Dielectric properties of healthy and chronically infarcted myocardium

Preliminary studies have shown that tissue resistivity may be useful in determining areas of chronically infarcted myocardiurn However, to date, no formal studies have been performed to examine the dielectric properties of chronic infarct. In this study, we measure and report the dielectric properties of healthy tissue and chronic infarct. This is done by measuring the complex admittance in vitro and in vivo using the capacitor cell and four-electrode techniques. We then determine how structural differences between healthy myocardium and chronic infarct affect the dielectric measurements of the two substrates. This is done by assessing the gross histology of healthy myocardium, chronic infarct, and of the transitional region in between. Our results show that the dielectric properties of healthy myocardium and chronic infarct are substantially different at low frequencies. The β-dispersion is prominent in healthy myocardium, but is small for chronically infarcted tissue. This observation is explained by the absence of cellular membranes in the fibrous matrix of infarcted tissue. Our results also show a gradual transition in the resistivity across the visible border between healthy and chronically infarcted tissue, starting 4mm in healthy myocardium and progressing to 2mm in the densely infarcted myocardium. This gradual change in tissue resistivity corresponds to a gradual change in tissue substrate and is highly correlated with the myocyte, and collagen content of cells. Although the data shows significant differences between the measured resistivity of healthy myocardium in vivo and in vitro, these differences were not seen in chronic infarct. We believe that the high collagen content of dense infarct renders it physiologically inert and therefore insensitive to the tissue preparation. We demonstrate this hypothesis by analyzing our data using well-established mixture models. The results of this analysis indicate that the majority of the current resides in the extracellular space when making dielectric measurements on chronically infarcted tissue