Myocardial impedance in the transplanted beating heart

Measurements of the electrical intramyocardial impedance for alternating current of different frequency can describe variations of intra- and extracellular space (ICS, ECS) and the functional state of cell membranes. The purpose of this study was to determine which cell compartment of the myocardium is most affected during allograft rejection. Furthermore we investigated, whether the rejection-induced inflammatory changes of the myocardium during acute allograft rejection can be diagnosed from alterations of electrical intramyocardial impedance. 12 beagle dogs underwent heterotopic heart transplantation and were immunosuppressed with cyclosporine and methylpred-nisolone. Electrical impedance was determined twice daily with the impulse method via screw-in electrodes. Transmural biopsies were performed under sufficient immunosuppression and after reduction of immunosuppression with consecutive Grade 1 (according to the grading system of the International Society for Heart and Lung Transplantation) and Grade 3 rejection. The animals were killed when mechanical function of the heterotopic heart deteriorated. All hearts showed a uniform impedance decrease of the ICS and the cell membranes immediately after implantation, reaching a stable plateau after 4 to 5 days. Impedance values then remained unchanged as long as rejection was absent. Biopsy findings of Grade 1 or Grade 3 rejection were reflected by statistically significant impedance increases of the ICS and the cell membrane of 13.2 ± 3.4% (Grade 1 rejection) and 27.6 ± 6.8% (Grade 3 rejection). Sensitivity and specificity were 100%. The impedance changes of the ECS were less pronounced. Three to 4 days after implantation we found a stable plateau, but the increase of 3.8 ± 1.6% in the course of Grade 1 rejection and of 8.27 ± 2.67% during Grade 3 rejection were statistically not significant. Impedance changes of the myocardium during rejection episodes are predominantly caused by chan- - ges in the ICS and the cell membrane. This seems to be the most specific and sensitive electrophysiologic parameter for rejection, precisely showing the inflammatory changes of the myocardium. The ECS does hardly influence the impedance during rejection episodes. This indicates that extracellular edema has no major influence on electrophysiologic changes during rejection.