Post-mortem ischemia in the non-infarcted area was also measured. Results: All subjects showed an increase of the 300 kHz relaxation from the beginning of the ischemia. After 25-30 minutes, the second relaxation at 7-10 kHz appears and becomes dominant. After 60-80 minutes, the impedance reaches a steady state and, in some cases, begins a slow decrease. The representation of the impedance spectrum in magnitude and phase vs. frequency or in the complex plane gives multiple plot graphs, with criss-crossed curves that are difficult to interpret. By comparing the alternatives, we have found that the phase spectrogram (time in the x-axis, log-frequency in the y axis and phase represented with a color scale) is a useful representation that synthesizes in a single graph the frequency dependence, the intensity and the time course of the impedance changes. Conclusion: The phase spectrogram allows the identification at a glance of the changes produced in the electrical impedance spectrum of the myocardium during acute ischemia. [1] H. Linhart et al. "The electrical impedance of the ischemic heart at low frequencies". IX ICEBI, Heidelberg, 1995. [2] R. Bragós et al. "Changes in myocardial impedance spectrum during acute ischemia in the in-situ pig-heart". 18th EMBS, Amsterdam, 1996.