It has been suggested that electrical bio-impedance can be a good marker of cell viability. The changes in bio-impedance of ischemic liver tissue, in vivo and ex vivo, at room temperature have been quantified (references). An increase in bio-impedance for all frequencies during the first two hours after removal of liver has been detected (Hemmilich). The changes are stronger at low frequencies (+180% at 1khz). This increase has been attributed to a decrease of extra-cellular space, cell swelling and closure of gap junctions (Hae 02, Konishi 95). After two hours, the bio-impedance decrease, probably owing to membrane breakdown.
Electric bio-impedance is defined as the electrical resistance offered by a biological material to conduct the electricity. The tissue is composed by the cells (cytosol surrounded by plasma membrane) and extra-cellular medium, which is also an ionic solution. In electrical terms, the plasma membrane, as a lipid by-layer membrane, has a capacitive behaviour. Therefore, an electric current will cross easier the extra-cellular medium than a cell, being the resistance through the cell higher. Under ischemic conditions the cells become swollen (by edema), and therefore the space occupied by cells, that means cytosol plus PM, increases with respect to the extra-cellular medium. Hence, the impedance should increase after ischemia.
There are few investigations into cold preservation of liver (2-4 degrees C with preservation solution). Some researches on rats show significant differences in bio-impedance evolution for several temperatures (Koisichi 95). Also, a stable behaviour of impedance after 6 hours of cold preservation (4 degreesC) has been detected (Raicu 00).
However to date there is not a commercially available effective system to monitor the impedance. Therefore, we have been developing recently a sensor, an instrumentation system that is suited to our experimental conditions and viable for future clinical application.
Furthermore, the first objective is to demonstrate that bio-impedance is able to follow the changes in the quality of an organ for transplant. To this end, livers were well procured and preserved, well procured and badly preserved or badly procured and preserved, and the impedance was monitored during the whole preservation period. In this way we induced increasing degree of lesion in the livers.
The results of the present study confirm this hypothesis. In all the livers the impedance increased progressively during preservation. Moreover, the impedance increased much more as more badly the livers were procured and preserved. Therefore, the most injured livers showed the highest impedance values during preservation.
On the other hand, ATP and EC levels as indicators of the cellular energy status also demonstrated differences between the study groups. Energy Charge gradually decreased during the preservation period, but well-preserved livers showed always-higher EC levels. In contrast UA, which is the final metabolite in the degradation pathway of ATP, accumulated more in the worst livers.
In addition XDH-XO is an important mechanism inducing ischemic lesion via superoxide radical production. This enzyme catalysis the production of xanthine from hypoxanthine and so the ratio Xanthine/hypoxanthine is an index of this activity. We previously have demonstrated that this index correlates with the viability of the liver graft and how the xanthine levels after warm ischemia also indicates the degree of ischemic lesion. Accordingly in the current study, the ratio X/HX was significantly higher in the worst livers during preservation.
Taking into account all these lesion parameters, if the initial cause of edema is the membrane imbalance promoted by ATP and EC depletion, they would inversely correlate with the degree of edema and to some extent with the raise of impedance. Effectively in the present study, the energy charge content inversely correlated with impedance. Therefore as more depleted is EC (ATP), more edema is produced and as a consequence impedance goes up. It suggests that at the end impedance measurement is an effective marker of the degree of injury, and more specifically it reflects the degree of edema as the cause of edema by ischemia correlates with impedance increase.
The electrical impedance techniques allow an early detection of irreversible tissue changes. Therefore, the method might be used in the future to monitor donor tissues before organ transplantation.